The Push Rod Rear Suspension is almost done.
With the arrival of some nearly new AVO coil-overs the big old DRZ400 motorbike shocks were consigned to the parts shelf.
I know I’m into Haynes Roadsters now, but I was looking at a feature on Geoff Cousins latest creation. He’s a legend of the hot rod scene. The cantilevers on the rear of his Ford Popular are works of art. It gave me the inspiration to get out in the garage and start rebuilding mine.
I still haven’t decided on whether I will box in the sides of the Bell Cranks with sheet tin, as for a strength point of view I don’t think they need it, yet from a looks standpoint they just don’t grab me like Geoff’s.
So why go to all of the trouble of bringing the suspension inboard?
- Reduction Un-sprung weight
- Rising spring rate
- No anti-roll bar
- Better aerodynamics
- IT LOOKS COOL!
Sprung weight is anything that is on the car that is carried by the shocks or springs. (This includes the bodywork, seats etc)
Unsprung weight is everything that touches the road before the springs. (This includes wheels, brakes, hubs and callipers and half of the shock itself).
Bringing the shock inboard means that the whole of the shock absorber and spring are supported by the chassis and are therefore ‘Sprung Weight’. In theory a car will corner better when there is less unsprung weight. With less inertia, the wheels can react more quickly to any changes in the road surface or vehicle roll angle. It must be remembered though, that bringing the shock absorber in-board will usually increase the sprung weight and indeed the total weight of the car. Although this is not desirable the gains are often out-weighed by increased suspension performance.
If I had used the usual strut arrangement on the rear, the shock absorber would have compressed with roughly 1:1 compression ratio. 1cm of vertical movement at the wheel would have resulted in 1cm of compression at the strut. This would have remained roughly constant across all angles of body roll. In fact due to the coil-over leaning over toward the car, the compression ratio actually drops slightly the more the wheel moves; this is not ideal.
By using a ‘Bell Crank’ or cantilever is possible to increase the amount of compression as the car leans or rolls. This has a similar effect to adding an anti-roll bar. The harder the car is worked the harder the suspension becomes.
At normal ride, a large movement at the wheel (Y) only produces a small change in spring compression (X).
However, once the suspension has been compressed near it limits, only a small change in wheel movement (Y) produces a large amount of spring compression (X).
I’ve lost the plot I did of the compression ratio, which is annoying, but from memory, at normal ride I was getting 1cm of movement at wheel : 0.75cm of spring compression. Close to full compression I was getting 1cm of movement at wheel : 3cm of spring compression.
I made the cantilevers for the front and rear suspensions at the same time as they are all identical. I’ve seen so many cars now where the cantilevers are done wrong, it utterly amazes me. The angle between the shock attachment and that of the push rod should ideally be less than 90°. Virtually all of the amateur builds I have seen recently have 180°. This means they will have a radically falling spring rate, which basically means either they will need the biggest anti-roll bar ever made or they have ruined their handling completely.
See my image gallery:
Push Rod Suspension Image Gallery
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