New FR-S Owner - AutoX Question

[ButteR]

Originally Posted by TrqlessWonder

I'm going to hope the /35 is a typo and you meant 45. In any event, you can run whatever size tire you can get to hang onto the wheel, that doesn't rub. The stock size wheels (with some small allowances for offset and +/- 1" diameter) are the rule, the tires, with the exception of Treadwear Rating, are pretty much completely open.

There's folks around here that run anywhere from 215's to 245's on CS legal cars.

People are running 245 tires on stock wheel specs eh? Forgive my ignorance here but I'm learning a lot very quickly on these forums vs the genesis coupe forums. MUCH more useful knowledge and advice.

CS rules state no changes to stock wheel specs, but allow for any tires that you can fit on there. It took me a while to understand wheel offset, camber/caster/toe. I think im down to two more pieces of the wheel & tire puzzle that ive been seeking answers to for months, and I think yiu guys can seal the deal on these last two for me

Tire size vs Wheel size: oversized tire on wheel and underside tire on wheel.
In order to fit a 245 tire on a 17x7 wheel (OEM spec wheel), the only unknownn variable is sidewall, yes? If I understand this correctly, by default, fitting a 245 tire onto a 17x7 wheel will require a very high sidewall, right? Where fitting a 205 onto a 17x7 wheel would naturally result in a shorter n stiffer sidewall. Couldn't one argue that putting 245 tires on the OEM wheels result in a decrease in performance due to having very tall and soft side walls?
I could see going with 225/45/17 on a 17x7 wheel, but any wider tire than that would result in diminishing returns.

the last piece of the puzzle that I've been trying to understand is how tire psi changes with all of this. Using the OEM Wheel is a constant variable here, how does the tire psi need to be adjusted when putting smaller tires on or putting oversized tires on? In my mind, the former will be stretching the tire requiring less psi where as the latter will have extra rubber and require more psi.

I realize that theres much more analysis that goes into wheel and tire fitment, but if someone could straighten me out on the last two pieces that I've been having a difficult time grasping, then a lot of other stuff is going to click and ill finally be able to link it all together.

Slightly off topic here, but on my genesis 2.0T, with a square set up, i always ran higher psi in the front than I did in the rears and loved the handling. I just did my first autoX in the FRS and did the same (prius tires will need to go soon). I thought I was going to miss the turbo, but the handling and overall feel of this car is amazing. I'm very happy I switched platforms.

[renfield90]

Originally Posted by ButteR

Couldn't one argue that putting 245 tires on the OEM wheels result in a decrease in performance due to having very tall and soft side walls?
I could see going with 225/45/17 on a 17x7 wheel, but any wider tire than that would result in diminishing returns.

With high performance tires and stiff enough sidewalls, you can get away with more pinching before diminishing returns (or physics preventing you from mounting the tire) come into effect.

DS national champion one year mounted 275 Hoosiers on a 6" wheel. The Hoosier has a ridiculously stiff sidewall, unmounted I can sit on the tire and take my feet off the ground without it collapsing.

Most CS folks run between a 225 and a 245. In general for autocross, the more rubber you can get in contact with the ground the better.

[ButteR]

Originally Posted by renfield90

With high performance tires and stiff enough sidewalls, you can get away with more pinching before diminishing returns (or physics preventing you from mounting the tire) come into effect.

DS national champion one year mounted 275 Hoosiers on a 6" wheel. The Hoosier has a ridiculously stiff sidewall, unmounted I can sit on the tire and take my feet off the ground without it collapsing.

Most CS folks run between a 225 and a 245. In general for autocross, the more rubber you can get in contact with the ground the better.

Ahh...I see I see. Very interesting. Stock class ain't as "stock" as one might think. They should make an OEM class for us new 86 owners to compete with and destroy these preius tires ;P

[justint5387]

You can run those tires if you want, no one is stopping you.

[Mad_Mike]

Originally Posted by ButteR

Ahh...I see I see. Very interesting. Stock class ain't as "stock" as one might think. They should make an OEM class for us new 86 owners to compete with and destroy these preius tires ;P

They changed the name recently so its not 'stock' anymore its 'street', which makes more sense with the allowed mods.

