rear brake drum not running true on 1973...

I can see that my rear drum does not run true and because of this the brake acts as though the drum is out of round (and I do know to apply the rear brake when tightening up the wheel to center the backing plate). The PO told me he replaced the wheel bearings and I suspect maybe he made some mistake on reassembly so that the hub does not seat properly against the drum/stub axle assembly correctly. Should I just take apart the hub and dummy axle or is there some common cause for this?? I did do a search here and read up on the spacers, washers, etc...

You MUST inspect the whole rear end thoroughly as un-trueness is very straining on the radioactive dumb axle fission issue. Suspect bad bearing by now and circlip failure. I'd sure have the axle mag fluxed myself now. I know you've nipped by the
DS nut of course, so dig in and hope for the best obvious things easy to fix back.

Thanks, I am checking everything out carefully and have read some of the broken axle threads....actually what I probably should have asked is what the normal gap is between the hub and the drum when everything is assembled correctly

OH Ugh bluto, that is a variable d/t the fit of the cushions standing proud of the hub holes. I trim mine about flush to get say 1/32-1/16" crush space but other wise expect like 1/8"+ cushion squashing to hinder even measuring what feels annoyingly sloppy fitting by hand till nipped w/o over doing it, just a bit more than enough to keep nuts on and not crush all the innards and strain the dumbassle stress riser surprise. I recommend finding some conveyor belt or tire case material to chop out your own for the DS loads. The factory ones seem to turn to mush after just one of my 40 miles commute w/o even kicking up full torque loads, just staying sane legal safe. I lightly glue my rubbers in the holes for obvious reasons after ya done it a few times otherwise.

When I ride with my buddies, who ride Commandos, it is pretty common for me to see their rear drums and sprockets wobbling from side to side. Recently I changed the rear drum on my 1972. When I rassembled it I noticed that if I spun the wheel the drum wobbled. I checked axle trueness (dummy and main axle), looked for restrictions in the cush rubbers and drive lugs, and made sure all the spacers were accounted for. Nothing seemed wrong. I used a lead mallet and by smacking the sprocket with the axle nuts lightly torqued, I could bring the wobble into a reasonable spec, which remained more or less true when I torqued the nuts home. My conclusion, interference of the drive lugs into the cush rubbers cause the drum to ride out of true. What would allow this, once the nuts are torqued? In my humble opinion, this is because the engineering in the quick disconnect wheel is flawed: the single bearing in the drum isn't up to the job of pulling the drum into concentric rotation. I also wonder if the pulsing sprocket causes the main axle to flex and load cyclically, and is the explanation for the axle breakage we keep hearing about?

Stephen Hill
Victoria, BC

I also wonder if the pulsing sprocket causes the main axle to flex and load cyclically, and is the explanation for the axle breakage we keep hearing about?

Click to expand...

You bet it does but even in those it don't dumb axles still fission randomly.
So does road seams, speed bumps, engine pulses, stones, etc etc etc. I saw one post about removing end of dumbassles so a longer same dia bolt-axle can be run through and nutted up. This is not the same level of robustness gained by the 17mm one piece conversions but may be enough just to remove the stress riser right where it matters most. Firmer more robust cushions may help a bit too. I have to trim cushions flush or they hold hub off drum enough to be prone to wobble instead of just shifting on each's faces.

found at least part of the problem, the cush drive rubber inserts were slightly squished and protruded a bit above the recesses, filed them down and the drum runs truer now and the rear brake works better too....thanks to all for the advice and it was helpful to learn that some drum wobble is typical on a post 71 Commando