In the first parts of our suspension series we have covered basic suspension stuff. Now it’s time to bury ourselves in tuning suspension geometry. Geometry tuning is a step above the usual bolt on street parts. Making changes on this fundamental level is what racecar and suspension engineers do for a living, but we’ve found that with the more popular cars in this market there are parts available to help with these mods. Some of you are also advanced enough to experiment with this as well.
The Roll Center
Roll center is the virtual pivot point in space that a car rotates around when subjected to cornering forces. The roll center is significant because its location determines how a car will handle and what factors must be considered when tuning its suspension.
|Finding the instant center is as easy as drawing some lines through the suspension pivots and extending them until they meet in space. The top example is typically what you would find in a modern multi-link car. The bottom example shows that the instant center can be on the outside of the wheel as well. The only type of car that I can think of being like that is a current F1 car. To find the roll center of a car like this you would have to draw a line bisecting the angle and extend it toward the center of the car. I don’t think any of us are going to design an F1 car anytime soon so we won’t worry about this for now.
The roll center is located by first finding the front view instant center of each side of a car’s suspension. The instant center is the point in space that the suspension’s links will rotate around. Drawing lines from the center of the ball joint through the inner pivots of the upper and lower control arms and extending them inward towards the center of the car until they meet will let you locate the instant centers. Now draw a line from the center of the tire’s contact patch to the instant center on both sides of the car. The point where these two lines intersect is the roll center. For a car with McPherson strut suspension, the upper line is found by drawing a line 90 degrees from the strut axis starting at the upper mounting point of the strut.
|The roll center is found by drawing lines from the center of the tire contact patch to the instant centers. When they intersect at the cars’ centerline is the roll center.
|In the case of extreme angularity of the links, the roll center can be under ground. Although this isn’t a show stopper and many race cars are like this in the front suspension due to a desire to have a lot of negative camber gain in the front suspension, it is not ideal.
|Finding the roll center for a live rear axle is very different from finding it on an IRS car. Live axles are common for domestic cars like Mustangs or trucks. For a leaf spring car, the roll center is where the leaf springs bolt to the axle. For a 4-link, it is the intersection of the links when viewed from the rear. For a panhard rod, the instant center is about in the middle of the rod. For a Watts linkage, it is at the center of the rocker.