Inerterential

The inerter <link> is an interesting component in automotive engineering. It was introduced to motorsport by McLaren where it was used to counteract the bouncy- oscillations in the suspension that are a big problem in F1 cars with big bouncy tires and aerodynamics that amplify bouncy behavior*. When an inerter is used within the suspension system, upward movement of the wheels is used to generate inertia - for example by spinning up a flywheel. If the suspension is to oscillate it must first slow, stop and then re-accelerate the flywheel in the opposite direction. In this way, it inhibits rate of change not velocity, like a conventional oil damper.

Oscillations also exist in the driveline, an educational example of this is to take a reasonably powerful front wheel drive car, stomp on the loud pedal and dump the clutch. In the absence of fancy electronics etc, what often happens is that the steering wheel will wobble accompanied by some telltale chirruping noises from the tires as you safely pull away**. This is because torque can oscillate between the two wheels dependent on the varying traction and type of differential fitted.

So, fit the differential with an inerter. To do this, each drive shaft drives an idler in the differential rotating cage assembly which in turn drives a flywheel that is essentially coaxial with the drive shaft/cage. With significant up- gearing, this would lead to the flywheel spinning up whenever there was relative movement between the drive shaft and rotating cage, i.e. one wheel is slipping.

This will be particularly good at damping driveline oscillations with the added benefit that it isn't just throwing energy away like viscous diffs, it's all there still in the flywheels. It would be interesting to know how it drives, I imagine it would cause initial turn-in understeer and very stable mid-corner handling on constant-radius corners***. Probably very suitable to a center differential, or something limousiney.

*If the car rises up slightly, the undertray develops less downforce. Conversely if the car squats down slightly, the aero package generates more force pushing it down further. Add in springy tires, energy and any form of initial disturbance and you have an expensive space hopper.

**That was confident, hesitation-free acceleration to the speed of traffic flow officer. I can assure you, any momentary loss of traction was well controlled and almost certainly attributable to the damp road surface at that particular junction, and the 6 previous junctions. The council really should do something.

*** Possibly some strange straightening-out induced oversteer behavior - send lots of budget, and a specced- out S-class, I'll find out.