The Ultimate Guide to Suspension and Handling: It’s All in the Geometry- Part One, The Roll Center

The Ultimate Guide to Suspension and Handling Part IX: It’s All in the Geometry – The Roll Center

By Mike Kojima

 

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 though 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 lake 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.

 

10 comments

  1. If i understood correctly, you want the roll centres close to the centres of gravity, but always below them? I looked at the rear axle multilink setup on my car and it looks like the roll centre, unloaded, is somewhere just above the boot floor. Im guessing the centre of gravity is somewhere there aswell. I have lowered the car about an inch at back because it got very light and skiddish under 0.8+ G braking, now i think the roll centre was going above the centre of gravity. Anyway, problem completely gone with lower and stiffer oem springs. No body roll in the car to speak off, so will not lower the front axle as the control arms are about horizontal already.

    1. You don’t, necessarily want the roll centers close to the CG and certainly not above them because that would induce jacking. Also checking the roll center unloaded is not very useful, it should be checked at ride height.

  2. Unloaded meant without load in the car, it was most certainly at ride height. The car is an alfa romeo giulietta, with a rear setup very close or even identical to a 2013 dodge dart. All the links on the body are higher than on the wheel side and just inches below the boot floor. I ordered the car with a dynamic chassis setup which had slightly stiffer and higher springs at the back. Cars with a heavy glass roof get the same springs to correct their ride height and attitude, but mine being lighter got very dynamic indeed. The front however seems very well sorted

  3. Ok, I looked at the Dart rear suspension and the roll center probably isn’t that high. It looks to be about 8-10″ off the ground on the Dart or just under the midline of the wheel.

    1. I ll try to work out the lines in detail from upper and lower mount, and from center of tyre contact next time its on a 4 post lift. Today i went around a roundabout a few times and between 1 and 1.05G lateral the rear right tyre starts squeeling. I can keep it there indefinitely and ESC doesnt quick in but i would have expected the squeel to start at the front. The rear tyres are new and the fronts are nearly worn so that will have an effect aswell. I will investigate the rear camber and toe settings next to see if i can raise the G limit some. Dont want to mess with it much as its very tolerant in changing direction right now. was thinking to modify the front lower control arms for a bit more track width and caster/camber but will leave it for now or just add a little caster as it would be easiest. Love to see your work here though Mike. Thanks for your time!

  4. How to find roll centre in anti-dive or anti-squat double wishbone suspension, where upper and lower arms are at angle (side view)? Thanks

    1. The roll center isn’t anti-dive or anti-squat and isn’t related. I think you need to read the article because I am not sure you understand what it is.

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