The brakes fitted to the new Audi A4 are an entirely new development and larger than those used on the previous model. Versions with a four-cylinder engine (1.8 TFSI and 2.0 TDI) have 314 mm diameter front discs and 300mm rear discs; the front brake discs are ventilated. V6-engined cars have composite aluminium brake callipers and larger 320 mm diameter front discs.
The pad surface area for the standard brake system has been increased by about 20 per cent. The newly developed high-performance pads combine high, stable friction values with low susceptibility to fading even when called upon to sustain severe loads. The new brake discs also increase the system’s retardation potential significantly.
The ventilated discs have been modified in detail to improve heat dissipation; this is achieved by optimal friction surface contact and a new design principle that dispenses with the conventional cooling channels. The two halves of the disc are connected by hundreds of small metal cubes, between which a high volume of heated air can dissipate in a very short time.
The brake callipers for the more powerful versions are also totally new: they use the floating-calliper principle and are of composite construction. Areas calling for maximum load resistance are made from high-strength spheroidal-graphite cast iron. The bolted-on piston housing is made of aluminium, and conducts heat away most effectively.
These brake callipers are light in weight but exceptionally rigid, promoting firm, precise pedal movement and accurate feedback for progressive brake sensitivity with low pedal effort. The brake-servo operating characteristic has also been modified with this aim in mind.
In the pursuit of minimal unsprung weight for optimum ride comfort and handling composure, Audi has fitted the A4 with a new type of brake disc which pares two kilograms from the overall weight, and has subtracted an additional one kilogram through the use of aluminium cover plates.
The latest generation ESP version - coded 8.1- incorporates high-precision hydraulic valves of a new type that control pressure build-up with greater accuracy, without unpleasant jerking or noticeable vibration.
The ESP operating concept is also new. As before, it can be switched off completely by holding the button pressed in, but a second, less complete shut-down stage is also now incorporated. If the driver presses the ESP button just once at speeds below 43mph, the wheel-slip control, or ‘ASR’ function will be de-activated. This largely prevents engine-management intervention and reduces the system’s brake applications slightly. If the dynamic steering option is ordered, the stabilising steering impulses remain active. A warning light comes on in the cockpit as a safety precaution.
When de-activated, ASR remains permanently off on the A4 with permanent four-wheel drive, whereas cars with front-wheel drive switch back automatically to the full ESP mode above 43mph.
ESP can also stabilise a trailer that starts to snake by applying the wheel brakes of the towing vehicle separately in a rhythm opposed to the outfit’s swinging movements. If a panic brake application at a rate of retardation exceeding 0.7 g is made, it switches on the hazard warning flashers automatically. In wet weather, it clears the water film off the brake discs by applying the brakes briefly and imperceptibly at regular intervals. In extreme situations involving very high brake loads, it compensates for the fading effect caused by heat build-up that can occur if a full brake application has to be made.
The innovative second-generation tyre pressure monitoring display fitted to the new A4 is both intelligent and reliable. It detects rapid loss of air from the tyre in the same way as a conventional indirect-measuring system, but unlike such systems also indicates which wheel is affected.
The tyre pressure monitoring display also detects situations – caused by diffusion – in which all four tyres lose their pressure very gradually. This may amount to no more than 0.1 bar per month, but if disregarded can lead to severe damage. The system does not only guard against this situation in the conventional way by comparing the four road wheels’ speeds of rotation, but also registers the torsional vibration generated by the road surface. If there is a loss of pressure, the tyre’s sidewall stiffness changes and with it its characteristic natural frequency.Published 9 February 2008