Italian Motorcycles

Aprilia Radial Calipers

Aprilia 2004 Models

Radial calipers are the hottest topic for supersport motorcycle riders and motorcycle mechanics. Here is what makes them different and why they are more effective than traditional brakes.

Everybody knows that radial calipers are the hottest technology in the sports bike world in 2003. Over recent years engines, frames, suspensions, and even tyres have been subject to continuous development. Constant technology transfer from the world of motorcycle racing (accelerated by competitions for series derived machines, like the Superbike championship) has given modern supersport machines exceptional performance and the sort of track handling that a GP machine would have envied ten years ago. Braking systems, however, have remained substantially unchanged.

The last major development in brake systems was the introduction of hydraulics and disks back in the sixties. Since then nothing much has happened. New friction linings have appeared and hydraulic pressure ratios between master and slave cylinders have been optimised, but the system itself is the same. Now the arrival of radial calipers has heralded if not a revolution at least a breath of fresh air for high performance sports bikes.

Maybe not everyone knows that Aprilia was the first manufacturer in the world to fit radial calipers to a competition machine. This was back in 1997. The machine was the RSW250 GP, the bike that won the world title for Aprilia with Biaggi, Harada, Rossi and Capirossi. April’s new system so quickly demonstrated its worth that today it is used on all competition machines.

If Aprilia was the first manufacturer to believe in radial caliper technology for competitions, then Aprilia was also the first to transfer that technology to production supersport machines in 2002, fitting it to the most powerful and prestigious machine currently made at Noale, the RSV Mille R.

The term “radial” describes the way in which the caliper is fixed to the foot of the fork leg. Whereas conventional disk brake calipers are fixed by bolts perpendicular to the surface of the disk, radial calipers are fixed by bolts arranged around the radius of the disk, from the circumference towards the centre. This fixing system is the only major technical innovation.

This one innovation nevertheless makes a significant difference, because radial calipers are fixed to mountings at both ends of the fork foot, whereas conventional fixing methods leave the caliper floating at the bottom, inevitably permitting a small amount of flexing to occur every time the brakes are applied. Also, with conventional calipers, correct pad positioning depends entirely on the rigidity of the caliper itself; radial calipers on the other hand are secured rigidly. This eliminates the possibility of flexing and allows the brakes to work in a more controlled, precise manner.

Since the force applied by the brakes is offset with respect to the caliper fixing points, when the brakes are applied a moment of torque is generated which tends to rotate the caliper (in practice it is as if the caliper tends to rotate towards the inside or outside of the wheel). The improved rigidity of radial calipers effectively opposes this moment. Because system rigidity is essential for good braking, radially fixed calipers are inevitably superior to conventionally fixed calipers.

But does the rider really notice any difference? Absolutely, yes. Even if not so much in terms of total braking power (even conventional systems can deliver excellent braking power), there is a clear improvement in precision, lever response, braking control, and fatigue resistance. Thanks to the excellent rigidity of radial caliper fixing systems, the brakes respond far more precisely to lever input, meaning absolute precision control. Minimised system deformation also means that efficiency remains constant even after repeated hard braking, as occurs under track and competition conditions. As an added extra, radial calipers hold the pads in perfect alignment with the disk. This distributes wear evenly and extends pad life. The rider therefore benefits not only from improved braking performance but also from reduced brake maintenance costs.

Source: John Sample Group

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