Pulsating rear drum brake

So I've noticed the rear drum brake on my `73 850 pulsates slightly during braking (seems to grab a little once per rotation). I've never had it apart during the 3 years I've owned it but I suspect the drum has some sort of warpage. The other thing I noticed is the brake shoes drag if I adjust the pedal where I like it. To eliminate the drag I have the pedal adjusted so it engages pretty far down. I was going to tear it all down and have a look but I suspect I need a new drum and shoes. Maybe there's a cheap pattern drum installed on this bike.
Or maybe there's a problem with the dummy axle? Are these drums prone to warpage?
Any thoughts?

If you chuck it up in a wheel stand without the shoes and backing plate, you can spin it and check it with a dial gauge for runout. The shoes may be worn out-of-round, since it is a single leading shoe brake and the leading shoe will tend to wear more since it is pulled by friction into the drum inner diameter. New shoes sometimes require a little sanding to get them concentric with the drum.

Cast iron is prone to developing hard spots randomly (heat-caused) that will become very glassy and friction-free. You can rough up the surface with coarse sandpaper, but, without removing the hard spot, it'll come back in short order. The only cure is to have it turned (at least .005" deep, .010" diameter larger), or replace the drum. Keep in mind that you should get the shoes re-arced if you opt for the drum turning.

Nathan

The condition of the shoes are unlikely to lead to the pulsing symptoms you describe. The drum rotates, not the shoes.

Stephen Hill

the brake drum on your 850 is cast steel not cast iron.

Have you centered it properly?? You should always spin the wheel with the axel loose, stomp and hold on the brake to center it and then tighten the axle. If you did it any other way, try it first. Might solve your problem.

Is it possible the bearings are contributing?

Anytime I had a pulsating brake pedal (4 wheeler), one or more brake drums were out of round.

Slick

Mark, before you go over the top, I would get in there and wire wheel the drum surface and clean everything really good with break cleaner. Maybe scuff up the shoes.
I do this every spring due to the freezing and thaw cycles here in Mich. over the long winters. My RGM hi friction shoes will cake up a little from the surface rust from the drum. If I recall, this is all of a 1/2 hour job from start to finish, so if it doesn't work out the investment in labor is minimal.

I'll pull it apart later this week and inspect and give everything a good going over and scuff up the drums and centralize the shoes.
Thanks for the ideas.

You should also check and see if the chain is getting tight and loose in sync with the pulsing brake. Mine warped a bit and I noticed the brake issue but then also noticed that the sprocket is no longer completely round so the chain pulses a bit as well. It was enough to make me order a newer drum.

I'm thinking of machining off the sprocket and mounting a 44 tooth ring on it. (after fixing the out of round issue of course.) That will give the same ratio as a 21/42 if I go up to a 22 countershaft sprocket. I don't ride my rear brake enough to have warped it so I think it was just the awesome power of my engine that snatched it out of round.
Dan.

Best wishes on only 44 T ring unless essentially welding each tooth on separately as the valleys would only be hair thick and there is not much meat on drum after removing its teeth. Norvil sells a ring of 47 teeth but never found out what kind of Commando created a market for it. Inking the drum then apply brakes might reveal high spots or distortion. I definitely depend on rear brakes especially on loose stuff and if they grab or pulse its splash down city even at walking speeds.

Would agree with trying to center the drum first. Three years is a long time to go without looking at it. Take a look at the rear brake threaded adjusting rod. If the nut is no longer fairly close to the end of the rod then the shoes are probably worn. (Some of the linings are too thin to begin with.)

There are two adjustments. One is pedal height where you adjust on the footrest arm where you want the top of the brake pedal to be at rest. The amount of downward movement when the pedal is pressed is determined by the rear brake adjustment at the rear and really comes down to pad thickness when the brake is adjusted so it just spins freely when the brake is off. I doubt the drum would wear that much to be out of round but just my opinion.

I just got a set of the Ferodo bonded rear shoes and will be putting them in this week. These are much meatier. Last time , about 30 years ago, I riveted new brake linings onto the shoes. I found these to now be crowned and I can see they don't make much contact with the drum. Also, they were evidently not very thick to begin with. (The crowning can be a problem with rivited linings according to the Vintage Brake website.) There is over an inch of threads past the nut on mine and the actuating arm is nearing vertical. Will take some before and after pictures. Want to have a good working rear brake for the hills of NC!

I still think it has to be a warped drum and not the brake shoes making it pulse. As hobot said, the sprocket is thin. A warped drum will also have a warped sprocket so the easiest diagnosis is to just spin it and watch the chain get tight and loose as it goes around.

hobot, about the 47 tooth sprocket from Andover, a 24/47 would be just a half a tooth higher ratio than a 21/42 and just a bit below a 22/42. I have a 24 tooth drive sprocket so that sounds really good to me! Thanks.
Dan.