# Thread: Myth Busted: 8" rotors on QR

1. ## Myth Busted: 8" rotors on QR

I have heard a lot on these forums about how it is unsafe to use 8" rotors on QR forks. This didnt make sense to me since the different between 7" and 8" is only 14.3% in terms of length. It didn't seem like a large increase to me.

I drew up this balance of forces (with gravity neglected and the assumption that the caliper is on the x-axis and the drop-out of the fork is on the y-axis) and it showed me that there isnt any force on the QR during braking that would confirm this myth.

It is shown that since the wheel and tire are basically a rigid body that the resultant force (moment) on the axle, which was supposedly forcing the wheel from the fork, is just distributed directly to the ground. This force is then balanced by the normal force of the ground pushing back on the tire. Not to mention that under braking circumstances the rider's weight is distributed forward to the fork which is clearly forcing the fork onto the axle which only helps the situation.

Under these circumstances it is clear that there is actually zero force on the QR/axle to leave its position in the drop-outs. Of course gravity is the only force freely pulling/pushing the QR/axle from the fork but this force would only magnify the normal force from the ground which would just equalize the forces.

This balance of forces is for the wheel being on the ground. I do not apply my brakes when I am in the air and am not interested in the force balance on that type of system.

2. OK. Now go out and test it yourself. Remove your front QR skewer entirely, get up to speed and grab a big handful brake. Be sure to have a buddy ready to call 911.

3. Originally Posted by shiggy
OK. Now go out and test it yourself. Remove your front QR skewer entirely, get up to speed and grab a big handful brake. Be sure to have a buddy ready to call 911.

Aside from it being a bit unsafe, why would I do an experiment when I already know the outcome? As long as I don't hit any large bumps, which would be a completely different situation, this analysis will stand true. Can't argue with the dynamics.

4. Originally Posted by psunuc
Aside from it being a bit unsafe, why would I do an experiment when I already know the outcome? As long as I don't hit any large bumps, which would be a completely different situation, this analysis will stand true. Can't argue with the dynamics.
If you are sure your figures are correct then you have nothing to lose by conducting a real-world test. Without the real-world test you do not know the results and have not busted the myth.

I say you are wrong. The axle will come out of the fork and toss you on your face.

5. Can't argue with the dynamics.
sure you can, when they're as wrong as yours are.

6. I'm new to disk brakes I didn't know there was a problem with running a 8in on the front with a QR hub. I was thinking about changing out my 160mm to a 203mm.

So this is not a good idea?

7. Haha, ok ferday, please show me a FBD (Free Body Diagram) which disproves mine. Aparently you have found flaws in my dynamics, I am interested in learning and seeing the flaws in my FBD.

8. Originally Posted by psunuc
I drew up this balance of forces (with gravity neglected and the assumption that the caliper is on the x-axis and the drop-out of the fork is on the y-axis) and it showed me that there isnt any force on the QR during braking that would confirm this myth.
This is where I stop reading.

psunuc, would you care to share with us your background in physics?

9. Interesting. I sort of agreed with your title but the rest of the post went off in another direction.

First the part I don't agree with:

The axle IS being forced out of the dropout. Actually, to be more acurate it is being forced in a direction downwards and perpendicular to a line drawn from the axle to the caliper. Whether or not that is the direction of the dropout depends on the fork and caliper type, but it is usually in the same general direction. Try this: Turn your bike upside down. Take out the qr. Step on the front brake lever. Now, grab the wheel and try to turn it in the forward direction. If the wheel does not actually come out it will at least be hitting up against the lawyer tabs.

Also, the downward force on the axle is the same as the upward force on the brake. If you don't believe this, try this: Do the same experiment as discribed above, but now tighten the qr and loosen the calliper bolts. This time you will see the caliper try to move in the opposite direction as the axle was trying to go. The two forces are the same.

The point here is that the axle and fork are being forced in two separate directions. With no other forces the two will go in opposite directions. So, what is keeping the two from actually parting ways if you take out the qr and lawyer tabs? A couple of things that you correctly mention. One is the ground pushing back against the wheel (an thus the axle). As you said, the wheel is not going anywhere, because no matter how much force is pushing down, the ground exerts the same force back.

However, the force on the caliper (and thus the rest of the bike, including the fork) is a different story. The only forces balancing the upward force from the rotor are normal gravity (which does not change) and the added front end loading due to weight being shifted forward. This is still just gravity, but this does change with braking forces. However the total of these two forces will never be more than the total weight of rider+bike.

Under light braking, the downward force from gravity will be greater than the upward force from the rotor. However, as the braking becomes harder and harder, the upward force on the caliper (and thus bike) can eventually become greater than the force of gravity on the bike, and the net direction of the caliper (and fork and bike) is up. The wheel stays on the ground where it is being forced, and the two part ways. What prevents this is the qr (or through axle). The point is that the qr DOES have a net force against it, once the force of gravity is overcome.

The point SORT of agree with you on is the myth of 8" rotors. They do not put more force on the axle. The downward force on the axle will be the same as the upward force on the caliper. Assuming you have the same caliper/braking system on the 6" and 8" rotor, the calipers will exert the same maximum force. In fact for a given amount of deceleration, I would argue that the caliper on an 8" rotor, do to the mechanical advantage of a longer lever arm needs to exert less force and thus less downward force in placed on the Axle. Anyone who doubts this, just try the above experiment with rim bakes (26" rotor) instead of discs.

However there IS difference between a 6" and 8" rotor in the maximum braking force available, and this puts rearward stess on the whole fork, especially the crown.

10. Originally Posted by collaborator
This is where I stop reading.

psunuc, would you care to share with us your background in physics?

I am a senior at Penn State University with a double major in Nuclear Engineering and Environmental Engineering. My physics background is best described by the courses I have taken and passed. Each entry is a different class I have taken that is heavily based on physics.

Calculus-based Physics: Mechanics
Calculus-based Physics: Electricity and Magnetism
Calculus-based Physics: Wave and Quantum Physics
Engineering Mechanics: Statics
Engineering Mechanics: Dynamics
Engineering Mechanics: Strength of Materials
Engineering Mechanics: Mechanical Responce of Materials
Mechanical Engineering: Thermodynamics
Mechanical Engineering: Fluid Flow
Mechanical Engineering: Heat and Mass Transfer
Nuclear Engineering: Reactor Physics
Nuclear Engineering: Reactor Design
Nuclear Engineering: Design Principles of Nuclear Reactors
Nuclear Engineering: Nuclear Reactor Core Design
Nuclear Engineering: (2 Labs in Penn State's Breazeale Nuclear Reactor)

11. ## And creationism is a valid theory

Look, I think you have some forces a bit off here. What you have modeled isn't geometrically correct either. Show me a set up where the tangent force is going parallel to leg. Doesn't exist. Second the reason most all manufacturers have some sort of clause about what rotor can be used on what fork has some to do with QR's slipping, but it's predominantly related to the casting of the dropout.
QR Dropout= less material
20mm dropout= more material
Not rocket science. The dropout picks up a shear force when the brake is acuated form the forward momentum of the bike and the angular momentum of the brake engaging. In simple terms, the hub no longer acts as the center of the wheel, the brake does. Think of it as becoming a pivot point. Now compare that to the pivot point of your cranks. the longer the crank arm the more leverage you have. Same applies at the moment the brake engages. The brake becomes the pivot point, the wheel still tries to turn, the dropout picks up the load, KA BLAMMO! Unless the dropout has been designed to cope with the load. Check out an 04 or earlier Fox fork and compare it to an 06. Dropout has been beefed up on the 05 and newer so that you can now use an 8" rotor where as pre 05 that was a big no-no. Compare the 06 Manitou R7 fork to the 06 Gold Label QR fork. R7 can't be used w/ the 8", Gold Label can. Again Material is the issue. It's material that is also the reason that fork makers are finally moving to a post mount standard rather than the IS. There's more meat over the bones to distribute the load of the brake, dig?

But enough salt, here's some sugar: I really like the fact that you put some thought into this and actually came up what you did. That is freaking killer! I haven't seen much of anyone try that route, so full on kudos! Just remember dynamic systems require wholistic thinking not just static force diagrams. Cheers!

