TooTurbos

was going to start my own thread in the near future but figured i might be able to piggy back on this one.

slight back story;
ran a couple autocross days on the P-crappos and will be doing more this year. PS4 is what is going on the car for sure but i wanted to dial out the understeer without getting too aggressive with alignment and camber. Just trying to balance daily driving feel/tire wear.

Keeping stock rims for now, so given that...what have people done to this?? below is the available tires and my thoughts, hoping people can chime in with some expertise.

Factory equipped tires - 245/35/20 305/30/20 ----> yields a 26.75" front tire diameter and a 27.20" rear tire diameter or a .983 percentage of front to rear ratio



Available 4s

Fronts:

1. 255/30/R20 -> 26.02
2. 255/35/R20 -> 27.02
3. 265/30/R20 -> 26.25
4. 265/35/R20 -> 27.30

Rears:

1. 305/30/R20 -> 27.20 (stock)
2. 305/35/R20 -> 28.40
3. 315/30/R20 -> 27.44
4. 325/25/R20 -> 26.40

Ratios of front to rear configurations: (stock 26.75/27.20=.983)

• 1. and 1. (255f 305r) yields 26.02/27.2=0.9566 ratio - less front sidewall, ratio would need confirmation it still worked, front and rear bias contact patch decreases 10mm more in front less understeer
• 3. and 1. (265f 305r) yields 26.25/27.2=0.9651 ratio - slightly less sidewall, ratio looks close should work, front and rear bias contact patch decreases 20mm more in front least understeer??
• 1. and 3. (255f 315r) yields 26.02/27.44=0.9483 ratio - less front sidewall, ratio would need confirmation it still worked, front and rear bias contact patch stays the same, same understeer characteristics?
• 2. and 3. (255f 315r ) yields 27.02/27.44=0.9847 ratio - more sidewall, perfect ratio, but the front to rear bias contact patch stays the same, same understeer characteristics?
• 3. and 4. (265f 325r) yields 26.25/26.40=0.9943 ratio- less sidewall, ratio is super close should work, front and rear bias contact patch stays the same, same understeer characteristics

onfireTTS

I'm not a road racer so take my comments with a grain of salt (although I really do enjoy Barber).

As a Drag Guy, we've found that putting the widest tire that will fit on a wheel doesn't always give better or equal traction even with the same tire family, compound, etc. A taller tire will almost always help since it will give more footprint if both tires have the same tread width. But if you have the same height, limiting the tread width (not section width) to the rim width tends to load the tread better and plant it better at launch. A tread that is wider than the rim width loads differently and might not plant as well. So that's why I selected the P4S in factory widths since the rear tread width was roughly 11.5" on the PS4 in the factory size and I have factory width BBS wheels. If I were factory racing and had to run a maximum 11.5" tread size, I would actually overstretch the tire onto a 12" rim for even better overall loading....but just for Drag Racing.

Now lateral loads on a road track is a different animal and those guys will advise you there.

onfireTTS

Some more useless tidbits. I used my tire durometer to test a group of tires I have in the garage at 55F.This is a "average" as rib readings can change across the tread :

Stock N1 Pirelli's = 75

P4S = 72

Pirelli Scorpion Winters = 64

X3M P4S = 76

Bruce H.

Do you think you are fully using driving technique to minimize under-steer?

shootamc58

Fantastic input and thank you all. I'm going with the Michelin ps4's. My friend owns a tire store so if i don't like them i can get different ones for a great deal and sell the lightly used ones and not be too much out of pocket. Thanks again for all the input and experience.

TooTurbos

On fuego:
Totally agree with everything you said and like a bone head i missed the final bullet and combination that i was most compelled to try. and that was the combo "2. and 1." with a 255/35/r20 and the stock rears. The 245 on the stock front rim looks a bit "stretched" and it really felt like there was no sidewall flex during hard cornering, and the 305 looks a touch bulged on the rear with the stock rim. seems like its kind of at the limit, but i have read of others tossing the 325 on there?

Bruce:
Likely not haha, but.....I had a friend with me that has a 2016 civic si, he's an instructor and won some nationals autocross comp last year (not sure what one). But driving his car i was 1.4 seconds off his best time and experienced very little understeer when managing it properly and with my car i felt like it was always teetering on understeer. He drove it and thought the same, and that it needed way more camber up front.



thanks for the replies and sorry to hijack a bit!!

TooTurbos

duplicate post**

onfireTTS

The 3xx has been used with success with the Road Track guys. Randy, Jean, etc will have to comment on it working on a stock size rim under lateral stresses.

crazy1eye

Ya where’s the 2 - 1 combo? That’s what I plan on putting on this spring, using P4S’

TooTurbos

ya....totally goofed when i copied it from my notes document and missed it because it was at the bottom as the most enticing combo haha.

but i think i'm going to go that route in the summer.