CW SPECIALTY FILE
Brake Discs
New solutions to old problems
KEVIN CAMERON
BRAKES SLOW VEHICLES BY FRICTION, converting the kinetic energy of mass at speed into heat that is then transferred to the surrounding air. During a full-effort stop, they also act as reservoirs, momentarily storing heat that is generated rapidly. As this happens, disc temperature spikes steeply. All solid materials have some temperature beyond which they begin to lose their essential properties. For each possible brake disc material-iron, carbon-carbon, etc.-this temperature is different.
The six discs examined here cover the range from production parts to an exotic carbon-carbon Brembo as used
in MotoGP. Somewhere in between are a Brembo Superbike race disc, required by rules to be iron, a Galfer high-performance iron “wave” disc and two discs embodying novel technologies.
Why does a stock 31 Omm-diameter Kawasaki ZX-14 disc weigh 2.9 pounds while the Brembo 290mm Superbike disc scales 3.7? Aren’t racing parts supposed to be light? That’s easy. Braking for corners makes race discs run hot, and when you add the “heat shot” of something like braking from top speed into Turn 1 at Daytona, the total energy would push a light disc’s temperature up into the warp zone. Peak tempera-
ture falls as disc mass is made greater. The Brembo Superbike disc’s extra weight is its heat storage reservoir.
The 13mm-thick, 290mm carbon disc is light at 1.75 pounds. How does that avoid a similar problem? Easy: Carbon continues to deliver smooth braking performance at extreme temperatures that would melt iron discs, which is why aircraft and Formula One cars use carbon brakes, as well. The material is called “carbon-carbon” because it consists of a framework of high-tensile carbon fibers solidly impregnated by amorphous (structure-less) carbon. Carbon discs are several thousand dollars each because their manufacture requires long residence in specialized high-temperature ovens already crowded with aircraft, missile and F-l parts. It’s a bit of a bidding war for space in there. Pads are carbon-carbon, as well.
Why the wave shape in the Galfer iron disc, which weighs 3.3 pounds? It is claimed that the gaps and irregular shape increase air turbulence around the disc and reject heat from the disc to the air. The gaps in its surface prevent gas, evolved from hot pads, from reducing friction.
Back in 1988, Honda tried to use the very least weight of iron in the rotors of its NSR500s. At fa*guna Seca, it was too little—the discs turned black and riders had almost no pad or stopping power left at race’s end. That kind of experience forced the general adoption of car-
bon discs during 1989 (carbon rears had been around since Honda put them on their 1982 FWS Daytona bikes). Superbike racers are still required to use iron, so the battle to get high-performance braking from the least possible weight of iron continues.
The dark BrakeTech disc with the textile look is a promising new material-a matrix of super-hard silicon carbide supported by high-tensile carbon fabric. Why the fabric? The silicon-carbide ceramic would be too brittle without it. A fabric pre-form is infiltrated with a polymer that is then pyrolyzed to yield silicon carbide. When this polymer infiltration and pyrolysis cycle fills the pre-form with silicon carbide, an extremely durable-yet-light (1.5 pounds!) part can be made of it. Some up-market autos are using this material now.
The last is the gleaming W81ess disc, which is made from the metal-matrix composite Al-SiC. This combines aluminum with a filling of nearly wear-impervious silicon-carbide particles, much as the cylinder-bore-plating Nikasil combines siliconcarbide particles with nickel. Weight is 1.75 pounds. The high heat conductivity of aluminum pulls heat into the disc from its friction surfaces quickly, limiting pad temperature. The makers tell us this material is “forged,” which probably refers to the process by which the particles are incorporated into the aluminum. Two types of pads appear in W81ess literature: One is a carbon-matrix
Kevlar pad and the other appears to contain a brassy material.
All these discs look good to me because when disc brakes first appeared in U.S. racing, each 400-series stainless disc was 7mm thick and weighed seven pounds. You’d think with all that mass that performance should have been good, but-combined with the rising grip of slick tires-by the later 1970s, discs were warping and coning. Why? One reason was that they were rigidly bolted to their carriers, which fought their expansion and contraction. See that all six of our subject rotors are on floating mounts, which permit free heat expansion.
A second reason was that early calipers gripped across a 50mm-wide pad track. On a 310mm disc, that meant that the outer edge of the disc was moving past friction material almost 50 percent faster than material at the inner edge of the pad track. The hotter-running outer material expanded more, stretching the cooler inner material. When the disc cooled, its inner portion was a bit too big for the outer. Result? A subtle cone shape that pushed the brake pads back, letting the lever come to the bar at the next corner. Riders struggle for words adequate to explain to crew just how undesirable this is.
On these modern discs, the pad track is narrowed to only 30-32mm. Pads are no longer circular but have become narrower arc-shaped
rectangles. Instead of just two caliper pistons, the modern number is four or even six.
The weights of tires, wheels and brake discs are more important than other masses on a motorcycle because they must be accelerated not only in a straight line but also around their axes of rotation. Lightening these parts has therefore a doubled effect on vehicle performance. Because these rotating parts resist the rider’s attempts to steer by virtue of their gyroscopic stability, lighter parts equal lighter, more responsive steering. When Yvon Duhamel (father of Miguel ) said in 1972 that his factory Kawasaki H2R “handled like a lumber wagon,” he might in part have been talking about its 14 pounds of firstgeneration brake discs.
Some things never change. When I was first working with disc brakes in the 1970s, I was told by Harry Hunt (then making a pioneering hard-coated aluminum brake disc) that discs had to rotate within .004 of an inch of perfectly flat in order that the rider not feel a thump in the lever. My experience confirmed this. When the discs you see here arrived at my shop, the instructions on the Brembo box said, “It is important that after fitting new discs the run-out is checked using the Brembo micrometer and run-out gauge. A value below 0.10mm should be obtained.” And .004 inches equals 0.10mm.
