Negative offset geometry - how much?

[johne]

S2s have negative offset geometry on the front suspension, by that I mean that an imaginary line drawn down the centre of the strut to the ground will lie outside of the centre of the tyre. Anybody know the distance between these two lines, i.e. how many mm's of offset is there?

Whilst related, this isn't the same as the oft talked about wheel offset which on a standard car with 16 inch Avus wheels is 37 mm.

John

[Knight Rider S4]

Hi John

Dont know if this is any help but have not long got my Coupe set up and is the best it has ever been ,,bottom is the final set up ..

[johne]

Interesting figures but unfortunately it doesn't help. The number I'm after is set by the design of the front strut angle to the vertical and can't be altered so doesn't come into any of the alignment settings.

John

[Keeper of The Kitten]

Difficult ...but not impossible to survey all the dimensions required to find the solution - although you'd have to rely on some belief in the bushes for each suspension point being correctly centred.

Furthermore you'll ideally have a complete strut surveyed to see how offset the bottom joint is in relation to the damper insert centreline, plus the offset of the wheel mounting face from the strut centreline at a known height (eg the hub centre), and the angular deviation between the strut centreline and the wheel mounting face.

You then have the top mounts centre to centre, you have the subframe wishbone centres and you have the wishbone inboard points on the subframe, plus the relative height between the top and bottom points.

Finally, you need the actual loaded height of the suspension to generate a dimension between the topmount and the bottom balljoint.

From there it's time to fire up the cad or get out the pencil and paper!

I think this is something which would definitely need to be recorded for posterity if the information can be mustered...

[Keeper of The Kitten]

Of course a lot of the information for the inboard points could be ignored and just a survey of the strut could be used (along with loaded height) ...but the additional information could be extremely useful especially as camber affects the contact patch and the effective dimension for the self-alignment offset.

[johne]

Of course a lot of the information for the inboard points could be ignored and just a survey of the strut could be used (along with loaded height) ...but the additional information could be extremely useful especially as camber affects the contact patch and the effective dimension for the self-alignment offset.

and when we know what the distance is, we'll then know by how much you can move the centre of the wheel outwards - due to spacers or different wheels - before negative offset becomes positive offset with consequential changes in self centereing & handling.

As you suggest, it could be worked out but as it's a figure designed in by Audi and set by the strut design for all the B3 & B4 cars I was hoping someone would already know the number.

I don't believe the loaded height matters as this wheel to hub to strut relationship doesn't change with load height.

John

[Skull]

the thing you are after is the negative roll radius. i have seen a diagram somewhere online, but i cant find it...

[Keeper of The Kitten]

John I think you're right, as the strut is a telescopic device the centreline will always be the same - I just figure that the camber variation is more important than people might realise - because it will change the effective centre of the contact patch ....but you start getting into tyre deflection then, and it's another magnitude of variables. For sure though, with more negative camber, the contact patch will move inboard.

[johne]

true but camber won't affect the negative offset since the strut - hub is fixed and you change the strut - lower wishbone dimension to adjust the camber.

John

[Keeper of The Kitten]

Yes from a nominal point of view and given a known (and quite minimal) camber, this is pretty much a fixed value with only tiny deviations caused by rolling radius. And a good baseline to start from.