1.6 Front Wheel Drive

MINI Front Wheel Drive

1.6.1 Types of Design
1.6.2 Advantages and Disadvantages of Front Wheel Drive

The engine, differential and gearbox form one unit, which can sit in front of, over, or behind the front axle. The design is very compact and, unlike the standard design, means that the vehicle can either be around 100–300 mm shorter, or the space for passengers and luggage can be larger. These are probably the main reasons why, worldwide, more and more car manufacturers have gone over to this design. In recent years only a few saloons of up to 2 l capacity without
front-wheel drive have come onto the market. Nowadays, front-wheel drive vehicles are manufactured with V6 and V8 engines and performances in excess of 150 kW.

However, this type of drive is not suitable for commercial vehicles as the rear wheels are highly loaded and the front wheels only slightly. Nevertheless, some ight commercial vehicle manufacturers accept this disadvantage so they can lower the load area and offer more space or better loading conditions. The propshafts necessary on standard passenger cars would not allow this.

The low cargo area on the Peugeot light commercial vehicle J 5/J 7 is achieved due to frontwheel drive and a semitrailing link axle to the rear.

Types of Design

Engine Mounted Longitudinally ‘North–South’ in front of the axle

In-line or V engines mounted in front of the axle – regardless of the wheelbase – give a high front axle load, whereby the vehicle centre of gravity is pushed a long way forwards. Good handling in side winds and good traction, especially in the winter, confirm the merits of a high front axle load, whereas the heavy steering from standing (which can be rectified by power-assisted steering), distinct understeering during cornering and poor braking force distribution
would be evidence against it.

This type of design, as opposed to transverse mounting, is preferred in the larger saloons as it allows for relatively large in-line engines. The first vehicles of this type were the Audi 80 and 100. Inclining the in-line engine and placing the radiator beside it means the front overhang length can be reduced. Automatic gearboxes need more space because of the torque converter. This space is readily available with a longitudinally mounted engine.

A disadvantage of longitudinal engines is the unfavorable position of the steering gear: this should be situated over the gearbox. Depending on the axle design, this results in long tie rods with spring strut (McPherson) front axles.

The large articulation angle of the short axle shaft can also limit the springIn front-wheel drive vehicles the engine can be mounted longitudinally in front of the front axle with the manual gearbox behind. The shaft goes over the transverse differential (illustration : Renault).

Transverse Engine Mounted in front of the axle

In spite of the advantage of the short front overhang, only limited space is available between the front wheel housings. This restriction means that engines larger than an in-line four cylinder or V6 cannot be fitted in a medium-sized passenger car. Transverse, asymmetric mounting of the engine and gearbox may also cause some performance problems. The unequal length of
the drive shafts affects the steering. During acceleration the vehicle rises and the drive shafts take on different angular positions, causing uneven moments around the steering axes. The difference between these moments to the left and to the right causes unintentional steering movements resulting in a noticeable pull to one side; drive shafts of equal length are therefore desirable. This also prevents different drilling angles in the drive shaft causing timing differences in drive torque build-up.

The large articulation angle of the short axle shaft can also limit the spring travel of the wheel. To eliminate the adverse effect of unequal length shafts, passenger cars with more powerful engines have an additional bearing next to the engine and an intermediate shaft, the ends of which take one of the two sliding CV joints with angular mobility. Moreover, ‘flexing vibration’ of the long drive shaft can occur in the main driving range. Its natural frequency can be shifted by clamping on a suppression weight.

Compact power train unit on the Vauxhall Corsa (1997). The engine is transverse mounted with the gearbox on the left. The McPherson front axle and safety steering column can be seen clearly.

Layout of transverse engine, manual gearbox and differential on the VW Polo. Because the arrangement is offset, the axle shaft leading to the left front wheel is shorter than that leading to the right one. The shifter shaft between the two can be seen clearly. The total mechanical efficiency should be around n ≈ 0.9.

Gearbox unit on the Lancia Thema, located beside the transverse engine and between the front axle McPherson struts. Owing to the high engine performance, the design features two equal-length axle shafts joined by an intermediate shaft. There are also internally ventilated disc brakes.

Front-wheel output shaft of GKN Automotive. A constant-velocity sliding joint is used on the gearbox side and a constant-velocity fixed joint is used on the wheel side. The maximum bending angles are 22 degrees for the sliding joint and 47 degrees for the fixed joint. For reasons of weight, the sliding joint is placed directly into the differential and fixed axially by a circlip. A central nut secures attachment on the wheel side. The intermediate shaft is designed as a carburized, shaped hollow shaft.

Arrangement of the gearbox beneath the motor, which is inclined towards the rear, and the differential gear placed behind it. A single oil-economy undertakes the supply, in this case, of the driving unit, narrow in its design.

Advantages and Disadvantages of Front-Wheel Drive

Advantage :

• there is load on the steered and driven wheels;
• good road-holding, especially on wet roads and in wintry conditions – the car is pulled and not pushed;
• good drive-off and sufficient climbing capacity with only few people in the
  vehicle;
• tendency to understeer in cornering;
• insensitive to side wind;
• although the front axle is loaded due to the weight of the drive unit, the steering
  is not necessarily heavier (in comparison with standard cars) during driving;
• axle adjustment values are required only to a limited degree for steering alignment;
• simple rear axle design – e.g. compound crank or rigid axles – possible;
• long wheelbase making high ride comfort possible;
• short power flow because the engine, gearbox and differential form a compact unit;
• good engine cooling (radiator in front), and an electric fan can be fitted;
• effective heating due to short paths;
• smooth car floor pan;
• exhaust system with long path (important on cars with catalytic converters);
• a large boot with a favourable crumple zone for rear end crash.

Disadvantage :

• under full load, poorer drive-off capacity on wet and icy roads and on inclines;
• with powerful engines, increasing influence on steering;
• engine length limited by available space;
• with high front axle load, high steering ratio or power steering is necessary;
• with high located, dash-panel mounted rack and pinion steering, centre takeoff
  tie rods become necessary or significant kinematic toe-in change practically inevitable geometrical difficult project definition of a favourable interference force lever arm and a favourable steering roll radius (scrub radius);
• engine gearbox unit renders more difficult the arrangement of the steering package;
• the power plant mounting has to absorb the engine moment times the total gear ratio 
• it is difficult to design the power plant mounting – booming noises, resonant frequencies in conjunction with the suspension, tip in and let
• off torque effects etc., need to be suppressed;
• with soft mountings, wavy road surfaces excite the power plant to natural frequency oscillation (so-called ‘front end shake’);
• there is bending stress on the exhaust system from the power plant movements
  during drive-off and braking (with the engine);
• there is a complex front axle, so inner drive shafts need a sliding CV joint;
• the turning and track circle is restricted due to the limited bending angle (up to 50°) of the drive joints;
• high sensitivity in the case of tyre imbalance and non-uniformity on the front wheels;
• higher tyre wear in front, because the highly loaded front wheels are both steered and driven;
• poor braking force distribution (about 75% to the front and 25% to the rear);
• complex gear shift mechanism which can also be influenced by power plant movements.

The disadvantage of the decreased climbing performance on wet roads and those with packed snow can be compensated with a drive slip control (ASR) or by shifting the weight to the front axle. On the XM models, Citroën moved the rear axle a long way to the rear resulting in an axle load distribution of about 65% to the front and 35% to the back. The greater the load on the
front wheels, the more the car tends to understeer, causing adverse steering angles and heavy steering, which makes power steering mandatory.