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  1. #1
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    Listen! Question about powerful brakes

    Ok so yesterday I was thinking about how some forks are limited to 160mm or 180mm rotors because they can not handle the power of bigger ones and that got me thinking what if you had a fork with a maximum rotor size of 160mm and you put a powerful brake like saint powerful but only used a 160mm rotor would it still have the some effect on the fork as using a larger rotor with a less powerful brake like a bb5 or xt?

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    Quote Originally Posted by edzwa
    Ok so yesterday I was thinking about how some forks are limited to 160mm or 180mm rotors because they can not handle the power of bigger ones and that got me thinking what if you had a fork with a maximum rotor size of 160mm and you put a powerful brake like saint powerful but only used a 160mm rotor would it still have the some effect on the fork as using a larger rotor with a less powerful brake like a bb5 or xt?
    I am interested to hear what others say about this.
    But I see where you are going with the more powerful caliper or a less powerful caliper and a larger rotor.
    It makes me kind of question the whole thing. A stronger caliper could in practice under the right circumstances put more torque on a fork with a 160mm rotor than a weaker caliper with a 203mm rotor. Assuming the wheel does not lock up and the rider doesn't endo.
    Heck you could also add the pad material considerations.

    I would like to see pictures of failed forks due to unapproved rotors on forks.
    No one ever mentions rotor size issues on wheel hubs. And the same effect happens to the hubs with larger rotors.

    One thing to consider is a 4 piston caliper running with 160mm rotor would probably not void warranty.

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    I think the adapter needed to support a larger rotor would put more force on the tabs or posts for the same amount of force exerted on the rotor. Since on most well-functioning disc brake systems (maybe not BB5s...) it's possible to lock up the front wheel, clearly even a 160mm system can get the wheel to exert maximum torque, at least in a realistic riding situation. However, the larger adapters send more torque to the tabs. Fork manufacturers may be afraid that too large a rotor will mean the tabs or posts get ripped off, or that their lightweight fork lowers might get warped.
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    Quote Originally Posted by AndrwSwitch
    I think the adapter needed to support a larger rotor would put more force on the tabs or posts for the same amount of force exerted on the rotor. Since on most well-functioning disc brake systems (maybe not BB5s...) it's possible to lock up the front wheel, clearly even a 160mm system can get the wheel to exert maximum torque, at least in a realistic riding situation. However, the larger adapters send more torque to the tabs. Fork manufacturers may be afraid that too large a rotor will mean the tabs or posts get ripped off, or that their lightweight fork lowers might get warped.
    Exactly right. It is the increased leverage due to the adapters needed to accommodate a larger rotor that can overload the fork caliper mounts and cause them to fail. Even if the mount is strong enough, the larger rotor can introduce flexing torque high enough to cause the fork to twist. On a lightweight XC fork this could cause brake lockup and the fork twist can put so much stress on the wheel hub as to cause spoke failure. Manufacturers are just being extra cautious by placing limits on their products that will keep them insulated from lawsuits.
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    Quote Originally Posted by AndrwSwitch
    I think the adapter needed to support a larger rotor would put more force on the tabs or posts for the same amount of force exerted on the rotor. Since on most well-functioning disc brake systems (maybe not BB5s...) it's possible to lock up the front wheel, clearly even a 160mm system can get the wheel to exert maximum torque, at least in a realistic riding situation. However, the larger adapters send more torque to the tabs. Fork manufacturers may be afraid that too large a rotor will mean the tabs or posts get ripped off, or that their lightweight fork lowers might get warped.
    That is a very interesting point I hadn't thought about the limits of traction affecting maximum torque.

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    Quote Originally Posted by AndrwSwitch
    I think the adapter needed to support a larger rotor would put more force on the tabs or posts for the same amount of force exerted on the rotor.
    Yes, but that amount of force exerted on the rotor would result in greater braking power. More braking means more stress.

    For the same amount of braking, the stress on the caliper mounts shouldn't change (though the balance might). The leverage is greater but the force at the caliper is less (and in proportion). If two brake systems are each able to lock the wheel, I'd like to see a technical explanation for how the one with the larger rotor will place greater stress on the bike.

    Niner recently upgraded their carbon fork rating from 160mm to 185mm. No structural change was made to the forks; all previously manufactured forks got the new rating.

    If you upgrade the rotor size in order to reduce lever effort and not to increase maximal braking power, then I don't personally see a problem. Thing is you will use maximal braking in a panic stop regardless, so if traction is good enough then you may exceed design specs.

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    Quote Originally Posted by craigsj
    For the same amount of braking, the stress on the caliper mounts shouldn't change (though the balance might). The leverage is greater but the force at the caliper is less (and in proportion). If two brake systems are each able to lock the wheel, I'd like to see a technical explanation for how the one with the larger rotor will place greater stress on the bike.
    Not sure what you're trying to say here. The leverage factor is definitely increased by moving the caliper further away from the mount. No, there's no increase in force at the caliper as far as the force exerted to the rotor by the pads, but the reciprocating torque transferred to the mount by the spinning wheel is definitely greater. Think of it this way: if you drew a line from the tire's contact patch to the center of the caliper and then to the point where the caliper mounts to the fork, the line is longer as the rotor size increases. Longer line=more leverage. This means that braking torque on the caliper mount is increased by a larger rotor. That reciprocating torque has to be absorbed by the caliper mount/fork leg. Too much torque for the fork design and you get twist, or worse, failure of the caliper mount.

    The main advantage to braking from a larger rotor is that for the same distance in travel of the wheel, a larger rotor will provide more swept area through the pads. More pad contact=better braking. This also results in more area to dissipate the heat from friction. so a larger rotor will run cooler in comparison, all things being equal.
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    Forks with a 160mm rating will most likely have a standard QR.
    The position of the disk calliper means that the frictional force of the brake pads on the disk acts largely to push the wheel downwards in the direction of the open fork ends.
    Larger rotors will give more leverage towards the axle.
    The more leverage from a larger rotor, the strength of the fork ends and QR axle might also have something to do with the manufacturers maximum rotor recommendation.
    Probably most if not all of modern forks with standard QR have 'lawyer tabs' on the fork ends as an extra safety measure to stop the wheel from popping out under braking.

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    Quote Originally Posted by craigsj
    For the same amount of braking, the stress on the caliper mounts shouldn't change (though the balance might). The leverage is greater but the force at the caliper is less (and in proportion). If two brake systems are each able to lock the wheel, I'd like to see a technical explanation for how the one with the larger rotor will place greater stress on the bike.
    If I read you right, you just answered your own question.

    It doesn't matter if total stress is the same (actually I think it's less.) What's important is that the force is distributed differently. If one post fails, the other one experiences a lot more force and likely fails too. At the very least, the bike's not much fun to ride anymore and the fork or frame has probably been totaled.

    <a href="https://www.flickr.com/photos/30867489@N08/4703160373/" title="rotor torque vectors by Andrew183, on Flickr"><img src="https://farm5.static.flickr.com/4026/4703160373_92cf224b19_b.jpg" width="533" height="513" alt="rotor torque vectors" /></a>

    As long as the little arrow travels between the posts, the forces exerted on the posts during braking will be almost entirely compression and shear. Most materials that someone would build with deal fairly well with compression.