12. ## Here is the flaw.

Originally Posted by psunuc
Haha, ok ferday, please show me a FBD (Free Body Diagram) which disproves mine. Aparently you have found flaws in my dynamics, I am interested in learning and seeing the flaws in my FBD.

Nothing really wrong with your diagram. But your interperetation is faulty. Fm and Fn will always cancel eachother out. I think this is what you are saying. However, what you are missing here is that Fs can, under hard braking overcome Frm, even though Frm becomes greater due to front end loading under braking. If you don't believe this, try Shiggy's suggestion.

There you go.

13. Originally Posted by psunuc
Haha, ok ferday, please show me a FBD (Free Body Diagram) which disproves mine. Aparently you have found flaws in my dynamics, I am interested in learning and seeing the flaws in my FBD.
Yes, your model is flawed. kapusta explains it pretty well in his reply. You include no forward/lateral momentum and no opposing lateral braking force. Add in that under hard braking the fulcrum point is where the brake pads grip the rotor and the force is applied to the "lever arm" at the tire contact point with the ground.

14. Originally Posted by psunuc
I am a senior at Penn State University with a double major in Nuclear Engineering and Environmental Engineering. My physics background is best described by the courses I have taken and passed. Each entry is a different class I have taken that is heavily based on physics.

Calculus-based Physics: Mechanics
Calculus-based Physics: Electricity and Magnetism
Calculus-based Physics: Wave and Quantum Physics
Engineering Mechanics: Statics
Engineering Mechanics: Dynamics
Engineering Mechanics: Strength of Materials
Engineering Mechanics: Mechanical Responce of Materials
Mechanical Engineering: Thermodynamics
Mechanical Engineering: Fluid Flow
Mechanical Engineering: Heat and Mass Transfer
Nuclear Engineering: Reactor Physics
Nuclear Engineering: Reactor Design
Nuclear Engineering: Design Principles of Nuclear Reactors
Nuclear Engineering: Nuclear Reactor Core Design
Nuclear Engineering: (2 Labs in Penn State's Breazeale Nuclear Reactor)
Well, it looks like you missed some lectures that apply to this subject.

15. Originally Posted by psunuc
I am a senior at Penn State University with a double major in Nuclear Engineering and Environmental Engineering. My physics background is best described by the courses I have taken and passed. Each entry is a different class I have taken that is heavily based on physics.

Calculus-based Physics: Mechanics
Calculus-based Physics: Electricity and Magnetism
Calculus-based Physics: Wave and Quantum Physics
Engineering Mechanics: Statics
Engineering Mechanics: Dynamics
Engineering Mechanics: Strength of Materials
Engineering Mechanics: Mechanical Responce of Materials
Mechanical Engineering: Thermodynamics
Mechanical Engineering: Fluid Flow
Mechanical Engineering: Heat and Mass Transfer
Nuclear Engineering: Reactor Physics
Nuclear Engineering: Reactor Design
Nuclear Engineering: Design Principles of Nuclear Reactors
Nuclear Engineering: Nuclear Reactor Core Design
Nuclear Engineering: (2 Labs in Penn State's Breazeale Nuclear Reactor)
13 engineering courses and you don't give us a computer rendered diagram?

17. Love it! All the bike geeks are running out of the woods! Loooooove it!

18. That is right.

But, I might add that the angle of the dropouts, does also provide a resisting force to the applied braking moment.... I ain't doing it. (I have when my QR lossened off a couple of times on a hill, although not completely out.)

But back to one part of the original question, I though that oh 90% of the axle load would be taken by the axle stubs rather than the quick release skewer?

I would think that the shock load of jumping a 3 foot ledge would equal or exceed the braking torque, (generally a relatively non shock load.) both held by axle stubs mostly.

19. shiggy, I can't be your physics teacher. I defined my system origin as the axle/QR and I included all forces that were significant, like momentum on this system would be represented by "rider momentum force" on an angle.

The only difference in your picture is that you included the force of friction is on the x-axis which has no bearing on the wheel coming out on the y-axis. Again, a negligible force to be honest. You say I didnt include anything for braking force, that makes it all too clear that you dont understand what a moment really is.

20. ## Impressive credentials but....

Originally Posted by psunuc
I am a senior at Penn State University with a double major in Nuclear Engineering and Environmental Engineering. My physics background is best described by the courses I have taken and passed. Each entry is a different class I have taken that is heavily based on physics.

Calculus-based Physics: Mechanics
Calculus-based Physics: Electricity and Magnetism
Calculus-based Physics: Wave and Quantum Physics
Engineering Mechanics: Statics
Engineering Mechanics: Dynamics
Engineering Mechanics: Strength of Materials
Engineering Mechanics: Mechanical Responce of Materials
Mechanical Engineering: Thermodynamics
Mechanical Engineering: Fluid Flow
Mechanical Engineering: Heat and Mass Transfer
Nuclear Engineering: Reactor Physics
Nuclear Engineering: Reactor Design
Nuclear Engineering: Design Principles of Nuclear Reactors
Nuclear Engineering: Nuclear Reactor Core Design
Nuclear Engineering: (2 Labs in Penn State's Breazeale Nuclear Reactor)
To put it in perspective, somewhere out there is an MD who is diagnosing illnesses and treating patients, that graduated at the bottom of his class. In addition to this, even the most brilliant surgeon will make a mistake.

Bob

21. You blew it, the rider momentum force is parellel to the ground... well unless you are taking a dive.... or bunny hoping etc.

The forks are in bending as well as compression, if appyling decceleration through the front wheel.

22. Psunuc,
You forgot to figure in that when you apply the brake, you create a moment at the dropout. Correct that in your free body diagram and I bet you'll see the error.

Everybody else, be nice! Cripes, this is a forum after all. No need for attacks and defensiveness. That's not the way he treated us coming into this (at least I don't recall him calling us a bunch of uneducated neandertals who are yto understand fire), and that's not the way we should act when we disagree (what is this, FOX News?) with him. Reason, people, REASON. Be it and show it.

And I was an engineering student at one point (mechanical). Decided I liked being a mechanic. Go figure.

23. ## The Bottom Line

I am not going to argue about my forces analysis because I am 99% sure it is correct. To confirm my FBD, I scanned and emailed it to 4 of my colleagues and they all agreed that it was correct. They also agreed that the forces which I have neglected (drag, friction, etc) are negligible in this case which only confirms my FBD.

After reading the responces to this thread it is clear to see who has a physics or engineering background. I am not this threads physics teacher. If you dont understand why this is correct then that is your own problem, read a physics book.

I thought it would be interesting to do an analysis on this and my results are as posted. They have been confirmed by 5 PSU senior engineering students. I stand by my results and analysis.

24. Originally Posted by psunuc
shiggy, I can't be your physics teacher. I defined my system origin as the axle/QR and I included all forces that were significant, like momentum on this system would be represented by "rider momentum force" on an angle.
The only difference in your picture is that you included the force of friction is on the x-axis which has no bearing on the wheel coming out on the y-axis. Again, a negligible force to be honest. You say I didnt include anything for braking force, that makes it all too clear that you dont understand what a moment really is.

arrgh.

the axle is NOT on a vertical axis. forks have angles and so do dropouts. you even drew your picture with the forces at angles, then you explain to shiggy that the FRM acts on an angle, then you proceed to pretend the axle force is purely vertical.

second, the braking force at the caliper points DOWN. if it pointed up the wheel speed would increase. that right there completely invalidates your stupid diagram.

if you actually did real, proper math, you'd find that the total braking force works out to point slightly backwards and downwards.

