What is special about the M compound brakes?

Klaus Dullinger: M compound brakes have a grey-cast friction ring in a floating arrangement that is connected by pins to the aluminium brake cover. The pin assembly means that as the temperature increases, it is able to expand freely in a radial direction and subsequently cool down again without any residual deformation. What is different about the new BMW M3 and BMW M4 is that fixed-calliper brakes are now fitted to the front and rear axles, with four pistons at the front and two at the rear. The predecessor had heavy single-piston sliding callipers.

Is it necessary to allow M compound brakes a running-in period when the components are new?

Yes. Drivers should be more restrained in their driving for the first tank full of fuel and avoid maximum deceleration wherever possible.

Why is it that with perforated brake discs in particular, you occasionally see slight cracks?

Cracks form in particular wherever relatively cold discs are suddenly subjected to a large load. The cracks that occur are limited to the surface areas. For instance, if you have been driving your car fully laden on the motorway for an extended period without braking and then you are suddenly called upon to perform an emergency braking manoeuvre, the brake disc will become very hot in a very short space of time – in such a case, the temperature can increase from around 20 degrees to 450 degrees. If the brake disc is required to perform in this manner several times in rapid succession, it is quite possible for surface cracks to form in the cast material of the friction ring – especially in the area around the perforation holes.

Will racing pads again be available for the M compound brakes of the new BMW M3 and BMW M4?

Yes, a new sport pad will be available under the name of ‘M Performance Sport Brake Pad’; this was developed directly alongside the new brakes.

What are the advantages and disadvantages of these sport pads?

The life expectancy of these pads is considerably longer at the kinds of high temperatures that typically occur on the racetrack. While the standard pad can display a drop in friction coefficient at high temperatures, known as fading, the friction coefficient value of sport pads is far more stable.

Why is it so important to perform a cooling-down lap after driving on the racetrack?

If you park your car while temperatures of 500 degrees are still prevailing on the brake discs, the considerable heat will be transferred throughout the entire brake system. The heat transfer progresses through the back plate and damping plates of standard pads into the brake callipers. In addition to the damping layer of the damping plates, the piston seals and dust shields can also be damaged, as can the surface coating of the brake callipers. Leaving brakes to cool in a stationary position is extremely unfavourable when brake discs are at a temperature of up to 500 degrees. By performing a cooling-down lap, the brake temperature generally falls to below 200 degrees.

What are the advantages of the M carbon brakes?

The rotating masses in M carbon ceramic brakes are around 12.5 kg lighter compared with standard M compound brakes. On the other hand, these brakes are dimensioned to offer significantly more in terms of performance. So on the racetrack in particular, the M carbon ceramic brake offers more stability.

A further advantage is that the disc is not so susceptible to corrosion, which means that no rust films form on the friction ring, a phenomenon that is well known in grey-cast constructions.
Corrosion-free: M carbon ceramic brake discs. The friction surfaces are already covered with a stress-relief coating when they are new.
Furthermore, they are less subject to wear. Under normal use, the carbon ceramic brake disc will last as long as the car itself; the silicon carbide friction coating is virtually non-wearing. However, on the racetrack, the brake discs may be subject to oxidative wear. When the discs are repeatedly heated to temperatures of 600 degrees and over, the fibres within the brake disc burn up.

This means that the disc does not become progressively thinner but lighter. To detect the level to which this may have occurred, three wear indicators per friction coating are applied to the disc, set apart at angles of 120 degrees. This allows the dealer to recognise when discs have been subjected to high levels of oxidative wear. The brake pad service team then takes out the brake disc and weighs it. If it weighs less than the minimum permitted weight, the disc must be replaced.

Are there any disadvantages with carbon ceramic brakes?

The production process is much more complex. One step involves siliconising the friction ring for a period of 24 hours at 1,300 degrees. The carbon fibre-reinforced silicon-carbide brake disc has a total throughput time of several weeks with the manufacturer before it is ready.
This explains why the purchase price of the M carbon ceramic brake is that much higher. However, this is partly balanced out by the lower level of wear.

Does it have a number of special characteristics, of which you should be aware?

Brake discs and brake pads in M carbon ceramic brakes only achieve a favourable wear and contact pattern after around 1,000 km.
Material-specific properties mean that increased operational noise may be experienced when braking, particularly in wet conditions, just before the vehicle comes to a halt. However, this does not affect the brakes' performance, operational safety or stability.

However, under the impact of moisture and road salt, the braking effect may be equivalent to that of a conventional brake system. This may be perceived as reduced braking performance, but it can be compensated for by applying more pressure with the brake pedal; this should be continued until the pads and discs have fully dried out.