Friday, 15 February 2013

Brakes

The F10 M5 is fitted with very large and efficient brakes for bringing its 4000+ lbs quickly from high speed to a standstill, or for dropping the speed quickly for corner entry.

The front brakes are M specific and use a large 15.7" ventilated and cross-drilled compound brake disk (aluminium centre, steel disk) combined with six-piston fixed calipers.


The rear brakes use the same type of disks (but a touch smaller at 15.5"), and have a single-piston floating calliper which includes the electromechanical parking brake. The rear is taken from the base 550i, but painted racy blue!


The front brakes do more work than the rears as the weight of the car shifts towards the front during hard braking and therefore there is more traction available at the front and more braking force needed there.

The steel outer ring is completely symmetrical, so when it expands due to heat it does so uniformly without introducing any bends or kinks that can rub against the brake callipers  As the brakes cool you hear them "ping ping ping" as the outer disk collapses back onto the inner aluminium ring via the pins.


The disks are ventilated, meaning that they are hollow with a plate on each side. They are also cross-drilled, which provides more ventilation and a lighter weight.

The front brakes are six-piston fixed callipers and the rear brakes are single piston floating calliper. 

Disk brakes have brake pads that squeeze against a brake rotor to slow the car door. There are two types of brake callipers at the wheels: fixed and floating. 

The floating calliper system is shown below.



It uses only a single piston that pushes against one side of the brake disk that then pulls the calliper over to make contact with the other (bottom left).

The fixed calliper system is as follows.


There are pairs of pistons that squeeze down on the brake pad from both sides simultaneously. the M5 has three such pairs, hence it is a six-calliper brake. Fixed calliper systems are more effective than floating, but more complex and expensive.

The brakes are power-assisted using the traditional time-honoured approach. Brakes use hydraulic lines to transfer force from the brake pedal to the pistons and callipers.



Leverage combined with hydraulic force multiplication translate a relatively longer travel on the brake pedal into a shorter travel at the brake pistons at a much higher force. The diagram above illustrates the basic mechanisms at work. In this example, the force at the brake pedal is multiplied by a factor of 3 by the leverage and by a further factor of 3 by the hydraulics.

By law, all brakes have two isolated subsystems, one for the front brakes and one for the rear, in case a brake line fails.


The master cylinder is a clever arrangement that ensures the system does not empty of hydraulic pressure and keeps functioning even when one or the other of the sub-systems leak.

Most power brake systems use a vacuum booster to assist braking. The brakes use a brake servo which is powered by the vacuum generated by the engine. In the M5, because it is turbocharged, vacuum is in short supply in the intake manifold, and so a special vacuum pump maintains a reservoir of vacuum in a can, ready to be used to assist breaking on demand.


When the brake pedal is depressed hard enough, an air valve is opened which allows atmospheric pressure into one side of a vacuum chamber that boosts the pressure applied to the master cylinder. 




Here we see a typical arrangement of pedal to power brake booster to master cylinder, and then off to the front and rear brakes respectively, with hydraulic fluid returning on the left.

For each brake there is also the Anti-Lock Braking system that contains electronically controlled valves and an electric pump that modules brake pressure when wheel lock-up is about to occur.

The purpose of ABS is to shorten the braking distance and to retain manoeuvring during braking so that obstacles can be avoided. When a brake is applied until it locks up, then the car starts sliding on its tires. Once the tires start sliding, they are actually less sticky. By pulsing the brakes, they are kept just on the threshold of lockup, which is the most effective for stopping. When driving without ABS, drivers must feel the point at which the brakes are just starting to lockup, and then ease off a bit to keep the wheels spinning. This is called "threshold braking", and is more effective than "pumping the brakes" but harder to master.

When brakes lock up, since there is no traction at all, there is certainly no traction for manoeuvring. The driver can turn the steering wheel round and round but the car will keep sliding in a straight line. With ABS, the car is kept on the threshold of traction, so traction is made available when the steering wheel is turned to steer the car away from obstacles during braking.

The system in the M5 pulses the brakes very quickly, can apply itself to the four wheels independently, and is completely under computer control. This system is used for a variety of additional stability control purposes in addition to the standard "Anti-Lock Braking" (ABS) function, all under the control of a sub-system calls "Dynamic Stability Control" (DSC).


Under normal braking conditions, hydraulic pressure from the master cylinder passes straight through to the brake pistons. The computer compares the wheel speeds against one another. If it detects a wheel locking it can isolate that brake from the driver's foot, and then bleed pressure off and then on again very rapidly. In order to recover pressure after the bleed, a pump is used to restore it. The operation of the pump and the valves is felt in the driver's foot as pressure pulsations when the system is regulating braking.

Additional braking functions in the M5 include the following.
  • Cornering Brake Control (CBC) which applies brakes differentially when cornering with light braking;
  • Dry Braking which applies 1 bar of pressure on the rotors for 1.5 s every 90s to dry the brakes when the windshield wipers are on continuous mode;
  • Brake Standby which looks for a quick release of the accelerator pedal and pre-tensions the brakes with 2.5 bar of pressure for 0.5 s in anticipation of hard braking;
  • Dynamic Brake Control which monitors speed and brake pedal pressure changes and goes to maximum braking pressure when warranted;
  • Automatic Soft-Stop which automatically reduces pressure at the rear axle just before the vehicle comes to a stop when braking lightly;
  • Fading Compensation which monitors brake effectiveness and provides additional pressure when brakes start fading;
  • Drive-off Assistant which holds the brakes until sufficient torque is available when on a hill.
  • The brakes are also requested to apply themselves to various wheels by the chassis dynamics system discussed later. 
The rear brakes incorporate an electromechanical parking brake that is an independent system for clamping the callipers down on the rotors. It will work when parked and when moving (as per government regulations).



A motor is used to turn a spindle that applies locking pressure. When the motor is off, the pressure is still held as it is "screwed down" tightly.

The system is operated from a switch on the centre console under the gear lever. Pull up on it to apply. Push down to release. It can also be released by pressing on the accelerator.


There is a manual release buried under the trunk.

There is an additional system in the car related to braking called "Brake Energy Regeneration". In most cars, the alternator is continuously run whenever the engine is turning over. In this car, when accelerating or coasting the alternator is disconnected leading to a smaller engine load and more efficiency.


As long as the battery stays above a certain threshold, the only time the alternator is connected and drains shaft energy is during braking, either by means of engine overrun or when applying the brakes directly.

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