1.4 Front Mounted Engine, Rear Mounted Drive

BMW M6 Cutaway.

1.4.1 Advantage and Disadvantage of the front mounted engine, rear mounted drive.

1.4.2 Non-driven front axles

1.4.3 Driven rear axles

V8 Front-Mid Engine and Transmission. Mercedes Benz SLS AMG.

Front-mounted engine, rear-mounted drive (BMW 3 series E46, 1998).

The manual transmission is flange-mounted on the engine, which is longitudinally positioned over the front axle. The rear-axle differential is driven by means of a propshaft. The fuel tank is situated in front of the rear axle for safety in case of an ancient. The battery was placed in the boot in order to achieve a balanced 50:50 axle-load distribution.

Chevrolet Corvette (1998).

In order to achieve balanced axle-load distribution, a more rigid overall system (necessary on amount of the greater flexibility of the plastic bodywork) and more leg room, the gearbox in integrated with the rear-axle differential. Compared with standard drives, the cardan shaft turns higher (with engine speed) but is subject to  correspondingly less torque. The front and rear axles have plastic (fiberglass) transverse leaf springs.

Chevrolet Corvette.

Advantage:

1. There is hardly any restriction on engine length, making it particularly suitable for more
   powerful vehicles, in other words for engines with 8-12 cylinders. 
2. There is low load on the engine mounting, as only the maximum engine torque times the conversion of the lowest gear without differential transmission has to be absorbed.
3. Insulation of engine noise is relatively easy.
4. Under full load most of the vehicle mass is on the driven rear axle.
5. A longer exhaust system with good silencing and catalytic converter configuration.
6. Good front crumple zone, together with the " submarining " power plant unit,i.e. one that goes underneath the floor panel during frontal collision.
7. Simple and varied front axle designs are possible irrespective of drive forces.
8. More even tire wear thanks to function distribution of steering/drive.
9. Uncomplicated gear shift mechanism.Optimum gearbox efficiency in direct gear because no force-transmitting bevel gear is in action.
10. Sufficient space for housing the steering system in the case of a recirculating
    ball steering gear.
11. Good cooling because the engine and radiator are at the front; a power-saving
    fan can be fitted.
12. Effective heating due to short hot-air and water paths. 

Disadvantage :

1. Unstable straight-running ability, which can be fully corrected by special front suspension  geometry settings, appropriate rear axle design and suitable tires.
2. The driven rear axle is slightly loaded when there are only two persons in the vehicle, leading to poor traction behavior in wet and wintry road conditions – linked to the risk of the rear wheels spinning, particularly when tight bends are being negotiated at speed. This can be improved by setting the unladen axle load distribution at 50%/50% which, however, is not always possible. It can be prevented by means of drive-slip control.
3. A tendency towards the torque steer effect.
4. complex rear independent wheel suspension with chassis subframe, differential gear case and axle drive causing.
5. The need for a propshaft between the manual gearbox and differential,a tunnel in the floor pan is inevitable, plus an unfavorable interior to vehicle – length ratio.

Rear axle (left side of wheel) of the Chevrolet Corvette (1998).

Links 1,2 and wheel carrier 3 of the multi-link suspension are made from aluminium in order
to reduce the unsprung masses. The plastic leaf spring 4 is mounted at two places on the right and left sides of the body (5) so that it also helps to make the body more resistant to roll. Roll spring stiffness is further increased by stabilizer 6. This is attached to subframe 7, which is also made of aluminium. The design of the wheel carrier 3 on the front and rear axles is the same, but not the wheel links 1 and 2. The toe-in control of the rear axle is exercised very stiffly and precisely, via tie rod 8.

Non-Driven Front Axles

The standard design for passenger cars that have come onto the market in recent years have McPherson struts on the front axle, as well as double wishbone or multi-link suspensions. The latter type of suspension is becoming more and more popular because of its low friction levels and kinematic advantages. Even some light commercial vehicles have McPherson struts or
double wishbone axles (Fig. 1.7). However, like almost all medium-sized and heavy commercial vehicles, most have rigid front axles. In order to be able to situate the engine lower, the axle subframe has to be offset downwards.

