Roll center calculator
For double-wishbone (SLA) suspension. Enter your front-view hardpoints, or drag the points on the drawing. The roll center, instant center and swing arm update as you go. Free, nothing to install, and nothing you enter leaves your browser.
Drag the numbered points; the numbers match the table. Drag the tire's blue point sideways to change the track. With a point selected, the arrow keys move it 1 mm (hold Shift for 10×).
Roll center height
32.8 mm
above the ground, on the car's centerline
- Instant center
- 2,844 mm past the centerline, on the other side · 186 mm up
- Swing arm length
- 3,459 mm
- contact patch to instant center
- Roll center in 25 mm of bump
- 8 mm (25 mm lower)
- Roll center in 25 mm of droop
- 58 mm (25 mm higher)
- Camber change in 25 mm of bump
- -0.51° more negative camber
Hardpoints
The corner on the right of the drawing (in a front view, that's the car's left). Out is the distance from the car's centerline; up is the height above the ground. The other corner is the mirror image.
Using CAD? For each chassis pivot, take the point where the arm's pivot axis crosses the front-view plane through the wheel center, not the front or rear bushing.
Center of one tire's contact patch to the other's.
How the roll center is found
- Extend the upper arm and the lower arm, in front view, until they cross. That point is the instant center: for small moves, the wheel swings about it.
- Draw a line from the middle of the tire's contact patch through the instant center.
- Do the same on the other side. The two lines cross at the roll center. On a symmetric car that is on the centerline.
If the arms are parallel there is no crossing. The line from the contact patch then runs parallel to the arms. Equal, level arms put the roll center on the ground.
Want the rest of the suspension math? The free Contact Patch cheat sheet puts the motion ratio, wheel rate, weight transfer and anti-dive formulas on one page, with a spreadsheet that does the math.
What the numbers mean
- Roll center height. The cornering force acts on the body through this point. The higher it is, the less the body rolls, but the more the tires push the car up in a corner (jacking).
- Swing arm length. A short swing arm gives more camber change as the wheel moves up and down. A long one gives less.
- In bump and droop. How far the roll center moves when the car rides lower or higher. A roll center that moves a lot makes the car's balance change with speed and load.
What moves it
Raise the lower arm's chassis pivot, or lower the upper arm's, and the arms meet closer to the car: the roll center goes up and the swing arm gets shorter. Do the opposite and it comes down. Moving both chassis pivots up together raises it more gently. Try each one on the drawing.
Lowering a car moves both chassis pivots down relative to the wheels, so the roll center drops too. See what lowering does to your roll center in the free setup lessons. For a full worked example of fixing a roll center, read the workbench guide's case study.
What this calculator leaves out
It's a 2D front view of a symmetric axle, with rigid parts and the tire as a point on the ground. Real pivots sit at different heights front to back, and in a corner the body rolls, so the roll center also moves sideways.
Questions
Does it work for the rear suspension?
Yes. It works for any double-wishbone (SLA) axle, front or rear: enter that axle's hardpoints and its track.
What's the difference between the instant center and the roll center?
Each corner has its own instant center: the point its wheel swings about, where the two arm lines cross. The roll center belongs to the whole axle. It's where the lines from each contact patch through its instant center meet, and the body rolls about it.
What is a good roll center height?
There's no single right number. A higher roll center means less body roll but more jacking, and the best trade depends on the rest of the car. Many race cars keep the front roll center low, a few tens of millimeters above the ground, and often a little lower than the rear. The example targets in our Vehicle Dynamics Workbench use 25 to 60 mm at the front. How far it moves in bump and roll matters as much as where it starts.
Why is my roll center below the ground?
When the arm lines cross below the ground, the line from the contact patch slopes down toward the centerline, so the roll center ends up below the ground. That's allowed, but the body will roll more. Raising the lower arm's chassis pivot brings it up.
Is my design saved or sent anywhere?
No. Your hardpoints stay in your browser. They're kept only in the page's address, so you can bookmark a design or share its link.
More free tools: Spring rate calculator · Weight transfer calculator