How to Tune Spring Rate and Ride Height
with Stock SpringsMulti-link suspension (I'll use MLS as acronym) consists of a number of pivoted links or "arms" to maintain a range of wheel alignment change throughout the entire range of suspension travel. In other words, each alignment parameter like toe-in or camber can be plotted in a curve against suspension travel. Furthermore, MLS can control roll centre and implement anti-squat/lift feature. The main advantage of MLS is creativity in design. By modifying link pivot locations, you can have virtually any wheel alignment change against suspension travel. Why do we care about wheel alignment change against suspension travel in the first place?
One obvious need is to create -ve camber in order to compensate +ve camber caused by body roll during cornering. Another trick is to create rear toe-out to improve turn-in response. However, these suspension tricks can induce unwanted side-effects. For example, some RWDs with MLS (and most live-axle suspended cars as well) implement an anti-squat/lift feature. In MLS case, at constant speed, engine power is transferred through a pair of offset half-shafts to the wheels like any regular half-shafts does. During acceleration, torque is asserted on the half-shafts to try to push the rear wheels onto the ground, preventing the rear to squat.
Sounds perfect! But reversing the thought means compressing the suspension will force the wheels to rotate backwards a bit. Result? When the rear wheels run over a speed bump, the car pushes backward a bit, potentially creating a back-and-forth rocking effect. More importantly, in the case of the 3 sensor ABS used in Nissan 240SX (200SX outside North America), the tiny backward push is transferred through the diff back to the ABS sensor along the diff input shaft. Result? An inaccurate rear wheel speed being measured when travelling on bumpy roads to incorrectly triggers the ABS.
Besides straight line performance, MLS also has potential problems in cornering. The geometry of the links in MLS is so nicely designed that altering any suspension parts like ride height or spring rate will definitely screw up the design in some degrees. Some rear MLS is designed such that for initial decrease in suspension travel (during turn-in), the outside rear wheel will be toed out to improve turn-in response. For further decrease in suspension travel (during high-speed cornering), the wheel becomes toed in for stability. If the ride height is lowered to pass the suspension travel zone for "toe out", the MLS will not toe out any more during turn in. Even re-doing wheel alignment will not help. The only remedy is to redesign the link pivot points which is quite impossible.
Legacy in-wheel double-wishbone suspension (I'll use DWS as acronym) like the ones used in Miatas has lower and upper "A" arms which look quite similar to the alphabet "A" where the name comes from. Rear DWS is simple but effective by changing only rear wheel camber without affecting any rear toe angle during cornering. I think everybody agrees that Miata's steering response is even better than many cars with MLS or active 4WS. If the geometry of a DWS is designed nicely, there is no need to get help from suspension tricks of MLS.
Next: How to Tune Spring Rate and Ride Height
with Stock Springs
