Rebuild Avid bb5 with bb7 internals?

[SergeL]

I don't have bb5 yet, only some broken bb7 calipers are lying around and I was wondering, if it is possible to replace internals such way. Piston & pull arm from 7 combined with everything else from 5.

[aener]

Never tried, but from poking around inside my BB5s, I'm going to say "probably".
The only thing I can think that might prevent it is the size of the hole that the column passes through, which could likely be enlarged with a drill.

 

I don't know, but I would be interested to know how you got on after you've tried!

[SergeL]

In short, piston internals are incompatible

It was time to service my BB5 brake, so decided to compare internals with BB7. Following photos aren't mine, as I forgot to take pictures, sorry.

Here is BB5 disassembled:


First of all, BB5's balls are bigger 😁 (1) and bearing base (4), which is pressed in caliper's body won't adopt BB7 drive cam correctly. Moreover BB7's drive cam is thicker in place (2), but few mm smaller in diameter (3). Basically, this is a dead end for a conversion. Possibly, you can press out bearing base (4), install new drive cam and put some thin wall tube in BB5 caliper to compensate diameter difference (3), but it is not worth the trouble.

Design issue with these brakes

It is a BB7 piston:


Drive Cam movement is fixed (it is pushing a pad into rotor while rotating), but Pressure Foot can independently rotate and tilt in every direction due to a small metal ball (default 4mm bearing ball) between Foot Screw and Pressure Foot. This design makes sure pad is always contacting rotor squarely under a load. Same goes for BB5 but without Foot Screw.

Everything here is made of hardened steel except Pressure Foot. Unfortunately, with time this ball imprints deeper in Pressure Footer's aluminum body. This directly affects on how outer pad is performing: deeper ball position makes pad further from a rotor, so more lever travel needed to initiate braking.

With BB7 this can be fixed with a couple of clicks of outer pad adjuster, but with BB5 you don't have many options: either put something under a ball, or between pad backing and pressure foot. Also this ball sits quite tight and can't be removed by bare hands, it needs some love from angle grinder to pop out.

In a very neglected condition Pressure Foot can rub Drive Cam under a load resulting in brake jamming. Normally, these brakes aren't tend to jam, but if it happens grease this gap to reduce friction on BB5 or screw outer pad adjuster inwards on BB7.


Another observation is uneven pads movement on BB5

On this pic Drive cam + Pressure Foot are red, pads are blue, retention spring is black and disc is green. 1 - piston axis, 2 - spring's contact point


BB5's retention spring design makes outer pad's idle position unparalleled to a rotor. Spring is pushing pads above Pressure Foot's rotating center, which is a metal ball, thus making lower part to stick out).
To avoid this situation put a rubber band or heat shrink tube on top of pads' handles. Note, that band shouldn't squeeze pads too much, as they can bite disc.

This is a piece of tube I used, fixed with sewing thread to prevent slipping:

Edited May 25, 2023 by SergeL