F Stock / Autocross R compounds

[Cone Sweeper]

Really? 275's will fit good on the stock rim without an issue? I got slack from people wanting to run that size lol

 

[Sam Strano]

Ah, yeah.... 2006 FS National Champion ran that size in his Mach 1 on 17x8's. The first National SCCA event an S197 was me, in a 2005 car (in 2006) on 275's on 8's.

 

[Cone Sweeper]

Nice to know lol. Kinda find it funny now that people were giving me crap when I was gonna do it when i needed new tires.

 

[Sam Strano]

Good advice is hard to find. What can I say?

 

[Cone Sweeper]

True Story Sam!!!!

 

[latch5]

Is there any chance that Ford put out 17 x 8.5"? I did a bit of research on measuring and it seems they are 8.5. Put tire on it's side - set a 4' level on the top and measured height from the floor (thru the center hole) - it's a bit more than 9.5". Using a level again - space of lip from the top is 1/2". With these measurements it should be 8.5" right???

 

[Sam Strano]

Nope. Rims are measured bead to bead width. If you measure anything but that you will get some other number. For what it's worth, the rim size is stamped or embossed on the back in one of the spokes.

 

[latch5]

Tks Sam

 

[jayel579]

Ah, yeah.... 2006 FS National Champion ran that size in his Mach 1 on 17x8's. The first National SCCA event an S197 was me, in a 2005 car (in 2006) on 275's on 8's.

And I own the rims he won on!

 

[Sam Strano]

Yes you do.

 

[giddyup]

Sam,what do you recommend on stock 18x8.5s?

 

[Sam Strano]

For what use? What tire? Are we talking about something like a Hoosier or V710 for competitive lap times? What I run to compete on vs. what I might use to screw around on are different things (the fastest setup wears faster).

 

[giddyup]

i guess you would call it screwing around...lol.im pretty much looking for something to start with on a road corse.i have the stock cali special wheels with 255 45s on front and 285 40s on rear.they are gt500 take off tires. i keep seeing varying opinions on tires,but i already know the rears seem a little to much for the stock 8.5s.

 

[Sam Strano]

Ok first I have to make sure we are talking about r-compound tires here. 285 street tires on 8.5's is way too big. This is a case where what we might do for outright speed and what we can do on stiff sidewalled r's is different than street tire

For r comps on 8.5's I'd have to recommend 275/35's. I started on those on the Shelby in 2007. They wore better, were slighty slower and when thousandths count for me that's important. Also to be frank I win a good number of tires back so the wear rate is diffused a good bit.

 

[Rapid]

OK,

We now have 18x 8s, 275/35 Hoosiers, and no experience on R compounds.

I have read the Hoosier website about brake in and such. My first autocross on the new setup will be a short one in Jan.

What advice would you offer?

 

[Sam Strano]

Pressures of about 45 front, 40 rear to start for decent balance and getting used to them. You need to watch the front tires on the edge of shoulder. If you start to get sort of a step (feel with your fingers too) you need more air. If you don't get it, you can come down in air (which is better anyway) until you start to get it then go back up a pound or two. Set rear pressures to balance the car. You could be all over on rear pressures depending on how the car is driven, sprung and what bar setup.

In FS trim with my '07 GT I actually ran about equal pressures, sometimes rear higher to help it turn. Same wheels and tires on the Shelby in FS trim and I ran similar front pressures (usually around 42 front, but on 8.5" wheels and I do not overturn the wheel like many do), but usually 35-37 rear because that car is much better balanced to start with. Your car is not setup like the Shelby or even my '11 Brembo car... Eventually I think you'll find sort of higher rear pressures (making the front and rear more even) will ultimately be a bit better. Of course, disclaimers apply. YMMV, etc.

 

[Rapid]

Thanks Sam,

When you say overturn the wheel you mean too much steering input?

Running more in the rear will induce some over steer?

 

[Sam Strano]

Yes, too much steering angle and that beats up the shoulders, badly (also makes you slower).

Tire pressure adds spring rate to the tire. More spring rate in the rear tire helps with balance in a tight car.