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Is this the result of wheel flex?

Road Cycling ďIt is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.Ē -- Ernest Hemingway

Is this the result of wheel flex?

Old 03-18-19, 10:17 AM
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FlashBazbo
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Is this the result of wheel flex?

In the rim brake days, you could quickly diagnose wheel flex because, under power, the rear brake pads would rub the wheel. Now, I've got disk brakes. And, under power, I get a rub on the rear brake disk that happens in time with my right leg power / over-the-top-and-down stroke. Is this the result of wheel flex? Hub flex?? And . . . are there any easy solutions short of re-lacing the rear wheel with sturdier spokes?

[EDIT: I'm also wondering if it might be caused by flex in the rear triangle of the bike. Has anyone else had the same problem on a road disk bike?]

Last edited by FlashBazbo; 03-18-19 at 11:00 AM.
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Old 03-18-19, 12:01 PM
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Originally Posted by FlashBazbo View Post
In the rim brake days, you could quickly diagnose wheel flex because, under power, the rear brake pads would rub the wheel. Now, I've got disk brakes. And, under power, I get a rub on the rear brake disk that happens in time with my right leg power / over-the-top-and-down stroke. Is this the result of wheel flex? Hub flex?? And . . . are there any easy solutions short of re-lacing the rear wheel with sturdier spokes?

[EDIT: I'm also wondering if it might be caused by flex in the rear triangle of the bike. Has anyone else had the same problem on a road disk bike?]
Is the frame damaged? I'd look all over the rear triangle for cracks.

Is there any play in the bearings?
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Old 03-18-19, 12:23 PM
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For rim brakes, rub is often caused by wheel too stiff.

https://www.slowtwitch.com/Tech/Debu...ness_3449.html


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Old 03-18-19, 01:52 PM
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It's probably not anything to do with the wheel. The disc is attached to the hub directly, and the hub is not going to deflect much from pedaling forces. It most likely is flex in the bike frame causing a slight movement of the caliper relative to the disc. Not sure what the solution is to that, as it's not as easy as just opening the calipers with the adjustment screw like you can on rim brakes. What you can do is to make sure the calipers are aligned to the discs well and that the discs are straight and not warped. Although, since you already point out that the rubbing is tied to pedal position and not rotational wheel position, it's probably not disc warpage.
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Old 03-18-19, 02:35 PM
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Originally Posted by TimothyH View Post
For rim brakes, rub is often caused by wheel too stiff.

https://www.slowtwitch.com/Tech/Debu...ness_3449.html


-Tim-
For my last set of rim brake wheels (ENVE) with this problem, the solution was a factory re-lace with beefier spokes. Problem immediately solved.

[EDIT: This is consistent with the article in the link. Stiff rim with too-light spokes. Solution: Stronger spokes.]

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Old 03-18-19, 02:50 PM
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Originally Posted by FlashBazbo View Post
Solution: Stronger spokes.
The solution is stiffer spokes, not stronger. Making a spoke thicker in the midsection - which makes it stiffer - can sometimes actually make the spoke as a whole more prone to fatigue failure. The higher spring rate means that it isn't stretched as far when under a given amount of tension, so less overall flex in the wheel is required to de-tension that spoke under load.

Stiffer spokes with thicker midsections do tend to have higher tensile strengths (if both spokes are otherwise made equal), but this isn't usually relevant, because tensile failure at the spoke midsection just about never happens in the real world. So "spoke strength" is kind of complicated.

Last edited by HTupolev; 03-18-19 at 06:53 PM.
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Old 03-18-19, 02:53 PM
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Originally Posted by HTupolev View Post
The solution is stiffer spokes, not stronger. Making a spoke thicker in the midsection - which makes it stiffer - can sometimes actually make the spoke as a whole more prone to fatigue failure. The higher spring rate means that it isn't stretched as far when under a given amount of tension, so less overall flex in the wheel is required to de-tension that spoke under load. Stiffer spokes with thicker midsections do tend to have higher tensile strengths (if both spokes are otherwise made equal), but this isn't usually relevant, because tensile failure at the spoke just about never happens in the real world. So "spoke strength" is kind of complicated.
At any rate, it fixed my problem.
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Old 03-18-19, 02:59 PM
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But that was then. That was rim brakes.

What about the problem with disk brakes? I plan to inspect the frame this evening, but I doubt I will find a crack. The bike doesn't do anything else untoward. But, as with a lot of high performing carbon framesets, the seat stays are extremely thin. Maybe they just flex enough to allow things to get out of alignment back there.
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Old 03-18-19, 06:45 PM
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I suspect the frame. I mean if it were me I'd check the rotors for true to rule it out, but the timing makes that pretty unlikely. Still, if the rotor is slightly warped and the frame is flexing slightly, fixing the rotor might be enough.
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Old 03-18-19, 07:36 PM
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Just loosen the caliper to frame mount bolts to reset the caliper position. Maybe first loosen the bolts squeeze the brake lever and re tighten the bolts.
You may need to try different techniques to get it right.
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Old 03-19-19, 05:23 AM
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Originally Posted by jbchybridrider View Post
Just loosen the caliper to frame mount bolts to reset the caliper position. Maybe first loosen the bolts squeeze the brake lever and re tighten the bolts.
You may need to try different techniques to get it right.
No, the brake doesn't rub 99.9% of the time. In the stand, it is perfectly centered in the caliper. Couldn't be better.

