Friday, August 10, 2012

1.6 Front Wheel Drive (2)

 Honda Civic 1992 - 1995. Rear Wheel Suspension.

1.6.4 Non Driven Rear Axles

If rear axles are not driven, use can frequently be made of more simple designs of suspension such as twist-beam or rigid. Twist-Beam Suspension

There are only two load available on each side of the wheel in the case of twist-beam axles. As a result of their design (superposed forces in the links, only two load paths), they suffer as a result of the conflicting aims of longitudinal springing - which is necessary for reasons of comfort - and high axle rigidity - which is requried for reason of driving precision and stability. This is particularly noticeable with the loss of comfort resulting from bumpy road surfaces.
If the guide bearings of the axle are pivoted, the superposition of longitudinal and lateral forces should particularly be taken into consideration. As a result of the design, twist-beam suspensions exhibit unwanted oversteer when subject to lateral forces as a result of deformation of the swinging arms. In order to reduce the tendency to oversteer, large guide bearings which, as 'toe-in correcting' bearings, permit lateral movements of the whole axle body towards understeer when subject to a lateral force provided. As the introduction of longitudinal and lateral forces into the body solely occurs by means of the guide bearings, it must be ensured that the structure of the bodywork is very rigid in these places.

Twist-beam Suspension of the Audi A6 (1997).

An advantageously large supoort width of the guide on the links - important for force application - was chosen becasue of the oberhung arrangement. The flexurallt resistent, but torsionally soft V section profile of the axle is in an upright position in order to ensure that the suspension has roll understeer properties through the high position of the center thrust of the profile.The instantaneous center height is 3.7mm and the toe-in alteration is 0.21min/mm. Breaking-torque compensation of 73% is reached. The stabilizer situated in front of the axis of rotation increase the lateral rigidity of the axle design,  because it accepts tension forces upon the occurence of lateral wheel forces. The linear coil springs monted on noise-insulating molded rubber elements on both sides are separated from the shock absorbers to allow the maximum loading width of the boot as a result of their location under the side rails. The gas-pressure shock absorbers support additional springs made of cellur which act softly, through specific rigidity balancing, to avoid uncomfortable changes in stiffness when reaching the limits of spring travel. Owing to the ridid attachment of the shock absorbers to the bodywork, there also work at low amplitudes; so called 'parasitic' springing resulting from the unwanted flexibility of wheel suspension or bodywork component is theryby
reduced. Rigid Axle

Their advantages outweigh the disadvantages because of non-variable track and camber values during drive.

Rear wheel bearing on the Fiat Panda with a third-generation, two-row angular (contact) ball bearing. The wheel hub and inner ball bearing ring are made of one part, and the square outer ring is fixed to the rigid axle casing with four bolts. Independent Wheel Suspension

An independent wheel suspension is not necessarily better than a rigid axle in terms of handling properties. The wheel may incline with the body and the lateral grip characteristics of the tires decrease, and there are hardly any advantages in terms of weight.

'Omega' rear wheel suspension on the Lancia Y10 and Fiat Panda.

A trailing axle with a U-shaped tube, drum brakes, inclined shock absorbers and additional elastomer springs seated inside the low positioned coil sprpings. The rubber element in the shaft axle bearing point, shown separatelt, has cut-outs to achieve the longitudinal ealsticity necessary for comfort reasons; the same is true for the front bearings of two longitudinal trailing links. The middle bearing point is also the body roll axis.

The body roll center is located in the center of the axle but is determined by the level of the three mounting points on the body. The lateral forces are absorbed here. The angled position of the longitudinal trailing links is chosen to reduce the lateral force oversteering that would otherwise occur. The coil springs are located in front of the axle center and so have to be harder, with the advantage that the body is better supported on bends.

Torsion Crank Axle on the Audi A6 (Audi 100, 1991)

With spring dampers fixed a long way out at points (6) and which largely suppress body roll vibration. The longitudinal control arms therefore had to be welded further in the U-profile acting as a cross-member and reinforced by shoe (5). The U-profile is also raised at the side to achieve higher torsional resistance. The anti-roll bar is located inside the U-profile.

Brace (2) distributes the lateral forces coming from the Panhard Rod (1) to the two body-side fixing points (3) and (4). Bar (1) is located behind the axle, and the lateral force understeering thus caused and could be largely suppressed by the length of the longitudinal controls arms. Furthermore, it was possible to increase the comfort and to house an 80 I fule tank as well as the main muffler in front of the axle.

The only disadvantage is that the link fixing points, and therefore the body roll axis Or, moves further forward and this reduce the 'anti-dive' and that the suspension requires much space whem assembled.

Top view of the double wishbone rear axle on the Honda Civic.

The trailing arm (2), which is stiff under flexure and torsion, and the wheel hub carrier (1) forma unit and, along with the two widely spaced lower transverse control arms (7) and (11), ensure precise wheel control and prevent  unintentional toe-in changes.

The rubber bearing in point (3), which presents the so-called 'Vehicle Roll Axle' Or, provides the real longitudinal wheel control of the axle. The lateral control of wheel carrier (1) is performed by the short upper transverse control arm (6) and the longer lower one (7), which accepts the spring shock absorber 8 in point (9). The length difference in the control arms give favorable camber and track width change.

During braking, bearing (3) yields in the longitudinal direction and, due to the angled positioned of the link (11) when viewed from the top, the front point (4) moves inwards and the wheel goes into toe-ie. Behavor during cornering is similar : the axle understeers due to lateral force and body roll. The wheel is carried by third generation angular (contact) ball bearings on which the outside ring is also designed as a wheel hub. In models with smaller engines, brake drums (10) are used which are fixed to the wheel hub.

Compact trailing arm real axle, fitted by Renault to less powerful medium-sized vehicles.

The short torsion bar springs grip into the guide tubes (2) and (3) in the center of the vehicle. Parts (2),(3) and (4) are jointly subjected to torsional stresses and so the torsional siffness of the transverse tubes contributes to the  spring rate. On the outside, the cast trailing arms (1) are welded to the transverse tubes, which (pushed into each other) support each on the torsionally elastic bearings (5) and (6). This creates a sufficiently long bearing basis, which largely prevents camber and toe-in changes when forces are generated.

The entire assembly is fixed bu the brackets (7) which permits better force transfer on the body side sill. Guide tubes (2) and (3) are mounted in the brackets and can rotate, as well as the outer sides of the two torsion bars (4). The two arms thus transfer all vertical forces plus the entire spring moment to the body. The anti-roll bar (8) is connected to the two trailing arms via two U-shaped tabs.  The two rubber bearings (5) and (6) located between tubes (2) and (3) also contribute to the stabilizing effect. The bump and rebound travel stops are fitted into the shock absorber (9).

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