Cantilever brake failure

fitzharrys said:

Recently the front brakes on my son's bike were repaired by a local
cycle shop. Within 2 days, they failed - a brake block became jammed
under the rim, throwing my son from the bike.

I am wondering how this can happen with cantilever brakes, if they are
properly set up.

My guess would be that the brake block was mounted too high in the
cantilever and therefore had to point downwards to line up with the
rim. After a bit of use, the block would squirm even further
downwards until it slipped under the rim.

When you install the blocks, look at where the pivot is - the boss on
the fork - and think what the direction of the force will be. You
want the block mounting to be at right angles (more or less) to the
cantilever arm.

fitzharrys said:

Recently the front brakes on my son's bike were repaired by a local
cycle shop. Within 2 days, they failed - a brake block became jammed
under the rim, throwing my son from the bike.

I am wondering how this can happen with cantilever brakes, if they are
properly set up.

poor setup, insufficient tightening, setup with frontwheel skewed,
riding with frontwheel skewed, levering wheel in/out fork without
releasing brake, excessive brakeblock wear (but in two days you'll need
a lot of mud to accomplish that)
--
---
Marten Gerritsen

INFOapestaartjeM-GINEERINGpuntNL
www.m-gineering.nl

fitzharrys <[email hidden]> wrote in
message news:[email hidden]...

Quoted message said:

Recently the front brakes on my son's bike were repaired by a local
cycle shop. Within 2 days, they failed - a brake block became jammed
under the rim, throwing my son from the bike.

I am wondering how this can happen with cantilever brakes, if they are
properly set up.

It can't, but setting them up and adjusting them as they wear is important.

Go back to that cycle shop ONCE. Tell them what happened, and that you will
not return. Despite the poor wages the cycle trade provides, setting a
brake block correctly is not difficult, and you have grounds for avoiding
them.

fitzharrys <[email hidden]> wrote in message news:<[email hidden]>...

Quoted message said:

Recently the front brakes on my son's bike were repaired by a local
cycle shop. Within 2 days, they failed - a brake block became jammed
under the rim, throwing my son from the bike.

I am wondering how this can happen with cantilever brakes, if they are
properly set up.

It's a known, and extremely dangerous, failure mode of badly adjusted
cantis. The usual reason is failing to realign them as the brake
blocks wear. Had they worn that much in only 2 days? The most probable
explanations would seem to be that either the shop adjusted them
badly, (or they failed to tighten them properly which is much the same
thing really), or something happened to them after the bike left the
shop. This could be someone fiddling with them, or it could be a fall,
for example.

--
Dave...

Dave Kahn said:

It's a known, and extremely dangerous, failure mode of badly adjusted
cantis. The usual reason is failing to realign them as the brake
blocks wear. Had they worn that much in only 2 days? The most probable
explanations would seem to be that either the shop adjusted them
badly, (or they failed to tighten them properly which is much the same
thing really), or something happened to them after the bike left the
shop. This could be someone fiddling with them, or it could be a fall,
for example.

No, the bike had hardly been used.

jtaylor said:

Go back to that cycle shop ONCE. Tell them what happened, and that you will
not return. Despite the poor wages the cycle trade provides, setting a
brake block correctly is not difficult, and you have grounds for avoiding
them.

I did go back to the shop (Rides on Air in Abingdon) but no joy. A bit worrying really.

Oh well, I guess there are other cycle shops in town.

fitzharrys <[email hidden]> wrote in
message news:[email hidden]...

Quoted message said:

jtaylor said:

Go back to that cycle shop ONCE. Tell them what happened, and that
you will
not return. Despite the poor wages the cycle trade provides, setting
a
brake block correctly is not difficult, and you have grounds for
avoiding
them.

I did go back to the shop (Rides on Air in Abingdon) but no joy. A bit
worrying really.

Oh well, I guess there are other cycle shops in town.

Braggs, as I remember. Much better.