Learning About Motorcycle Brakes

Desperate times called for desperate measures

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Technical Editor Kevin Cameron shares his wealth of motorcycle knowledge, experiences, insights, history, and much more.Cycle World

Not knowing any better, in the early spring of 1972 I put the 250mm Fontana four-shoe drum brake from our 500cc Kawasaki H1R on the front of our just-completed home-made 750 H2R. Our "program" took place in such haste that we didn't even start the engine until we arrived in Daytona. Fortunately, it started, ran, and was fast.

The brakes weren't so hot, though. Running with other bikes with disc brakes, my rider (the late Cliff Carr) was losing 100 feet on braking just into turn one – every lap. When we got home we wasted no time in building a twin disc front end. In those days, and up to and including the era of the Yamaha TZ750, each disc was solidly bolted to an aluminum carrier which in turn bolted to one face of the wheel hub.

The arrival of slick tires in 1974 allowed riders to brake much harder than they had in the previous flex-and-flab era of running tires with rain grooves in dry conditions. Hard times were coming again.

“So I set off for my shop that evening, 100 miles to the west. Once there I took desperate measures which I do not recommend. Using the bandsaw I made several radial cuts in each disc, each cut terminating in a drilled hole to discourage the cut from continuing as a fatigue crack.”

At Loudon, New Hampshire in 1978, Boston Cycles rider Rich Schlachter was using the brakes on his TZ750D very hard – so hard that with heat being pulled out of the discs’ inner edges by the high-conductivity aluminum carriers, their outer edges were running much hotter. When inner and outer edge temperatures are very different, the heat-expanded region stretches the cooler I.D. region permanently, so when the discs cool a bit the stretched I.D. region forces the disc into a slight cone shape – enough tilt to push the brake pads back in the calipers. When the rider next goes for the front brake, the whole lever pull is used up in squeezing the disc flat again. As Kawasaki technician Kazuhito Yoshida used to say, “Bad effect.” On corner approach there’s no time for taking a second squeeze on the lever.

MEASURE TWICE, CUT ONCE

So I set off for my shop that evening, 100 miles to the west. Once there I took desperate measures which I do not recommend. Using the bandsaw I made several radial cuts in each disc, each cut terminating in a drilled hole to discourage the cut from continuing as a fatigue crack. Thus relieved of hoop stress, the discs became flat again. I then beveled the edges of each cut to remove sharp edges which might accelerate brake pad wear.

The following day, Schlachter had serviceable brakes – no more pad knock-back – and in the race was able to lead for a time before finishing third. Was it a good idea? I subsequently saw factory bikes with radially-sawn discs, but it was (as noted above) a desperation measure. A better solution is what we have today – discs which “float” on their carriers with little thermal connection between them. This, by allowing the I.D. of each disc to become nearly as hot as the O.D., makes heat-driven disc coning less likely.

BIGGER AND NARROWER

Another difference between that long-ago weekend and today is in the width of the pad track on the disc. This can cause differential heating of disc I.D. and O.D. because of the sliding velocity difference. The big round pads we had in 1978 caused sliding velocity at the O.D. to be maybe 40 percent higher than at the I.D. Discs are bigger now and pads longer and narrower, reducing this to more like a 25-percent difference.

Nearly 40 years later, racers rely on just as much technology to brake as they do to accelerate, without having to rely on the black magic of a desperate mechanic. I wonder if there are still industrious souls working the pits to push the boundaries of man and speed?