Brake Drum Skimming and Machining

Brake drums are manufactured in several ways and are made employing different materials. Bike drum brakes have a cast iron shell cast into the hub but the shell may also be made of steel. Some drums are made of only cast iron so no shell is needed, but these types of drums are very heavy.

cast iron brake drum from BSA A65

Lighter drums are made of aluminium with the liner or shell cast or pressed into them. Quite a few bikes had the shell pressed in and then the shell was riveted into the drum. Why should drums be skimmed?. Drums have to put up with a lot of arduous work and different temperature changes. Temperature changes and moisture causes rust to form on the shell or liner. This can cause glazing on the braking surface thus impairing brake response. Rust patches can be caused by the vehicle standing for long periods of time with the brakes on. On a motorcycle rust patches are commonly formed by moisture getting into the drum and puddling on the bottom of the shell.

Rust caused by moisture

Rust patches cause a juddery braking action. Drums can also distort due to misuse such as having the brake shoe pads constantly rubbing on the liner. This can be caused by incorrect clearance between break shoe pads and liner causing too much heat. Riding with your foot or pushing on the brake can also cause heat. All of these aforementioned problems cause problems with the brake shoes pads. Brake shoe pads can be worn down to beyond the rivet head causing scoring of the break drum. Brake shoe pads can be glazed due to heavy usage causing glazing to the drum. Brake shoe pads can also brake away from the break shoes whether they be riveted or glued.

Brake lining detaching

Any of these problems can be cured by skimming or machining the hub. A more usual type of distortion is the over tightening or tensioning of the spokes. This problem would be compounded if the hub was made from thin wall metal such as found on many vintage bikes.Racing bikes such as AJS  7R have electron hubs that are fairly prone to distortion due to spoke over tightening. One such AJS 7R that came to our works was showing signs of brake judder. On inspection it was found that a pattern of wear due to incorrect spoke tension it was found that the pattern mirrored the points where the spoke heads were pulling on the hub. On measuring the hub it was found that a portion of the liner had distorted by .015″. On inspection of the wheel rim I found marks conducive of a bump or fall on the race circuit. I measured the spoke torque in the area of the rim distortion and found spoke torque up by 50% from all the other spokes.

In this case the spokes had been used to pull the rim straight and true to the detriment of the drum trueness. The rim had to be removed and straightened and then respoked to the hub with the correct spoke tension all round.The damage to the rim had obviously been done years ago and had permanently distorted the drum hence the need to skim the drum on our big gap bed lathe. We can turn or skim the drum with the tyre fitted to the wheel. When skimming the drum on the lathe.

Brake drum surface prior to machining

We have to watch out for vibrations caused by the action of the single pointed carbide cutting tip. Vibration causes an alligator skin appearance that would rip a brake shoe to bits. What we look for on a finished job is a constant smoothness of surface texture and trueness to the wheel spindle which is of upmost importance. When ever possible we leave a small witness mark to show we have removed the smallest amount of metal possible.

Glazed riveted brake shoes

Glazed bonded brake linings

We also restore and renovate brake linings to original or better employing our stock of modern friction materials.

Bellow is a email that was sent to us from a gentleman named Garry “we did his brakes”.

Hi,

Just a quick note to let you know wheel arrived ok last Friday, fitted new shoes, bed them in during practice and finished 2nd overall at end of race meeting.

Thanks for the work you did, it made a big difference!

Garry

Below are some breaks from a tank that we machined.

Below is an email sent to a customer called Colin

Hi Colin i need to find some wheel bearings out of my old stock to fit into your hub so i can machine the hub exactly to the spindle centers. i will machine minimum to clean and give you the measurements. What you will need to do is to chalk the linings and  gently apply the brakes and remove them and check the contact area.if you find some high points they can be rubbed down with 180 grit and you will have to go through the rigamarole again until you achieve full contact between the pads and drum. In the workshop we are usually given the brake plates with the shoes fitted and we set them to a, just brake on, condition and machine the linings to the same bore as the freshly machined hub so there is no running in. As we do a lot of racing brakes and the punter expects the brakes to work from the outset this is the only way it can be achieved. The linings are never accurately supplied to fit the worn drums so you will never achieve perfect braking. The perfect formula is to machine the drums so as just to clean and then measure the drums and set the liners on the brake plate in the open or brake position and then machine the liners to fit the freshly machined bore of the drum. This sequence of operations produces perfect braking symmetry with no bedding or running in and no lack of braking due to the heat induced in bedding the linings into the brake drum. cheers terry.