Rear Brake drum

Information relating to the Matchless G12 or AJS Model 31 650cc twin
bob121
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Location: WEST MIDLANDS UK

Rear Brake drum

Post by bob121 »

Has anyone, or would anyone bother skimming rear brake drum. Over the last few hundred miles mine has gone out of round. Bad enough that at slow speed the bike is pogoing like a bitch. I've been relying on it as the front such a waste of time.
alanengineer
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Re: Rear Brake drum

Post by alanengineer »

I have been doing some Norton ones recently. i cant see why ours cant come out a bit
Mollbhan
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Location: Perthshire UK

Re: Rear Brake drum

Post by Mollbhan »

Front brake shouldn't be as bad as all that, I have a g12 with standard front brake, it will never be as good as a modern disk or a 2ls but clean and properly adjusted it may squeal a bit but certainly dips the forks and stops the bike. Never had the back wheel in the air yet though.
Mick D
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Re: Rear Brake drum

Post by Mick D »

Hi

Not sure how the drum would develop an out of round whilst in use, have you had it apart and checked for oil or grease contamination?
Has the bike been laid up recently and allowed an area of the drum to become rusty?
Have you had the rear wheel off recently and not centred the brake assembly on re-fitment?

The drum could be trued up if needs dictate but I'd be surprised if it is the cause - do you have the means to measure the run out? It would be prudent to confirm if it is out of round before skimming.

Regards Mick
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ajscomboman
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Re: Rear Brake drum

Post by ajscomboman »

Mick D wrote:Hi

Not sure how the drum would develop an out of round whilst in use, have you had it apart and checked for oil or grease contamination?
Has the bike been laid up recently and allowed an area of the drum to become rusty?
Have you had the rear wheel off recently and not centred the brake assembly on re-fitment?

The drum could be trued up if needs dictate but I'd be surprised if it is the cause - do you have the means to measure the run out? It would be prudent to confirm if it is out of round before skimming.

Regards Mick
Common problem, I have 3 that are exactly the same, slightly out of round and I can feel the pedal slightly pumping, not drastic and all well within the brake tester limits despite the needle bouncing up and down on the brake tester dial.
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Rob Harknett
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Re: Rear Brake drum

Post by Rob Harknett »

I have a lightweight like that Rob, rear brake pedal thumps my foot on braking. Still not swapped the wheel for a spare I have.
bob121
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Location: WEST MIDLANDS UK

Re: Rear Brake drum

Post by bob121 »

Unfortunately mine is worse than that. At the right speed the pedal will dance a ridiculous amount. And if slow enough, the high spot will stop the bike dead. Probably best to replace. I'll have a look in there first. Cheers for the replies.
Andy51
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Re: Rear Brake drum

Post by Andy51 »

The rear drum is independent of the wheel/spokes, so shouldn't go out of round, but 60 years have passed so who knows? The rear shoul be capable of skimming on, say, a Myford lathe which takes 10 ins in the gap, but mounting it on a faceplate could be tricky. In the day, front hubs were laced to the rim and then trued on a lathe, but lathes that size ar e few and far between. The design of the '54 only full width hub is such that spoke tension has very little effect on roundness, although the '55 on design is not so good in this aspect. If you want to true the front brake then it is easy enough when the rim is off. I have done this myself on a BSA front hub but you need to make a stiff stub axle to mount it on in the chuck - if you use the normal axle between centres it flexes too much. A push fit mandrel would be the way to go on the AMC hub with its unusual bearings. Cheers, Andy
bob121
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Joined: Sat Jan 14, 2017 11:48 am
Location: WEST MIDLANDS UK

Re: Rear Brake drum

Post by bob121 »

I had the front done within the first few hundred miles. It had to be done as it made the front end almost uncomfortable. The rear has only become an issue after around 1400 miles. I've put just over 1700 miles on it now. Surrounded by lots of quick A roads with quick moving traffic. This is why the brakes are an issue at the moment. I'll get there. Just a ball ache keep taking it off the road to sort stuff. I was hoping the fettling would be done in a thousand miles. I think that was optimistic, more like 3-4000.
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Rob Harknett
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Re: Rear Brake drum

Post by Rob Harknett »

Just a thought, have you applied the brake and pushed it back and forth to try and see any movement of something that should not move, like brake anchor fitting. The movement you refer to seems to indicate some more at fault than the drum being a bit oval.
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