Tech Tips: Brakes, Drums, and Linings
This tech tip was published in the Vintage BMW Owners Bulletin Vol. 37 No. 1
If you can remove the wheels from your bike, you most likely be will be able to also inspect your brakes as well. It is a simple thing to do every time you change a tire. What to look for:
- Thin brake linings — are the rivets equal to the brake lining itself?
- Are there any cracks in the linings between the rivets? (Photo 1A)
- Is there scoring on the brake drum?
- Are the brake drums or lining wet with oil or grease?
- Do they have any hot spots or cracks in the drum lining where the brake material rubbed the drum?
The rear brake is the most likely to show grease or dampness from the rear hub seal, but the front can also leak wheel bearing grease onto the brake linings and drums. Sometimes people have been over-zealous with spline lube and this will get slung onto the brake and drum as well. Sometimes this will pack into the rear hub seal and will occlude the small hole at the base of the seal, which allows any oil leak from the hub seal to be drained away from the brake area (photos 1 and 2). You can find this hole at the bottom of the final drive on the outside away from the brake drum area, and you can see this without removing the wheel (photo 3). If you have oil or grease on your drums, you must first discover where it is coming from and fix it, or you will just be wasting time and money replacing the brake linings.
If you find the brake linings getting a bit thin (< 1.5mm thick or the rivet heads are at the lining surface) or oil-soaked, then it's time to replace them. There are three methods for doing this:
- re-rivet new lining material onto the old shoes;
- buy new shoes with bonded linings (for /5 and later models only — photo 4);
- work with a brake professional to bond or glue new brake lining material onto the old shoes.
Re-riveting pre-formed pre-drilled replacement linings (photo 5)
Sometimes replacement linings can be bought that are direct replacements — just drill out the old rivets, remove all high spots from drilling out the old rivets, check fit of the new rivets in the old holes, check the alignment of the lining holes to the holes in the shoe, carefully align the new linings and reset the new rivets. When doing this, start in the middle and work your way out to the last rivet. Beware that some of the replacement linings are very brittle and may crack when setting the rivet.
Replacement linings without pre-drilled holes (photo 6)
Some of the newer linings are available only as raw material and need to be cut to length using a utility knife or heavy shears. The new lining needs to be centered on the shoe and anchored in place. Starting in the middle, drill through the first pre-existing rivet hole from the brake shoe side through the new lining material. Then countersink the hole from the lining side to accept the new rivet, using a Forstner bit, so that the rivet will be below the lining surface and not touch the drum after wear has occurred. Repeat this process working from the middle to the outside edges of the shoe, until you've refastened the new lining using all of the original rivet holes. The linings should then be trimmed slightly wider than the aluminum brake shoes, and the toe and heal of the replacement lining should be sanded (tapered) away, copying the original lining so that the footprint of the brake has maximum surface contact with the lining of the brake drum (photo 7A). You can check this by checking the shoe against the drum to make sure the lining is not just touching the heal or toe (photos 7 and 8).
A small number of bike models may not have linings available. The old material must be removed, and new material bonded onto the braking surface — this can only be done by a professional brake bonder.
CAUTION: old brake linings (pre-1978) are asbestos-based — do not use air to clean the shoes or drums, and do not inhale the dust from them.
The final step to making the braking process work
Make sure the brake drum is square to get the maximum life and braking power from the linings . If it isn't square, it needs to be "turned" on a special lathe and all the out-of-roundness removed (photo 9 — checking for out-of-roundness with dial indicator). Note that there is a maximum amount that can be removed before the drum should be replaced. It has been stamped or cast into every drum brake /5 and on. I've only seen this stamp on a couple of late /2 drums, so if no stamp is visible, refer to the factory repair manual for maximum allowable wear.