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  1. #1
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    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.
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  2. #2
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    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.
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  3. #3
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    Quote 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.
    Last edited by psunuc; 02-15-2007 at 01:58 PM.

  4. #4
    Nouveau Retrogrouch SuperModerator
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    Quote 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.
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  5. #5
    conjoinicorned
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    Can't argue with the dynamics.
    sure you can, when they're as wrong as yours are.
    what would rainbow unicorn do?

  6. #6
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    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?
    Last edited by Moto Rider; 02-15-2007 at 02:27 PM.
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  7. #7
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    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. #8
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    Quote 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. #9
    Bicyclochondriac.
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    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. #10
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    Quote 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: Advanced 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. #11
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    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. #12
    Bicyclochondriac.
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    Here is the flaw.

    Quote 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.
    First, read my post below. Then read this.

    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. #13
    Nouveau Retrogrouch SuperModerator
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    Quote 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.
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  14. #14
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    Quote 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: Advanced 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. #15
    Bicyclochondriac.
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    Quote 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: Advanced 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?

  16. #16
    conjoinicorned
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    edit: not worth replying.
    what would rainbow unicorn do?

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

  18. #18
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    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. #19
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    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. #20
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    Impressive credentials but....

    Quote 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: Advanced 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
    'If Wal-Mart sold parachutes, who would jump?' Frank Havnoonian (quoting his father) Drexel Hill Cyclery

  21. #21
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    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. #22
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    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. #23
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    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. #24
    conjoinicorned
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    Quote 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.
    what would rainbow unicorn do?

  25. #25
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    Quote 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.

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