Bicycle Mechanics - disc rotor resonating?

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deerhoof
06-23-06, 03:50 PM
Im trying to figure this out: when im on uneven terrain I hear this occasional "twang" sound coming from the back rotor. If I hit the frame in certain spots I can reproduce this noise. Im pretty sure its not my pads moving around in the caliper, its more flex coming fromt he rear end of the bike. Im using avid bb5s on a aluminum hard tail. Any fixes for this?

free_pizza
06-23-06, 04:25 PM
There is probably flex in the wheel and not the frame. The flex in the wheel will cause the rotor to hit the caliper.
mtbikerinpa
06-23-06, 09:23 PM
Rotors are succeptible to flex, yes. Tire tread, terrain, rotor pattern all effect resonance. For instance, from 15-16.5 mph my rotors make a racket on the pads. above and below the range and its fine. Change to smoother tread or softer ground, and it either changes the speed or goes away.
Keep in mind the pad clearances are very small, so the least ammount of chatter can make an effect. This is seeminly amplified in aluminum(I have both in hardtails). Aluminum resonates sharper and crisper than steel, transferring the energy to anywhere it can dissipate it, which would end up as the rotors. When this is the case, it really is not something to be concerned about. The 'flex' associated with it is not something that is simply fixed, nor is it detrimental.

deerhoof
06-23-06, 10:11 PM
Rotors are succeptible to flex, yes. Tire tread, terrain, rotor pattern all effect resonance. For instance, from 15-16.5 mph my rotors make a racket on the pads. above and below the range and its fine. Change to smoother tread or softer ground, and it either changes the speed or goes away.
Keep in mind the pad clearances are very small, so the least ammount of chatter can make an effect. This is seeminly amplified in aluminum(I have both in hardtails). Aluminum resonates sharper and crisper than steel, transferring the energy to anywhere it can dissipate it, which would end up as the rotors. When this is the case, it really is not something to be concerned about. The 'flex' associated with it is not something that is simply fixed, nor is it detrimental.


the speed part of it makes sense, i only noticed it during certain times. thank for the response.