Ride Height
The ride height is the distance from the bottom of the car (or sometimes some other reference point) to the ground. This is a very powerful tool affecting the overall behavior of the car throughout the lap. It directly impacts the CG or center of gravity of the vehicle, the front and rear roll centers, and the rake of the car (generally difference between the front and rear ride height or ground clearance). It also has a less direct but important impact to the aerodynamic drag and downforce of the vehicle.
Adjustments
Spring perch offset - For cars with coil‐over spring/damper units and conventional suspension, the spring perch offset is the distance from the spring seat of the spring perch (or shock collar) on the shock body to the rod end of the shock body. With no other spring changes, reducing this offset will extend the shock (raising the ride height at that corner of the car), whereas increasing it will collapse the shock (lowering the ride height). This number simply represents the lengthening or shortening of the spring with zero being a baseline starting point.
Push rod length - The push rod is the component on some cars with an inboard suspension design that is used to adjust ride height. Lengthening the push rod at one corner of the car will increase the ride height at that corner. The advantage of push rod (and pull rod) suspensions are that they get the coil over assembly out of the airflow and reduce aerodynamic drag [especially for open wheeled cars], they allow for better shock/spring motion ratios, and they generally are arranged to position the coil over shock where it can be adjusted or changed much faster and easier than in a conventional suspension.
Pull rod length - The pull rod is the component on some cars with an inboard suspension design that is used to adjust ride height. Lengthening the pull rod at one corner of the car will decrease the ride height at that corner (note this is the opposite of a push rod suspension, also note that some cars will have a push rod suspension on one end of the car with a pull rod suspension on the other).
Tuning advice
In general, you want to run as low of a ride height as possible without frequently/excessively bottoming the car. This provides more mechanical grip caused by the associated lower CG and lower roll centers and typically decreases aerodynamic drag and increases downforce because of the enhanced ground effect. However there are a number of factors and interactions to consider. Positive rake (where the rear has a higher ride height than the front) is normally desired to improve downforce (generally under 1” max front to rear difference), and NEGATIVE rake needs to be avoided as it has bad effects on downforce and drag. Too low of a ride height (ground clearance) can hurt downforce by “choking” the underbody flow, reducing the amount of suspension travel (especially a problem on bumpy tracks), and may often put the suspension in an area where it has an unfavorable geometry. Normally you want the ride height to be symmetric from side to side, but on ovals, and especially banked ovals, it is often set to be somewhat asymmetrical. Part of this is to induce wedge or cross weight, and part is to compensate for the increased loads on the outer tires (thus the static ride height on the outside is increased, which gives an even ride height when the car is under cornering loads). See the cross weight section for further advice on oval tuning.
Front:
Lower ride height: Lowers CG, front roll center, and increases rake. This will shift more weight to front, improving front‐tire grip and thus shifting the balance to less UNDERsteer and/or more OVERsteer and typically reduce aero drag and increase downforce. Raising the right‐front ride height on an oval (when turning left) can increase front grip if the right‐front tire is being overloaded, resulting in an adverse camber effect. Optimum ride height can vary greatly depending on other chassis settings and the degree of banking at a particular track.
Increased ride height: Raises CG, front roll center, and reduces rake. The change will shift weight to the rear, improving the grip of the tires at that end of the car and shifting the handling balance toward UNDERsteer and typically increase aero drag and increase downforce.
Rear:
Lower ride height: Lowers CG, rear roll center, and reduces rake. This will shift more weight to rear, improving front‐tire grip and thus shifting the balance to less OVERsteer and/or more UNDERsteer .
Increased ride height: Raises CG, rear roll center, and increases rake. The change will shift weight to the front, improving the grip of the tires at that end of the car and shifting the handling balance toward OVERsteer and increase downforce. Note the positive downforce effect may dominate the negative mechanical grip impacts and reduce oversteer, especially in high speed corners. Another potential downside – less rear braking available – may need to be addressed with an increase in brake bias to the front of the car.
Interactions
Changing the spring rate will change the ride height, this is especially true for conventional (non pull rod or push rod) suspensions. Changing the vehicle rake changes the effective angle of attack seen by the wings (increasing the rake effectively increases the wing angle). Lower ride heights generally require stiffer springs and shocks (but not ARB’s) to control the car and keep it from bottoming excessively. Changing the ride height normally results in a change to the camber (generally it will go more negative as the car goes lower).