    The outer ring is about the size of a 203mm rotor. I moved the arrow out radially, and it's just starting to go outside the posts, meaning it will create a torque around the upper post. Assuming the adapter being used moves the caliper out radially or favors the vertical direction, I don't really see a problem with using up to a 203mm rotor - it certainly exerts more force on the upper post, but it doesn't seem like it should be enough to be a problem yet, at least as long as the caliper itself can deal with it. The magnitude of torque is the product of the length of the lever, the force, and the sine of the angle between the two. Since the angle is small, it's no big deal yet.

    But what if the adapter moved the rotor to the right, instead of diagonally up and to the right? Or worse yet, directly in line with the posts? In that case, the distance from the brake pads to the upper post would increase slightly, which isn't so bad, but the angle between the force direction and the direction of that imaginary line would be larger, so the torque would increase considerably. The distance between the two posts is such that not too much tension would be exerted on the lower post, but if it's designed to deal with compression and a little bit of shear, and many materials are stronger under those types of forces than they are in tension.

    I think as long as the design of the adapter isn't really stupid, any post mount fork that can accept a 160mm rotor ought to be able to accept a 185mm rotor, although I can imagine makers of superlight forks wanting to shave the last possible gram from the upper post and creating a problem. However, the fork manufacturers don't manufacture the adapters, and the designers of the adapters have an incentive to move the calipers along the line of the posts - it keeps them from getting the caliper tangled up in the adapter. So I can see why the manufacturers are concerned. If the fork leg is designed with the expectation that the posts will only be under compression, exerting force in one direction, it would also not be surprising to find that it warped when the posts exerted force in opposite directions. I think a 203mm rotor, depending on the design of the adapter, could truly be a problem for a fork in fairly normal usage.

    Since IS tabs put the eyelets closer together and closer to the dropout, the sine of the angle between the force and the displacement at the upper eyelet would be greater and the force on the lower eyelet would be greater at any given torque. So I could see how a 185mm rotor might be problematic on those and a 203mm rotor would be even worse.
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  10. #10
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    The main culprit here is torque twist of the fork. It is unlikely you will rip a caliper off a fork, but if the fork is not sufficiently stiff a larger rotor can cause it to twist under heavy braking loads. On lighter forks with standard QRs, twisting of the forks under heavy braking is a very real consequence. This twisting results in a loss of directional stability and sluggish response to rider input. Through-axles and larger QRs help stiffen the fork and resist torque twist. On a DH bike where braking is often under critical load, at speed and for sustained periods, a stiff fork is essential to keeping the wheel straight. This is why fork manufacturers specify max rotor sizes. A larger rotor probably won't break anything, but it sure can wreak havoc with how well the fork works under load.

    I'd add that torque twist of the fork legs, whether from braking or terrain, is the main reason modern competitive dirtbikes and street bikes use an upside-down fork design. It is much stiffer and resists twisting far better (not to mention the decrease in unsprung weight). I'm surprised no one has adapted this design to a MTB fork yet.
    Last edited by Miami_Son; 06-15-2010 at 10:41 AM.
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  11. #11
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    Quote Originally Posted by Miami_Son
    The leverage factor is definitely increased by moving the caliper further away from the mount. No, there's no increase in force at the caliper as far as the force exerted to the rotor by the pads, but the reciprocating torque transferred to the mount by the spinning wheel is definitely greater.
    No, for a given amount of braking force, there is a decrease in the force at the caliper for the larger rotor. The torque is the same since the braking is the same. The leverage is greater therefore the force at the caliper is less.

    Now, for the same caliper force the torque will be greater but so will the braking force. It comes as no surprise that there are greater stresses with greater braking forces.

    Quote Originally Posted by Miami_Son
    Think of it this way: if you drew a line from the tire's contact patch to the center of the caliper and then to the point where the caliper mounts to the fork, the line is longer as the rotor size increases. Longer line=more leverage. This means that braking torque on the caliper mount is increased by a larger rotor. That reciprocating torque has to be absorbed by the caliper mount/fork leg. Too much torque for the fork design and you get twist, or worse, failure of the caliper mount.
    The caliper mounting point to the fork or frame isn't changed by the rotor size. It is a fixed feature of the part. There is an adapter added but the fork/frame rating isn't applicable to the adapter.

    You can think of the braking force as a torque around the hub, so draw a line through the hub and the contact surface. Anywhere along that line you could compute an equivalent force, and the caliper mount would be a fixed point on that line which would always have the same force for a given torque. Of course, the caliper mount isn't necessarily on that line so their might be some slight effect there, but I mentioned that already.

    Quote Originally Posted by Miami_Son
    The main advantage to braking from a larger rotor is that for the same distance in travel of the wheel, a larger rotor will provide more swept area through the pads. More pad contact=better braking. This also results in more area to dissipate the heat from friction. so a larger rotor will run cooler in comparison, all things being equal.
    I agree with all that. In fact, I love that explanation and make it myself. It is clear that a stronger brake will create greater stresses on the frame. It's not clear, at least to me, that equal braking will also do that (to any significant degree). Because of that, I see little reason to be concerned with a +1 rotor size upgrade if the intent is to reduce lever pressure rather than increase brake power.

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    Quote Originally Posted by AndrwSwitch
    If I read you right, you just answered your own question.
    ...
    As long as the little arrow travels between the posts, the forces exerted on the posts during braking will be almost entirely compression and shear. Most materials that someone would build with deal fairly well with compression.

    ...
    Yes, you and I are thinking the same way here. I'd add that the fork/frame manufacturer provides a rating based on the part they provide, not on the adapter. It's possible that an adapter shifts the stresses inherently but I personally suspect that it's nothing more than the increased braking potential that's the concern.

    I suspect that rotor size ratings are dependent on the assumptions made in design and testing. Any change in the distribution of forces through an adapter that haven't been tested are going to cause concern with a manufacturer.

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    [QUOTE=Miami_Son] It is unlikely you will rip a caliper off a fork, [/QUOTE


    I've actually done that.
    http://forums.mtbr.com/showthread.php?t=591832

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    Quote Originally Posted by edzwa
    Quote Originally Posted by Miami_Son
    It is unlikely you will rip a caliper off a fork,

    I've actually done that.
    http://forums.mtbr.com/showthread.php?t=591832
    I'm glad you're okay.

    I notice that the fork used an IS tab - did the replacement fork have the same casting? (Did you keep the bike after all that?)
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    Quote Originally Posted by Miami_Son
    The main culprit here is torque twist of the fork. It is unlikely you will rip a caliper off a fork, but if the fork is not sufficiently stiff a larger rotor can cause it to twist under heavy braking loads. On lighter forks with standard QRs, twisting of the forks under heavy braking is a very real consequence. This twisting results in a loss of directional stability and sluggish response to rider input. Through-axles and larger QRs help stiffen the fork and resist torque twist. On a DH bike where braking is often under critical load, at speed and for sustained periods, a stiff fork is essential to keeping the wheel straight. This is why fork manufacturers specify max rotor sizes. A larger rotor probably won't break anything, but it sure can wreak havoc with how well the fork works under load.

    I'd add that torque twist of the fork legs, whether from braking or terrain, is the main reason modern competitive dirtbikes and street bikes use an upside-down fork design. It is much stiffer and resists twisting far better (not to mention the decrease in unsprung weight). I'm surprised no one has adapted this design to a MTB fork yet.

    That is what I am getting at I am sure that if you are just riding along and only braking lightly you will have no problem But if you are going fast down a hill and are braking hard I think you will at the least get a lot of flexing with a 160mm rotor and a powerful brake because you have more braking power to use which gives you more torque.
    I can feel my E150 flex a bit under very hard braking at 40Km/h on the road with a 203mm rotor and code brake but not with a xt brake and 203mm rotor.



