Brake drum skimming

Brake drum (front) skimming:
I have read where some seem to believe that you should turn your drum AFTER it is spoked and trued.
How important is this? 20 inch throw lathes are less common than 10 inch...

Actually that is the only right way to do it as the drum is prone to deform after tightening the spokes (not only out of round but also conical) :!:

Agree that it is probably correct to do it laced up but by how much?
In other words for the average street bike is it that important?

Turning after lacing was how I did mine. Not sure it is absolutely necessary, but I had the equipment for it. I put some new arc'ed shoes on the rebuild, along with this turning operation. Break performance was much better

Gidday All.

Madass must have done hundreds of these. listen to what he says about this. Experience matters.

Also my reasoning goes something like this. norton were a production shop. It is unlikely that the wheels were laced and trued, and then sent back to the machine shop for more turning. i am open to correction on this.

Also begs the question, do the drums need turning if just the job was just relacing. if the drums were fine, turning after truing would be unnecessary.

But if you have a lathe like G81, then u would be crazy not to if needed..

Aussie bradley

If you have ready access to a 27 inch lathe, then by all means skim the drum when it is laced. If not, then it comes down to how big a problem you are you dealing with. Is the drum scored? Is it egg shaped? Is it cone shaped? A low technology approach would be to put the wheel and axle on a set of saw horses, spin the drum, and see what you are dealing with. Is it running true? You can measure cone shaped. And you can see scoring. Assuming the drum is in the ballpark, roughing up the inside of the drum, taking the glaze off the shoes, greasing the pivots, adjusting the shoes, and centering the backplate, are probably more important than perfect concentricity of the drum.

BTW, if you are going to go to the trouble of skimmimg the drum, then you should arc the shoes.

Stephen Hill

In my experience it is very necessary.
I have done my own by mounting the wheel [tire and all] on an arbor in my mill and clamping a boring bar in the vice mounted to the table. Jim

Just to be clear: skimming the drum without the rim laced up is pointless?
The issue is availability of a lathe with enough throw.

Re; Stephen Hill quote “Assuming the drum is in the ballpark, roughing up the inside of the drum, taking the glaze off the shoes, greasing the pivots, adjusting the shoes, and centering the backplate, are probably more important than perfect concentricity of the drum.”
Concentricity of the drum is all relative as if the drum is NOT concentric then you WILL get brake judder :!: :!: :shock:

If you have a clock gauge, you can check on the bike if the drum runs true.

Re;

Onder said:

Agree that it is probably correct to do it laced up but by how much? quote]

Simple, until the drum is concentric-you are then left with the problem of the brake shoes being undersize for the drum , but don’t worry it’s not unsurmountable-you can either get oversize brake shoes fitted then skimmed down in a mandrel to fit the exact diameter of the drum ( with a .020” shims fitted under the flats ) –OR fit raising spacers under the brake shoe flats to raise them up.
The first method is much more preferred- if you want a dam good front brake.
Either way, remember to file a 30 degree chamfer on the leading edge of the brake shoes –or it WILL grab when applied :!:

Click to expand...

Onder said:

Just to be clear: skimming the drum without the rim laced up is pointless?
The issue is availability of a lathe with enough throw.

Click to expand...

The few times I have re-laced a drum brake wheel -it pulled the drum considerably out of round.
The only one I put on a bike without skimming it pulsed pretty bad. Jim

Ok i expect to get shot down for this but the way i have always done it is to use emory cloth cut just a little wider than the brake shoe and the full length of the friction material then i wrap a strip around each shoe trapping it by the cam,then i fit a piece of studding with a nylock nut to the brake plate ,i re assemble the wheel and clamp the spindle in my bench vice with the complete wheel horizontal then i use my pillar drill chuck to drive the outside of the tyre turning the whole wheel quite slowly whilst i adjust the brake arm adjuster nut until the cloth starts taking off the high spots in the hub ,i have a bar apprx 2ft that i hold to stop the brake plate revolving, then very slowly adjust the shoes until the hub stops jumping and becomes very smooth and i can hear a continuous hiss from the hub,i have done this many many times on many different bikes ,one time over 30 years ago i did this on my BSA a10 back wheel when it failed its MOT on the rear brake being oval i returned within 2 hours for the re test much to the testers amazement and i still have that bike ,,,,,,,,,,,,,cheers baz

Very clever technique, baz. I've been fortunate to have a gap bed lathe available that let me chuck up the complete wheel to turn the drum. I used it to turn the rear drum on my PR after I machined the teeth off and brazed on some fittings to take a removable sprocket. It worked great, and gave a smooth, powerful brake action. Course the catch 22 here is that back then I almost never used the rear brake unless I ran off the track.

Ken

Thx for the responses, I think I got the message!

Trueing the drum after building the wheel would be the right way to do it. I will bet that not 1 in 1000 Commandos are done that way and the brakes still seem to work. Put it together and see what you have got. Then decide if it needs skimming. And how to do it.

I might adopt Baz's idea to do my front drum.

Greg

I've skimmed brake drums on a number of wheels and always clamped them down flat on a milling machine and bored them with a bar mounted in the spindle. I find it easier to make sure they're set true like this and there's no need to go near them with even a soft mallet.