SA drum brake wtf?
 

I've been running a Sturmey-Archer X-FD drum for a few weeks now, and recently it's developed a very strange fault. The brake operates fine under light braking loads, but then when I brake harder, it locks on. The really weird part is that when I squeeze the lever while the bike is stationary, the brake releases fine no matter how hard I squeeze. When it locks on while I'm riding, it only releases when I stop, lift the front wheel up and rotate it backwards slightly. The cable is fine, I've unhooked the quick-release cable and checked it, so the problem isn't there. However, I recall reading somewhere that automotive drum brakes are partially self-energizing, and I'm wondering if the effect I'm seeing could be related to something similar happening in my brake, either by accident or because it's meant to. What do people think? Could that be it? More to the point, why has it developed recently, and what do I do about it?

When the brake is locked on, how much angular deflection from rest do you have on the brake arm? Have you got it adjusted so that the brake arm is not at a neutral position with no lever pull?

With regards to the angle, I'd say maybe 40 degrees, perhaps a bit less.

With no lever pull, the brake arm isn't quite as far back as it goes when I disconnect the cable, but it has to have some tension it in, because if I don't run it like that, the quick-release cable pops out.

What exactly do you mean by "neutral position"?

That's not normal, I'd guess one of the shoes broke loose. I'd open up the brake and see what's going on inside.

actually, on closer inspection, it's probably less than 40 degrees, it's more like 20. I didn't actually have the hub to hand when I wrote that post.

I'll still take a look inside though

OK, update

I've opened up the brake and there doesn't seem to be anything wrong inside. Both shoes are firmly attached and move the same amount when the brake arm is pulled. I wiped the drum clean and reassembled the whole thing, but the brake still locks on when there's a forward force on the wheel

The only thing I could think of that could be the cause is that when I first got the hub, the brake arm used to stick occasionally, which I remedied by greasing the cam that moves the two brake shoes outwards. I took great care to keep the grease off the shoes and drum, but could it have anything to do with the problem?

In my old High School daze I took auto shop and they drummed it into our brains, "Nothing but high temp grease in/on a brake." (pun intended). It could be an issue, I don't know enough to be sure. I bought an old SA 5 speed drum off ebay and the hub grease had leaked into the drum and onto the shoe. I cleaned everything with accetone before building up the hub and it works fine. What I'm trying to say is, grease in a drum is bad, I was able to clean my shoes off and the brake is working well. I'd get some accetone and clean everything inside the drum and see if that fixes it.