Introduction
Spring rate and anti roll bar adjustments are common as we tune our race car. Spring rates affect how the car reacts to both vertical and lateral loads, while anti roll bars affect how the car reacts to lateral cornering forces. These settings work together to alter the weight transfer and roll properties of the car in order to optimize its performance.
Spring Rates
Spring rates are critical for setting the correct ride height and are proportionate to the movement of the length of component travel in its stoke phases. If the spring rate is too soft, then the ride height will need to be increased to compensate, otherwise the car will bottom out during road irregularities or while reaching top end speed in high downforce race cars. If the spring rate is not correct, then the damper stiffness (bump and rebound) which is covered in the next tutorial will not be correct, leading to even more suspension irregularities. That is why it is critical to adjust spring rates and then ride height setting as a way to bench mark other suspension settings.
It is also important to understand that the overall spring rating of a car, should be as equally split both front and back as possible, depending on the weight distribution and drive line configuration. Otherwise we are setting the car up to either understeer or oversteer.
Spring ratings should equal the total weight of the car, with none down force producing aerodynamics vehicles. Down force levels have an effect on the overall spring rates and we need to add this onto the spring rating of vehicle mass. Otherwise the car will bottom out, which means the weight of the car isn’t supported by its tyres and thus, they will not do as much work.
If we have a high front spring rate (stiff) then the car is prone to understeer. The car will have a good response while at the entry of the corner but then understeer will gradually appear in mid and exit of the corner. In addition, too-high front spring rates will cause the front wheels to lock up under braking and understeer if we hit a bump while cornering. On the other hand, too-low front spring rates will cause the car to bottom out under braking which will again cause the front wheels to lock up and increase the braking distance. While cornering, the car will have increased body roll and we’ll experience understeer due to the front end not being able to handle the load that the cornering speed requires.
If we have a stiff rear spring rate the car is prone to oversteer. The car will become loose in the rear while exiting the corner due to wheel spin which also affects rear tyre wear. On the other hand, having low rear spring rates will increase the load on the rear tyres, changing the weight distribution so that the front is too light and prone to understeer mid-corner.
Now that we’ve covered the extremes of low and high spring rates on both front and rear axles of the car, let’s go ahead and see how we can find the balance between too low or too high by testing and adjusting with the image below.
Anti Roll Bars
Anti roll bars can provide adjustable settings in the suspension geometry, which are particularly useful in dealing with oversteer or understeer handling characteristics and getting a better balanced car. You can make adjustments of the front or the rear anti roll bar without affecting other suspension geometry settings – for this reason, suspension settings are usually done before moving on to changing your anti roll bars. Stiffer anti roll bars will reduce body roll as well as make weight transfer quicker. On the other hand, softer anti roll bars will increase body roll as well as make weight transfer more gradual, with less abrupt loading of the tyres, bending into corners rather then darting into them. Finding a balance between the front and the rear provides a direct effect on the handling of the car.
Let’s go ahead and see what each change does to the balance of the car in the image below.
