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Tires for Grip

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152
120
Exp. Type
Time Attack
Exp. Level
Under 3 Years
Greenbrier, TN
I‘m getting the ‘94 ready for TT racing next year.
I run mostly technical tracks without many top end straights, So I want to concentrate more on cornering than top speed.
While I’m working on the suspension I’ve been thinking a lot about wheels too.
I want to know what you guys think about the tire/wheel size.
I’ve read a lot of threads about sizes but mostly guys were concerned with looks not grip.
Ive also noticed that a lot of guys are running a staggered set up On the SN95. ( I felt like on my S550 it induced understeer. )
The fenders are rolled and I’m on coilovers, so how wide can I go without poke?
Right now we’ve got 17x9 wheels And 255/40/17 tires.
Will I see enough extra grip to justify the expense of going to a larger set up?
I know with my s550 It seems to be as much camber and tire as possible. So going to a 19x11 square was worth every penny.
 
152
120
Exp. Type
Time Attack
Exp. Level
Under 3 Years
Greenbrier, TN
Far from street legal, We always trailer. If my calculations are right, 315/40R17 and 295/40R18 are close to the same actual size but the 18 has a lower sidewall. Would that be a positive as it would give less deflection?
 

Bill Pemberton

0ld Ford Automotive Racing Terror
8,425
8,356
Exp. Type
Time Attack
Exp. Level
20+ Years
Blair, Nebraska
Are you only considering a 40 aspect ratio and not a 30? Sounds like you have room for some pretty wide meats? Is a 315/30/18 in your playbook? I remember when I was running a 97 Cobra and I was limited on tire size due to wheel well clearance yet with your mention of a 315/40/17 I am guessing you have some room?
 
152
120
Exp. Type
Time Attack
Exp. Level
Under 3 Years
Greenbrier, TN
The coilovers give me a little more room to play with and the rear is a ‘99 so I should have about an extra 3/4” backspace clearance. I may be able to make 30’s work?
 

PatientZero

@restless_performance
825
865
Exp. Type
Autocross
Exp. Level
10-20 Years
Kansas City, Missouri
Are you sure the coilovers give you more room? You can run 275 square on a 9.5 or 10 in wheel with no problems. Just steering rack limiters. With a little bit of work you can run 295's but 315's aren't going to fit without some poke or fender flares. You don't want a staggered setup, that's for the drag racers. Front grip is already a problem on these cars.
 
6,363
8,187
look at the tire sidewall as another suspension item, you can make some pretty dramatic changes just by increasing, or decreasing the sidewall. I would not just factor in tire size as my only consideration when it comes to aspect/size.
 
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Bill Pemberton

0ld Ford Automotive Racing Terror
8,425
8,356
Exp. Type
Time Attack
Exp. Level
20+ Years
Blair, Nebraska
BS1 , TMO Tire Guru, the spellcheck gods have you degreasing the sidewall instead of degreasing it --- just a pleasant morning note so you can edit ( great button on this site ) as the dang computers often have a mind of their own. Always appreciate when you jump in as your tire knowledge is more definitive and you live and breathe the dynamics all racing season!
 
152
120
Exp. Type
Time Attack
Exp. Level
Under 3 Years
Greenbrier, TN
Are you sure the coilovers give you more room? You can run 275 square on a 9.5 or 10 in wheel with no problems. Just steering rack limiters. With a little bit of work you can run 295's but 315's aren't going to fit without some poke or fender flares. You don't want a staggered setup, that's for the drag racers. Front grip is already a problem on these cars.
I‘m not too sure about anything on this car! lol.
all-of my track set up experience is from an ‘09 and ‘15 where coilovers did make some room.
On my ’15 PP, going to an 11” square set up pretty much cured the bad understeer. It just took some CC plates and 25mm spacers up front.
I started out by by looking at a tire size comparison.
* 255/40R17 = 10.0” contact patch.
* 295/30R18 = 11.8“
So I can get almost 2” more with the 295 without much work.
I’m assuming that an 18” tire would make that contact patch longer also.
Enough to matter, I don’t know.
BS-1 answered my sidewall question. It seemed to me that sidewall deflection has to be included in a good race set up. The suspension can be super stiff but if the tires are bouncing left & right. …Or if theres no give at all….
When you think about it, There really is a lot to consider with tires.
Thanks!
 
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6,363
8,187
It's always been my opinion, that you should start with the most soft suspension possible, back in the day there were 2 schools of thought, Dick Guldstrand, who thought all the parts should work in unison, and a guy named Adams, who's first name I forget, his idea was to use the softest suspension possible to follow the road, then tune with sway bars (this was way before current shock technology) I always like the Adams approach.
 
9
13
Exp. Type
HPDE
Exp. Level
Under 3 Years
MA
It's always been my opinion, that you should start with the most soft suspension possible, back in the day there were 2 schools of thought, Dick Guldstrand, who thought all the parts should work in unison, and a guy named Adams, who's first name I forget, his idea was to use the softest suspension possible to follow the road, then tune with sway bars (this was way before current shock technology) I always like the Adams approach.
Herb Adams
 

PatientZero

@restless_performance
825
865
Exp. Type
Autocross
Exp. Level
10-20 Years
Kansas City, Missouri
all-of my track set up experience is from an ‘09 and ‘15 where coilovers did make some room.

Both of those cars already had a strut on the front, when you went to a coilover with a smaller diameter spring of course it made more room.

On the SN95 the factory spring is on the control arm. When you switch to coilovers you're moving the spring to the outside of the front shock which is going to eat up the same space you're trying to occupy with a tire.
 

PatientZero

@restless_performance
825
865
Exp. Type
Autocross
Exp. Level
10-20 Years
Kansas City, Missouri
I switched to the shortest possible spring on the front of my car to create more room for the tire. You can clearly see how with a longer spring it would interfere (and did). This is a 295. I can't get my pinky between the tire/coilover and it's sitting flush with the fender on the outside with enough camber.
20190707_131634.jpg
_DSC0565.jpg
 

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