Great thread and very relevant to my own S197 build(s). I have a few data points that I can share that we've learned from our 2011 GT, that we autocross, run in NASA TT, and street drive.
We have been exploring the maximum tire and wheel widths inside the untouched S197 rear fender/chassis restrictions for the past year, and we think we have found the absolute limit without any cutting to be a 12" wide rear wheel. The rear wheel and
autocross tire on our red GT is shown below. I usually track on a different tire with these exact wheels, a 315/35/18 Kumho V170
That 12" wheel is a very tight fit in many places, and will rub on a chassis protrusion at full droop if it is located inboard far enough, so you have to pull the wheels before you can let the axle go to full droop (on the square of metal that protrudes with the elongated tie down hole, shown below). Not a huge PITA, but something you have to remember.
That picture above was after I tried to move the wheels inboard the maximum amount, because for an autocross car you want the LEAST track width possible, especially at the rear. This is after removing the stock rear swaybar restriction, which is the first thing it will hit. As you can see itss now rubbing on 4 places... the shock mount (eye-to-eye Moton doubles on a FRPP eye-to-pin mount) at the top, the rubberized cover at the back there, and all along the front of the inboard chassis. After this picture was taken we added a 1/8" wider wheel spacer and it cleared perfectly, and the tire barely stayed inboard at the fender to not rub outboard. There was not room for another 10mm of tire or a 1/2" of wheel without cutting something.
We went to these extreme lengths and tested many tires and track widths because a narrow track is always faster in slaloms, and autocross always has one or more slaloms. This is the opposite of a road course car, where a wider track helps add grip (but has a tradeoff of making passes more difficult and adding some aero drag). There are never slaloms on a road course, of course.
The axle also deflects laterally under load. A LOT. Even after switching from a Panhard to a Watts we are still seeing
"some" deflection. This is something I want to test under load, with a video camera mounted at the rear fender with some sort of scale.
We have tested a lot of tire sizes on our Mustang, in both street compounds and R compounds as well. With the R's we've used 275, 305. 315, and 345mm tires. The widest that fits front and rear without rub is a 315, but a sticky R compound 315mm tire is NOT enough to avoid wheelspin on this car, in autocross or on a road course. Our 5.0 car only makes 430 whp, but its enough to overpower a 315, from what I have seen. More easily in an autocross, where maneuvers are violent and you are on and off the throttle a lot, but even on a road course in slower corners it could use more rear tire.
Earlier this year we tested a 345/35/18 Hoosier, but it stuck out past the fender and rubbed everywhere under load. This has a 13.9" section width, so it is a frakkin big tire. Inboard, outboard, it didn't matter where we located the wheel relative to the hub, it rubbed terribly. Big clouds of smoke...
That's the tire rub on 345s. It was pretty bad. But damn, it could put the power down! The first places you will see tire rub if you use a BIG wheel and tire, if space the rear wheels inboard to avoid the fenders, is at the swaybar mounting location on the axle. This bent pieces sticks right where the inside sidewall will hit first...
That portion of the antiroll bar below the blue mounting link is where the tire will rub first. We went to the
Whiteline rear swaybar, which re-routes the bar completely and removes this restriction. Once that location is gone the next places are all over the chassis, in my first picture of this post. If your class rules allow you to cut/modify these areas you can go inboard even more, but most of the time that is heavy fab work that the rules don't allow for. In our case we cannot touch the inboard section, so we're left moving the wheel outboard, cutting the fenders, and adding flares.
That's what I plan to do to my 2013 GT, which I bought last week. My wife won't let me cut on her red 2011 GT, and I know it will be quicker on the 345 tire, so we will cut on this black one and flare it. And we'll use 335s up front to try to keep the front to rear grip balanced. Already sent a PM to 908ssp about the flared AIX Mustang posted earlier in this thread. Finding the source for those flares would save us a lot of time.
Speaking of these two cars, we've asked the owner of this forum if we can post our build thread on this forum. We aren't a "supporting vendor" but tried to be (he is not taking more at this time), but we'd still like to share our 2011 + 2013 builds on this thread. If you liked the tech in this thread, there's probably 100 more posts on our 2 year old project that I could bring here. Just need permission...
Thanks,
