Cantilever brakes gets very weak if it rains

Question

I have a bicycle with cantilever brakes.

Under light rain or dry weather the braking action is not very powerful, but good enough.

However, under heavy rain the braking action gets close to 0. This become especially problematic if I carry luggage or am going downhill (it can take > 100m to stop under theses conditions).

What can I do to improve this?

Note: the pads are a bit worn out and I will replace them soon, this is however not the source of my problem (encountered it while they was new/ well adjusted)

Edit:

Finally I got a heavy rain and could test out the proposed solutions. I fixed the brake angle on both front and back wheel. I changed only front brakes pas with newer ones (thicker, bigger water drain, but shorter ones).

1. Yoke angle did yield a noticeable improvement on the braking action, wether wet or dry
2. Changing the pads yielded the a huge difference in braking action under wet weather only (wear was not a factor here)

Conclusion: Both technique should be used together. Choosing an appropriate pad is VERY important.

Answer 1

The first step is to replace the brake pads with new ones. Even if they're not worn out, old pads harden and brake poorly. I like Kool Stop ones - they're black or a sort of salmon-coloured pad.

You should be able to press a thumbnail into the braking surface and leave a clear mark.

After that, check your alignment, and make sure the pads sit near to the rim with no pressure on the lever. A little lever movement should be all it takes to start braking, and then increased hand pressure increases the pad pressure.

Upshots - your rims need to be true, which means no wobbles as you spin them. I try to true wheels to within 0.5mm of completely flat, but 1mm of wobble is workable.

Answer 2

I think your transverse cable has got too great of an angle, and you can adjust it to provide more powerful braking force.

Much of the mechanical advantage of cantilever brakes comes from the "yoke angle", the angle the transverse cable makes from the horizontal. Sheldon Brown explains this in detail on his page about cantilever geometry (it's actually more complicated than Sheldon's explanation, but it's a good approximation)

(based off cantilever brake image from Shimano)

A transverse cable that pulls vertically upwards would have no additional mechanical advantage, so would have a mechanical advantage of "1" while a transverse cable that is completely horizontal would have in infinite mechanical advantage (it's impossible to have a completely horizontal cable since once you apply the brakes, the cable will deviate). The mechanical advantage is 1/sin(yoke angle) and he provides a handy chart:

Yoke Angle
(Degrees) Mechanical
Advantage
90°   1
80°   1.015
70°   1.063
60°   1.15
50°   1.31
40°   1.55
30°   2
20°   2.92
10°   5.76
5°    11.47
0°    Infinity!

It's hard to tell exactly what your angle is in that picture, but it looks like it's close to 60 degrees, so you're not getting much mechanical advantage from the yoke angle.

If you can decrease this angle either by shortening the tranverse cable, or by widening the cantilever arms by sliding the brake shoes closer to the rim (which would help "flatten" the transverse cable, decreasing the yoke angle), you can gain significantly more mechanical advantage. You don't want just go for the greatest amount of mechanical advantage since you'll run out of available cable travel if it's too high, but you can definitely tune the brakes for greater mechanical advantage.

There are additional cantilever adjustment tips on Shelden Brown's cantilever adjustment page.

Answer 3

In addition to your other answer, it's well worth checking/changing the cables. Rusty cables (or full of muck) have more friction which means more of the force on the lever goes into bending the cables. This is especially true if the cable routing has significant bends (more common at the back). New cables ran so much better for me that I could reduce the tension in the return springs as well. My bike with cantilevers has plastic brake levers that bend if you pull hard enough, and hit the grips, limiting the maximum force, so the cables have to be just right.

Answer 4

The stopping ability of the brakes is driven by friction coefficient of your brake-rim-lubrication setup and the force you, able to apply and friction of tire-ground setup. Suppose the third part is not a problem.

You probably lose the braking power by:

• Old and dirty pads - you can try to grind them sligtly or replace with new ones. (Criggie's answer)
• Old and greasy rim - try to clean them with ethanol, acetone, MEK or other (aggressive) solvent or replace with new one.
• Be sure the pads are properly aligned.

If the tribological properties are at their best and you still lack braking power you don't have applied enough force to the pads. This may be caused by:

• Poor yoke angle - try to set the cantilevers a bit wider. Or replace them with V-brakes. (Johnny's answer)
• Check and clean both cables and bowdens. Rusty cable and rusty bowden are the places where you lose the braking force. Also check whether the cable or bowdens are not bent or broken. If so, replace them and keep them lubed. Also check whether you don't have "soft" brake levers. (Chris' answer).

Answer 5

Just an idea here, but have you tried to see if you could put V Brakes on to your bike? Even if it's just the front, that would still improve your stopping power. I used to have cantilever brakes on one of my bikes a long time ago and when I went to V Brakes it was so much better. Cantilever brakes aren't all that good at giving you stopping power. Perhaps if you can change over to V brakes I think you will see an improvement. I hope this idea helps.
cheers