    The upside-down fork design has been used for mtb forks
    http://www.mtbr.com/cat/older-catego...7_1525crx.aspx


    https://www.maxmx.co.uk/upload/produ...87shiver50.jpg

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    Quote Originally Posted by AndrwSwitch
    I'm glad you're okay.

    I notice that the fork used an IS tab - did the replacement fork have the same casting? (Did you keep the bike after all that?)
    Yes as far as I can tell the replacement is identical. I do still have the bike but I am thinking about replacing the fork with a 2011 36 float.

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    I tryed a 4 piston caliper on the back of my 6 inch xc bike, just to break it in for my dh bike. On an adaptor for a 180mm rotor the caliper hits the adaptor and doesn't sit flat on the mounting points.That was for hayes strokers.

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    Quote Originally Posted by edzwa
    Yes as far as I can tell the replacement is identical. I do still have the bike but I am thinking about replacing the fork with a 2011 36 float.
    Before I posted on this thread the first time, I made a couple of assumptions about the results of using a larger rotor that didn't stand up to drawing pictures and doing some calculations based on ballpark numbers. One of the only cases in which I think that going to larger rotors actually creates a really big change in the force experienced by a part of the fork is when the fork has an IS tab. So I'm really surprised that a fork designed to go on an Enduro has one.

    I bet that bike will rock with a 36 Float on the front.
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    Quote Originally Posted by edzwa
    Quote Originally Posted by Miami_Son
    It is unlikely you will rip a caliper off a fork,

    I've actually done that.
    http://forums.mtbr.com/showthread.php?t=591832
    I didn't say it was impossible, just unlikely. If you read my posts on this I clearly said that overloading the mount from using a larger disc is a real consideration. Personally, I think the IS mount is insufficient for a 203mm rotor setup. A post mount is much sturdier and most manufacturers are now using it on their forks. Marzocchi has completely gone to post mount on all their forks. Glad it worked out for you and that you weren't injured. Can't imagine what would have happened if that had been on a trail, going downhill, or landing a jump.
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    Quote Originally Posted by edzwa
    I don't consider a dual-crown fork an MTB fork. They are more in the realm of a converted motorcycle design (Marzocchi makes OEM forks for several motorcycle manufacturers). I'd like to see an upside-down design on a single crown fork made specifically for MTB.
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    Quote Originally Posted by Miami_Son
    I didn't say it was impossible, just unlikely. If you read my posts on this I clearly said that overloading the mount from using a larger disc is a real consideration. Personally, I think the IS mount is insufficient for a 203mm rotor setup. A post mount is much sturdier and most manufacturers are now using it on their forks. Marzocchi has completely gone to post mount on all their forks. Glad it worked out for you and that you weren't injured. Can't imagine what would have happened if that had been on a trail, going downhill, or landing a jump.

    Landing a jump with the brakes on?????

    Loss of traction limits the amount of braking torque...

    Higher forces occur on the smaller rotors, with smaller lever arms.

    Lower forces occur on the larger rotors, with larger lever arms.

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    Quote Originally Posted by jeffscott
    Landing a jump with the brakes on?????
    No, my comment comes from the fact that in his original thread it was suggested that the mount was already damaged prior to it failing. If so, it could have failed at any time.

    Loss of traction limits the amount of braking torque...
    Actually, skidding can transfer more torque to the wheel than hard braking with the wheel still turning. It can also transfer more impact force to the mount since the connection from the wheel through the rotor and into the caliper will now be solid.

    Higher forces occur on the smaller rotors, with smaller lever arms.

    Lower forces occur on the larger rotors, with larger lever arms.
    What do you mean by forces? The leverage factor increases along with the distance the caliper is from the mount, i.e. the larger the rotor. Braking force and leverage force are two different things. Braking force is the force being applied by the caliper pistons to the pads and onto the rotor. Leverage force is the torque being applied by the spinning rotor onto the caliper body and into the mounting. One is linear, the other radial. While interconnected to a degree, they are different.
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    Cool, but with the exception of the first one they all appear to be discontinued models. I wonder why?
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    Great discussion!

    The effect of the different adapter sizes is an interesting point that I hadn't thought of. But I think, in a more basic sense, a larger rotor simply increases the max potential force that braking will transmit to the fork leg.
    Say you have an extreme condition of a 200 lb guy going 60 mph on a steep downhill with excellent traction. You slam on the brakes. The strength of your brake is limited and lets say at best it will take your wheel down from 1000 rpm to 0 in .5 seconds. Then you increase the rotor size and you can go from 1000 to 0 in .2 seconds.
    Impulse = change in momentum
    F^T=^MV (^ is supposed to be a delta)

    F=^MV/^T

    Time to stop goes down, force goes up.

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    Quote Originally Posted by Miami_Son
    No, my comment comes from the fact that in his original thread it was suggested that the mount was already damaged prior to it failing. If so, it could have failed at any time. Good so no braking landing a jump
    Actually, skidding can transfer more torque to the wheel than hard braking with the wheel still turning. It can also transfer more impact force to the mount since the connection from the wheel through the rotor and into the caliper will now be solid.Babble Babble traction limits the amount of braking torque not rotor size.


    What do you mean by forces? The leverage factor increases along with the distance the caliper is from the mount, i.e. the larger the rotor. Braking force and leverage force are two different things. Braking force is the force being applied by the caliper pistons to the pads and onto the rotor. Leverage force is the torque being applied by the spinning rotor onto the caliper body and into the mounting. One is linear, the other radial. While interconnected to a degree, they are different.
    The braking torque is applied to the larger rotor, for the same braking torque (limited by traction) the force exerted on the caliper by the rotor is smaller with a larger rotor.

    Of course the smaller rotor exerts a higher force on the caliper...

    In the end the same torque is applied to the mount regardless of the rotor diameter....cause braking torque is limited by traction.

    Since the mount is the same for a larger or smaller rotor the forces are the same.

    The only difference is the angles at which those forces act on the mount.

    Really quite simple.

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    Quote Originally Posted by smilinsteve
    Great discussion!

    The effect of the different adapter sizes is an interesting point that I hadn't thought of. But I think, in a more basic sense, a larger rotor simply increases the max potential force that braking will transmit to the fork leg.
    Say you have an extreme condition of a 200 lb guy going 60 mph on a steep downhill with excellent traction.Please put some real world into the discussion traction limits braking torque not rotor size You slam on the brakes. The strength of your brake is limited and lets say at best it will take your wheel down from 1000 rpm to 0 in .5 seconds. Then you increase the rotor size and you can go from 1000 to 0 in .2 seconds.
    Impulse = change in momentum
    F^T=^MV (^ is supposed to be a delta)

    F=^MV/^T

    Time to stop goes down, force goes up.
    I have always been able to skid a tire with a small rotor or a large rotor...

    Large rotors disappate heat better and prevent brake fade....

    Maximum braking torque will occur before brake fade while the brakes are still able to cause loss of traction.

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    I love jeffscott's posts. I think the insertion of red-colored text into other people's text is my favorite thing.

    Are we all working from the same definition of torque here? (I think it's the vector product of force and the displacement from where the force is applied to the axis of rotation of whatever the force is being applied to.)
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    Quote Originally Posted by jeffscott
    Of course the smaller rotor exerts a higher force on the caliper...
    Since the discussion was primarily about the force being able to rip the caliper from the fork, how does this fit in?

