Cantilever boss position

Zog The Undeniable

Should there be a standard vertical distance from the cantilever boss to
the top edge of the rim? The front brakes on my touring bike can't be
set up properly because, even with the pad at the very top of the slot
in the brake arm, it still contacts the rim at an angle so the bottom
edge of the pad hits the rim first. This means that the rim is only
wearing on the inner edge of the brake track. The rear brake is fine.
The cantilevers are Suntour XC SE and the bike was designed for, and
fitted with, 26" wheels.

Eventually I suppose the pad will wear to a wedge shape so it contacts
the whole brake track, but this takes forever on a front brake due to
the rim being much cleaner.

Jacobe Hazzard

Zog The Undeniable wrote:
> Should there be a standard vertical distance from the cantilever boss
> to the top edge of the rim? The front brakes on my touring bike
> can't be set up properly because, even with the pad at the very top
> of the slot in the brake arm, it still contacts the rim at an angle
> so the bottom edge of the pad hits the rim first. This means that
> the rim is only wearing on the inner edge of the brake track. The
> rear brake is fine. The cantilevers are Suntour XC SE and the bike
> was designed for, and fitted with, 26" wheels.
>
> Eventually I suppose the pad will wear to a wedge shape so it contacts
> the whole brake track, but this takes forever on a front brake due to
> the rim being much cleaner.

I would think the issue is the pad. Is there a spacer you can move so the
pad is further from the rim? If you move the pad away from the rim in the
brake arm, and then move the brake arm closer to compensate, you should
get a better angle of approach. Or perhaps one of those funky bowl shaped
nuts that let you adjust toe in would allow the pad to be angled downwards
a little?

I guess as a last resort you could sand down the pad into a wedge shape,
but only after you're sure the problem can't be solved with duct tape or
zip ties.

Mark Hickey

Zog The Undeniable <hrothgar19@yahoo.com> wrote:

>Should there be a standard vertical distance from the cantilever boss to
>the top edge of the rim? The front brakes on my touring bike can't be
>set up properly because, even with the pad at the very top of the slot
>in the brake arm, it still contacts the rim at an angle so the bottom
>edge of the pad hits the rim first. This means that the rim is only
>wearing on the inner edge of the brake track. The rear brake is fine.
>The cantilevers are Suntour XC SE and the bike was designed for, and
>fitted with, 26" wheels.

The spec for the brake bosses is that the distance from the wheel's
axle center to the brake boss center is 253.5mm (almost exactly 10").
It sounds like your fork probably has the bosses mounted a bit lower
than this.

>Eventually I suppose the pad will wear to a wedge shape so it contacts
>the whole brake track, but this takes forever on a front brake due to
>the rim being much cleaner.

You might consider finding a set of cantis with a bit more adjustment
range. If I recall correctly, the XC cantis are pretty short units -
there are quite a few options still available and most of them are
very reasonably priced.

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $695 ti frame

Sheldon Brown

Zog did not deny:
>
>>Should there be a standard vertical distance from the cantilever boss to
>>the top edge of the rim? The front brakes on my touring bike can't be
>>set up properly because, even with the pad at the very top of the slot
>>in the brake arm, it still contacts the rim at an angle so the bottom
>>edge of the pad hits the rim first. This means that the rim is only
>>wearing on the inner edge of the brake track. The rear brake is fine.
>>The cantilevers are Suntour XC SE and the bike was designed for, and
>>fitted with, 26" wheels.
>
Mark Hickey replied:
>
> The spec for the brake bosses is that the distance from the wheel's
> axle center to the brake boss center is 253.5mm (almost exactly 10").
> It sounds like your fork probably has the bosses mounted a bit lower
> than this.
>
The radial distance from the axle is not the only relevant parameter
here. The spacing between the bosses, and the width of the rim also
enter into it.

Zog's problem could also result from a rim being too wide for the
spacing of his fork.

> You might consider finding a set of cantis with a bit more adjustment
> range. If I recall correctly, the XC cantis are pretty short units -
> there are quite a few options still available and most of them are
> very reasonably priced.

That's good advice. The SE (Self-Energizing) cantis were not a great
design. They used a helical mechanism to cause them to create positive
feedback from the friction of the shoe against the rim, resulting in
very poor modulation. They are particularly not recommended for front
brake use due to the high risk of lockup.

By the way, there were front/rear specific versions of these, which had
opposited "handedness" of the helices. If you use one intended for rear
installation on the front, the helix will work _against_ you, and
braking will be correspondingly weak.

The "rear" version of these brakes is very much more common than the
front, because for much of the product cycle they were not selling the
front version at all, due to the danger of wheel lockup.

I'm sure a couple of people will come out of the woodwork about how
wonderful these are, but my advice is to ditch 'em, they're very fussy
to set up at best, and don't work nearly as well as cheap modern
direct-pull cantis.

Sheldon "An Idea Whose Time Has Gone" Brown
+----------------------------------------------+
| Every person who has mastered a profession |
| is a skeptic concerning it. |
| --George Bernard Shaw |
+----------------------------------------------+
Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041
http://harriscyclery.com
Hard-to-find parts shipped Worldwide
http://captainbike.com http://sheldonbrown.com

Zog The Undeniable

Sheldon Brown wrote:

> Zog's problem could also result from a rim being too wide for the
> spacing of his fork.

Could be. The rims are Sun Rhyno Lites, which are steamroller size,
although the fork has a pretty wide crown too.

Zog The Undeniable

Mark Hickey wrote:

> The spec for the brake bosses is that the distance from the wheel's
> axle center to the brake boss center is 253.5mm (almost exactly 10").
> It sounds like your fork probably has the bosses mounted a bit lower
> than this.

<snipped my OP>

> You might consider finding a set of cantis with a bit more adjustment
> range. If I recall correctly, the XC cantis are pretty short units -
> there are quite a few options still available and most of them are
> very reasonably priced.

I've just measured it and it is correct. Following Sheldon's comment, I
suspect the rim is unusually wide, which is causing a problem with this
particular brake.