25. Originally Posted by juancancook
Look, I think you have some forces a bit off here. What you have modeled isn't geometrically correct either. Show me a set up where the tangent force is going parallel to leg. Doesn't exist. Second the reason most all manufacturers have some sort of clause about what rotor can be used on what fork has some to do with QR's slipping, but it's predominantly related to the casting of the dropout.
QR Dropout= less material
20mm dropout= more material
Not rocket science. The dropout picks up a shear force when the brake is acuated form the forward momentum of the bike and the angular momentum of the brake engaging. In simple terms, the hub no longer acts as the center of the wheel, the brake does. Think of it as becoming a pivot point. Now compare that to the pivot point of your cranks. the longer the crank arm the more leverage you have. Same applies at the moment the brake engages. The brake becomes the pivot point, the wheel still tries to turn, the dropout picks up the load, KA BLAMMO! Unless the dropout has been designed to cope with the load. Check out an 04 or earlier Fox fork and compare it to an 06. Dropout has been beefed up on the 05 and newer so that you can now use an 8" rotor where as pre 05 that was a big no-no. Compare the 06 Manitou R7 fork to the 06 Gold Label QR fork. R7 can't be used w/ the 8", Gold Label can. Again Material is the issue. It's material that is also the reason that fork makers are finally moving to a post mount standard rather than the IS. There's more meat over the bones to distribute the load of the brake, dig?

But enough salt, here's some sugar: I really like the fact that you put some thought into this and actually came up what you did. That is freaking killer! I haven't seen much of anyone try that route, so full on kudos! Just remember dynamic systems require wholistic thinking not just static force diagrams. Cheers!
Yeah, the QR slot is at a different angle now too on some Manitou forks, rather than vertical like most QRs used to be, so it would be harder for the skewer to rotate out under braking.

26. Originally Posted by psunuc
shiggy, I can't be your physics teacher. I defined my system origin as the axle/QR and I included all forces that were significant, like momentum on this system would be represented by "rider momentum force" on an angle.

The only difference in your picture is that you included the force of friction is on the x-axis which has no bearing on the wheel coming out on the y-axis. Again, a negligible force to be honest. You say I didnt include anything for braking force, that makes it all too clear that you dont understand what a moment really is.
I kept my drawing simple. My contention is your model is wrong. It should not be based around the axle. Put in the wrong data and the result can not be right.

I have SEEN wheels come out of forks under hard braking. I have had wheels move in frame and fork because of braking forces.

Since you are so positive your figures are correct you have nothing to lose by doing a real-world test. All I ask is you have witnesses and do progressively harder stops. Do it. Take videos.

27. Originally Posted by psunuc
.......why would I do an experiment when I already know the outcome?
Whoah! I am in a PhD program in one of the sciences, and I would be shown the door if I ever said that!

Calculations are a wonderful tool, but EXPERIMENTS are where the proof of the pudding is. What you have is a lot of knowlege, but you will find that everything does not always work out in the real world the same way it does on paper. Every failed design by people with for more experience than you is proof of this. Why do you think designs are TESTED? Because calculations are only as good as the assumptions you are making when doing them.

Also, it is not up to anyone to disprove your calculations, it is up to YOU to provide proof. Shiggy suggested an experiment. THAT is where the proof is, NOT in your calculations.

28. Sorry you blew it and so did all your buddies.

What needs to be corrected is the rider momentum force

What needs to be added is the angle of the dropout slot.

29. Originally Posted by psunuc
I am not going to argue about my forces analysis because I am 99% sure it is correct. To confirm my FBD, I scanned and emailed it to 4 of my colleagues and they all agreed that it was correct. They also agreed that the forces which I have neglected (drag, friction, etc) are negligible in this case which only confirms my FBD.

After reading the responces to this thread it is clear to see who has a physics or engineering background. I am not this threads physics teacher. If you dont understand why this is correct then that is your own problem, read a physics book.

I thought it would be interesting to do an analysis on this and my results are as posted. They have been confirmed by 5 PSU senior engineering students. I stand by my results and analysis.
I clearly explained to you what is wrong with your calculation. Namely, that you are assuming the downward force of gravity on the front end of the bike will always be greater than the upward force on the rotor (and thus fork). I noticed that mine is one of the only posts you have not reponded to.

Anwer this: The combined weight of me and my bike is about 220 lbs. I'll let you figure that out in Newtons. Show me where you have calculated what the maximum possible upward force on a rotor is (I believe that is Fs in your diagram), and that for me it would always be less than 220 lbs. Until then, you are blowing air.

I stand by my analysis.

30. How about addressing the issue of flex and areas in which stress is concentrated?

31. Originally Posted by kapusta
I clearly explained to you what is wrong with your calculation. Namely, that you are assuming the downward force of gravity on the front end of the bike will always be greater than the upward force on the rotor (and thus fork). I noticed that mine is one of the only posts you have not reponded to.

Anwer this: The combined weight of me and my bike is about 220 lbs. I'll let you figure that out in Newtons. Show me where you have calculated what the maximum possible upward force on a rotor is (I believe that is Fs in your diagram), and that for me it would always be less than 220 lbs. Until then, you are blowing air.

I stand by my analysis.
See, this is a perfect example of someone who does understand this. I'm going to try to make this as easy as possible. The force downward away from the fork doesnt matter because that very force downward from braking (on the y-axis as I explained) is transferred right into the ground through the rigid body (aka the wheel). Now the ground pushes back with normal force to equal that force. All of you are looking at the situation without including the fact that the force on the axle from braking is transferred through the rigid body wheel to the ground thus no net force from braking to make the axle push downward. THE BRAKING FORCE IS ULTIMATELY TRANSFERRED TO THE GROUND BECAUSE THE WHEEL AND AXLE ARE A RIGID BODY SYSTEM. THE GROUND PUSHES BACK IN AN EQUAL AND OPPOSITE FORCE.

These concepts are not that difficult.

On this thread that follow, does what Johnny Hair Boy says seem odd to most of you considering not everyone puts the same clamping force on their skewers?
8" & my setup?

Also, don't you think the engineers at Specialized did the same analysis I did before they put out bikes with 8" front rotors with QR forks?

32. ## This is hilarious.

what the OP is suggesting may open him up to some serious legal issues.

I am not a Physics major, or student. I do not need to be, to KNOW that your analisys is WRONG. there is NO WAY that a rotor is not going to pull out of the dropout if there is no QR in place.

If you are that sure, I call you (and your colleagues) out on this. I say you are wrong, and request that you do a real world test prior to going any further down this path. Please, wear a full face helmet, and remember to keep your arms tucked in.

What you are suggesting is a pile of crap. Period.

ps, I wonder what Brian from Brake Therapy would say about all this.

Find out for yourself.

I have a cousin that is a genius, going to work for Mac Douglas and NASA straight out of college. He has no real world common sense. (and no social skills. He saw me dealing with my newborn, and related it to Goodall's Gorilla's in the mist) Are you the same way? I am not trying to be mean. just trying to understand. (if you are, you probably do not know, and may need someone to point it out.)

33. ## History lesson

Sometime in the past, when discs were first getting specced on bike and folks started throwing themselves off everything they could, a flaw was found. Seems in some cases frames were getting snapped across the axis of the seat & chain stays in front of the disc mount. What, if not the brake, would have caused this, psunuc? Where did this shear force originate? Why did, after sometime, frame makers start installing braces between the chain and seat stays or reorienting the disc mount all together, rotating the mount clockwise when looking at the left side of the bike, or raising the dropout and seat stay joint above the caliper?
So that instead of a shear force across both stays, the seat stay would become subjected to a compression force and the system better able to handle the load. It's the reason you see sooo many bikes around that have the 22mm caliper mount on the chainstay, 7 years later. And for the record I have seen a rear wheel reefed out of a dropout with a 6"rotor. Check your QR's kids!
If you forget about the angular momentum you create at the dropout when the bike is under load and the brake is engaged, your calcs will not simulate real world experience. It's a real force and as I've said in one other post, one I saw at Red Bull Bike Battle in Boston where a fork lost it's disc side dropout. 8" rotor.

Hey, any frame/fork/brake designers care to chime in? Could be silly fun to have someone with engineering experience from the industry chime in...

34. Originally Posted by psunuc
I am not going to argue about my forces analysis because I am 99% sure it is correct. To confirm my FBD, I scanned and emailed it to 4 of my colleagues and they all agreed that it was correct. They also agreed that the forces which I have neglected (drag, friction, etc) are negligible in this case which only confirms my FBD.

After reading the responces to this thread it is clear to see who has a physics or engineering background. I am not this threads physics teacher. If you dont understand why this is correct then that is your own problem, read a physics book.