The front rigid axle on the Mercedes-Benz light commercial vehicle of the 207 D/308 series with recirculating ball steering gear and steering rod 1 parallel to the two-layer parabolic spring.

This rod has to be slightly shorter than the front side of the spring, so that both parts
take on the same motion curve when the axle bottoms out (see also Fig. 4.6). The brace 3, running from the steering column jacket 2 to the body, bends on impact. The T-shaped axle casing 4, which is cranked downwards and to which the springs are fastened, can be seen in the section. The elastomer spring 5 sits on the longitudinal member of the frame and the
two front wheels are joined by the tie rod 6. The safety steering wheel has additional padding.

Front hub carrier (steering knuckle) on the Mercedes-Benz S class (W40, 1997) with a large
effective distance c. The upper transverse control arm 6 forms the casing for the ball pivot of
the guiding joint, whereas the lower supporting joint 7 is pressed into the hub carrier 5. The ventilated brake disc 34 (dished inwards), the wheel hub 9, the double hump rim 43 with asymmetrical drop center and the space for the brake caliper (not included in the picture) are
clearly shown.

Multi-link front suspension of the Mercedes-Benz model W220 (S class, 1998).

Based on a double wishbone axle, two individual links (tension strut and spring link) are used instead of the lower transverse link in order to control the steering axle nearer to the middle of the wheel. As a result, the kingpin offset and disturbing force lever arm are reduced and vibrations are caused by tire imbalances and brake-force fluctuations is consequently minimized. Crash performance is also improved by the more open design. The air-spring struts with integrated shock absorber proceed directly from the spring link. The laterally rigid rack and pinion steering in front of the middle of the wheel leads to the desired elastokinematic understeer effect during cornering owing to the laterally elastic spring link bearings. The manufacturing tolerances are kept so small by means of punched holes that the adjustment of camber and camber angles in production is not necessary.

Spring strut front axle of the BMW Roadster Z3, which Lemförder Fahrwerktechnik produce in the USA and supply directly to the assembly line there.

The additional springs 2 are positioned in the coil springs which are offset at an angle in order to reduce friction. The stabilizer 6 is connected to the lower links by the struts 3.The cross-member 7 which serves as the subframe takes the hydraulically supported rack and pinion steering 1 at the front and the transverse link 4 on its lower side.

The L-shape of the transverse link makes good decoupling of the lateral rigidity and longitudinal elasticity possible: lateral forces are introduced directly into the rigid front bearing, while longitudinal forces produce a rotational movement about the front bearing as a result of the laterally elastic rear bearing 5. These rubber elements ensure a defined lateral springing. The large-diameter internally ventilated brake discs (15 rim) and the third-generation, two-row
angular ball bearings, whose outside ring also acts as a wheel hub, are clearly shown.

Front axle of the Mercedes-Benz Sprinter series (1995).

The wheel controlling strut is screwed on to the wheel carrier, which is, in turn, connected to
the lower cross-member by means of a ball joint. Both the vehicle suspension and roll stabilization are ensured by means of a transverse plastic leaf spring mounted on rubber elements. Large rubber buffers with progressive rigidity act as additional springs and bump stops.

Driven Rear Axles

Because of their cost advantages, robustness and ease of repair rigid axles are fitted in practically all commercial and off-road vehicles in combination with leaf springs, coil springs or air springing. They are no longer found in saloons and coupés. In spite of the advantages the weight of the axle is noticeable on this type of vehicle.

For independent suspension, the semi-trailing arm axle is used as independent wheel suspension in passenger and light commercial vehicles. This suspension has a chassis subframe to which the differential is either fixed or, to a limited degree, elastically joined to give additional noise and vibration insulation. The springs sit on the suspension control arms. This gives a flat, more spacious boot, but with the disadvantage that the forces in all components become higher.

Because of its ride and handling advantages, more and more passenger cars have double wishbone suspension rear axles or so-called multi-link axles. Most independent wheel suspensions have an easy-to-assemble chassis subframe for better wheel control and noise insulation. However, all configurations (regardless of the design) require drive shafts with length compensation. This is carried out by the sliding CV (constant velocity) joints fitted both at the
wheel and the differential.