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Last Updated 12/09/00

Suspension

Sevens tend to have double wishbone suspension at the front but the rear can be one of 3 types of rear suspension:

I'll make a go at explaining these below. Beware that these explanations are for specific implementations of these suspension types. I won't even attempt to explain the fiendish multi-link IRS suspension that a Dax has (let me know if you understand it and I'll put your explanation here :) )



Live Axle

liveaxle.jpg
Cutaway drawing of a Caterham live axled rear

This is the simplest approach, The propshaft goes into the rear axle which contains the differential. The wheels are bolted onto the end of the axle with the drive shafts inside the axle. Since the whole axle moves up and down it is considered live. The axle is located by an A-frame (in red) and two radius arms (pale blue).

This set up is used on used Lotus 7s, Caterham classics and some Westfields. It has the advantage of being light and simple. However, it has the highest unsprung weight of any of the 3 tyres. Unsprung weight is the ratio of those parts of the car directly connected to the road (wheels, tyres, axle etc.) to the total weight of the car. The lower it is (i.e. light wheels etc.) the better the ride quality and road handling will be (all other things being equal). This means that on bumpy roads a live axled car is a bit more skippy, however, on a mirror smooth track the lighter car weight will pay dividends.

Since the wheels are bolted to a solid axle their camber is unaffected by the weight of the car (the whole axle moves up or down) but under cornering the inside wheel will take a less negative camber - i.e. it will lean outwards a bit as the axle lifts on the outside. This movement places a great strain on the connection between the A-frame and the axle - the a-frame bush must therefore be replaced regularly.

The movement of the axle can be exploited to produce passive rear steer. If the radius arm front mounting is higher than the A-frame mounting then an increased load on one side causes the axle to turn. For example in a left hand corner the left hand wheel has a greater load so the left of the axle moves up. The radius arms force the left end to also move forward and the right end to move back - thus the rear axle steers with the front wheels. This has the disadvantage of reducing stability under breaking as the unloaded rear tries to steer in response to bumps on the road surface.



Dedion

dd_diag.jpg dd_side_.jpg dd_below.jpg
Caterham dedion set-up, diagram, below and side views

This was developed for the Caterham (its a very old invention) in the 1986. It removes the diff and drives shafts from the axle to leave a hollow dedion tube. The diff is placed in a housing a bolted to the rear of the car, half shafts connect the diff to the wheels - these each have two universal joints

It is similar in operation to the live axle except that it has a much reduced unsprung weight (the half-shafts and diff are heavy!). The reduce unsprung weight allows much improved traction on bumpy surfaces and more stable cornering. It is the standard fitment on most Caterhams with the option of mounting the radius arms in a lower position to reduce rear steer.

The rear steer effect is shown in the first diagram.

The second shows the that radius arm is in the lower position. The horizontal bar obscuring some of the spring is the anti roll bar. This is a bar running across the rear that is twisted by the action of one wheel moving up while the other moves down, its torsional strength therefore tries to keep the car level - hence anti-roll. The bottom bar is part of the A-frame, the dedion ear is also visible to the left of the brake disc.



The final diagram is taken from underneath the rear of the car and shows all the components.
    1) dedion tube
    2) dedion ear, the shape of this controls toe-in (how much the wheels point inward, needed for high speed stability
    3) half-shaft, the bulge is a rubber gaiter over a universal joint before it connects to the diff
    4) radius arm
    5) A-frame


Independent Rear Suspension

irs_side.jpg irs_below.jpg
A westfield independent rear from the side and below

IRS totally removes the rear axle and replaces it with 4 wishbones as used in most racing cars. A wishbone is a V shaped construction made from 2 tubes of metal, sometimes with a brace to make a triangular shape instead of a V. The point of the V goes from the wheel (one just above, the second just below) to the centre of the car. In the Westfield a frame around the diff acts as the centre mounting point, a mid engined race car would mount them directly to the gearbox.

The main advantage of IRS is the much reduced unsprung weight sas the wishbones are not only lighter than an axle but are also only half unsprung. a simple IRS would also have the effect of maintaining a constant camber under cornering and conversely increasing the weight of the car would lead to a more negative camber. Unequal length wishbones are used to reduce these effects.

The first picture shows the IRS from the side, the two wishbones can be seen quite clearly.

The second picture shows the underside of the car.
    1) top wishbone
    2) half-shaft, the bulge is a rubber gaiter over a universal joint before it connects to the diff
    3) upright - connects the wishbone ends and the wheel together.
    4) lower wishbone
    5) an extra member that is needed to triangulate the lower wishbone to ensure that the wheel doesn't move in the toe axis.