Cool calipers

An innovative approach to cooling brake systems on high-performance cars developed by a UK company makes its debut in this season’s US NASCAR series. Jon Excell explains.

The disc brake has a tough enough time at the hands of the everyday motorist – listen to them click and pop after a long journey.

However, the transformation of kinetic energy to heat is perhaps at its most dramatic in motorsport, where the temperature generated during braking is a problem that can seriously affect braking performance.

Now, after three years of research and development, UK brake and clutchspecialist Alcon has launched an innovative race caliper that is claimed to make unprecedented use of available cooling air to prevent brake fluid from overheating.

Typically, a brake caliper, which straddles the brake pad and disc, contains hydraulic pistons. When the brakes are operated, these pistons push onto the pad, which contacts the disc and slows the car. The resulting friction coefficient converts kinetic energy into heat energy.

This heat – particularly in competition cars – can cause problems. If the temperature gets too high, the hydraulic fluid surrounding each piston begins to vaporise, which causes increased pedal travel. In a worst case scenario the pedal travel goes all the way to the floor and the brakes stop working.

The other effect of the heat – both radiated from the back of the pad and transmitted from one component to another – is that it gets to the seal in the caliper. This seal both retains the hydraulic fluid and controls piston position. It allows the piston to retract when the brakes are off and resists the piston being pushed back in when going over rumble strips or making high-speed turns. Deflection of components can push the piston back into the caliper which all leads to additional pedal travel.

In motor racing, where drivers need to leave braking until the last possible moment, unpredictable brake pedal travel can prove highly dangerous. So the less travel there is, the better.

Most race teams try to cool the brakes by forcing air from the front of the car through ducting and blowing it directly on to the caliper or disc. However,racing cars typically make use of an opposed piston caliper – which has pistons on both the inboard and outboard side of the caliper. But because of the space taken up by the disc and brake it is difficult to get air to the outboard side.

So designers usually content themselves with just directing air on to the inboard side. This lowers the temperature of the inboard side and creates a temperature gradient that ‘sucks’ heat out of the hotter outboard side. The trouble with this approach is that it’s not particularly efficient, and the outboard side is still likely to run hotter than the inboard side.

Alcon has developed an innovative approach that engineering director Phil Smith calls through-piston cooling (TPC). Instead of blowing air on to the caliper, the design blows air through tubes in the middle of the pistons. This, explained Smith, blows air into the area where it’s needed most – the bore of the pistons – to protect the fluid around the piston and the seal.

Air that comes into the caliper is blown through the middle of the pistons on the input side, across the caliper through tubes that straddle the disc and through the pistons on the outboard half of the caliper.

This means the temperature across the caliper is perfectly balanced by a splitter within the ducting that sends 33 per cent of the air to inner half and the remainder to the outer half of the caliper.

As a result, the sensitive film of brake fluid around each piston is cooled, helping to prevent vaporisation. Smith said that not only does the system improve safety and brake performance, allowing drivers to push harder for longer, but also, because of the much-improved cooling efficiency, designers are able to improve caraerodynamics by reducing the cooling duct entry size.

The first application of the system is on cars run by the Penske team in the US NASCAR stock car racing series. One of Penske’s drivers, Ryan Newman, is known to be particularly hard on the brakes and, according to Smith, normally has to back off to avoid ruining them.

Initial results are impressive. In track tests, fluid temperature in a conventional caliper stabilised at 207 degrees C – a reduction in bulk fluid temperature of 40-50 degrees C.

Smith expects the technology will be eventually rolled out across all of the teams, with a quarter of them using it by the end of this year.

However, he added that the application of the system extends beyond stock car racing.

It is, he claimed, currently being evaluated for use on an unspecified open-wheel racing car. Alcon is also in discussions with low-volume manufacturers about using the system on some high-performance road cars.

The caliper offers particular potential for use with the latest silicon carbide brake discs that operate for long periods of time at very high temperatures said Smith.

He reckons the secret is making the aerodynamics work. If manufacturers can be persuaded to provide forced air to the brake then Alcon has the technology to significantly reduce its temperature.

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