I thought it would be interesting to do an analysis on this and my results are as posted. They have been confirmed by 5 PSU senior engineering students. I stand by my results and analysis.
You are digging quite a deep hole there, man. You really are wrong, so don't go getting nasty. A little knowledge CAN be a dangerous thing.

Please don't bring Penn State into the middle of this, it is a fine school.

Go put whatever size brakes you want on your bike, ride it on horizontal ground, and keep the front wheel in absolute contact with the ground at all times-you'll be fine. The real world of mountain biking has rutted out, bumpy descents that would further invalidate your argument.

35. Originally Posted by psunuc
See, this is a perfect example of someone who does understand this. I'm going to try to make this as easy as possible. The force downward away from the fork doesnt matter because that very force downward from braking (on the y-axis as I explained) is transferred right into the ground through the rigid body (aka the wheel). Now the ground pushes back with normal force to equal that force. All of you are looking at the situation without including the fact that the force on the axle from braking is transferred through the rigid body wheel to the ground thus no net force from braking to make the axle push downward. THE BRAKING FORCE IS ULTIMATELY TRANSFERRED TO THE GROUND BECAUSE THE WHEEL AND AXLE ARE A RIGID BODY SYSTEM. THE GROUND PUSHES BACK IN AN EQUAL AND OPPOSITE FORCE.
?
Cool, we are talking physics again.

First of all, what you just said is correct: The wheel cannot be force downward becuase the force pushing down will always be countered by the planet. But I want you to think about where this downward force is coming from. It is coming from the caliper exerting force downward on the rotor. Again, loosen the bolts and see which way the caliper moves. But the rotor is pushing back on the caliper with the identical amount of force in an upward direction (one of Newtons laws about action and reaction), and this is the part you are not addressing, or if you are I am missing it.

You are absolutlly correct in saying their is no net force downward on the axle in qr-less example. However, there is (or can be under heavy braking) a net upward force on the caliper. So, the axle does not drop from the fork, but rather the fork is lifted off the axle. Result is the same.

You have correctly adressed the forces on the axle/wheel. You need to look at the forces on the caliper without a qr or lawyer tab.

36. Please, if you can tell me what it is that I forgot or neglected then I will gladly accept that I am wrong. You are the one being nasty. On what basis did you prove me wrong in your head to validate saying I am way off?

Just PLEASE explain to me what I did wrong. Everyone on this thread can tell me I am wrong but no one has yet proved me wrong.

By the way, I was asked where my educational background came from and I answered. How did I bring down Penn State? I am representing them well in a small field for which they are nationally recongnized. What is it that you do for a living yates1012?

37. 14.3% longer in length...In computing forces, do you use the radius??? 3.5"r to 4"r ?

38. Magura engineers beg to disagree... They changed the angle at which the dropout is oriented, as Marzocchi and others have done as of late... all this for safety reasons.

As a note, Magura forks are rated up to 210mm rotor diameters. These guys have been making forks AND brakes for a long time.

I will not question your physics, neither your background. But there's a reason why those that can get actually SUED for not making calculations properly, changed the angle at which their dropouts are oriented.

Peace, bro.

39. Originally Posted by juancancook

Hey, any frame/fork/brake designers care to chime in? Could be silly fun to have someone with engineering experience from the industry chime in...
Randy from Hayes has recommended against it on a few occasions.

40. Originally Posted by psunuc
Please, if you can tell me what it is that I forgot or neglected then I will gladly accept that I am wrong. You are the one being nasty. On what basis did you prove me wrong in your head to validate saying I am way off?

Just PLEASE explain to me what I did wrong. Everyone on this thread can tell me I am wrong but no one has yet proved me wrong.

By the way, I was asked where my educational background came from and I answered. How did I bring down Penn State? I am representing them well in a small field for which they are nationally recongnized. What is it that you do for a living yates1012?
I think Yates just explained it pretty well. I'm not an engineer but I did stay at a Holiday Inn Express last night...

41. Originally Posted by kapusta
Cool, we are talking physics again.

First of all, what you just said is correct: The wheel cannot be force downward becuase the force pushing down will always be countered by the planet. But I want you to think about where this downward force is coming from. It is coming from the caliper exerting force downward on the rotor. Again, loosen the bolts and see which way the caliper moves. But the rotor is pushing back on the caliper with the identical amount of force in an upward direction (one of Newtons laws about action and rection), and this is the part you are not addressing, or if you are I am missing it.

You are absolutlly correct in saying their is no net force downward on the axle in qr-less example. However, there is (or can be under heavy braking) a net upward force on the caliper. So, the axle does not drop from the fork, but rather the fork is lifted off the axle. Result is the same.

You have correctly adressed the forces on the axle/wheel. You need to look at the forces on the caliper without a qr or lawyer tab.
Thank you, kap, this is what I have been trying to point out.

42. Watch and learn feallas,

The first few stops are with a QR the last ones are with out a QR.

43. Originally Posted by psunuc
Please, if you can tell me what it is that I forgot or neglected then I will gladly accept that I am wrong. You are the one being nasty. On what basis did you prove me wrong in your head to validate saying I am way off?

Just PLEASE explain to me what I did wrong. Everyone on this thread can tell me I am wrong but no one has yet proved me wrong.

By the way, I was asked where my educational background came from and I answered. How did I bring down Penn State? I am representing them well in a small field for which they are nationally recongnized. What is it that you do for a living yates1012?
So, I believe Kap answered you and did tell you what is wrong. You ARE NOT accounting for the upward force being created by the brake rotor/caliper. The torque of the caliper/rotor is pulling the fork up while the wheel is staying in place, pretty simple, especially since a typical bike has the weight distributed 40/60 would make this possible. The next false statement that you have made is saying that the wheel is a rigid body, that is a big NO. A wheel is going to flex, enough said on that subject. Finally, if you are positive that your model is accurate then please conduct a test as part of a class project, also before you use statements such as "I have had 5 other people check my work" how about you go and have a PROF. check your work, then I will be satisfied.

P.S. You ask what yates1012 does for a living, does it matter? The answer to that is NO. It does not matter what he does for a living, because he has a valid point that you should address in a RESPECTFUL manner, regardless of his profession. Face it, you do not have to have a college degree in a field to understand things.

44. Originally Posted by bigfish
Watch and learn feallas,

The first few stops are with a QR the last ones are with out a QR.
Yup, that is how it works. The axle follows an arc pretty much as I drew it.

Mr. calculation seems to believe the rider's weight on the wheel will keep the axle in the fork, though.

45. Originally Posted by shiggy
Yup, that is how it works. The axle follows an arc pretty much as I drew it.

Mr. calculation seems to believe the rider's weight on the wheel will keep the axle in the fork, though.
and... the student would not make a good engineer because he has no concept of force paths and understanding how forces get from one member to another.

46. Someone explain the greek in the following to me:

http://www.ne.jp/asahi/julesandjames...annondale.html

47. Originally Posted by bigfish
Watch and learn feallas,

The first few stops are with a QR the last ones are with out a QR.

Excellent video! Now imagine that happening with the wheel down on the ground. The force from that tire and wheel transferring its force out of the drop-outs would be equalled by a force from the gound (normal force).

The force that you put on the fork to the axle (and ultimately the ground) when you brake as your momentum is accelerating negatively would be much greater than the force of the tire trying to pop out of the drop-outs. The larger force will prevail and the axle will stay in the drop-outs.

