1.3.2 Semi Rigid Crank Axle

Twist Beam Suspension of the VW Golf IV (1997), VW Bora (1999) and Audi A3 (1996).

About the Cover :

The rubber-metal bearings of the axle body are set at 25 degrees to the transverse suspension
of the vehicle in order to improve the self-steering properties of the suspension together
with the rigidity of the beariings which varies in three directions in space. Compares with the
previous model, it was possible to reduce unwanted lateral-force toe-out steer resulting from
link deformation by 30% to approximately 1 mm per 500N of lateral force.

The compound crank suspension could be described as the new rear axle design of the 1970s
and it is still used in today's small and medium-sized front-wheel drive vehicles.
It consists of two trailing arms that are welded to a twistable cross-member and fixed to
the body via trailing links. This member absorbs all vertical and lateral force moments and,
because of its offset to the wheel center, must be less torsionally stiff and function
simultaneously as an anti-roll bar. The axle has numerous advantages and is therefore found on
a number of passenger cars which have come onto the market.

From an installation point of view :

The whole axle is easy to assemble and dismantle; it needs little space; a spring damper
unit or the shock absorber and springs are easy to fit; no need for any control arms and rods;
thus only few components to handle.

From a suspension point of view :

There is a favorable wheel to spring damper ratio; there are only two bearing points which hardly affect the springing; low weight of the unsprung masses and the cross-member can also function as as an anti-roll bar.

From a kinematic point of view :

There is negligible toe-in and track width changes on reciprocal and parallel springing;
there is a low change of camber under lateral forces; there is low load-dependent body
roll understeerring of the whole axle; and gross radius axis locations which reduce tail-lift
during braking.

Disadvantages :

• A tendency to lateral force oversteer due to control arm deformation; torsion and shear stressin the cross-member; 
• high stress in the cross-member; 
• high stress in the weld seams; 
• which means the permissible real axle load is limited in terms of strength; 
• the limited kinematic and elastokinematic opportunities for determining the wheel position;
•  the establishment of the position of the instantaneous center by means of the axle kinematics and rigidity of the twist-beam axle;
•  the mutual effect on the wheel;
•  the difficult decoupling of the vibration and noise caused by the road surface; 
• and the considerable need for stability of the bodywork in the region of those points on the front bearings at which complex; 
• superposed forces have to be transmitted.