Front V-brake friction in pivots?

Brian Ray

First annual teardown and rebuild of my mountain bike. Mounted the rear
Deore XT V-brake arms to the frame bosses using the recommended torque (see
http://bike.shimano.com/product_ima...ges/BR-M760.pdf), the brake
arms pivot as if on glass. But when I mount the front brake arms to my
Marzocchi fork's bosses, after tightening the fixing bolts to the same
torque as that in the rear, the brake arms bind. There's no cables attached
yet so the friction's in the brake itself. I can move the arms but the
friction is pronounced, unlike the arms in the rear. Backing off the fixing
bolts to what seems like an unsafe, low torque removes the friction.

This doesn't make any sense to me. The bolts pass through, from what I can
tell, solid alloy knuckles. It doesn't seem like the bolts could deform the
knuckles to where they would bind the pivots. Or if they did, it doesn't
seem like it should happen to the front brakes but not the rears.

Any help is appreciated,

Brian Ray

Paul Southworth

In article <T3efc.36790$2H5.33536@newssvr29.news.prodigy.com>,
Brian Ray <be_ray@pacbell.net> wrote:
>First annual teardown and rebuild of my mountain bike. Mounted the rear
>Deore XT V-brake arms to the frame bosses using the recommended torque (see
>http://bike.shimano.com/product_ima...ges/BR-M760.pdf), the brake
>arms pivot as if on glass. But when I mount the front brake arms to my
>Marzocchi fork's bosses, after tightening the fixing bolts to the same
>torque as that in the rear, the brake arms bind. There's no cables attached
>yet so the friction's in the brake itself. I can move the arms but the
>friction is pronounced, unlike the arms in the rear. Backing off the fixing
>bolts to what seems like an unsafe, low torque removes the friction.
>
>This doesn't make any sense to me. The bolts pass through, from what I can
>tell, solid alloy knuckles. It doesn't seem like the bolts could deform the
>knuckles to where they would bind the pivots. Or if they did, it doesn't
>seem like it should happen to the front brakes but not the rears.

Sounds like the cantilever stud is shorter than the bushing in the
brake arm. You can file the bushing to shorten it. It's also easy
to screw up the brake arm this way.

--Paul

Chris Zacho The Wheelman

Is the stud on the fork shorter that the hole in the brake arm? If the
screw is contacting the arm itself, this will certainly bind. Did you
perchance forget something? A small washer, perhaps? Is there anything
preventing the brake arm from sliding on the stud all the way?

These things are usually pretty standard.

- -

"May you have the wind at your back.
And a really low gear for the hills!"

Chris Zacho ~ "Your Friendly Neighborhood Wheelman"

Chris'Z Corner
http://www.geocities.com/czcorner

Nick Kolodinsky

"Brian Ray" <be_ray@pacbell.net> wrote in message news:<T3efc.36790$2H5.33536@newssvr29.news.prodigy.com>...

> This doesn't make any sense to me. The bolts pass through, from what I can
> tell, solid alloy knuckles. It doesn't seem like the bolts could deform the
> knuckles to where they would bind the pivots. Or if they did, it doesn't
> seem like it should happen to the front brakes but not the rears.
>
> Any help is appreciated,
>
> Brian Ray

There is a pin (usually the spring end)which engages the hole in the
fork to hold the spring or spring adjustment housing in place. It
would be weird, but if inserted into a hole which was blocked up or
not drilled through - it would bind the whole arm... Usually there are
up to 3 holes to use... Did you use one not used last time?
Sorry if this isn't real clear, but anything keeping the arms from
seating on the fork completely would do it - (maybe that's obvious?)

Good Luck,
Nick

TheYvid

Brian Ray wrote:
> First annual teardown and rebuild of my mountain bike. Mounted the rear
> Deore XT V-brake arms to the frame bosses using the recommended torque
> (...)
> I can move the arms but the
> friction is pronounced, unlike the arms in the rear. Backing off the
> fixing bolts to what seems like an unsafe, low torque removes the friction.
>
> Any help is appreciated,

Hello,
AFAIR the bushing in the brakes' arms in this brake model doesn't
actually slide on the bosses, but have the "inner" bushing to
slide on - so the length of the bosses doesn't matter too much, as
others have suggested. The answer seems to lie somewhere else, then.

Of course not tightening the bolt enough isn't a good idea.
Maybe, as Nick suggested there's some problem with the spring holes?

Have you tried to change front brakes to the rear ones
- maybe the problem lies in the brake arms after all?

And how significant this friction is?
If only a little, there's a simple way to fix "the friction problem"
caused by too long bosses/overtightening the bolt (and deforming the
nut thus) on "classic" v-brakes (sliding directly on the bosses):

After tightening the bolt with a desired torque
you grab a brake lever, push it into the fork side,
and then (still pushing) move a few times to and fro
(just the way the brake works), and repeat it with pulling
the brake lever instead of pushing.

You should be careful of course not to damage anything
(you do it on your own responsibility
- depending how much force you apply it may help, but of
course not answering our question what caused the drag.

Greetings

--
TheYvid
I seek You at 49894592

TheYvid

TheYvid wrote:
> (...)
> And how significant this friction is?
> If only a little, there's a simple way to fix "the friction problem"
> caused by too long bosses/overtightening the bolt (and deforming the
> nut thus)

Of course I mean deforming the washers, not nuts.

> on "classic" v-brakes (sliding directly on the bosses):

So it _may_ also work on these.

--
TheYvid
I seek You at 49894592

Brian Ray

"TheYvid" <theyvid@fancol.DOT.com> wrote in message
news:c5lmcm$csr$1@nemesis.news.tpi.pl...
[...]
> Of course not tightening the bolt enough isn't a good idea.
> Maybe, as Nick suggested there's some problem with the spring holes?
>
> Have you tried to change front brakes to the rear ones
> - maybe the problem lies in the brake arms after all?
[...]

I will try a few more of these and other suggestions next time I wrench on
the bike. In the meantime the friction in the front arms seems to match
nicely the friction from the longer cable housing for the back brake. It
will become a real problem next time I need to scrub off a bunch of speed
and the front arms snap apart, hehehehe ...

Some more information after some troubleshooting: Hardware for the front and
back breaks is the same. There are the brake arms, a fixing bolt that screws
into the boss, and a tiny washer for each bolt. The mounting bosses on the
frame and on the fork are both the same length. The frame has three pin
holes around each boss, the fork has only one pin hole around each boss. But
I've got all brake arm pins inserted into the "middle" pin hole
regardless--the hole exactly horizontal to the boss. The part of the brake
assembly that slides onto the boss does not itself move during braking; it
forms the bottom of the parallelogram and its movement is restricted by the
pin.

Checking the pin holes sounds promising. Maybe they're obstructed; maybe
they are a little too far from the bosses an need to be reamed out. Flipping
the break arms front to back sounds good too. So does dropping the bike off
at the LBS. That'll be the course of action if nothing pans out here in my
garage.

Thanks again,

Brian Ray