The Mustang Forum for Track & Racing Enthusiasts

Taking your Mustang to an open track/HPDE event for the first time? Do you race competitively? This forum is for you! Log in to remove most ads.

  • Welcome to the Ford Mustang forum built for owners of the Mustang GT350, BOSS 302, GT500, and all other S550, S197, SN95, Fox Body and older Mustangs set up for open track days, road racing, and/or autocross. Join our forum, interact with others, share your build, and help us strengthen this community!

wheels and tires fitment guide

Norm Peterson

Corner Barstool Sitter
939
709
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
Why do these cars come staggered from Ford anyway?
It seems the general consensus is square is better, yet we get staggered wheels and tires.
It's about ensuring that the car remains understeerish under a wider range of driving (there probably are corporate standards for this), and may also make the job of certifying stability control systems somewhat easier. I suspect that staggering also tends to make the understeer slightly more progressive (read: you get a little more warning from the steering response 'softening'). Keep in mind that Ford (and the rest) have to set these cars up with a little extra margin for those buyers whose car control skills are, shall we say . . . open to question.

Anybody who is a member here is almost certainly more demanding of car cornering and handling capabilities than 99% of Mustang buyers overall. Hell, if all those C&C memes are any indication, half the Mustang world can't even tell the difference between understeer and underwear.


Norm
 

Norm Peterson

Corner Barstool Sitter
939
709
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
I’m just curious of the logic around the decision to stagger the wheels, when it seems to be the consensus that square is better. Why not save some money and run two 9.5” wheels instead of engineering a car around a 9” and 9.5
I'm a little confused here. I thought the Boss S197 cars ran 9's and 10's, and that it's the S550 GT PP1 that runs 9's and 9.5's.

Either way, they're too narrow given the weight of these cars and the front-bias to that weight.

Ford may be crutching a more understeerish wheel and tire package with slightly less understeerish suspension tuning than they'd have provided with a 'square' wheel and tire setup. But they haven't always done that for their own road course cars; their M-FR500-C Grand Am Cup cars were fitted with 18x10's all around and the M-FR500S Miller Cup cars ran 18x9.5's on all four.

Except where big power is involved - as in forced induction levels - I don't think that compensating for an understeerish tire and wheel package with looser or "less tight" suspension tuning is a better approach than starting with an inherently more balanced wheel & tire package.


Norm
 

Norm Peterson

Corner Barstool Sitter
939
709
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly

Grant 302

basic and well known psychic
Laguna Secas have 9"F/10"R and the rest of the Bosses are 9"F/9.5"R. both came with 255/40 and 285/35 tires. Corsas for the LS and P Zero for Boss. Later Performance Pack cars had 275/40 in the back.

Why do these cars come staggered from Ford anyway?
It seems the general consensus is square is better, yet we get staggered wheels and tires.

I think it was mostly a styling choice, but also the only way to inexpensively get more rubber under the car. '11 Brembo GTs (on 255/40 square P Zero) recorded 0.94 g on the skidpad and Bosses pulled over 1.0 g with the same compound tire, but 255/285. As noted previously, larger tires up front would likely have caused tramlining. And the 285 in the rear would greatly increase forward traction and corner exit abilities.

The '11-'12 Brembo GTs and Boss are all pretty neutral handling IMO. Very small suspension tuning differences were made to accommodate the
stagger in both LS and Boss. The LS reportedly has a bit more understeer tuned in.

Details on the springs and bars can be found here:
https://trackmustangsonline.com/threads/s197-spring-and-sway-bar-rates.13152/

Springs:
Boss 302 ................... 148F/186R lbs/in
Boss 302 Laguna Seca 137F/191R lbs/in
Brembo GT/ GTPP ...... 131F/167R lbs/in

Rear bars:
Ford 24mm Solid (2011+ 5.0, GT500): 193 lbs/in
Ford 25mm Solid (Boss 302):............. 250 lbs/in
Ford 26mm Solid (Laguna Seca):........ 275 lbs/in

The tuning strategies for all are very similar, but progressively stiffer rear bars are used to achieve balance with the added grip of the 285s.

On otherwise stock Boss or LS suspension, I would recommend dropping one bar size or more for going square.

Just keep in mind all of that goes out the window once changing to aftermarket spring rates and much different bar arrangements are needed to tune for balance.
 

TMO Supporting Vendors

Top