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315/35/18 on 18x10, fronts if S197 - doable?

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Duane Black

Curbs go brrrppp
567
401
Exp. Type
Time Attack
Exp. Level
5-10 Years
Durham, NC
So the quick run down, I've got McPherson struts at -2.5 camber.

I autocross some (as few as 3, as many as 18x a year ok very the past 4) and want to know if anyone has run the 315/35r18 on an 18x10 without massive spacers or modifications. I know am 18x11 is ideal, yet I'm not putting that kind of money into this endeavor. If theyll fit a set of 18x10 I already own, so be it.

The tire would be the BFG Rival S 1.5 btw.

If it's a known issue, theres always 285 bridgestones and 275 rivals
 

Norm Peterson

Corner Barstool Sitter
939
712
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
It's not been uncommon for Street class autocrossers to run tires on wheels half an inch narrower than min-recommended, and I think some have gone a full inch narrower. But if you're already on 10", you won't be running in Street and would be at a clear disadvantage in, say, CAM-C.

How much spacer you'd need would mostly depend on what the offset is for the wheels that you have, and whether or not your struts were designed for greater tire to strut clearance than Ford's OE struts (Cortex being one that I know of that has done that). Keep in mind that when you pinch a tire down onto narrower rims, the amount of the tire's own lateral flexibility under cornering loads tends to increase (part of why turn-in response then tends to be "softer").


Norm
 

Norm Peterson

Corner Barstool Sitter
939
712
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
I also suspect that you'd want camber to be further negative than -2.5°. You'd be trying to compensate for the additional tire flexibilities and the rolling of the outboard tires under. This may even be why people running on wheels at or closer to "measuring width" are running more camber than that.


Norm
 

Duane Black

Curbs go brrrppp
567
401
Exp. Type
Time Attack
Exp. Level
5-10 Years
Durham, NC
I understand the issues associated with a pinch, just curious if it fits.

It seems the autocross community thinks the overall grip gains are worth whatever "numb" feeling you ge .

I've never tried it though. 285s are a lot cheaper anyway
 

Bill Pemberton

0ld Ford Automotive Racing Terror
8,425
8,356
Exp. Type
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Exp. Level
20+ Years
Blair, Nebraska
305s are really, really tight ( on a 18x10 ) as I have done that. Best bet is a set of 18x11s from Apex and tires are the number one mod to go fast in Time Trials or Autocrossing.
 

Norm Peterson

Corner Barstool Sitter
939
712
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
It seems the autocross community thinks the overall grip gains are worth whatever "numb" feeling you get.
They've more or less been forced to think in terms of some gain being better than none by Street (nee Stock) category rulesets that limit wheel widths to OE but placed no such limits on tire sizing. It still comes up short of whatever an optimized fitment for unlimited tire sizing might be, but you and everybody else in the class has to live within the same rules. The playing field is at least leveled (more or less).


Further back in Production class road racing, a similar philosophy led to the development of "cantilever" tires where the lower sidewalls were made unusually rigid - essentially functioning like soft rim width extensions - in order to better support treads that were wider than the rims.


Norm
 

Bill Pemberton

0ld Ford Automotive Racing Terror
8,425
8,356
Exp. Type
Time Attack
Exp. Level
20+ Years
Blair, Nebraska
Simple key is if you want to be competitive in Time Trials you need to be on Hoosiers ( A7 or R7s ) , BFG gforce R1S models, or used Michelin or Pirelli Slicks if the penalty factor will allow ( usually for heavier vehicles ). For Autocrossing if you are in a CAM class then you have the BFG Rival S 1.5s, RE71Rs, or the new Yokohama A052. Frankly the A052 gets hot too fast to probably be effective to most Time Trialers ( except those that want just as few quick laps ), the RE71Rs do okay for some but will get greasy midway through a session, and the Rivals seems to be similar or lasting a bit longer. Some autocross classes the hot tickets are the A7s or BFG gforceR1S tires, though Hoosier seems to have the upper hand here - and one can not discount the A052 after it's stellar showing at the Solo Nationals.

What this all essentially means is if you want to be competitive in Time Trials you will likely need to make a choice , as the Hoosier of BFG Gforce rubber will walk over an equal driver in an equal car using a 200 TWR donut. If Autocrossing is your primary goal then a 200 TW tire might be your choice, but again depends on the Class you run.
 
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