Made a trip to the grocery store the other day and I noticed my rear brake wasn’t really working. Front was okay. I don’t use the brakes much but when I do it’s usually the rear brake I go to when I need to slow the bike down a little. So even without the vehicle inspection issue, I have become quite fond of having a rear brake and I’m sufficiently motivated to fix it.
I’m a shade tree mechanic until I can get my bike shed built, hopefully later this year. That’s right, heartland New Hampshire tactics. At least it’s not a BLUE tarp.
I’m getting a new tire and also lowering the bike and I have it up on the center stand and covered with a tarp in case of rain. I took the rear wheel out first and cleaned it up. I hope to take it to the BMW dealer and have them put a new tire on it, balance it, and check out the spokes and wheel run-out. I mounted the last tire myself, but thought I would let BMW do this one. I’ll get the wheel to the dealership and wait for it. That’s the plan. I want another Metzler Tourance tire. I put 15K miles on the last one.
Got the tire out by removing the mudguard, taping up the rear brake lever, loosening the chain tensioners, loosening the shaft, blocking under the wheel so it wouldn’t fall, slipping the shaft out, pushing the wheel forward and working the chain off the rear sprocket, and rolling the wheel straight back and letting the brake disk come out from between the pads.
Separated the sprocket and cush drive from the rest of the wheel. You have to be careful of the brake disk, although it would take a fair amount or force to bend it.
I look at the Clymer manual occasionally to make sure I’m not getting way off course. I’ve had it all apart more than once before.
With the wheel out I could see the inside pad looked pretty thin and I would likely have to replace the pads. With the wheel out, the brake caliper tends to fall off the bike, so I reinserted the shaft and clamped the caliper onto the swing arm so I could tap out the keeper pin after removing the spring clip retainer.
As you can see, the pin is fairly corroded and I should replace it, but I’m not going to. For one thing, even if the pin is corroded, you can still tap it out easily because it is the brass springy collar that is holding it it. Once you tap the pin about 1/8th inch towards the wheel, it just comes out easily the rest of the way. Also, the corrosion is superficial and it’s just a keeper pin to keep the brake pads from falling off the bike. The braking forces don’t go through the pin. Also, it’s easy enough to replace later when it’s convenient to pick up a new one.
With the pin out, the pads come right out. As suspected the inside pad was worn down to the minimum thickness, .060″, about 1/16th inch. (1.5 mm). I measured it with a dial caliper. I got .130″ for the metal part and .190″ overall.
The way the light is hitting the photo of the pad, the striations seem pronounced but the disk itself still seems reasonably smooth to me. I checked the disk thickness with a micrometer and got .180″ where the pads contact it. You can’t use the vernier because the outer edge of the disk is unworn and the straight caliper jaws would simply give you a reading of the unworn part. The outer edge was about .200″, which is the thickness of a new one.
Whoa! I checked the BMW spec and it says .180″ is the minimum thickness! Looks like I need a new disk (rotor).
I poked around the internet and decided the pads I want are these:
They are on order. Two days, prime. Tough to beat. $36.21
Here’s what I bought for a rear rotor replacement. $148.28. Prime shipping also. I got the MD651 part number off the EBC web site.
In the center of the caliper is the cylinder (piston) that pushes on the outer pad. The inner pad just rests against the stationary inner (wheel-side) wall of the caliper. The piston is the thing about 1.5″ in diameter.
As the pads wear, that cylinder moves more to the right and the level of fluid in the reservoir will drop a corresponding amount. When I install the new pads, I’ll have to force that cylinder to the left to make room and that will raise the reservoir level. Mine is about half full, so there should be enough room for it to back up in there. Brake fluid can damage your paint, so you don’t want it overflowing.
I think the master cylinder and caliper are fine because I don’t have any fluid leaking anywhere.
After I get it back together, I’m going to replace the brake fluid, but I’ll leave that to another post.