48. gentlemen, i know this individual well enough. he's a decent guy and stops in at my shop regularly. so please don't bust his balls. yes, he is wrong, wheels will come out of the drops when you don't have your skewer securing them. but he's still learning, let's not make it so he hates that process.

that aside, i've known far too many engineering students here at PSU, which i must admit is in the toilet as far as i'm concerned with education - but that's another topic, that think they know a lot because they take all these classes. the simple fact of it is, no one with an engineering degree should feel they know anything other than what they need to next become an engineer. that diploma is the beginning, not the end. an engineering student is NOT an engineer, despite the fact that i had a couple lame PSU students making that claim last weekend.

okay, now that that rant is aside, let's get it back on topic. whatever the forces involved are, the short of it is, 8" rotors have never caused an issue i've seen on q/r forks, PROVIDED that the skewer is secured. specialized equips all their bikes in size L and XL with big rotors up front, and i've never seen or heard of a problem.

if you guys want to debate the topic six ways from sunday, please do so with some even demeanor. and yes, it will only be solved with an experiment to prove one way or the other. there's a couple kids around the shop probably dumb enough for that to be arranged.

don

49. Originally Posted by bighitdon
gentlemen, i know this individual well enough. he's a decent guy and stops in at my shop regularly. so please don't bust his balls. yes, he is wrong, wheels will come out of the drops when you don't have your skewer securing them. but he's still learning, let's not make it so he hates that process.

that aside, i've known far too many engineering students here at PSU, which i must admit is in the toilet as far as i'm concerned with education - but that's another topic, that think they know a lot because they take all these classes. the simple fact of it is, no one with an engineering degree should feel they know anything other than what they need to next become an engineer. that diploma is the beginning, not the end. an engineering student is NOT an engineer, despite the fact that i had a couple lame PSU students making that claim last weekend.

okay, now that that rant is aside, let's get it back on topic. whatever the forces involved are, the short of it is, 8" rotors have never caused an issue i've seen on q/r forks, PROVIDED that the skewer is secured. specialized equips all their bikes in size L and XL with big rotors up front, and i've never seen or heard of a problem.

if you guys want to debate the topic six ways from sunday, please do so with some even demeanor. and yes, it will only be solved with an experiment to prove one way or the other. there's a couple kids around the shop probably dumb enough for that to be arranged.

don
Very nicely said about engineering students.

50. Good one Don!!!

FYI - I am an engineer...and I never profess to know anything but jacksh!t.

The point is moot to me. My 20mm thru-axle is just plain stiffer, and I like bolts. Besides...my 66RC2X doesn't come with a QR

51. Originally Posted by bigfish
Watch and learn feallas,

The first few stops are with a QR the last ones are with out a QR.
as we say on the internets, PWNED

all your brakes are belong to us

52. Originally Posted by psunuc
I have heard a lot on these forums about how it is unsafe to use 8" rotors on QR forks. This didnt make sense to me since the different between 7" and 8" is only 14.3% in terms of length. It didn't seem like a large increase to me.

I drew up this balance of forces (with gravity neglected and the assumption that the caliper is on the x-axis and the drop-out of the fork is on the y-axis) and it showed me that there isnt any force on the QR during braking that would confirm this myth.

It is shown that since the wheel and tire are basically a rigid body that the resultant force (moment) on the axle, which was supposedly forcing the wheel from the fork, is just distributed directly to the ground. This force is then balanced by the normal force of the ground pushing back on the tire. Not to mention that under braking circumstances the rider's weight is distributed forward to the fork which is clearly forcing the fork onto the axle which only helps the situation.

Under these circumstances it is clear that there is actually zero force on the QR/axle to leave its position in the drop-outs. Of course gravity is the only force freely pulling/pushing the QR/axle from the fork but this force would only magnify the normal force from the ground which would just equalize the forces.

This balance of forces is for the wheel being on the ground. I do not apply my brakes when I am in the air and am not interested in the force balance on that type of system.
Dude, you have to put some values to the forces if you think your fbd is even close to accurate. Just because the force vectors are pointing at each other doesn't mean they "cancel" each other out. Yes, they are forces that might be directed at each other, but it doesn't mean that they are equal and opposing forces.

Monte

53. I feel confident in my FBD, but if Don says its going to pop out then I will believe it. He's a top-tier mechanic in my book.

I will be putting the 8" rotor on my front QR fork. If it comes off I assure all of you that there will be a thread about it with pictures.

54. meh.

....

55. I aint got no fancy degree, aint never stepped foot in no uneevarsity. But, it would appear to me that unless braking forces some how increase the distance between my dropouts and the ground my axle could not be forced out of the dropouts. on the contrary application of the front brake would cause me to dive forcing my axle further up into the dropout. As far as that video goes, that only works with an upside down bike. What would happen in that video if a 100 pound person was hanging from the loosened QR?

56. Originally Posted by psunuc
I am a senior at Penn State University with a double major in Nuclear Engineering and Environmental Engineering. My physics background is best described by the courses I have taken and passed. Each entry is a different class I have taken that is heavily based on physics.

Calculus-based Physics: Mechanics
Calculus-based Physics: Electricity and Magnetism
Calculus-based Physics: Wave and Quantum Physics
Engineering Mechanics: Statics
Engineering Mechanics: Dynamics
Engineering Mechanics: Strength of Materials
Engineering Mechanics: Mechanical Responce of Materials
Mechanical Engineering: Thermodynamics
Mechanical Engineering: Fluid Flow
Mechanical Engineering: Heat and Mass Transfer
Nuclear Engineering: Reactor Physics
Nuclear Engineering: Reactor Design
Nuclear Engineering: Design Principles of Nuclear Reactors
Nuclear Engineering: Nuclear Reactor Core Design
Nuclear Engineering: (2 Labs in Penn State's Breazeale Nuclear Reactor)
Looks like the same credentials as Homer Simpson

57. I swaped out the 6" rotors for some 7" on my haro mary. after a short ride I found that the wheel was sitting cockeyed on the rotor side. the extra braking force and my qr not being tight enough had almost made the wheel pop out. the only thing stoping it was the little nubs at the bottom and the fact that I never realy hammered on the brakes hard. But now I keep that baby realy realy tight.

58. Originally Posted by psunuc
I feel confident in my FBD, but if Don says its going to pop out then I will believe it. He's a top-tier mechanic in my book.

I will be putting the 8" rotor on my front QR fork. If it comes off I assure all of you that there will be a thread about it with pictures.
I don't think anyone thinks the wheel is going to pop off if you do this. The part you are being challlenged on (and have never once addressed) is the part about there being no net forces on the axle without a qr.

BTW, I am calling BS on a bunch of senior enginneering students giving their OK on a sketch on a napkin with no figures or actual calculations anywhere. Any enginneering student that would put their name to this work without any actual values being calculated for Fs and Frm should be sent back to Freshman year.

59. PSUNUC -

You FBD appears correct, as presented (sorry, I had to throw that in there. It is the engineer in me that deals with legal actions). Please remember, though, that other loads are placed on that QR. Vibration, shear, fork leg deflection, yada, yada can contribute to the minute walking action shown at http://www.boltscience.com/pages/vibloose.htm that may (key word) loosen the QR or thru-axle bolt.

Now, given the information presented by boltscience, an interesting analysis would be what reciprocating load (and duration of time) applied to the fork legs caused by an 8" rotor, and thus causing deflection in the lowers, would cause a) a QR to loosen b) a 20mm thru-axle bolt to loosen.

And yes, I'm bored.

60. ## HOORAY! Someone with a background in this material comes along.

Originally Posted by Visicypher
PSUNUC -

You FBD appears correct, as presented (sorry, I had to throw that in there. It is the engineer in me that deals with legal actions). Please remember, though, that other loads are placed on that QR. Vibration, shear, fork leg deflection, yada, yada can contribute to the minute walking action shown at http://www.boltscience.com/pages/vibloose.htm that may (key word) loosen the QR or thru-axle bolt.

Now, given the information presented by boltscience, an interesting analysis would be what reciprocating load (and duration of time) applied to the fork legs caused by an 8" rotor, and thus causing deflection in the lowers, would cause a) a QR to loosen b) a 20mm thru-axle bolt to loosen.

And yes, I'm bored.
Visicypher, you are correct also. I am very happy that someone with background knowledge in this area finally came along. If I included things like vibrational forces, reciprocating loads, shear, etc. my FBD would look dramatically different (as you already know). I said in the originial post and a couple of posts thereafter that I didn't include all forces. This was a rough skematic of what is going on under braking and I neglected forces which I have been taught would be negligible in this situation. The whole point of me starting this thread was just to show people that it isnt as dangerous as everyone thought to put an 8" rotor on a QR fork.