    In the end the same torque is applied to the mount regardless of the rotor diameter....cause braking torque is limited by traction.

    Since the mount is the same for a larger or smaller rotor the forces are the same.

    The only difference is the angles at which those forces act on the mount.

    Really quite simple.
    More leverage=more force. If the angle changes the amount of force on the mount, then it is obviously increased by a larger rotor. And the mounts are not the same. A 74mm post mount caliper with a 160mm rotor mounts directly to the fork. On a 180mm or 200mm rotor an adapter is used thereby increasing the leverage factor on the mount and transmitted torque from braking. Are you arguing just for agument's sake?
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    Quote Originally Posted by AndrwSwitch
    I love jeffscott's posts. I think the insertion of red-colored text into other people's text is my favorite thing.

    Are we all working from the same definition of torque here? (I think it's the vector product of force and the displacement from where the force is applied to the axis of rotation of whatever the force is being applied to.)
    I really don't know where jeff is coming from. He seems to think a smaller rotor provides more braking force and that the only reason larger rotors are used is for heat dissipation. That totally ignores the fact that for a given amount of wheel travel, a larger rotor will provide more swept area of the rotor resulting in better braking with less effort compared to a smaller rotor.
    And he states that the larger rotor increases the caliper angle, but that it does not result in more force being applied to the mount.

    If the force to the mount is the same regardless of rotor size, then why do manufacturers need to spec max rotor sizes for their forks?
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    Quote Originally Posted by jeffscott
    I have always been able to skid a tire with a small rotor or a large rotor...

    Large rotors disappate heat better and prevent brake fade....

    Maximum braking torque will occur before brake fade while the brakes are still able to cause loss of traction.
    So what you are saying, in your charming way, is that all brakes will simply overcome the skid force, so are exerting equal force to the mount, since they all have more than sufficient power (either with less caliper force and a longer lever, or more caliper force with a shorter lever). So are you saying that rotor size will not effect the force to frame or fork, and that fork manufacturers are all wrong to assume they need to put an upper limit on rotor size?

    I think that since we are talking about forks, its not about skidding when you are talking about the front. It's about deceleration. I think it is possible that traction does not limit your rate of deceleration, the rotation of your center of mass over the handlebars would be your limiting factor. Your rate of deceleration is potentially greater with a larger rotor, and therefore, the potential force on the fork is greater.

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    Quote Originally Posted by smilinsteve
    So what you are saying, in your charming way, is that all brakes will simply overcome the skid force, so are exerting equal force to the mount,Yes braking torque is limited by traction (maybe front endo on pavement) since they all have more than sufficient power (either with less caliper force and a longer lever, or more caliper force with a shorter lever). So are you saying that rotor size will not effect the force to frame or fork, and that fork manufacturers are all wrong to assume they need to put an upper limit on rotor size? No the force and torque is the same...the angle that that force is applied with may change

    I think that since we are talking about forks, its not about skidding when you are talking about the front. It's about deceleration. I think it is possible that traction does not limit your rate of deceleration, the rotation of your center of mass over the handlebars would be your limiting factorYes on pavement the front endo is possible and still limits braking torque. Your rate of deceleration is potentially greater with a larger rotor, and therefore, the potential force on the fork is greater.No the max potential force is limited by either traction or front endo
    If you shift weight to the rear to avoid the front endo the front tire will skid...

    If you front endo the max braking torque is equal to your weight multiplied by the distance from you CG to the front axle...

    Really quite simple.

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    Quote Originally Posted by Miami_Son
    I really don't know where jeff is coming from. He seems to think a smaller rotor provides more braking force
    I think what he is saying is that a smaller rotor requires more force from the caliper to provide the same amount of torque to the hub.
    moment = FxD force goes up when distance goes down. He's right about that.

    a larger rotor will provide more swept area of the rotor resulting in better braking with less effort compared to a smaller rotor.
    A larger rotor provides betteer braking with less effort because again the moment =FxD
    bigger rotor means D is bigger so F is smaller. F is the force of the caliper which is proportional to the force your hand exerts.


    If the force to the mount is the same regardless of rotor size, then why do manufacturers need to spec max rotor sizes for their forks?
    Yes, that is the question. I tried to explain that in my last post.

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    Quote Originally Posted by jeffscott
    No the force and torque is the same...the angle that that force is applied with may change.
    So Jeff, you say that rotor size limits are in place only because of the angle of the force transmitted from caliper to the mount? In other words, the reason is based on the adapter size, as stated early on in this thread?

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    Quote Originally Posted by smilinsteve
    So Jeff, you say that rotor size limits are in place only because of the angle of the force transmitted from caliper to the mount? In other words, the reason is based on the adapter size, as stated early on in this thread?

    My view of andrwswitch rotor vector diagram is blocked ....

    I have not taken the time to reproduce it...

    But in a word Yes


    Braking torque limits (or fornt endos) the wheel torque and eventually mounting torques.

    I ran a 160 mm rotor for 4 years damn near front endo'd on pavement a couple of times, and lost traction on the steeps many times.

    Now I run a 203 mm rotor havn't front endo'e yet (no panic stops yet), but I have lost traction on the steeps many times.....

    The benifit of the bigger rotor is heat dissipation, I cooked the little rotor maybe three times a year....havn't cooked the big one yet.

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    I still disagree with your assertion that the only benefit to larger rotors is heat dissipation. I just went from 160mm to 200mm on my Enduro and there was a distinct improvement in lowered lever effort required to stop in the same distance.

    Still, my issue with your comments is that you keep ignoring that a larger rotor does indeed put more stress on not only the caliper mount, but on the braking force's ability to twist the fork. These are the main considerations why fork manufacturers have maximum rotor size specs for their forks.
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    Quote Originally Posted by Miami_Son
    I still disagree with your assertion that the only benefit to larger rotors is heat dissipationIt is for me, I have more than enough hand strenght to one finger brake either the 160 or the 203mm. I just went from 160mm to 200mm on my Enduro and there was a distinct improvement in lowered lever effort required to stop in the same distance.

    Still, my issue with your comments is that you keep ignoring that a larger rotor does indeed put more stress on not only the caliper mount, but on the braking force's ability to twist the fork.No the off center torque is not changed These are the main considerations why fork manufacturers have maximum rotor size specs for their forks.
    The stress on the mount brackets maybe moderately increased but very little.

    Manufactures must complete both calculations and tests to ensure the bikes meet safety standards....Why recalculate and retest when they can just say what they rated the fork for originally....

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    Quote Originally Posted by jeffscott
    The stress on the mount brackets maybe moderately increased but very little.

    Manufactures must complete both calculations and tests to ensure the bikes meet safety standards....Why recalculate and retest when they can just say what they rated the fork for originally....


    Keep dancing around the truth.
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    Quote Originally Posted by Miami_Son


    Keep dancing around the truth.

    Thanks I like to stay in reality and truth....

    BS is not a very good dance partner.

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    I think fork manufacturers may consider more than just the greater torque on the mount caused by the bigger adapter.

    You've got to look at a worst case scenario.
    2 guys 200 lbs each, racing down a very steep paved hill, 60 mph, seats low, butts way back, one has a 160mm rotor and the other has a 203 on their forks.

