The risks of running 203mm disc brake rotors?
06-07-11, 01:09 PM
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The risks of running 203mm disc brake rotors?
For my Big Dummy build I got a set of Avid BB7s with 203mm rotors. Surly said in their log that the BD can handle 203mm rotors.
Now, the Big Dummy is no more (didn't work out, too large, had to sell the frame, long story...) and I wanted to use these brakes on another bike: Nashbar aluminum MTB frame with a Surly 1x1 fork, XT hubs (9mm, QR).
I'm pretty sure the frame can handle these rotors: the indentation in the left chainstay aligns perfectly with the rotor, seems to be designed for that (I might be wrong though). The issue is the fork. Obviously, breaking forces are greater on the fork than the tail of the bike.
I've read some opinions that 203mm rotors are only safe on forks with 14mm or thicker bolt axles, not QR, due to the forces involved, that can exceed the forces caused by 160mm rotors. That there is a risk of the axle being "ejected" from the dropouts or twisting and breaking of the QR skewers or axles.
Is it really risky? I'd like to use those since I have them. I can always buy smaller rotors though to be on the safe side.
Thanks!
Now, the Big Dummy is no more (didn't work out, too large, had to sell the frame, long story...) and I wanted to use these brakes on another bike: Nashbar aluminum MTB frame with a Surly 1x1 fork, XT hubs (9mm, QR).
I'm pretty sure the frame can handle these rotors: the indentation in the left chainstay aligns perfectly with the rotor, seems to be designed for that (I might be wrong though). The issue is the fork. Obviously, breaking forces are greater on the fork than the tail of the bike.
I've read some opinions that 203mm rotors are only safe on forks with 14mm or thicker bolt axles, not QR, due to the forces involved, that can exceed the forces caused by 160mm rotors. That there is a risk of the axle being "ejected" from the dropouts or twisting and breaking of the QR skewers or axles.
Is it really risky? I'd like to use those since I have them. I can always buy smaller rotors though to be on the safe side.
Thanks!
06-07-11, 01:30 PM
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Co-Motion had 203mm rotors w/quck release skewers as an option front and rear for its Americano. Now they spec 160's I think because 203's are overkill for a loaded touring bike if not for a tandem. Call them and ask about it.
06-07-11, 05:24 PM
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For my Big Dummy build I got a set of Avid BB7s with 203mm rotors. Surly said in their log that the BD can handle 203mm rotors.
Now, the Big Dummy is no more (didn't work out, too large, had to sell the frame, long story...) and I wanted to use these brakes on another bike: Nashbar aluminum MTB frame with a Surly 1x1 fork, XT hubs (9mm, QR).
I'm pretty sure the frame can handle these rotors: the indentation in the left chainstay aligns perfectly with the rotor, seems to be designed for that (I might be wrong though). The issue is the fork. Obviously, breaking forces are greater on the fork than the tail of the bike.
I've read some opinions that 203mm rotors are only safe on forks with 14mm or thicker bolt axles, not QR, due to the forces involved, that can exceed the forces caused by 160mm rotors. That there is a risk of the axle being "ejected" from the dropouts or twisting and breaking of the QR skewers or axles.
Is it really risky? I'd like to use those since I have them. I can always buy smaller rotors though to be on the safe side.
Thanks!
Now, the Big Dummy is no more (didn't work out, too large, had to sell the frame, long story...) and I wanted to use these brakes on another bike: Nashbar aluminum MTB frame with a Surly 1x1 fork, XT hubs (9mm, QR).
I'm pretty sure the frame can handle these rotors: the indentation in the left chainstay aligns perfectly with the rotor, seems to be designed for that (I might be wrong though). The issue is the fork. Obviously, breaking forces are greater on the fork than the tail of the bike.
I've read some opinions that 203mm rotors are only safe on forks with 14mm or thicker bolt axles, not QR, due to the forces involved, that can exceed the forces caused by 160mm rotors. That there is a risk of the axle being "ejected" from the dropouts or twisting and breaking of the QR skewers or axles.
Is it really risky? I'd like to use those since I have them. I can always buy smaller rotors though to be on the safe side.
