1.6 Front Wheel Drive (1)

1.6.3 Driven Front Axles

Driven Front Axles

The following are fitted as front axles on passenger cars, estate cars and light commercial vehicles :

• double wishbone suspensions;
• multi-link axles;
• McPherson struts, and (only in very few cases);
• damper struts.

On double wishbone suspensions the drive shafts require free passage in those places where the coil springs are normally located on the lower suspension control arms. This means that the springs must be placed higher up with the disadvantage that (as on McPherson struts) vertical forces are introduced a long way up on the wheel house panel. It is better to leave the springs on the lower suspension control arms and to attach these to the stiffer body area where the
upper control arms are fixed. Shock absorbers and springs can be positioned behind the drive shafts or sit on split braces, which grip round the shafts and are jointed to the lower suspension control arms. The axle is flatter and the front end (bonnet contour) can be positioned further
down. The upper suspension control arms are relatively short and have mountings that are wide apart. This increases the width of the engine compartment and the spring shock absorber unit can also be taken through the suspension control arms; however, sufficient clearance to the axle shaft is a prerequisite.

Double wishbone front axle assembly of the Audi A4. The Audi A6 of 1997, the Audi A8 (1996) and the VW Passat of 1996 are similar.

Four individual transverse ‘arms’ on each side form what is effectively a double wishbone arrangement which provides lateral and longitudinal wheel location. The two upper members (1 and 2) are attached to the spheroidal graphite iron hollow-section stub-axle post (18) by low-friction ball and socket joints. The track rod (3) provides the steering input through a horizontal extension of this stub-axle post which forms a steering arm. The two lower suspension members consist of the radius arm (4) and the transverse arm (5). This latter must be capable of reacting high loads from the anti-roll bar (6) and spring/damper (7) attachment
points. The co-axial spring/damper assembly incorporates a polyurethane rubber bumpstop,
as well as the hydro-mechanical tension rod stop. The spring/damper unit (7) and the inner bearings of the upper members (1) and (2) are mounted on the upper suspension bracket.

The inner ends of suspension members (4 and 5) are located by substantial rubber mountings on the inside of the sub-frame (10). The rear mounting (11) is hydraulically damped to absorb any harshness associated with radial tyres. The vehicle body is mounted on four rubber mountings (12 to 15) of specified elasticity to ensure a high standard of ride comfort.

The inner drive shafts are located to the rear of the spring dampers (7) and are connected to the drive-line by ‘tripot’ flexible couplings (16). The outer ends of the drive shafts transmit the drive to the wheels through double-row angular contact bearings. The inner races of these bearings are integral with the wheel hubs.

The hydraulically assisted steering rack is mounted on the vehicle’s scuttle, with the steering damper (17) located on one side of the steering housing, and the other side attached to the steering rack.

The advantages of this type of four-link suspension include the location of the points
E and G of the paired arms 1 and 2, likewise 4 and 5, which are subjected to outward thrusts resulting from steering input to the steering-arm, which are thereby compressed through r = 10 mm. Moreover the high location of the point E – together with the negative steering roll radius
r = –7 mm – helps to reduce the loads in all components of the front suspension system. Other design parameters of the suspension arrangement are :

Double wishbone front suspension on the Honda models Prelude and Accord.

With short upper wishbones with widely spaced bearings, lower transverse control arms and longitudinal rods whose front mounts absorb the dynamic rolling stiffness of the radial tyres. The spring shock absorbers are supported via fork-shaped struts on the transverse control arms and are fixed within the upper link mounts. This point is a good force input node. Despite the fact that the upper wheel carrier joint is located high, which gives favourable wheel kinematics, the suspension is compact and the bonnet can be low to give aerodynamic advantages. The large effective distance c between the upper and lower wheel hub carrier joints results in low forces in all mounts and therefore less elastic deflection and better wheel control.

Lancia front axle. 

The McPherson strut consists of the wheel hub carrier 1 and the damping part 2; the two are connected by three screws. The lower spring seat 3 sits firmly on the outer tube and also acts as a buffer for the supplementary spring 4. This surrounds the outer tube 2 giving a longer bearing span. The supporting bearing 5 is arranged diagonally and thus matches the position of the coil spring which is offset to reduce damping friction. The rubber bearing 6 absorbs the

spring forces, and the rubber bearing 7 absorbs the forces generated by the damping.
Disc 8 acts as a compression buffer and plate 9 acts as a rebound buffer for this elastic
bearing. Both parts come into play if the damping forces exceed certain values.

The centre of the CV joint 10 lies in the steering axis and the wheel hub 11 fits onto a two-row angular (contact) ball bearing. Guiding joint 12 sits in a cone of the wheel hub carrier 1 and is bolted to the lower transverse control arm 13. Inelastic ball joints provide the connection to the anti-roll bar 14. The steering axis inclination between the centre point of the upper strut mount and guiding joint 12 and the (here slightly positive) kingpin offset at ground (scrub radius) r are included.

Due to the slight track width change, the change of camber becomes favourable. Furthermore, the inclination of the control arms provides an advantageous radius arm axis position and anti-dive when braking.

Most front-wheel drives coming on to the market today have McPherson struts. It was a long time after their use in standard design cars that McPherson struts were used at the front axle on front-wheel drive vehicles. The drive shaft requires passage under the damping part. This can lead to a shortening of the effective distance l–o, which is important for the axle, with the result that larger transverse forces FY,C and FY,K occur on the piston and rod guide and therefore increase friction.

On front-wheel drive vehicles there is little space available to fit rack and pinion steering. If the vehicle has spring dampers or damper struts, and if the steering gear is housed with short outer take-off tie rods, a toe-in change is almost inevitable. A high steering system can readily be
attached to the dash panel, but a centre take-off is then necessary and the steering system becomes more expensive. Moreover, the steering force applied to the strut is approximately halfway between mountings E and G. The inevitable, greater yield in the transverse direction increases the steering loss angle and makes the steering less responsive and imprecise.