The McPherson strut is a further development of double wishbone suspension. The upper transverse link is replaced by a pivot point on the wheel house panel, which takes the end of the piston rod and the coil spring. Forces from all directions are concentrated at this point and these cause bending stress in the piston rod. To avoid detrimental elastic camber and caster changes, the normal rod diameter of 11 mm (in the shock absorber) must be increased to at least 18 mm. With a piston diameter of usually 30 mm or 32 mm the damper works on the twin-tube system and can be non-pressurized or pressurized.
The main advantage of the McPherson strut is that all the parts providing the suspension and wheel control can be combined into one assembly. this includes:
- the spring seat 3 to take the underside of the coil spring;
- the auxiliary spring 11 or a bump stop;
- the rebound-travel stop;
- the underslung anti-roll bar 7 via rod 5;
- the steering knuckle.
The steering knuckle can be welded, brazed or bolted firmly to the outer tube . Further advantages are:
- lower forces in the body-side mounting points
- short distance ;
- long spring travel;
- three bearing positions no longer needed;
- better design options on the front crumple zone;
- space at the side permitting a wide engine compartment;
- which makes it easy to fit transverse engines .
Nowadays, design measures have ensured that the advantages are not outweighed by the inevitable disadvantages on the front axle. These disadvantages are:
- Less favourable kinematic characteristics.
- Introduction of forces and vibrations into the inner wheel house panel and therefore into a relatively elastic area of the front end of the vehicle.
- It is more difficult to insulate against road noise – an upper strut mount is necessary which should be as decoupled as possible.
- The friction between piston rod and guide impairs the springing effect .
- In the case of high-mounted rack and pinion steering, long tie rods and, consequently, more expensive steering systems are required ; in addition, there is the unfavourable introduction of tie-rod forces in the middle of the shockabsorbing strut plus additional steering elasticity.
- Greater sensitivity of the front axle to tyre imbalance and radial runout .
- Greater clearance height requirement.
- Sometimes the space between the tyres and the damping element is very limited.