Thanks!
If you can lock the front tire with 160's, 203's won't be putting any more pressure than that on the fork.
The maximum braking force is just before lockup, once you lock up the force goes way down.
That's the theory for ABS, keep the tires just short of locking up to maximize braking performance.
06-07-11, 05:57 PM
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I've read some opinions that 203mm rotors are only safe on forks with 14mm or thicker bolt axles, not QR, due to the forces involved, that can exceed the forces caused by 160mm rotors
Last edited by zzyzx_xyzzy; 06-07-11 at 06:01 PM.
06-07-11, 05:58 PM
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I recently put 203mm's on 2 of my MTB's, and love the result (one has a 9mm the other 15mm axle).
If you can lock the front tire with 160's, 203's won't be putting any more pressure than that on the fork.
The maximum braking force is just before lockup, once you lock up the force goes way down.
That's the theory for ABS, keep the tires just short of locking up to maximize braking performance.
If you can lock the front tire with 160's, 203's won't be putting any more pressure than that on the fork.
The maximum braking force is just before lockup, once you lock up the force goes way down.
That's the theory for ABS, keep the tires just short of locking up to maximize braking performance.
06-07-11, 06:03 PM
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I'll wear a helmet.
06-07-11, 09:01 PM
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I have been using 8" rotors with a standard QR on my mountain bike for years with no issues... make sure the QR is fully tightened before riding (or replace them with locking skewers if you're concerned still) and you should be good to go.
06-08-11, 05:27 AM
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Yup. I have skewers with hex heads.
06-08-11, 06:58 AM
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Just as bad. The trouble with disks and QR skewers is that the force put into the fork by the disk can cause the skewers to unscrew over time. Standard QR skewers tighten partially like a nut and finish off with a flip of the cam lever, which is likely to actually make them tighter than most people can get hex bolts.
Most people ride standard QRs with disks without problem, but the possibility does exist... as mentioned by igknighted, make sure they are tight before riding.
As for larger rotors... i don't know... I have Hayes hydraulic brakes that came with a 160mm rotor and I seem to recall when I was searching for the fork adapter that they reccomended against 8" or larger rotors with standard QRs, although I just looked at the hayes website and I don't see that warning...
Anyhoo, 8" rotors are considered by many to be overkill for anything other than DH racing or tandems, so if you have trouble fitting them and have to go to 6" don't worry.
06-08-11, 08:00 AM
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I bought these for a Big Dummy, they are probably an overkill for a regular bike. Although, 160mm rotors were a beat weak for a loaded touring bike from my experience. Besides, I already have these and the adapters so I'd like to use them rather than buy new rotors.
I've been running BB7s with 160mm rotors for many years and never had any problems with QR skewers loosening. But I do check my skewers tightness often.
BTW, I disagree that a QR lever can be tightened more than one with a hex head that is tightened with a wrench. I would be never able to tighten a QR the way I can the ones with hex heads.
I've been running BB7s with 160mm rotors for many years and never had any problems with QR skewers loosening. But I do check my skewers tightness often.
BTW, I disagree that a QR lever can be tightened more than one with a hex head that is tightened with a wrench. I would be never able to tighten a QR the way I can the ones with hex heads.
06-08-11, 08:09 AM
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As for larger rotors... i don't know... I have Hayes hydraulic brakes that came with a 160mm rotor and I seem to recall when I was searching for the fork adapter that they reccomended against 8" or larger rotors with standard QRs, although I just looked at the hayes website and I don't see that warning...
Avid still has a old PDF in their archives that doesn't recommend using large rotors with QR axles.
The trouble with disks and QR skewers is that the force put into the fork by the disk can cause the skewers to unscrew over time.
This is probably why a lot of mtb fork manufacturers have now changed the design of the QR dropouts to face more forward instead of downward to counter the downward force from the brake on the axle wanting to eject it from the dropout. The new dropout design is probably why you can now see mtb forks with QR axles rated to 210mm rotors where previously they were mostly rated to 160mm rotors with the occasional fork rated to the 185mm rotors.
Cotic came up with a simple solution to disc brake forces & possible problems.
