Airburst

About a year ago I posted on here asking for help with the SA front drum brake I'd recently bought. The brake used to stick on, but only when there was a forwards load on the brake. When the bike wasn't moving, it was fine. I was totally unable to fix the problem, I tried everything, and took the bike to various different shops. I finally found a mechanic who knew about drum brakes, and he suggested I disassemble the brake and check that the leading edges of the brake shoes were tapered, because if they weren't, the brake would experience a runaway servo effect and lock up. I did as he suggested, and lo and behold, the trailing edges had nicely-filed tapers on them, but the leading edges had nothing. After I filed on suitable tapers, the brake worked fine. Until now, that is. The problem has now returned. It's the same symptom: brake locks on when the bike's moving, but is fine when it's not. It started about a week ago, and as soon as it did, I dismantled the whole thing, cleaned the brake dust out, sanded the glaze off the pads and put it back together. The filing I did was still there, there was nothing unusual, so I put the whole thing together again and the problem went away for a few days. Earlier today, it happened again, and it now happens every time I brake. What is going on? I have a couple of theories.

1) I trued the wheel before I rode it today, and tightened some of the spokes considerably. Could the spoke tension be distorting the brake drum enough to cause this?

2) The brake unit may not be centred on the axle, and if it was, it would be off-centre in the brake drum. I did try to fix this tonight by loosening the locknut holding the brake on, pulling the cable as hard as possible and tightening the locknut again. It didn't help Would the brake unit being off-centre in the drum even cause the problem, and would what I did have fixed it?

Does anyone have any thoughts at all on this? Anything at all would be welcome.

Thanks in advance, and sorry for the massive post.

FBinNY 

It's been many decades since I worked on an SA drum brake, so this is just a guess that might point you in the right direction.

One possibility is that the brakes are worn enough that the actuator cam is rotating up to top dead center. If so it'll not want to go back.

The other is that the shoes are a bit sloppy on the pivot, especially the one which the rotating drum would push forward away from the pivot. That could cause a self actuation as it rides higher on the cam. The tip-off is if it realease promptly after a full stop, or if you roll back a hair, but not when just slowing down.

I hope this helps, but as I said it's a shot in the dark.

Airburst

Your second suggestion sounds like it could be right. That's exactly what happens, as soon as the bike stops, the brake releases perfectly normally. I don't think it's the cam, as I've made it go to top dead centre and stick like you say, but I've only done it with the brake unit removed from the hub, and the shoes stick out way past the edges of the brake plate when the cam does that.

The trouble is, I can't tell if the shoes have any play in the pivot, because the return spring is so strong. Assuming I manage to unhook the spring and check, what do I do if it is the pivot? Do I need to replace the whole unit?

fietsbob

This the all steel hub shell version, or the steel drum in an aluminum hub-shell?

the spreading of the actuating cam to its widest point, may be an indication
a new set of brake shoes , with thicker friction material may be in order .

Grease, just on the cam surface , a tiny amount, may help .
stick, less, one thinks.

I put a coil return spring, from the hardware store.
around the cable.. against the brake arm on the hub, and the adjuster.
Since no counter spring in the hand lever.. and a long cable to the rear..

FBinNY 

Use your hand to simulate the drum and torque the shoe assembly and see if you notice any shifting forward. Also check the area on the show that engages the cam for wear or burrs. If it's worn to form a bit of a pocket, that could be a factor.

You'll have to use your eyes and look for anything that could hang up the shoe when it's pushed forward. As for how to fix, that depends on parts availability. If the pivots or pin is worn you might be able to make a bushing to take up the slack.

I could probably help you more if I had it in my hands, but it's kinda tough working blind, so all I can do is point you in a direction.

Airburst

OK, there's a tiny bit of play in the pivot, and I'm trying to dismantle the mechanism further to see if I can put something in there as a shim. The trouble is, I can't get the clip thing off the pivot shaft. It looks like it's just been pressed on, but I can't seem to pry it off. It looks like a metal disc, with a star-shaped hole in the middle. How do I remove it to get at the shoe pivot?

FBinNY 

It sounds like you're describing a "speed cap" which is the female equal to the star nut used in forks. They can sometimes be pried off but usually removing them destroys them.

The play seems to minor to be the problem anyway. Try to cause the hang up by squeezing the shoes and rotating forward and seeing if you can cause them to hang up. It might help if you can find a ring to function as the drum so you can see the operation under load. Possibly a tin can, or maybe an oil filter wrench might help you see exactly what is happening.

The place to focus is where the forward turning shoe meets the cam. BTW since filing the edge helped for a while before, you might try filing a longer angle on the cam end of the shoes and seeing if it helps.

Airburst

OK, my problems have just multiplied. I got the speed cap off without doing it any damage, but I managed to snap one of the backing plates for the shoes in half.....

I just superglued it back together. Is this a stupid idea? I figure the tensile strength of the aluminium casting is so low that the joint won't make a difference, but correct me if I'm wrong. It did break under very, very little force though, just trying to pull the shoe out did it. Strength issues aside, the repair is in perfect alignment though, so my plan is to do as you suggested and try and simulate the drum to work out what's going on. Even if I do replace the shoes, it would be nice to know what's going on in the drum in case the problem persists.

Edit: Scratch the above, the glued joint failed as I was trying to re-fit the shoes, so I've ordered a set of replacement shoes, hopefully they'll fix the problem once and for all.