Original author: Jakub Zawada
The ALK effectively modifies the position of the side view instant centre on the front suspension. The side view instant centre is the pivot point for the side view swing arm (also the pivot point for the suspension at that instant), which is a line drawn from the tire contact patch to the side view instant centre (under braking – for acceleration it is drawn from the wheel centre). The slope or angle of this swing arm (effectively the position of the instant centre) describes the amount of anti-dive and anti-lift present in the front suspension.
The instant centre is found by the intersection of the two lines. The first is the projection of the lower control arm, say through the chassis mounts, behind the front wheel the second is the normal to the axis of the strut tower at the top of the strut, projected behind the front wheel in the case of the WRX.
(Note: To be absolutely correct these lines should be projected onto the wheel centre plane, so any lateral angles in the lower control arm will affect the instant centre position. However the lower control arm in the WRX is relatively flat which will give minimal effect).
The diagram below shows the instant centre and swing arm.
Together with the position of the instant centre, the WRX’s wheelbase, CG height above the ground, % front torque (for anti-lift) and % front braking (for anti-dive) are required to calculate the anti features of the front suspension.
With the ALK fitted, the rear mount of the front lower control arm is lower by about 20mm. There is also a castor change by moving the mount outwards (however this has not been taken into account). This has the effect of lowering the instant centre and decreasing the angle or slope of the swing arm resulting in the following anti-dive and anti-lift coefficients, expressed as percentages.
As can be seen in the above spreadsheet, with the ALK fitted the anti-lift and anti-dive coefficients reduce to 0%.
The effect of lowering the % anti-lift / anti-dive
If the suspension has 100% anti-dive / anti-lift, then all the longitudinal load transfer experienced when braking and accelerating is carried through the control arm, leaving the springs unloaded and no deflection present. If there is 0% then the springs take the entire load, giving full spring deflection.
By lowering the % the front suspension becomes “softer” under acceleration or braking. This gives rise to the higher diving and lifting that has been experimentally shown. These results are shown below:
A softer front suspension during acceleration and braking will even out the load on the front tyres, giving a higher total cornering load available or more front grip.This will lead to less under steer when cornering under power or brakes.
Softer front rate will also allow better wheel tracking over rough roads, keeping the wheels in contact with the ground.
The drawback of these is the amount of pitch and roll increase that will be seen. This can effect the front suspension geometry if there is a very large amount of pitch and roll.