From Cotic:
Our ROADHOG forks caused a bit of a stir when they were released to compliment our ROADRAT frame. Why? Well, as you can see from the photo, the disc mount is positioned on the front right of the fork rather than the more usual left rear. As always with Cotic products, there were firm engineering reasons for placing it where it is, and here's the lowdown:
Forces
Axle movement under disc braking is now a fairly well known phenomenon in mountain biking. Basically with the caliper positioned left rear (usual place), and a vertical dropout, there is a reaction of the braking force which tends to push the axle out of the dropout, hence you're relying on the friction of your QR to counteract this rather than a completely mechanical stop. Most manufacturers have moved to a slightly forward facing dropout to counteract this.
We've actually never experienced any trouble with this on our mountain bikes, but during prototype testing of the ROADHOG fork - which had the disc mount in the usual left rear position - we did actually experience some axle movement. Nothing actually came undone, but after some serious downhill braking on fast roads, the rotors would be rubbing a touch and the wheel would 'clunk' home in the dropout when we undid the axle nuts. This was clearly unacceptable, so we set about finding a solution. Our first port of call was forward facing dropouts, but these would have required custom cut version which would have increased the fork cost disproportionately. Therefore the design problem became how to contain the disc forces whilst using an off the shelf vertical dropout. The solution, as you can see, was to place the disc mounting on the front. This puts all the forces into the dropout, and is still a completely normal International Standard setup, so it takes any IS caliper. It also gets the brake away from the back of the fork which brings us to....
Mudguards
With the ROADRAT's do-it-all brief, it was clearly important to provide mudguard mountings. However, trying to figure out if it was possible to mount a mudguard regardless of the type of disc brake used when the disc mount was on the back of the fork was turning into a quest, and a futile one at that. The beauty of the front right disc mounting was that it killed two birds with one stone: The disc forces were contained, and no matter what brake system you used, you'd always be able to get a mudguard stay fitted.
Conclusion
The front right disc mount position solved a number of problems we were faced with during the design phase:
* Disc forces contained
* IS disc mount
* Full length mudguards always usable
*Standard QR compatible dropout retained
Which gives you a fantastically versatile fork with no drawbacks.
Forces
Axle movement under disc braking is now a fairly well known phenomenon in mountain biking. Basically with the caliper positioned left rear (usual place), and a vertical dropout, there is a reaction of the braking force which tends to push the axle out of the dropout, hence you're relying on the friction of your QR to counteract this rather than a completely mechanical stop. Most manufacturers have moved to a slightly forward facing dropout to counteract this.
We've actually never experienced any trouble with this on our mountain bikes, but during prototype testing of the ROADHOG fork - which had the disc mount in the usual left rear position - we did actually experience some axle movement. Nothing actually came undone, but after some serious downhill braking on fast roads, the rotors would be rubbing a touch and the wheel would 'clunk' home in the dropout when we undid the axle nuts. This was clearly unacceptable, so we set about finding a solution. Our first port of call was forward facing dropouts, but these would have required custom cut version which would have increased the fork cost disproportionately. Therefore the design problem became how to contain the disc forces whilst using an off the shelf vertical dropout. The solution, as you can see, was to place the disc mounting on the front. This puts all the forces into the dropout, and is still a completely normal International Standard setup, so it takes any IS caliper. It also gets the brake away from the back of the fork which brings us to....
Mudguards
With the ROADRAT's do-it-all brief, it was clearly important to provide mudguard mountings. However, trying to figure out if it was possible to mount a mudguard regardless of the type of disc brake used when the disc mount was on the back of the fork was turning into a quest, and a futile one at that. The beauty of the front right disc mounting was that it killed two birds with one stone: The disc forces were contained, and no matter what brake system you used, you'd always be able to get a mudguard stay fitted.
Conclusion
The front right disc mount position solved a number of problems we were faced with during the design phase:
* Disc forces contained
* IS disc mount
* Full length mudguards always usable
*Standard QR compatible dropout retained
Which gives you a fantastically versatile fork with no drawbacks.
06-08-11, 08:16 AM
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Ha! So they put the disk brake in the front, on the right leg? That certainly solves the problem but I wonder if it introduces other, new problems. After all, if it was that simple most manufactures would do the same, IMHO.
