[LSmith353]

This began to bother me back when all the offset discussions were taking place. Then when SeanBlur commented that he was not happy with 305's on the front with his new MRR FS-01 setup, it really made me wonder about the geometry of the car and how wheel sizes and offsets affect the track width ratio front to rear. EricSS commented in another thread about 10mm more track width in the front (because of spacers) was a good thing, it pushed me to share my thinking.

It was well documented on the 5th Gen that a square setup greatly reduced understeer. However, even on that car, the 1LE and ZL1 ran a higher offset rear wheel. I think right out of the box, the 6th Gen does not suffer from near as much understeer as the 5th Gen.

Using the 1LE stock setup as a baseline (+23 FR and +43RR), here is the math I came up with for a square setup. By going with +25 on the back, we increased the rear track by 10mm based on a 1LE's +43 offset. Stock 1LE is +20mm on the front so with a +25 on the front, we decreased front track by 10mm. So, now we have effectively decreased the front to rear ratio by 20mm (rear now being 20mm wider comparatively than a stock 1LE setup). Assuming you run a 5mm spacer on the front for camber purposes, you get 10 mm back. That leaves the back track width at 10mm greater than the 1LE. With or without out the spacers, theoretically, our square setup would actually increase understeer.....a wider rear track would cause push. Its just not as bad with spacers.

Is my math wrong? Is my thinking wrong? Or both?

[BradfordCamaro]

How about accounting for the tire width.?

[LSmith353]

I don't know that tire width matters as much as where the outside edge of the tire is physically. If the rear track, which is outside to outside edge is wider than the front track, you get push. And I think it would push even if the rear tires are narrower than the fronts.

[BradfordCamaro]

My opinion is that a couple of mm is insignificant.

[LSmith353]

You may be right. Obviously, it looks like I'm the only one worried about it!

[grnflash]

I have a square setup on 19x10's ET25 with 285/35R19. Your math involving rear track seems correct (or at least close), but the torque arm (during a turn, inside rear tire to outside front) is going to change by even less than the difference in tire track.

You're also dismissing... or at least minimizing... a big component of this issue. By going square (I'm going to use my tires above as a baseline), you're increasing rear contact patch by 10mm or +3.6%, however, you're increasing the front's by by 40mm or +16.3%. No matter what tire size you use, you are shifting traction forward vs. the stock setup when you go square. This is a significant increase in front traction.

My own experience has been very positive with this setup. The stock SS doesn't understeer badly, but it's definitely there. The square setup is an improvement, and it's remarkably neutral. What surprises me is actually that there isn't more oversteer. You *can* drive with the throttle with this setup, but you don't *have* to.

[LSmith353]

You're also dismissing... or at least minimizing... a big component of this issue. By going square (I'm going to use my tires above as a baseline), you're increasing rear contact patch by 10mm or +3.6%, however, you're increasing the front's by by 40mm or +16.3%. No matter what tire size you use, you are shifting traction forwardvs. the stock setup*when you go square. This is a significant increase in front traction.

No doubt it's better than a stock SS setup, but I was referring to choosing a square setup vs. the 1LE setup. If I'm going to spend money on a track setup (I already did and am running same setup as you are), I would prefer to get the most performance from them as possible. However, as Bradford stated, at least at my skill level, 10mm is probably so negligible, it's unnoticeable. At the highest levels though, my guess is that it does matter.

[glamcem]

I think you guys are overthinking about the rear and front track. I've decided to run the car with 5mm spacer and then with 8mm spacer back to back and noticed 0 difference. It is very hard to tell the effect of a few mm, well at least for me it was the case

Speaking of understeer, in theory having a square setup vs a staggered setup should reduce the understeer but not directly but by inducing more oversteer. I don't feel like it's productive to address one issue by making the other worse.

Understeer was really bad with the Trofeo Rs (305/30/19 sq setup) that I previously had and surprisingly wasn't as bad with the 305/325 NT01s. This tells me that there are other factors that come in play such as having a softer sidewall.

I still have some understeer after -2.9 camber up front but it feels definitely better than before. I think the Multimatic suspension kit (if they ever offer that as they did for the Gen 5 Z28) will be the only real solution to address the understeer as the front is height adjustable and adjustable rear sway bar helps dial in the car

[WhiteNight]

Why not get adjustable BMR swaybars to dial out the understeer?

[BradfordCamaro]

Originally Posted by LSmith353

However, as Bradford stated, at least at my skill level, 10mm is probably so negligible, it's unnoticeable. At the highest levels though, my guess is that it does matter.

This is something I am honest about. Skill level.
As we know there are some very talented and skilled owners here that share their knowledge and set-ups.
We all want the 'hot set-up' and run what the big dogs do.
We all want (or think) to be 'That' guy. But I know I cannot drive like 'That' guy and problaby would not know what understeer feels like when it happens. Lol.

Basic thinking would lead you to believe that more width out back would add understeer....but so much more goes on when adding front Width, contact patch, tire compounds, etc.
And not all tires are the same width.....

Anyway... myself, I wil Drive and Adjust with what I have. Currently my simple 19 x 9.5 square meets my needs and I have not been able to find fault with it because for the way I drive I have not found the limits where I might conclude something needs changing.

[LSmith353]

Anyway... myself, I wil Drive and Adjust with what I have. Currently my simple 19 x 9.5 square meets my needs*

Now the truth is revealed.... I was looking for justification for a new set of wheels! Thought i had figured out a sure thing. Now I'm gonna have to think of another angle. Can't y'all help a brother out?!

[Redlinez]

What kind of choices that aren't crazy money are there in 19x10 ET 25 wheels? Some that compare to Apex?

[PolynesianPowerhouse]

a staggered wider rear setup would increase understeer. running a square would decrease it.

on the 5th gen, both JPSS (32mm) & GM on the 1LE and Z28 developed the huge rear bar, to help make the car more neutral to rotatable. that was the main advantage to going to such a larger bar in the rear.

[WhiteNight]

Hence the adjustable BMR sway bar.