[7thgear]

Originally Posted by ButteR

Ahh...I see I see. Very interesting. Stock class ain't as "stock" as one might think. They should make an OEM class for us new 86 owners to compete with and destroy these preius tires ;P

no.. it's pretty stock. You have to buy tires either way, so this argument is moot. The only thing that changed is that before you had to buy Hoosier A6s, and change them at the event... and now you don't.

[cactus]

I ran 245/40/17 ZIIs on stock wheels and crash bolts for CS last year. I drove a car with 225/45/17s last weekend and it feels less vague on turn in, but not nearly as much grip as the 245s.

[TrqlessWonder]

Tire size vs Wheel size: oversized tire on wheel and underside tire on
wheel.


In order to fit a 245 tire on a 17x7 wheel (OEM spec wheel), the only
unknownn variable is sidewall, yes? If I understand this correctly, by default,
fitting a 245 tire onto a 17x7 wheel will require a very high sidewall, right?
Where fitting a 205 onto a 17x7 wheel would naturally result in a shorter n
stiffer sidewall. Couldn't one argue that putting 245 tires on the OEM wheels
result in a decrease in performance due to having very tall and soft side
walls?


I could see going with 225/45/17 on a 17x7 wheel, but any wider tire than
that would result in diminishing returns.

In a 245 on a 7, the only variable left is the sidewall. But doing so doesn't automatically necessitate a tall sidewall. Remember, you can also use tire size to make small changes to the effective gearing of the car (different schools of thought on that).

For diminishing returns, yes, probably after 225, you aren't getting all of the extra width actually converted into contact patch. But you're getting some of it, and a little better is usually preferable to no better. Although you may get a small amount of that lost extra width with a taller sidewall. Would it be of consequence? Don't know.

the last piece of the puzzle that I've been trying to understand is how tire psi
changes with all of this. Using the OEM Wheel is a constant variable here, how
does the tire psi need to be adjusted when putting smaller tires on or putting
oversized tires on? In my mind, the former will be stretching the tire requiring
less psi where as the latter will have extra rubber and require more psi.

What tire pressures work have more variables than width. Buy the set you like, hope that your research on the pressures will at least get you in the ballpark, and then tune to suit. You'll have to do that with any tire. A good air gauge and 12v compressor will be your friend.

[ButteR]

Originally Posted by cactus

I ran 245/40/17 ZIIs on stock wheels and crash bolts for CS last year. I drove a car with 225/45/17s last weekend and it feels less vague on turn in, but not nearly as much grip as the 245s.

You fit 245/40 ZIIs on the stock wheels?? Didn't even know that was possible.
I figured you'd need at least an 8 inch wheel to fit 245's.

Did you have to do anything special to get a good fit?

[Kostamojen]

Are you competing locally or nationally?

If its just locally in C-stock, stick with 225/45's... Maybe 235's at most.

That and the front swaybar with the camber bolts, thats all you need for C-stock.

[G_Ride]

Originally Posted by ButteR

You fit 245/40 ZIIs on the stock wheels?? Didn't even know that was possible.
I figured you'd need at least an 8 inch wheel to fit 245's.

Did you have to do anything special to get a good fit?

I've seen his tires, as well as 245 Rivals and RS3s on stock size wheels. I ran 245 RS3s on the 17x7 RPF1. There really isn't anything special you need to do to make them fit. You just someone that'll do it. Some tire shops won't mount that size tire on a 7" wide wheel.

[G_Ride]

Originally Posted by Kostamojen

Are you competing locally or nationally?

If its just locally in C-stock, stick with 225/45's... Maybe 235's at most.

That and the front swaybar with the camber bolts, thats all you need for C-stock.

245s will work. I decided to try it when Des drove his car with 235s and another car with 245s back to back. The 245s were faster. My codriver and I have tried 225 and 245. We liked the 245.

[Kostamojen]

Originally Posted by G_Ride

245s will work. I decided to try it when Des drove his car with 235s and another car with 245s back to back. The 245s were faster. My codriver and I have tried 225 and 245. We liked the 245.

Its still risky and I wouldn't recommend it... I've seen 235's on the stock wheels and thats more than I would even push it, unless you were seriously competing in nationals and ONLY using these wheels/tires in Autox and immediately removing them.

[ButteR]

What do you mean by risky?