Bicycle Mechanics - Rear wheel 'buckles' slightly when braking. Why?
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03-23-11, 02:52 PM
I have an issue with my rear wheel on my nearly nearly new Trek Hybrid 7100 which i suspect is just annoying rather than anything dangerous or serious.
I got the bike just for commuting 8 mile a day, and the odd spot of family touring and I'm liking the bike so far. However, I noticed when I was cleaning the frame that the rear wheel is 'buckling' ever so slightly when the brakes are applied.
Now, the wheel is running 100% true, I've checked and so have my LBS, so its only becoming 'misaligned' when the brakes are squeezed. Its as if the brakes are pushing it out of line. I could actually feel the play in the wheel by pulling it side to side.
So I took it back to my LBS as I suspected it was the axle or just the cones. They adjusted the cones and checked the wheel for trueness, which was fine.
Theres no more play in the wheel, but I'm still getting the issue with the wheel when I brake. Is it the brakes causing it? Is it normal? Its hardly noticeable but thats not the point - why is there lateral movement when braking?
Any ideas other than cones or trueing? :cry:
There's movement because bike frames flex -- nature of the beast. I've watched the seatstays on V-brake bikes flex out during braking as much as 5-6mm.
Sounds to me like there's a LITTLE BIT of inequality in the stays. No two metal tubes will be exactly identical. There may just be a bit of a mismatch.
Don't get worked up -- sounds like the issue, at least to the degree that it would affect the ride, has been fixed.
If you're talking about the rim being pushed sideways by the brakes try this.
Have someone apply the brakes while you stand directly behind the bike. If the caliper pushes the wheel sideways a bit it isn't an issue, it shouldn't me more than 1/8" tops.
You can possibly eliminate it entirely by loosening the brakes mounting nut, applying the brake which will now float to the rim's position then re-tightening the bolt while holding the brake there. Release and see if it opens evenly. If not perfectly centered open, but clears the spinning wheel you're good. Try again and see if the brakes move the wheel, which they now shouldn't
If you're really finicky you can adjust the balancing screw, so the brake stays centered on the same location both open and closed.
BTW- Dual pivots mounted uncentered is extremely common, with most mechanics just bolting them on then using the balancing screw to center the open position. You can often see the mis-adjustment reflected in shoes which are at different heights in the slots on either side.
03-23-11, 03:48 PM
Thanks for these replies - really helpful
I've just checked the brakes again, it definitely is the calliper on one side pushing the rim slightly but it's only by about 1/8" as you said.
LBS weren't worried about it and I trust them but it was just that I'd never seen this before on either my roadie or my MTB. The brakes on this bike are terrible stock v brakes that have constantly needed readjusting so I think I'll have a bit of a play with them again to try and minimise it. Otherwise I'll try not to worry over it
03-23-11, 03:53 PM
BTW, Wouldn't the brakes be toed in incorrectly if I were to loosen the brake nuts and retighten with the brakes applied?
yes, the centering method I gave you earlier was for dual pivot calipers, which it turns out you don't have. I haven't seen the problem as often on V-type brakes. These would require a decent amount of imbalance between the springs in either arm to deflect the rim. They can also be pushed to either side by too short or long a housing loop coming from the frame, which is then overcompensated for in the spring settings.
It'll take a bit of detective work to get these to be neutral on the rim when closed but it can be done.
As to the toe in question. It will move when you loosen the shoes, but rear brakes require little or no toe-in anyway, because the rear pointing bosses don't flex to a toe out position under load as much as fronts do. If you want toe-in wrap a rubber band around the back of each shoe to hold the heel out as you position and tighten the shoe. You want no more than the thickness of a dime.
03-23-11, 04:23 PM
Just had a look again and the centering of the rear brakes can definitely be improved... The right pad hits the rim first... and that is what is causing the wheel to be pushed 'off' its true alignment when the brakes are tightly closed.
It took me a long time to be happy with the centering of the front brakes but I have always been happy with rear brakes - should have checked more closely.
As I said before, its 1/8" movement at worst, so I'm thinking correct centering is at least best practice if not a complete remedy...
03-23-11, 04:38 PM
Right, centred as best as i can by eye, the rim bending is now minimal but still visible. Its a $400 bike, so I'm not gonna cry over it, I'll just ignore it and take it as normal behaviour. The most important thing is that the brakes are, in general, pretty good at stopping me on this bike. A minimal amount of movement is ok, plus - wheels are true and cones are fine. Thats all that matters
Right, centred as best as i can by eye, the rim bending is now minimal but still visible. Its a $400 bike, so I'm not gonna cry over it, ...
Glad you got it better. It's a shame that the mechanic couldn't be bothered.
Brakes are the poor stepchild of the bike world. Mechanics will spend any amount of time to get the shifting right, but use a "good enough for government work" attitude when it comes to the brakes. It isn't because yours is a $400 bike, I see the same thing in $4,000 and more bikes. If it doesn't rub on the tires, and clears the moving wheel when off that's good enough.
03-23-11, 05:16 PM
You're right, theres nothing technically wrong with it, it just isn't 100% right - thats why it is so annoying.
I need to completely tighten the brakes when I get more time, one of the brake arms is hardly moving, its almost like its getting left behind when the cable tightens. It's the other arm that is putting most of the pressure on the rim, causing the slight displacement.
Its all in the learning process i suppose :rolleyes:
03-23-11, 09:18 PM
V brakes have an adjustment screw on each arm that allows the pads to be centered on the rim. Some canti.'s do too.
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