Assessment of Concepts
In contrast to single-axle drives, all AWD concepts are associated with significantly higher production costs.
The lowest additional expense is associated with mechanically engaged all-wheel drive. It distinguishes itself through its simple and robust design. At poor road contact, the rigid engagement of the front and rear axles produce satisfactory traction. The rigid engagement of the axles, however leads to stress on the drivetrain, slip on the wheels (loss in side forces) and to a change in the brake-force distribution in curved roads with high coefficients of friction. If dealt with properly, large benefits can however be achieved with these systems.
In permanent AWD, the possibility of balancing the speeds between the axles using intermediate differentials exists. The differences between the various concepts of permanent all-wheel drive cannot be noticed in normal driving. Only extreme situations expose the advantages of one or the other systems. The electronic-hydraulic control is currently considered the most optimum of all production all-wheel drive systems. A safe driving behavior during acceleration is common to all permanent all-wheel drives, since even at low coefficients of friction, the slip of traction forces on the wheels and hence resulting loss in cornering force is smaller compared to a single-axle driven concept. At high locking values in the differentials, the braking behavior is influenced. Even the use of ABS cannot compensate the discrepancy between driving and braking behavior. This is also applicable to self-regulating differential locks such as the Torsen-differential or the visco-hydraulic clutch, whose locking effect hinders independent wheel braking which is necessary for ABS. Integrated free-wheeling or adjusted "soft" locking characteristics are required.
The disadvantages of additional weight and increased friction losses compared to rear and front-wheel drives is common to all all-wheel drive concepts. This leads to an increased fuel consumption, which is however negligible in modern concepts equipped with electronically controlled locks. Hence Porsche quotes the identical 1/3-mix consumption for the Carrera 4 as well as the Carrera 2. The additional space requirement has to be created by reducing the size of the fuel tank and trunk or by locating the vehicle higher. In addition, the second driven axle requires a considerably more constructional expense compared to its undriven counterpart, particularly in originally front-wheel driven vehicles combined with a simple twist-beam rear axle (AE-II).
This brings us to the end of the Module "Driving Performance limited by Power". The next module deals with the effects of friction on driving performance.