Disc Brake torque - misaligning forks

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Disc Brake torque - misaligning forks

Postby glawrence2000 » Fri Jun 27, 2008 8:17 am

Hi lads and ladettes,

I ride a 29er mountain bike of which I am very proud.

I do not know whether it is a function of the larger wheels exaggerating an issue which may be evident on 26” wheeled bikes or it is due to the fact that I have large discs and very large callipered brakes over them; that I apply with great gusto with my very strong forearm muscles, considerable braking force.

The issue is that I have a misalignment of the front wheel in it’s even spacing between the LHS and RHS shock stanchions: that is to say it is closer to the left stanchion than the right.
Taking it to my LBS, he checked the ‘Dish’ of the wheel to ensure that it is symmetrical around the hub - that it is. He suggested that the forks may have a very slight bend in them with one bracket being ever so slightly higher that the other, giving the front wheel a mis-alignment within the forks.

There is no grit or obstructions in the brackets or on the axle that would make the wheel not sit in its position correctly.

LBS thought it may be a torque issue with the disc brake applying force on one side of the wheel only; effectively forcing it to try and rotate around the disc pads rather than the axle when braking heavily. This in turn apply a non-symmetrical load on the LHS shock stanchion and possibly causing a load on the shock cross member deforming it ever so slightly.

I am running Hayes Stroker Trail Hydro discs (new model brakes with very large brake pads) and 7” Hayes discs.
My wheels are Mavic 719 and the shocks are White Brothers Magic 29s.

Anyone come across this issue and got any ideas on remediation / consequence? :idea:
Cheers, Gerard
Thanks heaps.
BMC 4Stroke 03. (web nazis won't let me put a third picture up :()
Voodoo Dambala 29er.
FELT AR4 Carbon.


Postby Hawkeye » Fri Jun 27, 2008 9:11 pm

If the rotational torque of the brakes is doing it you'd see either the middle of the left fork leg flexing forward or the bottom of the left leg flexed back.

This would cause the left leg to have a verly slightly shorter distance from the dropout to the fork crown, which would tip it the other way, moving the top of the rim slightly away from the left stanchion, while twisting the axle slightly left.

While it's possible, I think torque from the brakes would be unlikely to be the culprit. I suspect your front dropouts are the reason, and a very light trimming of the bottom of the dropout cup (viewed with the bike inverted) on the disc brake side with a round file should do the trick.

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