I am fixing up a BSO where the rear cantilever brake is missing. I want to keep this project green and cheap, and there is a good supply of used cantilever brakes online. But they often come without the screws or washers for mounting to the frame, and my original ones are missing too.
It's easy enough to pick some screws and washers from my box of odds and sods, but it's a brake, not the place for a mistake (check out the rhyme - if a brake company use this line for advertising, I want a free brake).
The front brake is good so I observed it in action - it's even captured in this video I took. I pulled the lever several times and I noticed that the washer doesn't move when the brake pivots.
The photo below is just a snippet from the video. The photo is not very clear but the video is better.
Speculation about design intentions
It's a simple matter and it could be accidental but I can easily see this being important: it seems that the washer is designed to stay still, to reduce the risk of the screw becoming undone. It seems that the sizes, materials etc were picked so that the washer has more friction against the screw than the caliper. I believe the caliper face of the washer starts slipping under very low forces, therefore very little torque is then passed onto the screw.
Hypothetically, if the washer's friction was higher against the caliper than the screw, then every application of the brake would gradually loosen the screw on one side and tighten it on the other.
I don't want to get into the physics - I assume the factors at play are the materials' static and dynamic friction coefficients, contact area and it's distance from the pivot axis etc etc.
So how do I pick the right screws and washers from the parts bin?
My guess at a strategy
My guess would be to use a trial and error strategy:
- Get a combination of screw and washer that physically fits the frame and caliper.
- Check if the washer moves with the caliper. If it does, go back to step 1. If the washer stays put, happy days!
My guess at selection criteria for step 1
And here is the list I can think of for getting a combination that fits:
A. Washer internal diameter needs to be close to screw thread external diameter.
B. Washer external diameter needs to be close to the size of the recess in the caliper.
C. Screw thread length needs to just under the depth of the female thread in the pivot/stud/boss.
D. Screw head size is a compromise between:
- the contact area with the washer wants to be big,
- it wants to keep clear of the caliper itself.
E. Get some threadlocker just to be on the safe side.
Is there something I can put on the caliper face of the washer? I wonder if it would encourage movement to the correct face of the washer. Just grease?
Finally, I'm sure someone will spot two issues with this bike, but please let me know if you see more
- I know the brake pivot is missing on the non-drive side but I'm planning to use this answer for this.
- The front brakes seem to be set up for a poor mechanical advantage at the moment.