Friction Varies With Temperature

[Hugh Janus]

In the early days of carbon-carbon brakes, friction would increase so quickly with temperature that they required covers to keep the heat up for a consistent feel and actuation. If cooled too much and then reheated quickly with a sharp application, the rider risked seizure and wheel locking. (Ducati /)

In 1989 at Laguna Seca, Wayne Gardner was holding some lever to bring his new-technology carbon-carbon brakes up to operating temperature when their friction-versus-temperature curve shot up so steeply that his front wheel locked, throwing him up the road and breaking a leg.

This was an extreme example of how friction can depend upon temperature. Graph out the relationship between temperature and friction coefficient, and with early carbon brakes it could rise too steeply for human reactions to prevent seizure and wheel locking. While improved carbon-carbon disc and pad materials were in development, bikes often wore carbon-fiber disc covers to keep heat in the discs when they weren’t being used. Such disc covers are seen occasionally today in wet conditions. The covers notionally prevented the discs from cooling off down a straightaway enough that when applied too slowly for the next turn, their material would again have to heat up through the danger zone of steeply increasing friction.

Just for review, carbon-carbon is a solid material made by repeatedly impregnating a disc-shaped carbon-fiber preform with a resin, tar, or other carbon-bearing material, then reducing that filler to amorphous form in an oven until the preform is carbon-filled to the desired density. This process takes considerable time.

The steep rise in carbon friction when heated is the reverse analog of the problem riders had with the torque output of two-stroke racing engines. As engine rpm rose into the zone in which the tuned exhaust pipes began strongly pumping mixture, torque could rise more steeply than human reactions could keep up with, resulting in corner-exit high-side crashes.

Yamaha tackled a related problem with clutches in MotoGP. At the start of a race, all the bikes are stationary with engines running, and at the start signal a mass drag race occurs. Not surprisingly, this is the most dangerous few seconds in each event, as riders seek to get maximum drive from revving engines and slipping clutches while maneuvering for the top positions entering the first turn. Fine control of the slippage taking place is the clutch is essential, for if the friction discs act anything like Gardner’s carbon brakes in 1989, the front wheel will pop up, the launch control system will intervene, and time will be lost in one or more corrections.

Yamaha’s testing was effective, apparently resulting in selection or development of friction material whose temperature response was “flat” enough to be more easily controlled by the rider. The result was fast, controllable, and uneventful starts.

Many will remember the high song of a 250 two-stroke racebike making a race start; the rider pinned the throttle and controlled the launch entirely with left-hand pressure on the clutch lever. Front end coming up? Add lever pressure to settle it. Engine dropping below its best torque range (11,000)? Pressure on the lever pops the tach needle back up into “the zone.” This was the method taught by tester and drag racer Jay Gleason and by roadracer Randy Renfrow. It was one of Randy’s great pleasures at the riding schools he taught to challenge his students (many of whom considered themselves pretty hot) to a match race to second gear. He showed them how much there was to learn.

Clearly, for this to work well, the torque transmitted between friction and steel plates in the clutch must remain consistent, neither shooting up with temperature (tending to lock up the clutch and wheelie) nor “fading”—decreasing as the friction material grows hotter.

The front brake of a 305 Honda Superhawk that I tried to “build” into a racer was lined with friction material of the latter kind. Through a single practice, I could feel that brake fading as I had to pull harder and harder for each corner. Once back in the paddock, there I’d be, front wheel out, brake backing plate in my lap as I sanded off the shiny layer of binder resin that hard brake use had boiled to the surface of the stock “linoleum” lining bonded to my brake shoes. I had a lot to learn.

The most common situation of temperature-dependent friction is tires. Before 1974, roadrace bikes ran only on all-weather tires, their tread surfaces crisscrossed by many water drainage grooves. The extravagant flexing of the resulting tread elements as they passed into and out of the loaded tire footprint quickly brought such tires up to operating temperature. Rubber is not 100-percent elastic. Deform it with 100 units of energy and when you release it, you may get back only 75 units. The other 25 units become heat in the flexing rubber itself. That’s how tires warm up.

Street tires achieve operating temps quickly thanks to flexing of tread elements and carcass. (Bridgestone/)

Slick tires were different because their tread was not weakened by any drainage grooves. Flexing very little, they warmed up much more slowly, revealing that racing tread compounds are essentially useless until hot enough for their rubber-to-road dynamics to function properly. Lap one crashes were many until racers were given warm-up laps (I remember bone-headed officials, reciting their tiresome mantra that “The throttle turns both ways”). Soon, tire warmers brought added protection: electric blankets that wrapped circumferentially around the tires to keep them at around 176 degrees Fahrenheit/80 degrees Celsius.

Slick tires warm up more slowly and need tire warmers to keep them at operating temp before heading out on the track. (Pirelli/)

Why should friction vary with temperature? A primary mechanism in friction is the continuous formation, stretching, fracture, and re-formation of adhesive bonds between myriad tiny points of contact on surfaces moving past one another. Think of a ship, its deck laden with people, gliding slowly along a pier on which stand hundreds of others. Departing travelers on the ship and well-wishers on the pier hold hands as long as they can, holding on, exerting pier-to-ship forces. But then their hands are pulled apart by the ship’s continued motion, and they reach out to grasp other hands in turn.

If we imagine rubber and pavement instead of people, it becomes clear that the process of rapidly forming new “ship-to-pier” bonds has to be temperature dependent—because rubber becomes stiffer the lower its temperature. At its so-called “glass point” (Tg) it becomes a rigid solid! Too stiff to keep up with the adhesion, stretch, fracture, and re-adhere cycle that is a primary mechanism of friction. That is why racing tread rubber needs tire warmers to help it function during the tricky first three laps of races.

Why don’t street tires act so temperamentally? The Tg, or glass point, of racing rubber must be quite high to maximize grip in the desired temperature range, but that of street rubber is set low enough to assure safe all-weather operation.

The same adhesion, stretch, fracture, and re-adhere behavior goes on between brake pad and brake disc, and between clutch friction and steel discs. The details of how friction varies with temperature depend upon the blends of materials used. Early friction materials contained such ingredients as cloth, horsehair, and tar. Film of the 1906 auto Grand Prix of France shows an explosion of smoke from one car’s clutch at the start, as rapid heating of the friction material vaporized its binder. Considerable progress in friction materials has been made in the intervening 114 years.

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