Motorcycle Brake Overheating Has A Bad Effect

Kawasaki World Superbike rider and current series point leader Jonathan Rea suffered boiling of the brake fluid in his calipers earlier this year in Thailand. When the brakes are not applied, the return port from the system to the reservoir is open, so when fluid boils, the expanding vapor pushes fluid back into the reservoir. When the rider next applies the brake, much or all of the lever stroke is used up in compressing the vaporized fluid. Having low or no lever approaching a corner, the rider pumps the lever a second time. As Kawasaki’s Mr. Yoshida used to say, “Bad effect.”

Rea almost pulled in that day, but later the team built scoops to direct streams of air right onto the calipers to lower their temperature. Other teams, faced with the same thing, have made wider fork crowns to provide room to space the discs and calipers farther out of the “wind shadow” of the front tire. Brake manufacturers have created calipers with more open structure to present more of their surface to cooling air and to facilitate its free circulation.

It is normal for there to be some form of insulation, often perforated, between the caliper’s hydraulic pistons and the very hot backing plates of the friction pads, encouraging the rapid changing of the air inside the pistons (which insert into their bores closed end first, creating another air gap between the pad and the part of the piston in contact with the fluid).

The fluid itself is made with a high boiling temperature, which brings us to the reason users are advised to use fresh fluid from previously unopened containers: Atmospheric moisture is fairly soluble in brake fluid, so any time the container is open, moisture is being absorbed from the surrounding air into the fluid, reducing its boiling point (water boils at 212 degrees Fahrenheit at sea level).

Noticing that every team mounts its calipers behind the fork legs, you might ask, “Why not reverse the legs to put the calipers out front in higher-velocity air?” The answer is the closer to the fork pivot axis the steered mass can be located, the easier the steering is to stabilize against wobble.

In the earlier era of drum instead of disc brakes, the friction material riveted or bonded to the brake shoes could fade if overheated, and overheated drums could expand into slightly conical form that narrowed the contact between shoes and drum. The brake linings of my 1964 Honda Super Hawk were, in my ignorance, the stock linoleum. When hot, their binder resin melted and oozed to the surface, forming a shiny slick glaze. Even good racing linings (Ferodo “green stuff”) could overheat, emitting a smell like burning pot handles or overheated electrical equipment. Gas or dust released from shoe or pad friction surfaces can also reduce braking power.

In World Superbike, modern brake pads are mysterious mixtures of heat-conductive metal wire or flakes, strong reinforcing fibers (used to be asbestos, since replaced by Kevlar), squeaky, scratchy ceramics, and solid lubricants, requiring discs of trick stainless steel that can be made hard enough to resist rapid wear.

In MotoGP, both discs and pads are carbon-carbon composite. This is amorphous carbon pyrolyzed into a preform of super-strength crystalline carbon fibers in a heat process requiring six months. This material was originally developed for the nose cones of re-entry vehicles and ICBM warheads, and was later adopted in aircraft brakes, greatly increasing their performance. Carbon-carbon was adopted on Grand Prix motorcycles in the late 1980s. Operating temperature can range as high as 1,475 to 1,835 degrees Fahrenheit.

In both metal and carbon disc systems, brake fluid needs effective protection from temperature!