The inerter

Inerters first surfaced in the secretive world of Formula One when McLaren began using a suspension component dubbed the J-Damper. This was in fact an inerter, a component that while elementary in theory, provides a powerful additional tool when controlling chassis movement.

The inerter was developed by a Professor Smith, of Cambridge University, England, as a solution to the age-old problem of balancing ride compliance with stiffness. A regular suspension set-up, be it on a car or motorcycle, is based around two components – springs and shock absorbers (dampers). Together these contribute to the car’s ride and handling, but no matter how the system is tuned, there is always a compromise between handling, comfort and grip. Even in racing machines, where comfort is less important, the suspension needs to be set to allow both sensitive handling, which requires a harder suspension, and a good mechanical grip, which requires suspension compliance.

The upshot is that there is oscillation of the suspension as the load on the tyres varies. If these oscillations reach a high enough frequency, the damper and spring package will not be able to react fast enough to damp them out, so tyre contact with the track surface will be reduced.

Prof Smith realised that this poor trade-off between handling, comfort and grip could be better resolved if a third type of component was added to a suspension system to make it more flexible – hence the inerter. Superficially it looks like a conventional shock absorber, with an attachment point at each end. In the case of a Formula One unit, the inerter will be attached to the suspension rocker and the chassis. A plunger slides in and out of the inerter’s main body as the car moves up and down. As this happens, a flywheel mounted on the plunger rotates in proportion to the relative displacement between the attachment points. The result is that the flywheel stores rotational energy as it spins.

In combination with the springs and dampers, the inerter reduces the effect of oscillations in the suspension and tyre, and thus helps the tyre retain a better grip on the track surface. By changing the weighting of the flywheel, the level of ‘inertance’ can be varied, allowing the effect on chassis behaviour to be fine-tuned; the latest systems even allow for the inerter to be completely disengaged in certain circumstances.

One area where inerters are of particular use is on vehicles with large tyre sidewalls, such as those in Formula One. A large sidewall is more prone to uncontrolled deflection at high suspension frequencies, which an inerter can be highly effective at combating, the result being a more consistent tyre contact patch and thus greater mechanical grip.

Speaking at the time the technology came into the public domain, in 2008, Prof Smith was surprised that no-one had thought of the idea before. “I was nervous about talking about the idea at first because it seemed so elementary a concept,” he said. “It was very difficult to believe that nobody had thought of it before, and I presumed that either it had been done already or there was some sort of snag.

“As I discussed the idea with colleagues, however, I began to realise it hadn’t been done and that it was possible to achieve this trade-off to improve vehicle suspension. The next question was whether it could actually be done, and once I had worked out what it should look like, that was a fairly simple matter."

Since these initial developments a decade ago, inerter technology has been refined to the point where it is now an indispensable part of a Formula One car’s chassis package. The technology has also begun to filter down into other series – for example, Porsche’s factory LM P1 is thought to use inerters, while they have also been widely adopted in Pro Stock drag racing, where they have allowed tuners to obtain much more consistent chassis set-ups.

Written by Lawrence Butcher