Thank you very much Visicypher for confirming that I am correct in my FBD because I was starting to go crazy here with the non-stop ridicule from people who are probably intelligent but don't have a background in these concepts. As you said, you are an engineer that deals with legal actions and I am sure you are more than educated on this matter since you are a professional, I am only a student.

To all of you who have ridiculed me on this thread, no hard feelings. I never meant any malice towards anyone (even though I was accused of it) and I hope that ultimately my point can get across; that an 8" rotor on a QR is not completely unsafe. I hope that atleast the wisdom of someone who is a professional in this area (visicypher) will help you see that this isn't some off the wall concept I just throught up one day.

For anyone who is so interested in the actual calculations:
2) Calculate the figures
3) You will probably understand the concept better at this point

61. Originally Posted by kapusta
I don't think anyone thinks the wheel is going to pop off if you do this. The part you are being challenged on (and have never once addressed) is the part about there being no net forces on the axle without a qr.

BTW, I am calling BS on a bunch of senior engineering students giving their OK on a sketch on a napkin with no figures or actual calculations anywhere. Any engineering student that would put their name to this work without any actual values being calculated for Fs and Frm should be sent back to Freshman year.

I believe they were referred to as "colleagues" which until this thread I'd assumed you had to collect a salary to have?

62. Visicypher for confirming that I am correct in my FBD
please (both of you if you like) explain to me why the braking force Fs is pointing upwards. a force in that direction will tend to increase wheel speed.

63. The problem I see with the original FBD is that when the brake is clamped, the axis of rotation is no longer the axle or skewer, but the brake caliper itself. The axle rotates around the brake caliper in a counterclockwise direction. The force of the CCW rotation can overcome the force of the riders weight pushing the axle to the ground with a large enough rotor.

64. You guys really think its unsafe to run a 8" on a QR? As long as the QR is tight, its as solid and safe as any vertical drop out locking system. I run 8" on QR and i never had any problems, neither did my friends. I never saw or heard any one complain the wheel fell off as he was braking. The only problem that could happen is if the QR was unsafely closed. The way the drop outs are slotted on forks, the QR's bolts sit a little IN the dropouts thus making it even more safe. There is no problem with 8"/QR combo as long as you close it tight!

65. ## In some cases, it IS unsafe.

Originally Posted by IonicRipper
You guys really think its unsafe to run a 8" on a QR? As long as the QR is tight, its as solid and safe as any vertical drop out locking system. I run 8" on QR and i never had any problems, neither did my friends. I never saw or heard any one complain the wheel fell off as he was braking. The only problem that could happen is if the QR was unsafely closed. The way the drop outs are made on forks, the QR's bolts sit a little IN the dropouts thus making it even more safe. There is no problem with 8"/QR combo as long as you close it tight!
There have been a few cases of the lowers/dropouts not being able to handle the additional forces, and have cracked/failed. To broadly state that there is no problems with that setup because you have never had a issue is over the top. Feel free to risk your own safety, but stick to the mfg warnings as to what size rotors the forks can take if you are going to help others with their setup.

66. Originally Posted by damion
There have been a few cases of the lowers/dropouts not being able to handle the additional forces, and have cracked/failed. To broadly state that there is no problems with that setup because you have never had a issue is over the top. Feel free to risk your own safety, but stick to the mfg warnings as to what size rotors the forks can take if you are going to help others with their setup.
I personally never have seen any problems with QRs breaking caused by additional forces of disc brakes. But my bike is stock with QR and 8" rotors, if it really was that unsafe, do you think they would sell bikes with that combo?

67. Originally Posted by ferday
please (both of you if you like) explain to me why the braking force Fs is pointing upwards. a force in that direction will tend to increase wheel speed.
The force he is indicating is the force being applied to the CALIPER by the rotor. The caliper is applying the opposite force to the rotor. He has not indicated that force at that location, but I think it is understood to be there. The important part here is that the downward force is transfered to the axle/wheel and to the ground, and that is indicated in the diagram.

This guy is not an idiot, and his diagram, though simplified, is basically correct. It is his interpretation of this in the absence of a few critical numbers, and his insistance on digging his whole deeper and deeper rather than actually listening to what some people are telling him that makes him sound like an idiot.

The ironic thing is that his assertion that an 8" rotor is not any more unsafe than a 6" rotor (in terms of the axle slipping) is, IMO, completly correct. Th eproblem is that the arguement he is making for it has nothing to do with why this is true, and is basically flawed, but he will not even acknowlege the legit critizism offered, and instead only engages the people making weak arguements. He has still never answered my question.

What is truely amazing is the fact that I am still drawn to this nonsense

68. Kapusta stay away from the light. I got sucked in also.

as was stated. it isn't completely unsafe. it is just not safe as some, or all, would like. Shiggy pointed out the arc of the wheel in the youtube vid. The arc will have x & y vectors which to varying degrees load the drop outs of the fork...hence changes in fork drop out designs to address those loads, and a handleful of others. (Damn...is that light getting closer...must stay away!!!)

I think we should lay this one to rest. I still prefer my 20mm thru-axle. It just handles loads way better than the 9mm. I prefer to blow my day wondering which tire to get...to which I end up asking Shiggy questions any way.

69. ## I am sure it is fine.

Originally Posted by IonicRipper
I personally never have seen any problems with QRs breaking caused by additional forces of disc brakes. But my bike is stock with QR and 8" rotors, if it really was that unsafe, do you think they would sell bikes with that combo?
A couple of years ago, Specialized started speccing 8" rotors on Fox forks, prior to Fox ok'ing it. THey did a recall of the 8" rotors, and put 6" rotors on for free.

Now, most modern forks are designed to accept an 8" rotor, but older ones are not ok.

I am sure your setup is fine, as long as your QR is checked regularly.

70. Originally Posted by psunuc
Please, if you can tell me what it is that I forgot or neglected then I will gladly accept that I am wrong. You are the one being nasty. On what basis did you prove me wrong in your head to validate saying I am way off?

Just PLEASE explain to me what I did wrong. Everyone on this thread can tell me I am wrong but no one has yet proved me wrong.

By the way, I was asked where my educational background came from and I answered. How did I bring down Penn State? I am representing them well in a small field for which they are nationally recongnized. What is it that you do for a living yates1012?
Proof---

1) Turn your bike upside down
2) Loosen QR
3) Spin tire
4) Apply brakes

I just did this last night and the faster the tire spun and the harder I applied the brakes, the harder the axle jumped out of the dropouts.

An object at rest (the axle) will stay at rest unless acted on by another force. The force that ejected my axle from the dropout is the same force you neglected.

That is proof that your diagram is flawed.

You could probably do it with no problems, but don't come on this forum and preach like an expert that anyone can do this-someone could be hurt or void a warrantee.

71. Originally Posted by Johnny M
on the contrary application of the front brake would cause me to dive forcing my axle further up into the dropout. As far as that video goes, that only works with an upside down bike. What would happen in that video if a 100 pound person was hanging from the loosened QR?
All right, enough of this pointless arguing. The results are in. I shot a short video in my basement of what happens when you ride with a loose quick release, on a disc brake equipped bike. I set the QR was loose enough so the wheel could be taken out of the fork, uninterrupted by the lawyer tabs. By the way, I now have a very tweaked rotor, just a warning for those who might reenact this stunt

So once again, watch and learn. http://www.youtube.com/watch?v=losWKtO69q4

72. Originally Posted by bigfish
All right, enough of this pointless arguing. The results are in. I shot a short video in my basement of what happens when you ride with a loose quick release, on a disc brake equipped bike. I set the QR was loose enough so the wheel could be taken out of the fork, uninterrupted by the lawyer tabs. By the way, I now have a very tweaked rotor, just a warning for those who might reenact this stunt

So once again, watch and learn. http://www.youtube.com/watch?v=losWKtO69q4
Thats on a loose QR. Now can you prove that running a 8" rotor on a decent QR is unsafe?

P.S. how bent is that disc now?

73. angular momentum and torque at its best...

Nice vid.

74. OK, after looking at BigFish's video, do you still stand by your analysis? You can be comforted knowing there is no net force on the axle as you get your teeth replaced and your jaw wired after paritng ways with your front wheel

"why would I do an experiment when I already know the outcome?............Can't argue with the dynamics."

