What is Roll Center? | Revv.ly Glossary

What is Roll Center?

I'm going to ask you to imagine something that doesn't actually exist. The roll center is a virtual point--you can't see it, touch it, or point to it on your car--but it fundamentally determines how your suspension behaves when you turn the steering wheel. Understanding this imaginary point explains more about suspension tuning than perhaps any other single concept.
When a car corners, it rotates around an axis that runs longitudinally through the vehicle. The front suspension rolls around the front roll center; the rear suspension rolls around the rear roll center. Connect these two points and you have the roll axis. The height of the roll center relative to the center of gravity determines how much the body leans during cornering.

Finding the Invisible Point

Roll center is determined by suspension geometry--specifically, by extending lines through the control arm pivot points and their intersections. For a double-wishbone suspension:

1. Draw a line through the inner and outer pivot points of the upper control arm
2. Draw a line through the inner and outer pivot points of the lower control arm
3. These lines intersect at a point called the "instantaneous center"
4. Draw a line from the tire contact patch through the instantaneous center
5. Where this line crosses the car's centerline is the roll center
   On strut suspension, the process is similar but uses the strut axis and lower arm geometry.
   The maths work regardless of the suspension being symmetric. Each side of the car has its own geometric relationship, and the roll center represents where these relationships meet at the centerline.

Why Height Matters

Roll center height affects several handling characteristics:
Body Roll -- Higher roll centers reduce body roll during cornering because the roll center is closer to the center of gravity. Lower roll centers increase roll because the lever arm between roll center and CG grows longer.
Weight Transfer -- How quickly and through which path weight transfers during cornering depends partly on roll center height. Geometric weight transfer (through the suspension links) versus elastic weight transfer (through springs and anti-roll bars) is affected.
Jacking -- When roll centers are above ground level, lateral forces during cornering create a vertical component that tends to lift the body. High roll centers can cause noticeable "jacking" behavior where the car rises through corners.
Camber Change -- Roll center height and suspension geometry interact to determine how camber changes during body roll. Getting this relationship right helps maintain tire contact patch through corners.

The Lowering Problem

When you lower a car, you change the control arm angles. This changes where those geometric lines intersect, which changes the roll center height--usually lowering it significantly, sometimes even below ground level.
A roll center below ground creates interesting (not necessarily good) dynamics:

• Increased body roll tendency
• Different weight transfer characteristics
• Potential jacking in the opposite direction
• Camber curves that may not suit the new ride height
  Roll center correction involves either:
• Adjustable control arms that restore approximate original geometry at the new height
• Roll center adjusters that relocate ball joint positions
• Purpose-designed coilover systems that account for geometry changes

The Practical Application

For most enthusiasts, roll center remains theoretical knowledge that informs modification decisions rather than something actively measured and adjusted. But understanding the concept explains why:

• Lowering affects more than just ride height and appearance
• Adjustable control arms are recommended for significant drops
• Some lowered cars feel vague or unsettled despite quality coilovers
• Suspension geometry is a complete system, not isolated components
  Engineers spend enormous effort optimizing roll center behavior at factory ride height. Changing that height without considering geometry throws away their work.
  Explore suspension theory and practice with the Revvly community--where understanding the "why" improves the "what."
  Related: Control Arms, Coilovers, Camber
  Vehicles with Discussed Roll Center Geometry: Honda S2000, Mazda MX-5, BMW E46 M3