Take the 2-minute tour ×
Bicycles Stack Exchange is a question and answer site for people who build and repair bicycles, people who train cycling, or commute on bicycles. It's 100% free, no registration required.

I have a 2005 Specialized HardRock Sport with disc brakes.

When I'm slowing down from a reasonable speed with the front brake, I noticed that the top of the front wheel appears to deflect sideways toward the disc. I can't tell when I'm riding but I suspect that the whole wheel is twisting. This doesn't seem to be safe to me, and I'm don't remember that happening before when I ride.

I tightened the QR on the front already. What else could be the cause of this or where should I start to look? Or has it always done this and I never noticed before?

share|improve this question
    
Are you sure it is the wheel and not the fork that deflects? If the fork twists, it may look like the wheel moves sideways. –  posipiet Mar 20 '11 at 16:49
    
I'm not sure. How could I tell? If I'm not riding it, you can't see the deflection. –  Jay R. Mar 20 '11 at 22:12
    
Can you quantify how much your wheel is moving to the side? Would you say that it is a couple mm or several cm? –  sixtyfootersdude Mar 21 '11 at 2:33
    
I'd say deflects maybe 6 to 10 mm or maybe little more if I'm traveling downhill at speed and trying to come to a stop. If I brake very lightly, it barely deflects at all. –  Jay R. Mar 21 '11 at 2:41
    
A deflection of 10mm or more to one side? That sounds unacceptable, surely you'd be able to notice that in the handling? –  lantius Mar 21 '11 at 22:41
show 4 more comments

4 Answers 4

up vote 5 down vote accepted

I'd take a close look at your dropout and your QR skewer. The rotating disc exerts an upward force on the disc caliper, which tends to want to push the dropout up and away from the axle (or as normally stated, pushes the axle down and out of the dropout). Some calculations at Jules & James' comprehensive site computes the force generated to be as much as 3800N.

To achieve a 6mm offset at the top of the tire you'd need the axle to move about 1mm within the dropout. Try spinning the wheel in a bike stand with the skewer loose enough to allow free sliding but not wheel ejection, and see if you can replicate the same effect you're seeing out in the real world.

share|improve this answer
    
I tried spinning the wheel and it did move just a tiny bit. I then tried to hold the brake and just push down on the wheel and I could see the QR nut moving up and down. I made the QR tighter. It seems to be better. –  Jay R. Mar 29 '11 at 3:47
    
Wow that is an amazing diagnosis. Nice work! –  Mac Jun 5 '11 at 10:21
add comment

It could be wear or poor adjustment of your hub bearings. Try lifting the bike up at the front and try moving the wheel from one side to the other. There should be little or no movement.

share|improve this answer
add comment

I suspect it's the fork that's deforming and causing the "twist". When you brake with disk brakes, enormous force is placed on one side of the fork and essentially none on the other. The fork is going to deform -- it's just a question of how much.

Caliper brakes, of course, distribute the force evenly, so you don't notice any deformation, even though there is some from the wheel pushing backwards on the fork.

share|improve this answer
add comment

It will be your spokes... they will all need tightening (on the drive side at least but get someone to check them all). This can happen if your bike comes assembled from a manufacturer.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.