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Wheel and tire choice question

2K views 20 replies 8 participants last post by  Jack Hidley 
#1 ·
I'm looking at getting new rims and tires for my car. I'm not sure how much track time I will see because I've got full mm suspension front and rear(ta, panhard bar) and a supercharger. I'm also a vert without a role bar yet. I'm not sure if there is an auto-x class I qualify for.

For mostly street would I notice the weight saving of the enkei rpf1? If not and I get some factory replica wheels should I stay with a squared out stance to help avoid under steer?
 
#5 ·
If not and I get some factory replica wheels should I stay with a squared out stance to help avoid under steer?
I’ve never understood this line of thinking. Maybe with the full MM package the rear has more grip than the front, but I still don’t see why you would hinder the rear(with less tire) because of the short comings of the front.
 
#8 ·
The new SCCA CAM class. American made, RWD. Any motor, suspension, etc. Just has to weigh - 3200?



If rear grip exceeds front grip, the front pushes in a corner. So, if you increase rear grip, you can't steer around a corner - at least at any decent speed. The Fox chassis is an under-steer car with the occasional snap over-steer. So, you either increase front grip to match, or decrease rear grip. Better to increase front grip by using a square configuration and tweak your rear bar as needed.
 
#10 ·
With a supercharger you are out for CP class. With your mods could run Street Mod (R-comps), XP (slicks), CAM (200 tread wear and 3000 lb minimum weight), or maybe STX. Study the rules, pick a class, then mod your car, legal rims/tires, and go have some fun.
 
#16 ·
That’s how I see it. I have a Mach with exhaust, 4.10’s, Bilstein shocks up front and Tokico’s in the rear (hope to get my Bilstein’s on after our next event), H&R Springs, and adjustable RCLAs. Most any time I want I could 100% throttle and break the rear loose and that usually means that I have to stay out of max throttle, which leads to slower entry speeds, which leads to needing less grip in the front. Maybe when I get my torque arm on, for my slightly more than stock power level, I might not need huge honking tires to almost always have 100% traction in the rear. As I set right now, traction in the rear is an issue, but traction in the front isn’t…but that’s mainly due to the rear traction issue.

But I'll be the first to let y'all know if I get slower putting a good 315 on in place of the 245. I'm not afraid to eat some crow!
 
#19 ·
Derk,

With a car that has a 55-58% front weight bias, if larger rear tires are installed on the car, this is going to make the car understeer. The understeer is caused by the difference in front and rear slip angle that the tires will operate at. The front tires will always be operating at a higher slip angle than the rears, so the car will always understeer, even when not at the cornering limit. Having the car understeer makes it not fun to drive.

With too large a rear tire, at any cornering force, the front tires will always have not enough grip compared to the rears. It isn't the case that the front tires will just suddenly slide when they run out of grip. If that were the case, putting larger tires in the rear wouldn't hurt the handling. A rubber tire has some portion of the contact patch sliding on the ground at any cornering force. This is not an easy subject to explain without drawings of force vectors and such.

Since it appears that all of your Mustangs have a Quadra Bind rear suspension, they are going to unpredictably oversteer and understeer when pushed to the cornering limits. Once the rear suspension is fixed, the handling will become much more consistent.
 
#20 ·
Jack, I greatly appreciate your response. Over the past year and a half, every time I ask why, I either get no response or "because". I've read this, a solid, four times now and I still can't get my mind around it though. I can see how it would become tougher to drive and not as "fun", but just not slower. If someone could point me to some more in depth literature I would greatly appreciate it.

And yes, Jack, they are all Quadra Binds until I can find someone semi reputable to install your Panhard bar and torque arm I have currently setting in my buddies garage!
 
#21 ·
Derk,

I would buy a copy of "How to make your car handle" by Fred Puhn. I think it is the best beginner on vehicle dynamics. The first 10-15 pages are mostly on the subject of tires. I would read this section over and over again until you understand it completely. It isn't glamorous, but without really understanding it, you won't be able to really understand how changes in the suspension affect the handling.
 
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