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Brake Arcing Service by Race Tech

| 4/11/2017 12:00:00 AM

Race Tech’s proprietary tool in place to machine the 1973 BMW R75/5 front drum. Photo courtesy Race Tech

Race Tech brake arcing service

After reading my vent a few issues back about the poor front brake performance on my 1973 BMW R75/5, vintage suspension specialist Matt Wiley at Race Tech contacted me, asking if I knew about Race Tech’s brake arcing service for drum brakes. Briefly, brake arcing  involves machining the brake drum so that it’s perfectly round, then matching the brake shoes to the drum for optimum contact. Back when drum brakes were the norm, brake arcing was common, but with the advent of disc brakes it’s become an increasingly rare and specialized service. After talking with Matt, I decided to test the concept.

Before sending my wheel off for service, I checked it against Race Tech’s requirements. First up was inspecting for any loose spokes; they must be properly tensioned and the wheel must run straight and true. Re-tensioning the wheel after machining runs the risk of pulling the drum out of shape, negating any improvements. Next was thoroughly cleaning the brake backing plate before installing new shoes (you can use your old ones if they’re good, but why would you?) and making sure the brake cams were properly lubed. Following that I inspected both wheel bearings (if they’re old or suspect, replace them), then I confirmed the wheel axle was straight before shipping the wheel — complete with brake backing plate and axle — to Race Tech in Corona, California.

Black lines act as cutting guide; when they’re gone, the drum’s done. Photo courtesy Race Tech

Getting true

At Race Tech, the wheel is set in a work stand supported by its axle. The inside face of the drum is then marked with black squiggly lines. Next, a specially made cutting tool designed by Race Tech founder Paul Thede is centered on the wheel axle. An air-powered rotary cutting head attached to the tool is adjusted to just touch the drum face. As the cutting head spins, the tool slowly revolves around the drum, the cutter removing metal as it does. The black squiggles act as a guide, showing high spots as material is removed; wherever black still shows, the drum hasn’t been cut. This is done in multiple passes, the cutter removing as little material as possible with each pass until a clean surface remains.

6/26/2020 5:20:50 AM

In the 1960s when servicing drum brakes on cars a good repair shop would first machine the inside surface of the drums (if they had enough material left) and then match the new brake shoes to match the new surface of the drums. The resulting drum brakes then worked quite well with no pulsing or premature failure due to high spots on either mating surface.

John Nutting
6/26/2020 2:33:54 AM

Setting up drum brakes so they operate correctly is a skill that's increasingly being lost. Many riders of classics seem to accept that drums are poor, and that's it. My 1958 650 Thunderbird also suffered from poor brakes: the previous owner had just fitted new shoes but they were only touching for part of the friction surface. Clearly their diameter was incorrect. I checked the drums for ovality with a dial test indicator set up on a mill and they were within limits. I also measured the internal diameter of the drums, and they had been skimmed oversize. So I needed the shoes machined the the correct diameter, and eventually found a specialist in Derbyshire here in the UK who was set up to provide the service. I sent the shoes with the diameter needed and once returned they worked fine, with no more heart-stopping moments. Back when drums were the norm in racing, the red sticker on a back plate from Joe Dunphy was a badge of honour and an assurance that you'd be able race safe. I also remember seeing a photo of the bloke in the 1950s works Norton team whose job was to set up all the brakes with 30 thou clearance. The Race Tech machine is a clever and simple way of doing the job. Well done Jim for bringing it to our attention.

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