Roll stiffness of D bushes, making it better.

[johnl]

Regular readers may recall that my car is fitted with a custom made (sounds better than home made...) rear ARB (a greatly reshaped front ARB from a Holden Rodeo truck, a very stiff 20mm bar compared to the wimpy  14mm stock 147 rear bar, which I think might be made from coat hanger wire...).

When I fitted this I used the Rodeo rubber 'D' bushes and the associated clamps (that came with the bar), attached to custom made brackets. The rubber bushes were a bit old and worn, but not worn out. There was no clearance between any of the hardware, the ARB was snug in the bushes. However the bar was not tight in the bushes. The rubber bushes were not somewhat compressed as they really should be (at most only slightly compressed, enough to prevent the bar from rattling...).

So a few days ago I finally got around to doing what I had been intending to do for months, shim the D bushes. This just means placing some squares of metal plate (in this case 1/8th" aluminium) between the base of each D bush and the bracket to which it attaches (i.e. beneath the rubber only, not the clamps). This means that when the retaining clamp is tightened the bush becomes somewhat compressed, and as a result the bar is clamped more tightly, but more importantly the D bush rubber is 'pre-compressed' and will further compress a lot less in use. It's a bit like fitting stiff poly D bushes, except it costs 5c (guesstimate of material costs...). The affect is to stiffen the 'ARB system' in roll, to whatever degree, because bush compliance impacts on roll stiffness.

What I expected this to do for the cars' handling was - not a lot. I expected anything from a minor but noticeable improvement (lessened roll motion, sharper steering and handling response), to nothing noticeable at all. What actually happened was quite different.

It is a big change, much more than I thought remotely likely. With such a beefy rear ARB, body roll wasn't a lot before, but now it is a lot less. Steering response is very noticeably sharper, handling responsive as well. Steering feel is now firmer. There is now a feeling that the steering wheel is more 'directly' attached to the front tyres. On centre feel is significantly better. Less steering wheel rotation is needed in any given corner, the steering feels more direct, as if the ratio has been changed.

I am super happy with this. The handling is now very much improved, much 'sportier' with a lot less understeer and much more 'playful'. The rear end is a bit 'loose' on a gravel road, which I do like, but it might frighten some drivers...

I think a large part of why this has been such a dramatic improvement is that the ARB is such a stiff one. The same thing done to the stock ARB mounting would I expect make very little difference. With such a stiff ARB most of the initial deflection in the 'system' is likely to be in the D bushes rather than in the bar itself, so the chassis doesn't 'see' the full bar rate until roll motion has reached X° and the bushes have compressed under that loading.

With softer D bushes initial roll stiffness will be relatively weak as the bushes are easily 'squashed' by the stiff ARB. This means that initial weight transfer will be relatively weak, and relatively slow. At some point as body roll occurs there will be a sudden rise in roll stiffness as the bushes become compressed, and the full stiffness of the bar comes into play (this would be a non linearity, generally not a good thing for handling predictability). With a much stiffer D bush the full bar stiffness will come into play much earlier, the non linearity will be less, roll motion will be less, weight transfers will occur more rapidly, and as a result the handling response will be sharper.

With a soft ARB, the bar and bushes will both be deflecting very easily, so stiffening the bushes a bit - or a lot - will have a minimal effect.

If anyone asked me what to look for in a rear ARB, my advice would be fit the stiffest one you can find, and use stiff D bushes. I don't find the ride harsh with the stiff ARB, but this is very subjective judgement, and others may disagree.

Interestingly, with the stiffened D bushes, the rear end feels better damped. It's as if the damper rates have been somewhat increased. I recall feeling this same effect when I initially fitted the stiffer rear ARB. My theory is that in single wheel bump (or droop) some of the bump force is passed from the 'bumped' side of the car to the other through the stiffer ARB, so if say the left rear wheel hits a bump it is damped by the damper on the left side and also to some degree by the damper on the right side. If so then this must occur in a manner not really significant with a soft ARB, since the effect would be lost in the easy twisting of the soft bar. With a more stiffly located stiff ARB this effect would occur more strongly more immediately and for smaller bumps...

Now I'm wanting to do a similar thing to the front ARB D bushes (my front ARB is a GTA item, fitted by previous owner). But it's such a pain to get at...

Regards,
John.