It's just when climbing a steep climb out of the saddle that it rubs during my right leg power stroke. I'm beginning to be convinced that it's a frame flex issue.
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Old 03-19-19, 08:58 AM
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Welcome to disc brakes. The clearances are way too small for roadies. We donít like disc rub. Mtb guys have always just ignored it.

Ive always been of the mind that a dual system is needed. Spring retract with a hi/low hydraulic circuit.
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Old 03-19-19, 09:03 AM
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Originally Posted by Psimet2001 View Post
Welcome to disc brakes. The clearances are way too small for roadies. We donít like disc rub. Mtb guys have always just ignored it.

Ive always been of the mind that a dual system is needed. Spring retract with a hi/low hydraulic circuit.
That makes sense. I can't feel the rub, it's just a quiet brushing sound. I can live with it. It doesn't happen that often.
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Old 03-19-19, 09:29 AM
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Originally Posted by FlashBazbo View Post
In the rim brake days, you could quickly diagnose wheel flex because, under power, the rear brake pads would rub the wheel. Now, I've got disk brakes. And, under power, I get a rub on the rear brake disk that happens in time with my right leg power / over-the-top-and-down stroke. Is this the result of wheel flex? Hub flex?? And . . . are there any easy solutions short of re-lacing the rear wheel with sturdier spokes?

[EDIT: I'm also wondering if it might be caused by flex in the rear triangle of the bike. Has anyone else had the same problem on a road disk bike?]
Which bike is this?
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Old 03-19-19, 09:31 AM
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Originally Posted by noodle soup View Post
Which bike is this?
BMC SLR01 TeamMachine. As with a lot of current bikes, you can very easily flex the seat stays with moderate hand pressure.
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Old 03-19-19, 09:31 AM
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Originally Posted by Psimet2001 View Post
Welcome to disc brakes. The clearances are way too small for roadies. We donít like disc rub. Mtb guys have always just ignored it.

Ive always been of the mind that a dual system is needed. Spring retract with a hi/low hydraulic circuit.
This really isn't typical of disc brakes and is probably the frame. Lots of people complained about breaker run with rim brakes, there's an article about it in this very thread.
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Old 03-19-19, 10:38 AM
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Originally Posted by Seattle Forrest View Post
This really isn't typical of disc brakes and is probably the frame. Lots of people complained about breaker run with rim brakes, there's an article about it in this very thread.
It's way more common on disc brakes than rim brakes by far. In some instances With new pads and new rotors you sometimes have to tear the system down and machine the mounts because the tiny amount of un-parallelism in the mounts will cause rotor rub on the pads and uneven wear. Even with the pistons all the way back in. In a lot of systems the pads only really come back as much as the seals flex when they push out around the piston.
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Old 03-19-19, 04:24 PM
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I'm in to MTB'ing so we have all kinds of disc issues/solutions.
1- Inspect the rear hub bearings to ensure there is proper preload and that everything is correct. Many times a disc rub issue can be traced to the hub.
2- Make sure that there is the proper amount of fluid in the reservoir, too much is typically worse. You need some air volume "above" the fluid to ensure suction to retract the pads. Not only do the piston seals distort (upon brake activation) and then return to shape (to retract slightly) but the volume of air "above" the system also helps.
3- Use the bleed blocks! Many people do the bleed with a disc in, and the system gets overfilled (see above). This causes minimal or virtually non-existent retraction.
4- Caliper has to be DEAD straight, and PERFECTLY centered! If it's not, the pistons will search for equalization and it can/will squeal (one of the causes) or rub on the disc runout spot.
5-No disc is perfectly straight, and there will always be a "high" spot. If your pads do not retract uniformly or enough, then disc rub is inevitable

Check your hub first, that's the easiest check. There is more to disc brake troubleshooting than this, but these are the common issues that I see.

ETA- My disc brake experience is currently limited to MTB, so I have not personally witnessed frame flex induced disc rub. With road frames, this is entirely possible. MTB frames are pretty stout (even the light ones).

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Old 03-19-19, 04:42 PM
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if you were running 160mm rotor try 140mm as the caliper will be closer to the axle, from the picture, the chain stays looks pretty beefy, how much does the frame weight?
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Old 03-19-19, 08:51 PM
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Originally Posted by TimothyH View Post
For rim brakes, rub is often caused by wheel too stiff.

https://www.slowtwitch.com/Tech/Debu...ness_3449.html


-Tim-
Or rather a stiff rim combined with a not "equally" stiff lacing.
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Old 03-19-19, 09:12 PM
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Originally Posted by Racing Dan View Post
Or rather a stiff rim combined with a not "equally" stiff lacing.
some people get confused by this.
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Old 03-22-19, 05:11 PM
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The rear disc brake on my mountain bike rubbed when jamming hills or hard acceleration. The rear end of the bike flexed enough to cause this. Wheel flex should not cause any issues with disc brakes.
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