plentywahalla
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| posted on 12/10/10 at 08:11 PM |
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Trailing arm geometry - opinions please
My rear end set-up is nearly complete. 7.5" Sierra 2WD Cosworth diff and half shafts, Inboard Compbrake R2 disc brakes, De dion tube, Watts
linkage etc.
I am now cosidering the trailing links geometry.
I know there are advantages in longer than standard arms. They reduce the foreshortening of the arms as they swing around a small arc. This can have
the effect of inducing oversteer/understeer depending on the ride height. There is a limit however created by the rear arches.
I have been working on the idea of non-parallel arms. i.e the vertical spacing of the axle mounts are wider than the chassis mounts. This has the
effect of making the hub arc about a much longer 'virtual' axis as one arm lengthens as the other shortens.
The problem with this set-up is that with body roll when cornering the hub twists forward on the inside and backward on the outside.
But ... this could be a good thing as it would have the same effect as an anti-roll bar limiting body roll whilst allowing full normal deflection.
I could employ this geometry as the inboard disc brakes have removed any torque reaction from the De dion tube, and the tube itself is a one-piece
57mm Dia CDS tube mandrel bent and would have some natural torsion spring.
Has this been done before? Am I treading a well trodden path ... or barking up a blind alley?s
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craig1410
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| posted on 12/10/10 at 08:46 PM |
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Hi,
I've got a similar de-dion setup albeit with outboard brakes.
My advice would be to stick to equal length parallel trailing arms and just ensure that they slope slightly upwards from front to back at normal ride
height. This gives you a little bit of stabilising rear wheel steering under roll where the outboard (wrt corner) wheel moves slightly forward and the
inboard wheel moves slightly backwards due to foreshortening/lengthening of the arms respectively. This will give you mid corner stability and allow
you to get on the power early on exit.
I would worry about fatiguing the axle with the twist induced by the diverging arms. If you need an anti roll bar then fit one. If it breaks then you
get a bit more roll but if the axle breaks then you might end up upside down! Also, you can buy different torsion bars with different stiffness but it
would be pot luck if your axle was the correct stiffness for your needs.
If you do decide to use the axle as a torsion bar then look very closely at the structure and make sure that any critical joints are protected from
fatigue. I think you would want virtually all of the twist to occur in the tubes, not near the joints.
Good luck,
Craig.
[Edited on 12/10/2010 by craig1410]
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Peteff
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| posted on 12/10/10 at 10:32 PM |
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For unequal length you need a lot of compliance in the bushes. If they are too hard it mashes them in no time.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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plentywahalla
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| posted on 13/10/10 at 08:09 AM |
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Thanks guys ... points are helpfully noted and I will keep thinking!
I know that unequal length arms and non-parallel arms have been discussed before and employed.
But no mention has been made of the effects of inevitable torsion in the beam or ways to avoid it
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MikeRJ
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| posted on 13/10/10 at 10:32 AM |
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Several of the old live axle Sylva cars use a Watts type arrangement to locate the rear axle instead of four trailing arms which gives the same effect
of twisting the rear axle in roll. Plenty of compliance in the bushes is needed - i.e. proper rubber bushes rather than hard poly and definitely not
rod ends. Even so, cracked axle brackets are not unknown on the Sylva cars.
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