Cantilever boss position

Zog The Undeniable said:

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.

Zog The Undeniable said:

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.

Quoted message said:

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 habcycles.comhabcycles.comOpen ↗ Home of
the $695 ti frame

Zog did not deny:

Quoted message said:

Quoted message said:

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:

Quoted message said:

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.

Quoted message said:

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
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Sheldon Brown said:

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.

Mark Hickey said:

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>

Quoted message said:

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.

Zog The Undeniable said:

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.

And there are no spacers to remove like I suggested? That
has fixed the problem for me in the past, but I guess it
depends on how wide the rim is and how far apart the bosses.