wheel offsets, handling, and audi front track?...

[Davi]

Just looking for confirmation here to make sure I'm not about to make a daft mistake in my design...

Basically, if I'm taking the complete set-up from a front wheel drive car (audi A6) to use in a mid engined car, but want to use substantially wider wheels than would have been used, in order to not cause geometery changes and severe problems with camber angles on bump and drop I need to take the standard ET for the offset, then make sure that the new wheels offset results in the centerline of the wheel remaining exactly where it originally was, correct?! nothing else I need to take into account is there?

I'm sure this is right but thought I best check! It's a bit of a bugger as the track is slightly narrower than I wanted, and I think it would be a bit overkill to go above 11J with 295's on, but unless I fit some 14J wheels on the rear there would be nothing to fill the arches so the cars is going to have to go narrower!!

I'm also trying to find out where the difference in track comes from between the 2002 A6 and the 2004 A8 as I've been told they use the same 6 speed 'box - with any luck it might just be longer driveshafts on the A8 which would help get me another 3 inches closer to the required width.

[Sam_68]

Wheel offset has no influence whatsoever on camber angles!

edited to add...
and if you are intending to use the whole suspension geometry from the front, you might find that it puts the geometric roll centre in entirely the wrong place for correct weight transfer characteristics.



>> Edited by Sam_68 on Monday 9th January 22:54

[andygtt]

I agree with Sam.... and when you say entire set up, does this include hubs as the fronts will be designed to turn which you will have to elimiant at the rear.

If you are making hubs, then why not make the wishbones and add a spacer into the drive shafts at the gearbox end?

[Davi]

well the steering element of the hubs is an easy one to overcome and will also allow adjustment. The wishbones are going to be home made, but I was trying to avoid any modification to the driveshafts at all, be it spacers or having them professionally lengthened.
As for the comment on camber, my apologies I really didn't explain what I was getting at very well (at all in fact!) there :P In the books and reference material I've been using for the design they all mention the effect of moving the center point of tyre due to the effective coincident point with the hub centerline - from my understanding of all these notes, when taking into account the camber changing the track moving the offset can change the load point of the tyre and change grip. actually maybe I should just get the technical terms out the book later, I know what I'm trying to say but not doing a very good job of putting it in words!!!

Re the roll center... one of the reasons for using the whole setup is going by the suspension thread responses and my pages of scribblings it should actually be quite close to what I need!!!

>> Edited by Davi on Tuesday 10th January 09:52

[Sam_68]

Davi said:

In the books and reference material I've been using for the design they all mention the effect of moving the center point of tyre due to the effective coincident point with the hub centerline

If I understand you correctly, this time, it's still completely irrelevant. What you are talking about (I think!) is 'king pin offset' or 'steering axis offset' - the distance between the centre of the tyre contact patch and the point at which the axis the steering rotates around intersects the ground. This has a critical influence on steering feel at the front of the car, but unless you are planning to steer the rear wheels it ought not to bother you too much at the back.

Last time I took any interest in Audis, they used negative steering axis offset (i.e. the steering axis intersects the ground outboard of the centre of the tyre contact patch), which gives good stability and safe characteristics in the event of a front tyre blowout, but very 'dead' feeling steering.

Whatever...it's not a factor on the rear suspension, though of course if you are using the Audi front uprights with the steering 'locked' by means of a fixed trackrod, you will need to take great care to ensure no bump or roll steer.

[Davi]

LOL oh man do I feel a total pillock now. You are of course totally correct, I've been trying to work on both front and rear suspension at the same time and referenced the wrong info - think I need a break!

The bump/roll steer element I believe I have overcome by adding brackets to the upright so I can use 2 link arms of equal lengths to the wishbones, rather than relying on the steering arm. Haven't finished putting it in CAD yet to get it animated in CAD but should work in theory!

Thanks for the help mate, much appreciated!

>> Edited by Davi on Tuesday 10th January 14:57