The most common material used for brake disc rotors are of the cast-iron type. Materials like ceramic and carbon fibre have found their way into brakes as well. Carbon-ceramic disc brakes have been in use for a long time in motor racing but the Porsche Carrera GT is the first road car to use these brakes. Initially, carbon-fibre discs would not last long and were also expensive. They also worked well only in a narrow temperature range between 200-300 degree Celsius. Brembo finally came up with a carbon compound that was less sensitive to wear and suitable for road cars. Again, the advantages of these brakes were a reduction in weight and good performance even after hard use.
Carbon-ceramic brakes weigh much less and work much better at high temperatures than cast iron brakes. Mercedes-Benz claims its ceramic composite rotors can withstand temperatures upto 1400 degree Celsius, at the same time exhibiting low levels of thermal expansion for less brake judder under hard use. The biggest advantage that ceramic rotors offer is weight reduction. One rotor weighs six kilograms less than a conventional cast-iron disc. This can greatly improve vehicle dynamics and steering because of the reduction in unsprung mass.
Anti-lock Braking Systems
The greatest advantage that an anti-lock braking system (ABS) offers is that it allows the driver to maintain directional control while simultaneously braking as hard as possible. When the brakes of a car are locked during emergency braking, steering ability is lost because of the loss of rolling friction of the tyres. The car will simply plough straight on, regardless of the angle in which the wheels are turned. An anti-lock braking system will detect when a wheel is about to lock up and release brake pressure on that wheel and re-apply pressure many times a second.
The system has an electronic processor, four sensors (one for each wheel), an electrically-driven hydraulic pump and a pressure accumulator. If it senses one wheel slowing down faster than the rest, the hydraulic pressure to that wheel will be released and re-applied many times per second, via a servo valve system. When this happens, the pulse, which happens with the rapid opening and closing of the valves, will be felt at the brake pedal. The pressure accumulator is necessary as without it, the brake pedal would sink to the floor every time the valves open and close. ABS is extremely useful on low friction surfaces such as a rain-soaked road or on ice, where wheels tend to lock even on minimal braking.
It has been seen that many drivers fail to maintain maximum possible braking effort during emergency braking, thereby increasing their stopping distances. Mercedes-Benz introduced ‘Brake Assist’ (BA) where the system detects any pattern of pedal movement indicating an emergency stop (speed at which the driver lifts his foot from the throttle and stomps the brakes). It then applies maximum servo braking force till the car has come to a complete stop or till the driver has removed his foot from the brake pedal. EBA ensures maximum possible braking throughout an emergency stop. EBA can only be employed when ABS is fitted.
One of the problems faced by ABS engineers is how to deal with Four-Wheel-Drive systems that have a limited slip differential or lockable differentials. The mechanical linkages (through the gearbox) can be difficult to control. Some transmissions solve this problem by disconnecting the four-wheel- drive system when ABS braking takes place. Some manufacturers even add extra sensors to give the system more flexibility.
Dynamic braking is essentially a system controlling braking force to individual wheels at all times. It works on the principle that when a car is moving, different wheels carry different loads at different times. Wheels carrying higher loads can handle more braking than unloaded wheels. An example: imagine a car taking a left turn at speed on a dry surface. The outer wheels ie, those on the right hand side of the car are carrying higher loads because of the weight transfer. If the car has its engine mounted in the front, the load carried by the front right tyre will be the greatest. The front right tyre will be able to handle the strongest braking, followed by the rear right tyre, front left tyre and the rear left tyre. A dynamic braking system will work out which wheels can handle higher braking and accordingly allocate brake forces. Distributing braking force according to wheel loads can aid vehicle stability.
While ABS begins operating only when the wheels are about to lock, a dynamic braking system will monitor individual wheel speeds and wheel loads at all times, and brake different wheels accordingly. To achieve this, the system must be able to work out loads on individual wheels and also monitor and control braking force at all times, not just when the wheels are about to lock up.
To achieve this, sensors are placed at each wheel and share information between themselves and the central control unit. Dynamic braking systems are more expensive than ABS and hence, for now, are sold only on high-end luxury cars. But as it is with all new technology, time cuts costs and we may see them in smaller cars in the not too very distant future.
Every time you use your car brakes, energy used to propel the car forward is lost in the form of heat dissipated from the brakes. Some of this kinetic energy that the car loses as it brakes can be tapped and converted into electric energy to charge the batteries. This is called regenerative braking.
In a conventional braking system, friction between the brake pads and the rotors stop a car when the brakes are applied. Regenerative braking on the other hand relies on the system to do the braking. When the brakes are applied, the system makes the electric motor run in reverse mode which slows down the vehicle. This motor also generates electricity in the process and stores it in the batteries. The system works at optimum level in stop-start driving conditions, but cars cannot rely only on regenerative braking since it doesn’t supply enough stopping power. Hence cars with these systems need to be fitted with conventional brakes too.
Controllers in the system calculate the torque available to generate electricity to be fed into the batteries. This system works similarly to ABS controllers that monitor the rotational speed of a wheel in relation to the other wheels. Regenerative braking systems are mostly used in hybrids and electric vehicles such as the Toyota Prius and the Tesla Roadster.
Brake By Wire
Another recent advance in braking technology has been Brake By Wire (BBW). As the term implies, there is no mechanical link between the brake pedal and the device that applies the brake. In BBW, the system takes the driver’s signal ie, pressure on the brake pedal and speed of application and splits the signal into four channels (one for each wheel), and applies the brakes in the best possible way.
The benefits of this technology are that it can interact with an intelligent cruise control system to maintain preset distances from the car in front and also brake to a complete halt if necessary, all without driver intervention. Also, there is no need for holes in the bulkhead for mechanical linkages to pass through (reduction of NVH). The only challenge is providing the artificial feel in the brake pedal for the driver to push against.
Brake by wire can be either electric or electro-hydraulic at the wheel end. In the electric system, the brake pad is forced into place and released by an electric screw jack arrangement. In the electro-hydraulic system, a motor driving a pump pressurises the brake fluid accumulator. However, with both there is likely to be a reduction in weight as well as ease of repair.
Mercedes-Benz’s patented Sensotronic Brake Control (SBC)?brake-by-wire system — now employed on most Mercs including the E, SL 500 and new Maybach — is a world first for a passenger car. Pressing the brake pedal sends a signal to the SBC computer which applies the brakes, using hydraulic pressure generated by a pump rather than the driver’s foot. SBC also links up with the anti-lock brakes and ESP (stability control) systems, taking into account steering angle, lateral acceleration and individual wheel rotation speed when applying the brakes.
A sudden movement of the right foot from accelerator to brake signals an emergency and SBC pressurises the brake system for action and brings brake pads into contact with the discs. During braking, SBC applies just the right amount of pressure to each wheel and in corners, slightly more force to the outer tyres with the most grip. Another neat touch is the way SBC applies regular light brake impulses to remove the film of water from the discs.
Previously, brakes were regarded only as a means of slowing down and stopping the vehicle. But with the introduction of systems like ABS, BBW and SBC, brakes are helping improve the handling characteristics of cars, in turn making them safer and more fun to drive. Cars have never been so safe to drive on the limit and on this count, Ettore, you were so very wrong.