I hope this has taught you how dangerous it is to say those words in your field.

75. Originally Posted by IonicRipper
Thats on a loose QR. Now can you prove that running a 8" rotor on a decent QR is unsafe?

P.S. how bent is that disc now?
That's not what he is trying to proove.

76. Originally Posted by IonicRipper
Thats on a loose QR. Now can you prove that running a 8" rotor on a decent QR is unsafe?
No, I can’t prove that, because it is perfectly safe. I have seen plenty of bikes through our shop with QR and 8 inch rotors together (Gary fisher biggens series all have 8 inch discs and QR, except for the PHD). As long as the QR is tight there will be no problem, and if the QR is not tight enough the worst thing that will happen is the wheel will become angled in the dropout under hard braking. (assuming the fork has lawyer tabs)

The rotor required a good 30 min of truing and still has a little tweak to it.

77. I posted this to the Mythbusters message board, just for fun . I'd love to see those guys test this one out. Personally I've always thought that it makes perfect sense, that the braking force will pull the wheel out of the dropout seems obvious. I've also thought that since it never seems to actually happen to me in real life, A) I'm wrong, and B) it doesn't really matter one way or the other.

oh, and
why would I do an experiment when I already know the outcome?............Can't argue with the dynamics.
you were doing so well, up until this little gem! This is about the most ridiculously arrogant thing I've ever read.

78. I must have way too much time on my hands to reply to this post - or I'm just too much a geek.

There are 2 questions here that are different:

1) are 8" rotor bad for some forks?
2) do brake forces pull the wheel out of the dropout?

They are unrelated in the sense that larger rotors put less force on the dropout QR.

Here's our OP's diagram with some extra crap drawn on it.

To determine the forces at the dropout from wheel torque, you must first pick a point that is not the point of interest.

fN is the friction(f) time the normal force (N). f is usually <1, but with rubber knobby tires on rock, it can be >1, but lets use "1". N is maxed out as your weight. Because the wheel has a radius of ~13" and the rotor has a radius of 3" (actually a little less, but lets ignor that), then the whole shebang acts like a lever & fulcrum magnifying the forces at the dropout by 13/3 = 4.3X. That means that D6 = 4.3N or there is 4.3X your weight pulling on the dropout. So if you subtract the normal force, or your weight pushing down on the axle, you can see why bigfish ejected his wheel.

With an 8" rotor, the ratio is 13/4 = 3.25, ie: D8=3.25N, or only 3.25x your weight, - less force at the axle than a 6" rotor.

The same forces are acting at the caliper, but that's what they were meant for.

To put it in perspective, it you hang your bike by the wheel from the ceiling with the dropout facing up, and take 3 of your buddies and all hang on the bike, would you feel confident that the QR interface is going to hold you up? Because that's what's happening at the dropout when you are in a critical braking situation. That's why facing the dropout slot away from the caliper is a good thing.

Now comes the question as to why some fork makers don't like 8" rotors. The ISO (or post) mount is always located at the same place on the fork leg. Thus the further you move the caliper out, the larger the peak loads on the caliper mount. Trials boys are able to shear the bolts holding on rear ISO mounted calipers by bouncing on the rear tire.

Too much geek - time to go for a ride.

79. Originally Posted by itsdoable
I must have way too much time on my hands to reply to this post - or I'm just too much a geek.

There are 2 questions here that are different:

1) are 8" rotor bad for some forks?
2) do brake forces pull the wheel out of the dropout?

They are unrelated in the sense that larger rotor put less force on the dropout.

Here's our OP's diagram with some extra crap drawn on it.

To determine the forces at the dropout from wheel torque, you must first pick a point that is not the point of interest.

fN is the friction(f) time the normal force (N). f is usually <1, but with rubber knobby tires on rock, it can be >1, but lets use "1". N is maxed out as your weight. Because the wheel has a radius of ~13" and the rotor has a radius of 3" (actually a little less, but lets ignor that), then the whole shebang acts like a lever & fulcrum magnifying the forces at the dropout by 13/3 = 4.3X. That means that D6 = 4.3N or there is 4.3X your weight pulling on the dropout. So if you subtract the normal force, or your weight pushing down on the axle, you can see why bigfish ejected his wheel.

With an 8" rotor, the ratio is 13/4 = 3.25, ie: D8=3.25N, or only 3.25x your weight, - less force at the axle than a 6" rotor.

The same forces are acting at the caliper, but that's what they were meant for.

To put it in perspective, it you hang your bike by the wheel from the ceiling with the dropout facing up, and take 3 of your buddies and all hang on the bike, would you feel confident that the QR interface is going to hold you up? Because that's what's happening at the dropout when you are in a critical braking situation. That's why facing the dropout slot away from the caliper is a good thing.

Now comes the question as to why some fork makers don't like 8" rotors. The ISO (or post) mount is always located at the same place on the fork leg. Thus the further you move the caliper out, the larger the peak loads on the caliper mount. Trials boys are able to shear the bolts holding on rear ISO mounted calipers by bouncing on the rear tire.

Too much geek - time to go for a ride.
Great explaination. I just have one question: You say facing the dropout slot away from the caliper is a good thing. Does that mean that dropouts should face in a more forward direction than they usually do? Because it seems to me that most forks have the slot going almost perpendicular to the caliper.

80. ## Here'sa the deal...

I dont have years of schooling about forces but here's how I see it. The bike has inertia and when you're moving and the wheel is on the ground. So we can say that the wheel wants to keep spinning. When you pull on the brake really hard, the wheel is forced to stop spinning aroung the axle, BUT it will find the easiest way to keep spinning. So it begins to rotate around the point where the caliper is squeezing the rotor. That forces the axle down. I've had that happen once when braking hard. It went down, but didn't completely fall out.

81. Originally Posted by kapusta
...Does that mean that dropouts should face in a more forward direction than they usually do? Because it seems to me that most forks have the slot going almost perpendicular to the caliper.

Some vendors are starting to do it.

The caliper location varies with make, adapter, rotor size, etc..., very rarely does the dropout slot align exactly 90 deg to the caliper normal. Any small deviation from 90 deg goes a long way in preventing the axle from being ejected (or you would have heard a lot more about it), but it doesn't stop the axle from shifting enough to create rotor rub.

82. ## Dead horse This is a really old post

The original FBD is correct for a bike sitting still, not a rotating wheel.

The real problem is not what brake system or size of the rotor.
Assumptions:
1) Larger rotor = faster braking
2) Any 8" rotor would be mounted with a bracket that mounts to what the OEM thought would be a 6" rotor
3) Frames are made of Aluminum

The larger rotor besides being cool to look at also gives a longer contact area per revoution. Resulting in the same pad pressure(as a smaller rotor) over a longer path per wheel revolution. More work(braking) over the same time. This component of time is what is the (main) difference.
Despite what anyone says about forces or torque, the brake does NOT apply a torque to the wheel. The brake pads apply a frictional normal force tangential to the rotor. With the wheel spinning, this creates a reaction to the normal force, causing the wheel to want to spin around the center of the brake pad and pushing the axle up into the dropout. The resultant force to hold the caliper in place is tangential to the rotor and if this is not directly inline with the line of force, it will create a torque around the mount bolts in the stays. This is bad, and gets worse with a longer bracket. As someone said if they had different caliper mounts for larger rotors, this could be reduced.
The other issue is that aluminum is bad in repeated flexing (fatigue). Ti seatstays would also help.

Any ME like to add somthing I missed or pick out an error?

83. ## I'm convinced

Originally Posted by bigfish
Watch and learn feallas,

The first few stops are with a QR the last ones are with out a QR.
The vid seems to be proof that "in theory" and "in reality" are often two different worlds.

Johnny? What lovely parting gifts do we have for our contestants?

Bob

84. Originally Posted by Johnny M
I aint got no fancy degree, aint never stepped foot in no uneevarsity. But, it would appear to me that unless braking forces some how increase the distance between my dropouts and the ground my axle could not be forced out of the dropouts. on the contrary application of the front brake would cause me to dive forcing my axle further up into the dropout. As far as that video goes, that only works with an upside down bike. What would happen in that video if a 100 pound person was hanging from the loosened QR?
Things are not always as they appear.

