TECH UPDATE: Trying to Stop

The idea of disc brakes is very old, going back at least to the strange and wonderful Frederick Lanchester who, like Luis Alvarez half a century later, shed useful and practical ideas like trees shed leaves in fall.

The attractive thing about disc brakes is that thermal expansion cannot distort them away from the pressure of the friction material against them; they have no place to go. By contrast, the part of a drum brake in contact with the friction material can expand freely only at its open end, but its other end—the web of the wheel hub—operates at a lower temperature and so doesn’t allow much expansion. The result? The drum opens into a slight cone shape, such that the brake shoes, with their friction material, press outward against it strongly only near the closed end of the drum. With less friction material in strong contact with the rotating drum, brake torque falls; this is brake fade.

This could be complicated by the fact that many less expensive bikes had press-formed steel drums that remained full of forming stress; they longed to become any shape other than that of a brake drum. Connoisseurs of technology past are surprised to discover that the drum brake fitted to sporting BSA Gold Stars was a great, clunky piece of cast iron. Stability!

Early drums were entirely separate from the wheel itself, and were bolted to one face of the hub. Later, the brake drum became an aluminum casting bearing the two spoke flanges around its outer diameter. Yes, you guessed it; as the brake became really hot in use, the spokes could become loose. Kawasaki tried a few special magnesium rear brakes back in their two-stroke triple days that were famous for this, and there were surely others.

Various tricks were applied to mitigate these problems, such as making a double-sided hub with two narrow shoes on the right and two shoes on the left: the “four-shoe drum brake.” The shoes were made “leading,” so that friction tended to rotate them around their pivots to press harder against the drum. Those who played the drum brake game hardest experimented with combinations of grabby and less grabby friction materials, riveted to the shoes in various positions and lengths. Their goal was to achieve high brake torque yet avoid creating a beast with so much self-servo effect that the rider couldn’t control it.

The last hurrah of the drum brake occurred as large military aircraft were fast outgrowing such devices. This was the “bag brake,” which had no shoes at all. Instead, a ring of small, separate brake blocks was expanded out against the inside of the drum by a ring-shaped bag, inflated by hydraulic pressure. As the blocks were free to tilt but kept from rotating with the drum by buttresses, the blocks could accommodate drum expansion. Disc brakes were next, culminating at the present in clutch-like stacks of carbon-carbon discs, pressed against each other by a ring array of hydraulic cylinders.

When Jaguar’s C-type race car won Le Mans in 1953 with Dunlop disc brakes, it was clear that drum brakes could now be converted into decorative bases for floor lamps. Many experiments were made, such as those of the creative Peter Williams in England, but it was Honda who put discs into production.

Soon we entered the “era of the 50-pound front wheel,” as the more powerful production bikes were given 12-inch brake discs 7mm thick (0.276 in.), bolted to aluminum-slab carriers.

The disc brakes on the 1974 Yamaha TZ750A employed heavy but quite stiff iron calipers weighing 4.5 pounds. The 5.5mm stainless discs bolted rigidly to aluminum disc carriers, which in turn bolted to the end faces of the front wheel hub. On tracks like Loudon or Sonoma where brake use was heavy, discs expanded, permanently stretching their cooler inner edges (which were cooled by conduction to their carriers). When the discs cooled down the next straight, there was too much middle for the contracting OD region, driving the disc into a flat cone shape that was enough to push the pads back. It's not nice to have the lever come to the bar.

Next step was carriers allowing free disc expansion, which were a great improvement and come in a variety of styles. In the “7mm era,” there was a big fad for drilled discs. This started when Steve Whitelock, Yvon DuHamel’s Kawasaki factory mechanic, decided to lighten those monsters. He drove over to Dan Gurney’s shop, where a pattern of holes was decided upon and executed. People saw Yvon’s holey discs and liked them. I’m sure attentive tool salesmen wondered at the sudden spike in orders for half-in. drill bits (because stainless work-hardens, it is hard on tool edges).

The holes triggered a long outpouring of grooves, slots, and lately, wavy openings, all of which are said to benefit the user by scraping off brake dust, improving cooling, and strumming the heartstrings of the universe. But as so many riders have suggested in the past, it’s good to take a look at what’s on the podium bikes.

Early US 1025cc Superbikes (1976-82) dumped a lot of heat into their production brakes, resulting in disc distortion that wouldn’t quit. One problem: At very high temperature, phase transformations can occur in the metal as one crystal arrangement (more stable at low temperature) transforms into another (stable at higher temperature) with a slightly different volume. This can also lead to the late-1980s phenomenon of disc hard-spotting; the hard spots stuck up very slightly from the disc surface, producing a thumping at the lever. Users were advised to use abrasive paper to correct this. Good luck. Kawasaki resorted at one point to having inch-thick iron disc blanks cast, after which each machined reduction in thickness was followed by a stress relief cycle, finally resulting in fairly stable discs.

Whatever the hard stainless used for discs in World Superbike is, it seems to avoid the usual problems for long enough to get through a race. In MotoGP, carbon-carbon discs and pads are standard. Nothing that works well is cheap! Problems are never solved, just pushed to a higher level of function, where they reappear, often in new forms.