    Now they both squeeze the front brake lever as hard as possible.
    In fast motion, they probably both go over the bars and it doesn't look too much different between the two.
    But in slow motion:
    Force on the lever is ramping up from zero to whatever force is sufficient to lock the wheel.
    Force for the 160 is A, for the 203 is B.
    A>B.
    So the time it takes to reach force A for the smaller rotor is longer than to reach force B for the larger rotor.
    The large rotor guy therefore goes flying a split second sooner than the other guy.
    But more importantly, the deceleration is not instantaneous. A hand does not reach full clamping power instantaneously, it ramps up to it, and it has to ramp higher with the small rotor. The deceleration from full speed to zero may not even be a perceptible difference to a casual observer, but the deceleration is greater with the large rotor, so the force on the fork is greater. Half the time means twice the force, even if the times are very small, .1 seconds vs .05 seconds for example.

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    Quote Originally Posted by smilinsteve
    I think fork manufacturers may consider more than just the greater torque on the mount caused by the bigger adapter.

    You've got to look at a worst case scenario.
    2 guys 200 lbs each, racing down a very steep paved hill, 60 mph, seats low, butts way back, one has a 160mm rotor and the other has a 203 on their forks.

    Now they both squeeze the front brake lever as hard as possible.
    In fast motion, they probably both go over the bars and it doesn't look too much different between the two.
    But in slow motion:
    Force on the lever is ramping up from zero to whatever force is sufficient to lock the wheel. Get it straight either the wheel locks up or the endo not both
    Force for the 160 is A, for the 203 is B.
    A>B.
    So the time it takes to reach force A for the smaller rotor is longer than to reach force B for the larger rotor. I'll give you a micro second or two diffence in time but the both endo....and that sets the max brake torque
    The large rotor guy therefore goes flying a split second sooner than the other guy.
    But more importantly, the deceleration is not instantaneous. A hand does not reach full clamping power instantaneously,bingo it ramps up to it, and it has to ramp higher with the small rotor. The deceleration from full speed to zero may not even be a perceptible difference to a casual observer, but the deceleration is greater with the large rotor, so the force on the fork is greater. Half the time means twice the force, even if the times are very small, .1 seconds vs .05 seconds for example.
    Sorry not buying it at all once the the guy starts to endo peak braking torque is past.

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    I'm sure there are a lot of variables that go into what a manufacturer specs as max rotor size for their forks. But the geometry of where the caliper mounts in relation to distance from the fork is static and the physics of how much stress will be applied to the mount in relation to rotor size can be calculated from that. I'm sure the manufacturers calculate in all kinds of variable like rider weight, fork travel, intended use, etc in coming to their max specs.
    If heat dissipation was the only benefit of a larger rotor you could easily get the same benefit by just increasing rotor thickness and spreading the caliper out to accommodate. The extra material in the rotor would handle more heat without introducing a higher stress to the mount from increasing rotor size.
    In the automotive world, increasing brake size (pad contact area) increases braking efficiency. That's why my 1-ton pickup has 12" drums with 4" wide shoes and a Toyota Corolla only has 7" drums with 2.5" wide shoes. It isn't just about heat dissipation.
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    Quote Originally Posted by jeffscott
    Sorry not buying it at all once the the guy starts to endo peak braking torque is past.
    Completely unwilling to see anyone elses viewpoint but your own.

    By the way, I've endoed while skidding the front wheel several times. Sometimes forward momentum is greater than traction or loss of such.
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    Get it straight either the wheel locks up or the endo not both
    Both could happen, lock up causing enough deceleration to send you over the bars.

    I'll give you a micro second or two diffence in time but the both endo....and that sets the max brake torque
    They both endo, but one decelerated faster than the other, leading up to the endo. Thats when max force is seen by the fork.
    Time matters. Just like stopping your bike in 15 seconds takes less hand force than stopping it in 5 seconds.
    What I'm not sure of, is if that micro second you "give me", is a practical consideration, or theoretical mumbo jumbo.

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    Quote Originally Posted by Miami_Son
    I'm sure there are a lot of variables that go into what a manufacturer specs as max rotor size for their forks. But the geometry of where the caliper mounts in relation to distance from the fork is static and the physics of how much stress will be applied to the mount in relation to rotor size can be calculated from that. I'm sure the manufacturers calculate in all kinds of variable like rider weight, fork travel, intended use, etc in coming to their max specs.
    If heat dissipation was the only benefit of a larger rotor you could easily get the same benefit by just increasing rotor thickness and spreading the caliper out to accommodate.What! Get a grip heat transfer is increased by increasing the surface area a thicker disc ain't gonna do it The extra material in the rotor would handle more heat without introducing a higher stress to the mount from increasing rotor size.
    In the automotive world, increasing brake size (pad contact area) increases braking efficiency. That's why my 1-ton pickup has 12" drums with 4" wide shoes and a Toyota Corolla only has 7" drums with 2.5" wide shoes. It isn't just about heat dissipation.

    You are starting to lose it....

    So your driving around with drum brakes???

    We are talking disk brakes here right.

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    Quote Originally Posted by Miami_Son
    Completely unwilling to see anyone elses viewpoint but your own.

    By the way, I've endoed while skidding the front wheel several times. Sometimes forward momentum is greater than traction or loss of such.

    Not that I believe you were in any type of control when that happened but if you endo'd than that limited the braking torque.

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    Quote Originally Posted by smilinsteve
    Both could happen, lock up causing enough deceleration to send you over the bars. Maybe if the front is turning as well


    They both endo, but one decelerated faster than the other, leading up to the endo. Thats when max force is seen by the fork.Still gotta squeeze the lever that is the control
    Time matters. Just like stopping your bike in 15 seconds takes less hand force than stopping it in 5 seconds.
    What I'm not sure of, is if that micro second you "give me", is a practical consideration, or theoretical mumbo jumbo.

    Micro seconds would be the difference between the rates at which the braking torque is applied to the rotor, all controlled by how fast the hands can move.

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    Quote Originally Posted by jeffscott
    You are starting to lose it....

    So your driving around with drum brakes???

    We are talking disk brakes here tight.
    More material also absorbs heat, genius. It doesn't have to be just surface area.

    And yes, my 1-ton 4X4 Dodge has rear drum brakes as do most heavy duty trucks and semis. You get way more brake pad surface area with drums than with discs.

    Losing it? It's worth noting that you are alone on your side of the argument.
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    Quote Originally Posted by Miami_Son
    More material also absorbs heat, genius. It doesn't have to be just surface area.That is heat absorption not dissipation

    And yes, my 1-ton 4X4 Dodge has rear drum brakes as do most heavy duty trucks and semis. You get way more brake pad surface area with drums than with discs.Depends on howw big you make the pads

    Losing it? It's worth noting that you are alone on your side of the argument.
    I don't think you have ever cooked a brake going down a long hill...

    It usually takes 3 to 5 minutes to turn the rotors blue and get the pads smoking....that length of time means heat transfer is the limiting effect....

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    Quote Originally Posted by jeffscott
    Not that I believe you were in any type of control when that happened but if you endo'd than that limited the braking torque.
    You're right, because as a motorcycle magazine test rider for a dozen years I know little about front brake control.
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    "More material also absorbs heat, genius. It doesn't have to be just surface area.That is heat absorption not dissipation."

    Did I say differently?^^^^^^^^
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    Quote Originally Posted by Miami_Son
    You're rightOF COURSE, because as a motorcycle magazine test rider for a dozen years I know little about front brake control.



