1.5 Rear and Mid Engine Drive

Porsche Carrera GT

The rear-mounted power plant consists of the engine and the differential and manual gearbox in one assembly unit, and it drives the rear wheels. The power plant can sit behind the axle  or in front of it. This configuration makes it impossible to have a rear seat as the engine occupies this space. The resulting two-seater is only suitable as a sports or rally car.

Advantage :

• very agile handling properties as a result of the small yawing moment;
• very good drive-off and climbing capacity, almost irrespective of load ;
• a short power flow because the engine, gearbox and differential form one compact unit;
• light steering due to low front axle load;
• good braking force distribution;
• simple front axle design;
• easy engine dismantling (only on rear engine);
• no tunnel or only a small tunnel in the floor pan;
• a small overhang to the front is possible.

Disadvantage :

• sensitivity to side winds;
• indifferent cornering behaviour at the stability limit (central engine);
• oversteering behaviour on bends (rear-mounted engine);
• difficult to steer on ice because of low weight on the front wheels;
• uneven tyre wear front to rear (high rear axle load);
• the engine mounting must absorb the engine moment times the total gear ratio;
• the exhaust system is difficult to design because of short paths;
• the engine noise suppression is problematic;
• complex gear shift mechanism;
• long water paths with front radiators;
• high radiator performance requirement because of forced air cooling, the electric
  fan can only be used on the front radiator;
• the heating system has long paths for hot water or warm air;
• the fuel tank is difficult to house in safe zones;
• the boot size is very limited.

VW Transporter, a light truck which could be used either as an eight-seater bus or for transporting goods, and which has the optimal axle load distribution of 50%/50% in almost all loading conditions. The double wishbone suspension at the front, the semi-trailing link rear axle and the rack and pinion steering, which is operated via an additional gear set in front, can be seen clearly. To achieve a flat load floor throughout, VW changed the Transporter to front-wheel drive in 1990.

The Porsche Boxster (1996) has a water-cooled engine which is longitudinally installed in front of the rear axle. The front axle is designed as a spring strut-type axle. The transverse link is arranged almost in extension of the wheel axle; it is connected to the longitudinal link by a strut bush which is soft for reasons of comfort. This open design and link geometry make it possible to combine a high level of driving precision, a result of rigid wheel control, with riding comfort, owing to the longitudinal elasticity of the axle. At a camber angle of 8 degrees, good straight running results from the large castor displacement of 41 mm. The kingpin offset is –7 mm and the disturbing force lever arm is 83 mm. The pitch centre of the front axle was located near
to the road to achieve kinematic wheel recession of the axle, which is important for riding comfort, with the result that braking-torque compensation is only 10%.

The rear axle is also a spring strut-type axle in an open link design; the wheel carrier, hub and bushes as well as the transverse link are the same as those found on the front axle. The open design makes it possible to have an inwardly inclined elastokinematic axis of rotation, so that a stabilizing toe-in position of the rear wheels is produced during braking. The axle can also be designed to understeer when subject to lateral forces. The main disadvantages of the mid-engine design are apparent from the boot space: only 130 l are available at both the front and back.