06-08-11, 08:41 AM
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Who knows the answer because I certainly don't.
Canyon designed a ridiculous looking bike with a disc brake on the front of the right fork leg and a disc brake on the back of the left leg.
https://www.canyon.com/_en/technology/project68.html
Last edited by cobba; 06-08-11 at 11:37 AM.
06-08-11, 09:24 AM
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At that point you no longer have a QR skewer. If it were me I'd consider either a regular lever operated "toolless" skewer or just bite the bullet and go with a solid axle and regular axle nuts. Using a tool to operate a skewer takes away all the advantages of using a skewer in the first place and leaves you with a wheel that can't be tightened as effectively as a solid axle with track nuts. For the wrench get as light a 15mm box end as you can find then cut it off to be about 5 inches long and clean up the cut end with a grinder or Dremel.
All the physics you and others have quoted in this thread are valid. But the forces are effectively contained as shown by igknighted, myself, and the legions of serious off roaders that did fine with 8 inch discs before the beefy front axles became an option. Basically you're obsessing over issues which are not issues at all. The only factor that can come into play is that if a fork leg is a bit spindly you can end up with it vibrating and causeing some bad shuddering from the rotor forces being contained.
There's no reason at all not to mount the caliper on the right fork leg in front of the leg as shown. I strongly suspect the "traditional" mounting behind the leg was and still is inspired by motorcycles which all mount the calipers behind the leg. In their case it's to keep the typically much larger mass of the caliper and mount in closer to the pivoting axis and avoid the swinging mass becoming any heavier than it already is. But bicycle components are so light that it's not an issue. And as mentioned in that article it cures a lot of woes for mounting other parts.
And besides, in the end the forks these days all have those "lawyer lip" tabs. So a loosened wheel should still give some warning before it leaps away to freedom.
All the physics you and others have quoted in this thread are valid. But the forces are effectively contained as shown by igknighted, myself, and the legions of serious off roaders that did fine with 8 inch discs before the beefy front axles became an option. Basically you're obsessing over issues which are not issues at all. The only factor that can come into play is that if a fork leg is a bit spindly you can end up with it vibrating and causeing some bad shuddering from the rotor forces being contained.
There's no reason at all not to mount the caliper on the right fork leg in front of the leg as shown. I strongly suspect the "traditional" mounting behind the leg was and still is inspired by motorcycles which all mount the calipers behind the leg. In their case it's to keep the typically much larger mass of the caliper and mount in closer to the pivoting axis and avoid the swinging mass becoming any heavier than it already is. But bicycle components are so light that it's not an issue. And as mentioned in that article it cures a lot of woes for mounting other parts.
And besides, in the end the forks these days all have those "lawyer lip" tabs. So a loosened wheel should still give some warning before it leaps away to freedom.
06-08-11, 09:36 AM
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I use the hex skewers mainly as a theft deterrent, some added security. I don't want to rebuild my wheels and the XT hubs I use don't allow for axle replacement, I was told. Otherwise I would have definitely gone with solid axles and nuts. I never liked QR anyway.
06-08-11, 09:37 AM
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Cobba, I'm curious why you think the Canyon bike is ridiculous? Is it the idea of using discs at all on a road bike or is it the fact that Canyon chose to go with a dual disc setup? If you read the reasoning for this in the writeup it makes perfect sense. Having burned my hand on even a single 8 inch rotor after stopping at the end of a long'ish downhill trail one time I can certainly confirm that discs generate a lot of heat. Getting rid of that heat effectively is very much a consideration. And given the suppleness of road forks in order to generate the comfort level they need using two discs to balance the forces makes fine sense. And until the bike brake makers produce mirror image sets of calipers using one on the back and the other on the front of the leg is the only way to get around the issue. For my part I think their solution looks fine. I'm not a road racer that does long climbs and descents though so I'm not in any position to know if the idea of disc brakes for road bikes is needed or not.
06-08-11, 09:38 AM
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Ah, that makes sense.
Last edited by BCRider; 06-08-11 at 09:45 AM.
