04Green

iTrader: (1)

I played the search game, was either too dumb to do it right, or the question has not been asked, I guess both is an option...

Say I take an RX8 with H-Tech springs and factory wheels. Then I get it aligned to somewhere around the factory specs for a GT, which is pretty straight, very little negative camber in the front, and about a degree and a half negative in the rear. The car rides nice, corners nice, and is solid going down the road.

Then, I take the same car, and swap for a rim with a different offset (35 mm vs factory 45 mm). Now the tires stick out a tad more (about 10 mm) and the handling is much more aggressive. The car is a touch twitchy in a straight line, and is wonderful on the track. I do not think it is all due to the different tires.

What I think might be happening is:
The greater offset give me a bit wider track, not sure what that effects though.
The greater offset changes how much the suspension defects (although I cannot imagine it is much) because the center of the tire has moved out a bit.
The wider track, and possibly different suspension deflection, make minor changes to the overall alignment by compressing the springs a tiny bit more and maybe providing a bit of toe-in front and back.
I may be looking at more negative camber all around, again due to the greater offset.

The result is a car that handles very well on the track and shows a significant amount of negative camber tire wear both front and back (2/32 more on the inside than on the outside after one weekend).

So, the question is, what does this offset change really do to the suspension, if all things were equal? Is it the equivalent of "track mode", providing a bit more negative camber and a bit of toe in?

Ducking and Covering,

Thanks,

9krpmrx8

iTrader: (46)

Factory is 50mm. And back when I was on shocks and Tein S techs I had 25mm spacers and the handling was improved IN MY OPINION.

Boeuf

iTrader: (2)

I agree, a few weeks before I put on my coilovers I replaced my OEM wheels with 18x9 +35 PF01's and even though I didn't like the look of the more aggressive wheel on the stock suspension (looked like a truck) I felt that the handling was improved and steering felt more direct. I ran the same tires on both sets of wheels.

04Green

iTrader: (1)

So, that is 3 of us. Any suspension gods out there that can explain what is going on?

9krpmrx8

iTrader: (46)

I found the explanation.

Boeuf

iTrader: (2)

Can't argue with Coyote!

Mr_Pieper

She is always telling me wider is better. Now I know she wants a Pontiac.

My guess would be it might change toe slightly but that's only because mine became twitchy on the highway until I had the front zeroed out again.

olddragger

iTrader: (3)

read about this. King pin angle.
http://www.mgf.ultimatemg.com/group2...ering_axis.gif

Atilla

iTrader: (1)

wow...really surprised Team hasn't come in here and verbally body slammed somone.

JCrane82

iTrader: (3)

Like OD said....King Pin Inclination and scrub radius.

A change in wheel offset will directly affect scrub radius. Increased scrub radius will give a "boosted" steering response (due to the wheel rolling under turning instead of pivoting on the contact patch), but will also increase sensitivity to braking inputs.

In short....a larger scrub radius will give an increase in steering sensitivity, at the expense of an increase in feedback under braking.

04Green

iTrader: (1)

Soooo.

More offset increases the Scrub or Pivot Angle Radius. I take it that a shorter tire will also increase this a bit as well. So, I have an increased Scrub or Pivot Angle radius when the track wheels are on. Check.

The additional offset also will slightly compress the suspension due to the slightly longer lever arm (tire is out half an inch or more, center point is out half an inch or more). This will SLIGHTLY increase the king pin angle, but not a lot. This will also generate a tiny bit of toe in, because as the suspension compresses, and the arms rotate, the tires move a tiny bit apart while the rack stays the same length.

It does not look like caster is effected at all.

It looks like, as the compression occurs, the camber will go slightly more negative, due to how the suspension is designed. the RX8 geometry is not too different to the picture OD posted.

Conclusion:
The increased offset and shorter tire I use for the track alter the Kingpin angle a bit, increase my negative camber a bit, and give me a slight bit of toe-in. This is reflected by a car that is a tad twitchy on the street, dives for the apex, provides significant feedback under very heavy braking and is creating a set of track tires with significant camber wear on the insides when I did not think I was carrying much negative camber.

All of this is good, I just need to lower the air pressure a bit (1 to 2 psi) to get more of the outside of the tire more involved in corners. I have very little wear on the outside.

For everyone, and I really hope this gets vetted by folks that understand this, if you want a track alignment that increases scrub radius a bit, increases toe a bit, and adds some negative camber, then go with a wheel that has a lower offset than the factory 50 mm wheel. When you put it on, you get a track alignment, when you take it off, you are back to a street alignment.



Interesting links from reading assignment:

http://www.mgf.ultimatemg.com/group2...ering_axis.gif
Car Suspension, The basics


Thanks everyone.