    And still 203mm disc dissipate heat better than 160mm discs....

    By about 203/160 times or 127% better.....

    Hey maybe if you rode your pedal bike up and down some hills you might develop enough finger strength to brake with one finger even with a 160mm rotor.

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    Quote Originally Posted by Miami_Son
    "More material also absorbs heat, genius. It doesn't have to be just surface area.That is heat absorption not dissipation."

    Did I say differently?^^^^^^^^

    The heat absorped in very light bike rotors is mimimal compared to the heat dissipated...

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    Quote Originally Posted by Miami_Son
    The main culprit here is torque twist of the fork. It is unlikely you will rip a caliper off a fork, but if the fork is not sufficiently stiff a larger rotor can cause it to twist under heavy braking loads. On lighter forks with standard QRs, twisting of the forks under heavy braking is a very real consequence. This twisting results in a loss of directional stability and sluggish response to rider input. Through-axles and larger QRs help stiffen the fork and resist torque twist. On a DH bike where braking is often under critical load, at speed and for sustained periods, a stiff fork is essential to keeping the wheel straight. This is why fork manufacturers specify max rotor sizes. A larger rotor probably won't break anything, but it sure can wreak havoc with how well the fork works under load.

    I'd add that torque twist of the fork legs, whether from braking or terrain, is the main reason modern competitive dirtbikes and street bikes use an upside-down fork design. It is much stiffer and resists twisting far better (not to mention the decrease in unsprung weight). I'm surprised no one has adapted this design to a MTB fork yet.
    Brakes are limited by two things, heat dissipation and the compressive strength of the pad/shoe material.

    First we will kill off your drum brake examples; one reason drum brakes are used on trucks and large vehicles is that they are to an extent self actuating, if you look at the pivot point of the shoe and the direction of the drum rotation you will notice that one shoe is forced onto the drum surface, this reduces the required pressure in the hydraulic/air circuit. As for the shoe size, that is a balance between the required stopping torque, required heat dissipation and maximum allowable contact stress.

    Now back to MTBs, yes a 203mm rotor will require less lever force to give the same braking torque as a 160mm rotor, it will also dissipate significantly more heat (probably more than that estimated by Jeff as heat transfer is rather strongly influenced by free stream velocity which is higher with a bigger rotor). As has been pointed out, the maximum braking torque is limited by the tyre, therefore the torque transmitted from the calliper to the fork will be the same (assuming that in both cases it is capable of locking a wheel), again like Jeff pointed out this may produce forces in slightly different directions depending on the exact geometry of the adapter.

    It may be that fork manufacturers limit the rotor size so they don't have to look into these different forces. My personal take on it though is that they limit rotor size to discourage certain types of riding, ie someone puts a 203mm rotor on a cheap low end fork so they can do a couple of downhill runs and the fork gets destroyed because its not strong enough for downhill while it is still under warranty.

    Now your single crown USD forks, one word.... flexi
    A conventional fork allows the use of a bridge, this gives much more rigidity than you get from the larger tubes of a USD fork.

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    I don't even understand what you guys are arguing about at this point.

    I think we can all agree that bigger rotors are bigger and on most or all forks, use of a rotor larger than 160mm requires an adapter.

    It seems like everything gets acrimonious from there. I'm not even sure everyone's using the same definition of torque (I know that smilinsteve and I are, only he calls it moment. Which is fine.) But it seems like there's some disagreement as to whether or not the length of the lever arm effects the amount of torque?? And maybe as to what, exactly, is rotating about what, which forces are involved in this whole mess, and where they come from.
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    My main point is, was and always has been that the further away the caliper is from the fork, the more stress is put on the caliper mount and the more likely the fork is to twist under heavy braking.Larger rotors require the use of an adapter, which means more mounting point stress. The other issues (heat, traction, et al) were brought in by someone else, perhaps to obfuscate the obvious. Period...I'm out!
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    Quote Originally Posted by Miami_Son
    My main point is, was and always has been that the further away the caliper is from the fork, the more stress is put on the caliper mount and the more likely the fork is to twist under heavy braking.
    Well, duh.
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    Quote Originally Posted by Miami_Son
    I didn't say it was impossible, just unlikely. If you read my posts on this I clearly said that overloading the mount from using a larger disc is a real consideration. Personally, I think the IS mount is insufficient for a 203mm rotor setup. A post mount is much sturdier and most manufacturers are now using it on their forks. Marzocchi has completely gone to post mount on all their forks. Glad it worked out for you and that you weren't injured. Can't imagine what would have happened if that had been on a trail, going downhill, or landing a jump.
    I know what you wear saying I was just saying that I have done that anyway it's all good.

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    Quote Originally Posted by Miami_Son
    My main point is, was and always has been that the further away the caliper is from the fork, the more stress is put on the caliper mount and the more likely the fork is to twist under heavy braking.
    And your main point is wrong as jeff, and others, have repeatedly pointed out. The larger rotor only places more stress on the mount when the system is capable of greater overall braking force and it rarely is. Who's the one unable to see the other side of the argument?

    I also believe it's a matter of what the manufacturer is willing to test. I already gave an example where a manufacturer expanded their testing and changed the rating on forks they had already shipped.

    As for current, inverted single crown forks "made for MTB", Maverick SC32.

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    Quote Originally Posted by craigsj
    And your main point is wrong as jeff, and others, have repeatedly pointed out. The larger rotor only places more stress on the mount when the system is capable of greater overall braking force and it rarely is. Who's the one unable to see the other side of the argument?
    You. Braking force capable of stopping the wheel is all that's needed to create enough stress to twist the fork and stress the mount (see post above you). Why do you think manufacturers have steadily changed their fork/hub/axle designs over the years? To make them stiffer (the holy grail) and better able to resist not only the forces of trail impacts, but from braking as rotor sizes and caliper power have increased.

    I also believe it's a matter of what the manufacturer is willing to test. I already gave an example where a manufacturer expanded their testing and changed the rating on forks they had already shipped.
    Probably done more out of a desire to be able to sell their forks to a wider audience.

    As for current, inverted single crown forks "made for MTB", Maverick SC32.
    Nice setup, but far too proprietary.
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    Quote Originally Posted by Miami_Son
    You. Braking force capable of stopping the wheel is all that's needed to create enough stress to twist the fork and stress the mount (see post above you). Why do you think manufacturers have steadily changed their fork/hub/axle designs over the years? To make them stiffer (the holy grail) and better able to resist not only the forces of trail impacts, but from braking as rotor sizes and caliper power have increased.
    None of that argues your point, of course. Without increasing braking force, where is the increase in stress coming from?

    Quote Originally Posted by Miami_Son
    Probably done more out of a desire to be able to sell their forks to a wider audience.
    Absolutely, yet they didn't have to make any engineering changes to the fork. How is that if the stresses are so much greater?

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    I think, as was discussed yesterday, that the main difference is the larger adapter putting caliper farther away from the fork leg. Andrew posted a good diagram of it. So the stresses on the mount are different, and that is probably what they need to test.

    PS> If you look at Edzwa's pictures of his failure, it is a failure at the fork mounting tabs, while using a 203 mm rotor. I don't know if that is the typical type of failure you would see, but I would suspect that it is.

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    Quote Originally Posted by craigsj
    None of that argues your point, of course. Without increasing braking force, where is the increase in stress coming from?
    At the risk of repeating myself: leverage.