06-08-11, 09:46 AM
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In any event if you want better disc braking performance the ONLY disadvantage to a 203 over a 160 is some slight extra weight. As zzyzx_xyzzy mentioned above the actual forces at the pads and axle is less with bigger rotors. It's actually least of all with rim brakes and the forces needed to stop multiply as the position of the brake track moves inwards, away from the tire contact patch. So yeah, the smaller the disc the more force is needed to stop at any given rate and the more lever force the axle sees at the dropout. It's all about leverage. Consider a lever and pivot where the one side is as long as the distance from the axle to the tire contact patch. The other side is as long as the distance from the axle to the center of the brake pads. You're going to push on the short "disc" side hard enough to lift up 10 lbs on the "tire" side. The longer the lever on the "disc" side the easier it is. Does that help?
06-08-11, 09:47 AM
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Yeah, with my next builds I will definitely look into solid axle hubs with nuts. It's a bit unknown area for me though, I'll need to learn about these standards more and not all frames/forks can take larger diameter axles. I'm afraid though that QR are too popular and took over most frame designs and bolt on through axles are mainly found in BMX and DH frames and forks.
I take wheels off so infrequently and I usually carry lots of crap anyway with me so I don't need QR levers and the additional safety of a bolt-on axle sounds like something worth for me to consider.
I take wheels off so infrequently and I usually carry lots of crap anyway with me so I don't need QR levers and the additional safety of a bolt-on axle sounds like something worth for me to consider.
06-08-11, 09:50 AM
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In any event if you want better disc braking performance the ONLY disadvantage to a 203 over a 160 is some slight extra weight. As zzyzx_xyzzy mentioned above the actual forces at the pads and axle is less with bigger rotors. It's actually least of all with rim brakes and the forces needed to stop multiply as the position of the brake track moves inwards, away from the tire contact patch. So yeah, the smaller the disc the more force is needed to stop at any given rate and the more lever force the axle sees at the dropout. It's all about leverage. Consider a lever and pivot where the one side is as long as the distance from the axle to the tire contact patch. The other side is as long as the distance from the axle to the center of the brake pads. You're going to push on the short "disc" side hard enough to lift up 10 lbs on the "tire" side. The longer the lever on the "disc" side the easier it is. Does that help?
06-08-11, 09:50 AM
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Another disadvantage of a 203 on the rear is getting the rear wheel on without nicking the paint on the chainstay.
06-08-11, 09:55 AM
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One disadvnatge is that I can't fit a Grenfield rear kickstand with a rotor larger than 160mm
A dual V kickstand mounted under the BB should solve the problem. Lucky, this frame doesn't run cables under the BB.
06-08-11, 09:58 AM
#24
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Did you see the example for leverage I posted above?
I'm not sure why you can't rebuild an XT hub to have a solid axle. I've had a few that I stripped, cleaned and lubed over the years and all of them used a fairly regular axle. Assuming that you can find a quality solid axle with the same threading to accept the cones and locking nuts I can't figure out what the issue would be. Looking over a few of the various parts diagrams for the Shimano XT hubs at https://techdocs.shimano.com/techdocs...&bmUID=iLhU.Rp I'm not seeing any special axles other than the large axle styles that would preclude such a switch.
I'm not sure why you can't rebuild an XT hub to have a solid axle. I've had a few that I stripped, cleaned and lubed over the years and all of them used a fairly regular axle. Assuming that you can find a quality solid axle with the same threading to accept the cones and locking nuts I can't figure out what the issue would be. Looking over a few of the various parts diagrams for the Shimano XT hubs at https://techdocs.shimano.com/techdocs...&bmUID=iLhU.Rp I'm not seeing any special axles other than the large axle styles that would preclude such a switch.
06-08-11, 10:02 AM
#25
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I've got some woodman mt bike hubs with solid axles. You should be able to replace a standard shimano xt/ deore qr axle with solid bolt on axle. Just get the right diameter "9mm?" thread pitch, and long enough to go outside of the fork/dropout. There are also a variety of mt bike through axles these days. My hadley rear hub has a hollow 9mm through axle with a 6 mm hex wrench to fasten the other side.