inerter
An active suspension component that is the dual of the spring. A device that reacts similarly to a spring but 180-degrees out of phase.
inerter: the mass that makes suspension symmetric
An inerter is a two-terminal suspension component that generates a force proportional to the relative acceleration between its two attachment points. In essence, it behaves like a mechanical mass, but one that can be mounted anywhere in the car without adding conventional weight to the chassis. The force follows the equation F = b(a1 - a2), where b is the inertance (measured in kilograms) and a1, a2 are the accelerations of each end.
The device is the mathematical dual of a spring. Where a spring generates force proportional to displacement, an inerter generates force proportional to acceleration. This duality is fundamental to suspension tuning. A spring creates a phase lag between input and response; an inerter creates a phase lead. By combining springs, dampers, and inerters in the same linkage, engineers can achieve suspension responses that were previously impossible with springs and dampers alone.
How it works in practice
Most modern inerters use a geared flywheel mechanism. One suspension arm connects to a gear that spins a heavy rotor; the other arm connects through a different gear ratio. When the two arms accelerate at different rates, the rotor accelerates, storing kinetic energy and resisting the acceleration difference. The inertance value depends on the rotor mass and the gear ratios. Values typically range from 5 kg to 30 kg on road cars, though racing applications sometimes use 50 kg or higher.
Inerters sit in the suspension triangle or within the damper housing, often alongside the traditional spring and damper. They are particularly effective at controlling unsprung mass response and body roll compliance. Formula One teams have used inerters in front and rear anti-roll bar systems since their introduction in the mid-2000s. Road car applications are growing, especially in performance vehicles where they help decouple chassis control from suspension stiffness.
The main limitation is packaging and cost. A geared inerter requires precision machining and sealed bearings to function reliably over thousands of suspension cycles. Friction and seal leakage are failure modes; regular maintenance is needed in racing. For road cars, the additional weight and complexity of the mechanism must justify the tuning advantage gained. In off-road and commercial vehicles, where suspension travel is large and simplicity is valued, inerters remain rare.