    Absolutely, yet they didn't have to make any engineering changes to the fork. How is that if the stresses are so much greater?
    In the case of Marzocchi, the switch from IS to post mounts was enough. When they retroactively tested them they found them to be stronger than the intial spec. Still, there are some forks where a larger rotor is not recommended, even ones with post mounts. It may have to do with the mount and it may have to do with the fork not being sufficiently stiff to prevent flexing from the extra twist under hard braking. The point is that a larger rotor not only requires a sturdier mount, but a significantly stiffer fork design overall.
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    Quote Originally Posted by Miami_Son
    At the risk of repeating myself: leverage.



    In the case of Marzocchi, the switch from IS to post mounts was enough. When they retroactively tested them they found them to be stronger than the intial spec. Still, there are some forks where a larger rotor is not recommended, even ones with post mounts. It may have to do with the mount and it may have to do with the fork not being sufficiently stiff to prevent flexing from the extra twist under hard braking. The point is that a larger rotor not only requires a sturdier mount, but a significantly stiffer fork design overall.

    Okay buddy sit back and relax....

    The braking torque is limited by traction or front endo...

    The force required to make that happen is smaller for a larger rotor...

    The twisting force is lower as well....and guess what the leverarm is the same (cause the rotor is the same distance away form the fork leg....

    So guess what lower force same leverarm equals less torque on the fork leg.

  65. #65
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    I'm not your buddy. Jabber all you want. Less hard-headed people understand what's being discussed. I'm done wasting my time with you. Please follow my lead.
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  66. #66
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    I have a chemistry test in a couple hours and don't feel like drawing a picture right now...

    However, there's a really big difference in what a fork mount does to the leg it's on when the position of the caliper changes the direction of force from going in the same(ish) direction at both posts to pushing on one post and pulling on another. Instead of the pushing force being distributed across both posts and a fair amount of the length of the fork, it's all concentrated into the one post. If the caliper is far enough from the posts to exert a torque about one of them instead of just a compression on both of them, the amount of force going through that one post starts getting pretty high. We all know from personal experience that braking force gets high enough to make parts of the bike flex a little bit. Most bikes that accept a rear disc brake have a brace for that reason.

    Here's an experiment to demonstrate the effect of pressure. Finish your domestic beer so the can's empty. Gently squeeze it with both hands - try not to dent it right now. Now repeat, but use only one finger on one side and try to match the amount of force. It's a lot easier to dent the can.

    I don't see how fork companies retroactively re-rating their forks for larger rotors really changes any of the fundamental issues. These things are usually overbuilt. They probably just did some more testing and decided their fork was strong enough in the right places, even if they hadn't necessarily designed with that in mind.

    I haven't drawn a picture from the perspective that I think would give me some insight into the fork twisting, but Miami_Son's thought certainly seems plausible to me.

    And let's not forget that one of the three things determining the magnitude of a torque is the angle between force and displacement. Change the angle and the magnitude of torque changes too. Anyone who uses a wrench has noticed that if you push perpendicular to the wrench handle, it works better.
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    Something I found on the internets, a discussion of how quick releases should never be used with disc brakes because the downward forces can pull the wheel out of the drop outs.
    Not exactly on topic, but I will post some of it here because it has a pretty good static force analysis applicable to what we have been talking about:


    The relevant forces and dimensions on the wheel are indicated. We have the braking force B and the ground reaction force R acting at the contact patch. The disk calliper exerts a force D as indicated, tangential to the disc at the point of calliper contact. The radii of the disk and wheel are r1 and r2, and finally the angle of the dropout exit is 'a' in front of vertical. For this fork and calliper, the force D is virtually vertical. If it wasn't, there would be another angle 'b' for the angle the disk force makes behind the vertical.


    Now for the simple sums. Let's assume we have a bike + rider weighing 90kg in total, braking hard and decelerating at 0.6g (6m/s^2) with the front brake alone (this is a reasonable estimate for maximal braking). The rearward force is 90 x 0.6 x g = 540N, and the vertical reaction force is 90 x g =900N (all the weight is on the front wheel). Taking moments around the axle, the force D exerted by the disk is given by D = 540 x r2 / r1. Here r2 = 13 x 25.4mm is the radius of the wheel and r1 = 72.5mm is the effective radius of the disk (ie to the centre of the force at the pads, rather than the outer edge = 10mm less than the full radius of 165mm / 2). So D = 2460N, acting vertically downwards. This acts entirely on the left hand side of the wheel, but the ground reaction force and braking forces are split equally between the two sides. So we are left with resultant forces of 2460 - 900/2 = 2010N vertically down and 540/2 = 270N rearward. The sum of those is equivalent to a single force of 2030N acting downwards at an angle of 8 degrees behind vertical. Resolving this parallel to the dropout opening angle (= 18 degrees ahead of vertical for a head tube angle of 72 degrees) leaves a force of 2030N x cos(18+8) = 1825N out of the dropout on the left hand side!

    http://www.ne.jp/asahi/julesandjames...quick_release/
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    Last edited by smilinsteve; 06-17-2010 at 02:13 PM.

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    A couple of things to note on the above analysis:
    1: Forces downward at the dropout are asymettrical, and greater on the caliper side.
    2. Downward Force generated by the disc = braking force x the ratio of wheel radius/disc radius.

    This shows that a larger disc results in a smaller downward force. The conclusion here is that possible wheel pullout or quick release stress is not a reason to limit rotor size.

    It is funny however, because elsewhere on that website they quote a Marzocchi letter telling a customer not to use a larger rotor because of the risk of wheel pull out.

    Its a pretty interesting website.

  69. #69
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    "Give me a lever big enough, and I can lift the world."; Archimedes ...
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    Quote Originally Posted by jeffscott
    Okay buddy sit back and relax....

    The braking torque is limited by traction or front endo...

    The force required to make that happen is smaller for a larger rotor...

    The twisting force is lower as well....and guess what the leverarm is the same (cause the rotor is the same distance away form the fork leg....

    So guess what lower force same leverarm equals less torque on the fork leg.

    Here... really rough drawing, but I think if you are as mechanically inclined as you are trying to sound it should make sense. Very different forces when you locate the caliper a long ways out on an adapter for a big rotor.

    I'm aware that it's not a FBD, and that I don't have the forces applied properly. I sketched it with a pen in two minutes between customers. sorry to all the engineers out there
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    OK this thread is getting a bit off topic what we are supposed to be discussing hear is the effects of a powerful caliper being used on a fork limited to a 160mm rotor when you are using a 160mm rotor.
    All of you that are saying a larger rotor has the same effect on a fork as a smaller one are fired clean out your desks ant get out of this thread

  72. #72
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    Quote Originally Posted by edzwa
    OK this thread is getting a bit off topic what we are supposed to be discussing hear is the effects of a powerful caliper being used on a fork limited to a 160mm rotor when you are using a 160mm rotor.
    All of you that are saying a larger rotor has the same effect on a fork as a smaller one are fired clean out your desks ant get out of this thread
    Please, give me one more chance!

    In the case of strong calipers, I think reading the comments above leads me to conclude that the forces in general will be same everywhere, with the exception of less hand force needed for a given amount of braking. That is similar to the effect of a large rotor, but without the large rotor benefit of heat dissipation. Caliper mounting point stress probably doesn't change, because max brake force doesn't change, and the mounting position of the caliper doesn't change as it does with a larger rotor.

