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Radial-Mount Brake Calipers – Another Perspective

Radially mounted brake calipers (“radial brakes”) seem to be The Next Big Thing. Coming (eventually) to a showroom near you. Kawasaki’s redesigned ZX-6R and RR models and Suzuki’s King Kong GSX-R1000 will be sporting radial brakes for 2003.

Why radial brakes? Aren’t the ones we have good enough? Is this a marketing gimmick? All are good questions . . . and there are probably more than a few of you who are asking other questions about radial brakes that I haven’t thought of. Let’s talk about the basics.

A radially mounted caliper is stiffer, and is better at maintaining alignment with the rotor. This means the pads stay in alignment better, making better contact with the rotor and exerting less lateral force on the rotor while generating less heat for a given pad pressure.

With traditional caliper mount points on the trailing end of the caliper, pad pressure at the leading edge of the caliper has considerable leverage to misalign the caliper in relation to the rotor. Radial mounting takes care of that. Putting a mount at both the leading and trailing end of the caliper, leverage at the leading edge of the pads is reduced to a level that it becomes insignificant.

Increased caliper stiffness can be achieved without radial mounting, but the costs can be high. Materials like high-tensile billet aluminum, magnesium, etc., can be used. Manufacturing processes, like CNC machining one piece calipers. Design details, such as adding a ‘bridge’ over the gap where the pads are inserted, as Brembo has done. All are effective, and in the racing world where cost is no object, these design and manufacturing elements are combined to maximum effectiveness, which may partly explain why Honda’s AMA and WSB spec RC51s still use the traditional mounting method (though the MotoGP Honda uses radial mount calipers).

I recently read the opinion of an individual (one in the brake retailing business) that a street rider would not be able to appreciate the increased performance of radial brakes, saying the finer points of caliper stiffness would be lost on the street rider. Huh?! How can this person summarily dismiss the entire street riding public as being unable to appreciate the improved performance of radially mounted brakes?

The brakes we have are good enough, I suppose, since they get the job done, but the new ones are better. Applying the “average Joe can’t tell the difference” logic would have halted motorcycle development long ago, and, moreover, radial brakes will make a difference for the average Joe.

For the price-conscious street-riding public, trickle-down race technology must be affordable, and so while street bike calipers will not be made stiffer via machined billet aluminum or magnesium (too costly), they are going to be stiffer due to more efficient, radial mounting. They will also employ another trick – for each caliper piston, there will be a separate brake pad. This means that there are four, instead of two, leading edges on the pads (of a four-piston caliper), creating more/better initial bite, and since each piston has its own pad, the need for differentially sized pistons to equalize pad wear on a “long” pad is eliminated, and power is increased.

So effective is this new design, Kawasaki saw fit to reduce rotor diameter to 280mm on the 2003 ZX-6R, down from 300mm on the ’02 ZX-6R. Doing so reduces gyroscopic effect away from the axle centerline, reducing steering effort. Concerns about thermal capacity and warpage due to excessive heat are addressed with 1mm thicker rotors on the ’03. In this day and age of 330mm rotors, making rotors smaller had better have more than marketing gimmickry behind it. As you have read from our Spanish counterparts, the brakes of the new middleweight from Kawasaki have already elicited excited comment from them, and the Japanese testers as well, so the performance is discernable and reality-based. Even average Joe on the street can benefit from this reduced turn-in effort, added power, and increased control (even though his knee won’t end up on the pavement mid-corner) and, heck, it will be a lot easier to clean the front wheel with those small rotors.