Fork-mount V-Brake problem: common or bizarre?

On 10 Aug 2006 04:25:04 -0700, "Borrall Wonnell" <[email hidden]>

Quoted message said:

Setup:
Front brake is centered, with a reasonable gap between pad and rim. By
"reasonable gap", I'd say you could fit two quarters in there...pulling
the brake lever doesn't give you instant braking power.

Trail Symptom / Quick Fix:
When riding, the pad sometimes audibly brushes against the rim,
particularly when turning in one direction. Brakes appear to be
off-center when riding. Adjusting the centering did not 100% fix the
problem. Had to increase the gap, resulting in a very weak front
brake.

Investigation:
When compressing the fork, the brake arm with the cable fixing bolt is
being pulled towards the rim. At first glance, this looks very
strange. When under compression, the brake cable housing has a tighter
bend radius. Would this increase the effective length of the housing
enough to pull the left brake arm significantly towards the rim? If
so, shouldn't this also pull the right brake arm?

I checked the pivots and everything seems smooth. In the end I went
for compromise...I increased the fork pre-load to maximum to minimize
the amount of sag when just sitting on the bike, and as a result I was
able to reduce the pad-rim gap enough to provide acceptable braking
power without any rubbing. Still not close to an ideal setup.

Has anyone else experienced this? Of the three other MTB's I've used,
this is the first time I've seen this particular problem. The V-Brakes
in question are some Tektro stuff on a $500 hardtail.

I'd look at the fork (especially at this price point). It is very common for
fork flex to cause a brake pad to brush the rim during turns and out of the
saddle cranking.

There is a bit of slop in every component involved. Brake centering is never
perfect, cables do push, shove and hang up a bit, forks and rims flex relative
to one another. The bottom line is that occasional pad rub against a rim is
completely unimportant in the overall scheme of things. You aren't exactly
scrubbing off speed with it and it is far less harmful than adjusting your
brakes into a nonfunctional condition to avoid it.

Ron

Borrall Wonnell said:

Setup:
Front brake is centered, with a reasonable gap between pad and rim. By
"reasonable gap", I'd say you could fit two quarters in there...pulling
the brake lever doesn't give you instant braking power.

Trail Symptom / Quick Fix:
When riding, the pad sometimes audibly brushes against the rim,
particularly when turning in one direction. Brakes appear to be
off-center when riding. Adjusting the centering did not 100% fix the
problem. Had to increase the gap, resulting in a very weak front
brake.

Investigation:
When compressing the fork, the brake arm with the cable fixing bolt is
being pulled towards the rim. At first glance, this looks very
strange. When under compression, the brake cable housing has a tighter
bend radius. Would this increase the effective length of the housing
enough to pull the left brake arm significantly towards the rim?

It shouldn't, in my opinion. I think you are looking in the wrong
place for the problem. Closely examine the arch that connects the two
fork legs together. I believe that you will find that it is cracking
at the point where it joins to one of the fork tubes. When this
occurs, the forks compress unequally, and the V-brake will react to
the motion by the greater-compression side's brake pulling against the
rim as that leg compresses. Running the preload up changed the
results; this, in my opinion, is very strong evidence that you have a
fork failure in progress. This is not a "ride it and see" situation;
if you do not feel qualified to examine the fork, take it to someone
who is. Continuing to ride it could result in the loss of the wheel
at speed.

--
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Werehatrack said:

Borrall Wonnell said:

Setup:
Front brake is centered, with a reasonable gap between pad and rim.
By "reasonable gap", I'd say you could fit two quarters in
there...pulling the brake lever doesn't give you instant braking
power.

Trail Symptom / Quick Fix:
When riding, the pad sometimes audibly brushes against the rim,
particularly when turning in one direction. Brakes appear to be
off-center when riding. Adjusting the centering did not 100% fix the
problem. Had to increase the gap, resulting in a very weak front
brake.

Investigation:
When compressing the fork, the brake arm with the cable fixing bolt
is being pulled towards the rim. At first glance, this looks very
strange. When under compression, the brake cable housing has a
tighter bend radius. Would this increase the effective length of
the housing enough to pull the left brake arm significantly towards
the rim?

It shouldn't, in my opinion. I think you are looking in the wrong
place for the problem. Closely examine the arch that connects the two
fork legs together. I believe that you will find that it is cracking
at the point where it joins to one of the fork tubes. When this
occurs, the forks compress unequally, and the V-brake will react to
the motion by the greater-compression side's brake pulling against the
rim as that leg compresses. Running the preload up changed the
results; this, in my opinion, is very strong evidence that you have a
fork failure in progress. This is not a "ride it and see" situation;
if you do not feel qualified to examine the fork, take it to someone
who is. Continuing to ride it could result in the loss of the wheel
at speed.

Yes; my old Manitou Skareb with the old casting began to crack at the arch,
with similar results to what you're getting. The fork would flex laterally
when compressed, causing the whole brake assembly to [censored] over to one side.

Otherwise, the situation looks pretty normal. Flexy wheels and lightweight
stuff will cause rim rub in turns on MTBs, which is pretty normal.
--
Phil Lee, Squid