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Complementary air suspension


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While staring out of the window studying this afternoon I had an idea for offroad suspension, on the following principle:


On the road, I want to minimise body (chassis) roll relative to the axle for cornering etc, to make it less like a boat.

When offroad in a cross axle, the vehicle is only as good as the heaviest wheel. When getting towards a crossaxle situation, the drooping wheel is already going light and reducing the available traction. Imagine a coilsprung vehicle crossaxled with the droop wheels nearly lifting off the ground - there's nearly no traction available to drive the vehicle. Now picture that same situation with a single centrally-mounted coilspring - the weight on the two wheels is the same, right until the linkage runs out of travel and lifts a wheel. Problem with this is, there's no roll stiffness at all so the vehicle would spend its whole life on the road resting on one or other side's bump stops (and corners where it flopped from one to the other would be "interesting"). Practically, keeping two coil springs I want to be able to force more body roll than the suspension achieves naturally.

In my mind, air suspension can be made to emulate good road manners and balanced cross-axle performance by the right combination of cross-linking, and SimonR has proved that it can be feature-packed and very clever for active cornering. However, he's also had trips home on the bumpstops where an airbag has punctured.


So I want to be able to force more or less body roll (under different conditions) than a simple coil spring system, but without adding the fallible complexity of airbags. This suggests I want something passive, so the primary springing is still coils.


My idea is one of three options:

  • Active anti-roll bars - the Disco2 uses these for active cornering enhancement. The vehicle's got no anti-roll at the moment, so a similar setup grafted into a Defender could reduce or force body roll at each end of the vehicle, either in phase for cornering on the road, or out of phase for 'helping' cross axles. The latter especially would need quite a high frequency ability.
  • Air springs in parallel - mounted inboard of the chassis (for lower roll resistance), and working alongside short, stiff coil springs for low and tidy road manners. The air springs could be permanently cross linked to give good offroad performance, and the coil springs would just dislocate when the vehicle was raised to offroad height. The airbag piston could be adjusted to compensate for the extra springing at lower ride heights when the coil re-locates (in cross-axles).
  • Air springs in series - as above with short coils, but mounted something like the X-spring. I'm still getting my head around the relative benefits and shortcomings of this compared with parallel mounting.

Thoughts / ideas / shoot-down-in-flames ?

Disclaimer - it's taken me over a month to put a new turbo on my Ninety - it'll be a looong time before I actually implement any of this, but it provokes discussion.

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