The real question is:

Is this happening to people? Head to ground facials from 8" rotors on a QR fork?

I don't see it happening around here, more force on the QR or not.

85. Originally Posted by 29Colossus
Things are not always as they appear.

The real question is:

Is this happening to people? Head to ground facials from 8" rotors on a QR fork?

I don't see it happening around here, more force on the QR or not.
Just today I had my front QR wheel/185 rotor slip in my fork during a low/moderate speed trail ride. The skewer did not open (was clamped and hard as I could get it--not a Shimano) and the hub stayed in the dropouts because of the lawyer lips. No crashes and the wheel as not seated completely in the fork.

86. Originally Posted by shiggy
Just today I had my front QR wheel/185 rotor slip in my fork during a low/moderate speed trail ride. The skewer did not open (was clamped and hard as I could get it--not a Shimano) and the hub stayed in the dropouts because of the lawyer lips. No crashes and the wheel as not seated completely in the fork.
I ride a 203 on the Reba and it has seen some decent riding. No worries on a few Slickrock loops the otherday. That is usually the place that failure happens if it will happen.

If my wheel EVER shifted with the QR as tight as I have it, I would be changing components that day. Of couse it does happen. It has happened to me. Just happened to you. But I still don't see anyone showing off their broken bodies from 8" rotors causing catastrophic failure with quick release set-ups. I did swap out my Ameican Classic QR for an XT. The American Classic was inferior and would not hold well even when tight. The XT feels 100% more solid.

I want to be in control of as much safety as I can, and do run large rotors one the One9, so I am interested. I don't want to go back to smaller rotors, but I don't want to goon it either.

Maybe I just need to throw a Lefty on the One9 and be done with it. I'm ODing on that Kool-Aid right now. The Lefty embarrasses Reba.

87. hahaha, very funny, good picture too!

anyway there must be a force acting down under heavy braking because you see brake dive. There is also a larger force acting backward coz if u lock ur front wheel when the tire is flat the tire can actually spin round and chop the valve off.

88. i was test riding a bike a few months ago, checking for a creaking sound, and a skipping freehub. grabbed a handful of rear, 160mm brake, and laid into the pedals. just about ate **** when the rear wheel slipped out of the dropouts. bad skewers = no bueno. good times, good times.

89. Originally Posted by 29Colossus
...But I still don't see anyone showing off their broken bodies from 8" rotors causing catastrophic failure with quick release set-ups...
I think we have been ignoring that there are two different things in this discussion:

1. That it is possible for a QR axle to move in/come out of the dropouts of a fork when using disc brakes (with any rotor size).

2. That many fork manufacturers do not endorse the use of larger rotors with some model QR dropout forks. The reason for this may or may not have anything to do with #1 and may be because of possible increased stress on the fork's caliper mounts and/or other parts of the fork.

90. ## 3rd point....

Originally Posted by shiggy
I think we have been ignoring that there are two different things in this discussion:

1. That it is possible for a QR axle to move in/come out of the dropouts of a fork when using disc brakes (with any rotor size).

2. That many fork manufacturers do not endorse the use of larger rotors with some model QR dropout forks. The reason for this may or may not have anything to do with #1 and may be because of possible increased stress on the fork's caliper mounts and/or other parts of the fork.
the 3rd point is that the original FBD is invalid.

91. Originally Posted by Monte
the 3rd point is that the original FBD is invalid.
Which we did not ignore.

92. Now folks...any word on whether or not a 9mm thru bolt is safer than a standard QR? I have a 185mm rotor with a Hadley thru bolt on a Revelation. I can't imagine it going anywhere. Input?

93. don't know. the specialized stout hubs used a solid 9mm for a few years. i don't recall them having any real effect.

94. Originally Posted by juancancook
Look, I think you have some forces a bit off here. What you have modeled isn't geometrically correct either. Show me a set up where the tangent force is going parallel to leg. Doesn't exist. Second the reason most all manufacturers have some sort of clause about what rotor can be used on what fork has some to do with QR's slipping, but it's predominantly related to the casting of the dropout.
QR Dropout= less material
20mm dropout= more material
Not rocket science. The dropout picks up a shear force when the brake is acuated form the forward momentum of the bike and the angular momentum of the brake engaging. In simple terms, the hub no longer acts as the center of the wheel, the brake does. Think of it as becoming a pivot point. Now compare that to the pivot point of your cranks. the longer the crank arm the more leverage you have. Same applies at the moment the brake engages. The brake becomes the pivot point, the wheel still tries to turn, the dropout picks up the load, KA BLAMMO! Unless the dropout has been designed to cope with the load. Check out an 04 or earlier Fox fork and compare it to an 06. Dropout has been beefed up on the 05 and newer so that you can now use an 8" rotor where as pre 05 that was a big no-no. Compare the 06 Manitou R7 fork to the 06 Gold Label QR fork. R7 can't be used w/ the 8", Gold Label can. Again Material is the issue. It's material that is also the reason that fork makers are finally moving to a post mount standard rather than the IS. There's more meat over the bones to distribute the load of the brake, dig?

But enough salt, here's some sugar: I really like the fact that you put some thought into this and actually came up what you did. That is freaking killer! I haven't seen much of anyone try that route, so full on kudos! Just remember dynamic systems require wholistic thinking not just static force diagrams. Cheers!
Bingo.. I also have an experiment that can back this up, I used a hayes hfx HD 8 inch brake in front with a 04 fox talas rlc. I never had the hub come off but the flex of both fork less caused the rotor to twist and wear off an edge of the caliper. I now use a 6 inch.

95. Originally Posted by SLX
Bingo.. I also have an experiment that can back this up, I used a hayes hfx HD 8 inch brake in front with a 04 fox talas rlc. I never had the hub come off but the flex of both fork less caused the rotor to twist and wear off an edge of the caliper. I now use a 6 inch.
Are you sure that was not an alignment issue?

96. Originally Posted by indyfab25
Now folks...any word on whether or not a 9mm thru bolt is safer than a standard QR? I have a 185mm rotor with a Hadley thru bolt on a Revelation. I can't imagine it going anywhere. Input?
It is possible to tighten the bolt-on skewer/axle more than a QR so it "should" hold better.

97. Originally Posted by psunuc
why would I do an experiment when I already know the outcome?
Young sir, for this statement of yours alone, you should be drummed out of the scientific community. Calculations alone do not ascertain what is real. Science is about PROVING summations like you are proposing, and that is done with experimentation.
I believe the world is flat. Therefore, it is.
Same thing. Your belief is blinding you. Your science is not science.

98. What you should take in to acount is the rotating mass of the wheel and how you are braking it in a point that is nog it center of mass. This results in a momentum.

The bigger the disc, the further away you put you braking point from the center of mass, them bigger the momentum.

Bigger disc = more force on your QR (if all else is equal)

99. Originally Posted by TomG
What you should take in to acount is the rotating mass of the wheel and how you are braking it in a point that is nog it center of mass. This results in a momentum.

The bigger the disc, the further away you put you braking point from the center of mass, them bigger the momentum.

Bigger disc = more force on your QR (if all else is equal)
I may be misunderstanding what you are trying to say, but I don't think this is correct. If we are talking about the momentum from spinning, then the whole wheel has a certain momentum, regardless of where the brake is. No matter if you put the brake 6", 8", 2" or 26", you are dealing with the same momentum. The difference is that when you get closer to the axle, it takes more force to bring the momentum of the wheel to zero. The farther away you get from the axle, the less force is required to bring the wheel to a stop in the same amount of time. This is why large rotors are better at stopping you. It also means that less force is placed on the caliper as well as the axle. In other words, for a given amount of stopping force, LESS force is aplied to the dropout (and qr) with a larger rotor.

I think the reason for some forks not being suited for 8" rotors has more to do with the maximum possible horizontal stopping force (the force that is actually slowing you down), via the axle/dropout interface. This would obviously be greater with an 8" rotor.

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