  73. #73
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    "A lever is a simple machine that makes work easier for use; it involves moving a load around a pivot using a force. Many of our basic tools use levers, including scissors (2 class 1 levers), pliers (2 class 1 levers), hammer claws (a single class 2 lever), nut crackers (2 class 2 levers), and tongs (2 class 3 levers). "

    Force and levers

    The force applied (at end points of the lever) is proportional to the ratio of the length of the lever arm measured between the fulcrum (pivoting point) and application point of the force applied at each end of the lever.
    Mathematically, this is expressed by M = Fd.
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  74. #74
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    Quote Originally Posted by edzwa
    OK this thread is getting a bit off topic what we are supposed to be discussing hear is the effects of a powerful caliper being used on a fork limited to a 160mm rotor when you are using a 160mm rotor.
    All of you that are saying a larger rotor has the same effect on a fork as a smaller one are fired clean out your desks ant get out of this thread
    What about armchair engineers? I'm still a couple years away from getting a piece of paper that says I know what I'm talking about.

    My BB5s couldn't lock up my front wheel, really. So I think that they exerted less force on the fork than my current brakes, which can. smilinsteve's discussion of impulse could be used to prove that. With the new brakes, I notice my front end shudders a lot if I grab a lot of brake all at once and release and grab it and release it. My BB5s were much more tolerant of bad braking technique like that. I'd rather have to brake like a smart person than have brakes that don't work, though. I couldn't say whether what I was feeling had more to do with the fork flexing or just me rocking forward on the bike. If I use the brakes correctly, though, they don't seem to effect fork travel or tracking.

    Are you considering getting a more powerful brake?
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    Quote Originally Posted by AndrwSwitch
    Are you considering getting a more powerful brake?
    No this was just a thought that popped into my head.

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    Quote Originally Posted by AndrwSwitch
    What about armchair engineers? I'm still a couple years away from getting a piece of paper that says I know what I'm talking about.
    Ok but you are officially on notice

  77. #77
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    Another trouble-making engineer. Just what the world needs.
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  78. #78
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    Quote Originally Posted by AndrwSwitch
    My BB5s couldn't lock up my front wheel, really. So I think that they exerted less force on the fork than my current brakes, which can. smilinsteve's discussion of impulse could be used to prove that. With the new brakes, I notice my front end shudders a lot if I grab a lot of brake all at once and release and grab it and release it. My BB5s were much more tolerant of bad braking technique like that. I'd rather have to brake like a smart person than have brakes that don't work, though. I couldn't say whether what I was feeling had more to do with the fork flexing or just me rocking forward on the bike. If I use the brakes correctly, though, they don't seem to effect fork travel or tracking.

    This is what I am talking about did you use the same rotor size with the new brake?

    I have had the same thing happen with my E150 I am a big guy 125Kg and when I am going downhill fast on the road and brake very hard stopping as fast as I can with my code brake and 203mm rotor I can feel the fork shudder and flex and the bike will endo but when I put a xt brake and 203mm rotor there is no shuddering and no flexing I cannot endo it takes longer to stop and the brake just feels weak compared to the code (no surprise really). So from this I must conclude that the more powerful brake is capable of putting more stress on the fork regardless of rotor size.

  79. #79
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    Quote Originally Posted by wormvine
    "A lever is a simple machine that makes work easier for use; it involves moving a load around a pivot using a force. Many of our basic tools use levers, including scissors (2 class 1 levers), pliers (2 class 1 levers), hammer claws (a single class 2 lever), nut crackers (2 class 2 levers), and tongs (2 class 3 levers). "

    Force and levers

    The force applied (at end points of the lever) is proportional to the ratio of the length of the lever arm measured between the fulcrum (pivoting point) and application point of the force applied at each end of the lever.
    Mathematically, this is expressed by M = Fd.


    You're fired

  80. #80
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    Quote Originally Posted by edzwa
    This is what I am talking about did you use the same rotor size with the new brake?

    I have had the same thing happen with my E150 I am a big guy 125Kg and when I am going downhill fast on the road and brake very hard stopping as fast as I can with my code brake and 203mm rotor I can feel the fork shudder and flex and the bike will endo but when I put a xt brake and 203mm rotor there is no shuddering and no flexing I cannot endo it takes longer to stop and the brake just feels weak compared to the code (no surprise really). So from this I must conclude that the more powerful brake is capable of putting more stress on the fork regardless of rotor size.
    I used the same rotor size. Replaced a 160mm Roundagon with a 160mm G3. I haven't noticed shuddering or flexing problems with my fork and the new brakes when I apply pressure steadily - only if I'm using poor technique. I've braked hard enough to skid the front wheel a couple of times with no fork issue, so I'm not inclined to worry about it. Haven't tried for maximum braking on the road, though. I haven't had the brakes that long, and I usually ride a road bike when I'm going to be on pavement.

    Steve mentioned the issue of stopping time in one of his early posts. His point is that if the stopping time is less, more force has to have been exerted. So that's consistent with your experience.

    No braking force can compare with plowing into a tree, though, and I think some compressions also exert a lot more force in funkier places than the brake. People certainly break their forks and frames in bad crashes, but on a mountain bike they are designed to get through compressions, water bars, etc. without breaking. That's why I'm inclined to think that on a fork that has a problem with too large a rotor it's not because of net force pushing back on the fork but because of the forces doing something weird, like exerting a torque about one of the mount points.
    "Don't buy upgrades; ride up grades." -Eddy Merckx

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    Quote Originally Posted by AndrwSwitch
    I used the same rotor size. Replaced a 160mm Roundagon with a 160mm G3. I haven't noticed shuddering or flexing problems with my fork and the new brakes when I apply pressure steadily - only if I'm using poor technique. I've braked hard enough to skid the front wheel a couple of times with no fork issue, so I'm not inclined to worry about it. Haven't tried for maximum braking on the road, though. I haven't had the brakes that long, and I usually ride a road bike when I'm going to be on pavement.

    Steve mentioned the issue of stopping time in one of his early posts. His point is that if the stopping time is less, more force has to have been exerted. So that's consistent with your experience.

    No braking force can compare with plowing into a tree, though, and I think some compressions also exert a lot more force in funkier places than the brake. People certainly break their forks and frames in bad crashes, but on a mountain bike they are designed to get through compressions, water bars, etc. without breaking. That's why I'm inclined to think that on a fork that has a problem with too large a rotor it's not because of net force pushing back on the fork but because of the forces doing something weird, like exerting a torque about one of the mount points.
    I totally agree with what you are saying hear most forks are designed extremely well and will take a hell of a beating but some forks are not so well designed (RST, Suntour and I'm sure there are more). I bet if you got one of the cheapest forks you can get put a saint brake and the largest rotor size recommended for the fork it would flex a lot and eventually fail under continues hard braking.

  82. #82
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    Quote Originally Posted by edzwa
    This is what I am talking about did you use the same rotor size with the new brake?

    I have had the same thing happen with my E150 I am a big guy 125Kg and when I am going downhill fast on the road and brake very hard stopping as fast as I can with my code brake and 203mm rotor I can feel the fork shudder and flex and the bike will endo but when I put a xt brake and 203mm rotor there is no shuddering and no flexing I cannot endo it takes longer to stop and the brake just feels weak compared to the code (no surprise really). So from this I must conclude that the more powerful brake is capable of putting more stress on the fork regardless of rotor size.

    Yup if one brake can cause and endo and the other can't....well the one that can cause the endo puts more braking torque on the fork....

    Doh....

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