The BSA A7-A10 Forum

Technical => Frame => Topic started by: Slymo on 06.05. 2019 04:47

Title: Truing Brake drums
Post by: Slymo on 06.05. 2019 04:47
My 58SR has what appears on a DTI to be a minor runout of around .004" on my front brake drum but it feels dreadful under braking. I checkout youtube for suggestiins on methodology to repair and up comes a brilliant vid of a man truing up exactly my wheel using a milling machine. Clearly these full width hubs with sraight pull spokes have a bit of an issue this way.
Title: Re: Truing Brake drums
Post by: Joolstacho on 06.05. 2019 05:34
I can't see that just a mere 4 thou discrepancy would cause the nasty symptoms you describe. Personally I'd look elsewhere... assembly issue, brake shoes etc.
Not saying I'm right but...
Title: Re: Truing Brake drums
Post by: Slymo on 06.05. 2019 08:32
No I completely agree but Ive pulled them apart endless times and put them together that many plus 1 and they still feel awful. This time I will leave nothing to chance.
Title: Re: Truing Brake drums
Post by: Tomcat on 06.05. 2019 09:32
One of the members on this forum glues sand paper to the brake shoes and lightly applies the brakes whilst spinning the wheel (on the bike).
Title: Re: Truing Brake drums
Post by: Rex on 06.05. 2019 09:40
It would be some sandpaper to take 4 thou off an iron drum...
Title: Re: Truing Brake drums
Post by: RDfella on 06.05. 2019 11:38
Had this problem a few times after fitting new rims. Some of the rims of the last 20 yrs or so are more pear shaped than round. Have even had to take some off again and do a little panel beating in the area of the weld, otherwise some spokes are over-tight with the rest almost loose just the pull the rim into round. Of course, the result is a distorted drum.
Check the spokes to make sure they all ring the same. To machine them with rim on would need a larger lathe or mill than most workshops have. Anyone got an old DSG railway lathe?
Title: Re: Truing Brake drums
Post by: Slymo on 06.05. 2019 12:56
Yes, the rims were an issue, the Akronts whilst in perfect new condition have a decided lump on the join. Ive got new stainless Dunlop pattern rims now and will lace them up to see what effect that has. There is apparently a chap in the Waikato who has the setup to machine drums on a complete wheel so might resort to him. Alternatively Ill see what changes in the drum profile when the rim is laced up and if nothing changes (I took a video of the DTI) I'll machine it unlaced in a pals Colchester.
Title: Re: Truing Brake drums
Post by: worntorn on 06.05. 2019 16:13
4 thou is easy peasy to remove with some 120 grit stick on emory. Stick the Emory on the shoes, put the brake assembly in the hub, axle in and start turning while gradually applying pressure to the brake arm.
You can do this with the wheel on the bike or setup on the bench.
It might require a couple of emory changes and might take 30 minutes or so.

Title: Re: Truing Brake drums
Post by: morris on 06.05. 2019 20:30
Tried the emery trick on my FWH. Turned the wheel back and forth until my arm almost fell off but still it was as bad as before.
It wasn’t until I took the wheel to a machinist that made a perfect brake. He had to skim 0.3mm which is about .012 inch. Of course a lot more than your .004 but for the mere €25 it cost me I won’t even think of getting the emery out next time.
Title: Re: Truing Brake drums
Post by: worntorn on 06.05. 2019 21:56
I could see 12 thou being a bit much for the Emery.
I measured my out of round drum at 5 thou variation.
With 5 minutes of work I could feel the contact area getting greater.
The whole operation took perhaps an hour, which doesn't seem much to fix a problem, without having to start the car or leave the shop.
And with just 5 thou out of round the low speed pulsation was really harsh, so it doesn't take much to make bad ( or good) braking.