QUESTION: Would you please explain why it’s better to mount disc brake calipers radially instead of the traditional bolting them to the side of the fork slider?
ANSWER: When Yamaha’s TZ750s showed up at Daytona in 1974, their twin 4.5-lb. cast-iron calipers were mounted ahead of the fork legs. This was done for the obvious reason that this best exposed the hot calipers to cool air. After first practice, riders began moving the calipers to the back of the fork legs (turning the sliders around) because that moved the calipers closer to the steering axis, thereby reducing the polar moment of the steered assembly.
And there they stayed until a new problem arose. Those bikes had been designed for 1970s tires of only moderate grip—Dunlop Triangulars—but the slick-tire revolution hit in 1974. By the early 1980s, slick-tire traction was working the big TZ's twin-loop steel tube chassis pretty hard (they all cracked in the same places!) and one result was that rolling leaned over through a bumpy corner would tilt the front wheel (by flexing the slender solid-steel axle), causing the sideways motion of the brake discs to knock the brake pads back into their calipers. When the riders would pull the lever at the next braking point, it would come to the bar. If the rider was lucky, there was time for a second pull that would put the pads against the discs, a working brake. In the car-racing world this is called “pad knock-off.”
If you look at a motorcycle front end from this perspective, the best place for the calipers to avoid pad knock-off is directly behind the discs, where the disc motion is minimum. When this was combined with the new male-slider forks and the variety of discs sizes being used, it made sense to build calipers for radial mounting behind the axle. When a larger disc has to be used (as in the case of the 340mm carbons now mandatory at Motegi), moving the calipers to the larger radius is a simple matter of adding a pair of cylindrical spacers between caliper and mounting, and using correspondingly longer caliper mounting bolts. And with male slider forks, it is not possible to add a fork brace, so such forks depend for their lateral stiffness on the axle alone.
There is nothing wrong with the old way of mounting calipers unless the above problems are encountered. Few street riders run hard enough to experience pad knock-off, but it did become quite common in the closely production-based Supersport racing classes.
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