Rear offset adjustment effects???

[dvs_dave]

Found plently of info about the effects of altering the front offset and how it affects scrub radius etc, however I can't find anything useful on the effects of altering the rear offset/scrub radius?

How does it affect handling, and what on a powerful RWD drive car is generally regarded as best practice for the rear? Positive, negative, or zero scrub?

[garagewidow]

is your car rear wheel steer?

[dvs_dave]

Nope, double wishbone. TVR Tuscan.

[Sam_68]

Then it doesn't have any rear scrub radius or steering axis offset (on account of it not having a steering axis at the back).

Or perhaps I should say it shouldn't have... with a TVR, anything's possible. Have you checked that all your suspension bolts are there?

[dvs_dave]

I know it doesn't have a steering axis on the rear, but will adjusting the rear wheel offset make any difference to the handling?

The wheel offset will alter the distance between the centre of the tyre contact patch and the actual load centre position (is this not scrub radius?) so in theory will influence performance.

More or less under/oversteery? Better or worse directional stability under power/braking or are the effects negligable unless you get into silly offsets?

[Sam_68]

dvs_dave said:

...will adjusting the rear wheel offset make any difference to the handling?

It will change the track. If you change the offset of the rear wheels only, it will therefore influence the diagonal weight transfer slightly and hence change the understeer/oversteer balance.

If you change the wheels front and rear (ie. so that the front and rear tracks both increase equally, then the basic balance will remain pretty much unchanged (there will be an overall reduction in weight transfer, but it will be pretty negligible compared to the effects the changed offset will have on the steering feel at the front).

dvs_dave said:

...(is this not scrub radius?)...

No, scrub radius is the the radius of the arc described by the centre of the tyre contact patch around the steering axis, when you turn the wheel. If you don't have a steering axis, you don't have a scrub radius.

dvs_dave said:

More or less under/oversteery?

As above... the effects are as you would expect from increasing or reducing the track.

dvs_dave said:

Better or worse directional stability under power/braking or are the effects negligable unless you get into silly offsets?

Difficult to say... it will slightly change the amount of leverage on the wishbone mounting bushes (and hence their deflection), but unless you have a tricksy suspension that uses substantial compliance to introduce a measured amount of passive rear-wheel steer (which the TVR doesn't), or your bushes are shot (which is always a possibility), then the difference is likely to be negligible. If you change the offset to increase the rear track, it would be the difference between braking or accelerating just a little bit harder, so unless you are suffering instability under braking/ acceleration at the moment you'll probably not notice any difference. In the unlikely event you do feel a difference, a marginal increase in the rear toe-in will dial it out again.