When changing between wheels with different offsets does setup change?
 

I am currently running a 1973 RSR with Goodyear slicks in 10.5 inch x 16 front and 13inch x 16 rear. I have 2 sets of BBS rims that are both 10 inch and 12 inch but the offsets are slightly different between the one set to the other. At the front it is a 5mm difference and the rear difference is 13mm.

When I change from 1 set of rims to the other I have been told that I need to ensure that the offsets remain exactly the same, via spacers, otherwise the setup geometry of the car will change and the handling will be altered.

I am battling to see the logic behind this and these are my reasons why.

1. Toe - No matter what the offset the toe should remain the same?
2. Camber - Again would not change?
3. Ride height might change very slightly due to the offset difference changing track width and if there is some camber the car might rise or drop slightly, but I cant see this being anything significant.

Am I missing the point here? I do realize that changing spacers is not a real problem, but I am interested in the logic behind this.

In the front the scrub radius changes.

Wheel rate can change.

I would argue that you don't need to change spacers, but you should expect slight differnces in handling. Any time you change the track width you change the amount of weight transfer. In this case you are widening the rear so you will get more transfer in the rear and will create more oversteer (probably very small effect).

-Andy

Incorrect. If you widen a track you decrease the change in load at that axle and (due to tire load sensitivity) increase grip.

Andy - Flieger is right. Moving track out with spacers or offsets lengthens the lever arm which the spring has to resist. Got to make effective spring rate lower, and softer spring means that end gains grip at the expense of the other end of the car.

However, I would posit that very few of us would be able to tell the difference. Physical conditions limit how much you can move the track (tire centerline) around within a wheel well for our cars. Too far out, you rub fenders, too far in you rub suspension components (inner fenderwells, too, but that only happens when maneuvering around in the paddock, really, or desperate attempts to countersteer if you've lost it). This assumnes you are already using a tire and rim combination which pretty much fills up the fender space.

I agree that the ride height change would be inconsequential, despite the fact that you will change your spring rate. Just not enough to alter ride height with any practical effect.

May, of course, just be me. I've got some rims with different offsets in my fleet.

.http://forums.pelicanparts.com/uploa...1349050515.jpg
http://forums.pelicanparts.com/uploa...1349050553.jpg

I've used them somewhat interchangeably (as pairs), and can't say I have ever noticed a difference. But then again, some drivers, especially some pro drivers, can tell if, say, corner balance is off. And other things the rest of us would not notice.

Walt, you certainly seem to have a vast selection there!

Barring any other posts to the contrary it seems that it does not make any noticeable difference.

I was racing at Kyalami here in Johannesburg (old F1 track) over the weekend and spoke to various people and got the same opinions as on the forum.

Thanks for your feedback guys.

Keep Max's statement in mind:

I altered the front to rear wheel/tire stagger on my a/x 911 to get rid of some understeer. It worked all too well. So, I took the 10mm spacers off the front and put them on the back. Voila! A little less tail happy (and I am by no means a pro driver).

So, if your changes in offset are close front and rear - probably no big deal. If only on one end or a lot more one one end, you may get a noticeable change.

Also, what might change is tire clearance, so be sure to check that inside and out.

Thanks Bob,

What you are saying makes absolute sense. I think I will make up a spacer for the rear to not have the 13mm (1/2 inch) difference and just leave the front as it is only 5mm.

Careful, now, with terminology. Wheel Rate is a static calculation involving ACTUAL spring rate and motion ratio on a particular corner of the car. Simply increasing track does nothing to effect Wheel Rate.

Ed

It depends on the suspension geometry. On a McPherson/Chapman strut there is almost no effect on it. On a swing axle, the extra track goes directly into making the wheel rate softer because the motion ratio changes.

Of course that is right, BUT -- We are working here with a '73 RSR (assumed torsion bar) car.

Obviously, if we were discussing a prototype or formula car with variable length rocker-arms, or different length pushrods, or variable ratio bell cranks; then altering the track will change Wheel Rate.

Ed

Right, but I just wanted to make sure that people did not get false ideas about these principles.

You are seriously saying that increasing the distance of the track from the spring doesn't impact the wheel rate? How can changing a vehicle's track without moving the spring not change the calculated motion ratio?

I haven't looked at the rear geometry close enough to say whether wider track would change the effective lever. But I did say "can", not "will".

It depends on the suspension geometry. Mostly on roll center/FVSA length. If the wheel moves vertically and you space out the wheel then the wheel rate stays the same (assuming you don't move the spring pivot points). The roll angle goes down due to the increase in track width, however. On a trailing arm setup the wheel moves vertically if you disregard bushing deflection and link deflection.

Obviously, they already have them! I do appreciate your clarification - I should have mentioned that myself.

Ed