Hello,

Car: 1978 911 SC Rolled Fenders up front.


I'm trying to purchase a set of Kumho tires for Autocross events and can't decide which tire size to go with. The new V710 look like the best bet in the Kumho line.

I run 205/55/16 front and 245/45/16 rear on Fuch 7/8 inch wide rims for street tires. Kumho doesn't make either of these size tires in the 710.

Can anyone tell me what whould be a good alternative?

Also I have read a lot about people using 15" rims for autocross. Is this a better idea than the 16" rims?

Thanks

 

I would run 225/50-15s front and rear on 7/8 inch rims. The circumference is 4% less than the 225/50-16s. The 15s would be less likely to contact the fenders when cornering as well.

Leeds

 

Why not run bigger tires in the back though. Is this to take out some understeer?

I have adjustable sway bars on the car, would that be a better option than running the same tires front and rear?

 

911s push when autocrossed unless properly set up, especially in slow corners. If you're going to the effort of Kumho 710s you should have a very well balanced car for the speeds you're driving, especially with how quickly the 710s wear. You don't want to abuse them unnecessarily.

To compensate for the larger rear tires you will have to increase the roll resistance in the rear or reduce it in the front to reduce push. This will increase the difference in vertical load on your rear tires in corners and increase the likelyhood of spinning your inside rear tire accelerating out of a corner. Then you would really need a limited slip and even then you would be more likely to have corner exit oversteer. You may already have a limited slip, but most 911SCs were delivered without. You will probably hurt your car's turn-in at the beginning of the corner as well.

When I autocrossed my 1978 911SC 16+ years ago I ran 225s on 7 and 8x15 rims, but ran 16s at the track with larger tires in the rear (205s and 225s). My current autocross cars, an A Stock (SCCA class) 2003 Boxster S (M030, larger adjustable front bar and 245/35-18 Hoosier A3S04s) and F Prepared 914/6 (295 hp 2.8 liter, 2100 lbs, 10x16 rims, Hoosier R35 compound slicks), both run on the same size tires front and rear even though they have significant horsepower compared to a 911SC. I do admit that my 1993 RS America, 1995 993 and 1999 996 all ran on larger rear than front tires, but they all had less inherent push when properly aligned with their more sophisticated suspensions.

Leeds

 

What about the Avon Tec-R tires?

I want something that is a little easier to deal with than the Hoosier A3SO4 tires and last longer. I’m not trying to win nationals or anything. I’ve been running on street tires (Dunlop SP9000), and would like more stick on dry pavement.

I would also give up a little time on the autocross for the ability to drive 2-4 miles on the street to our local events.

My car is setup real well with stiffer t-bars, lowered, and corner balanced. I also have adjustable Smart Racing bars so I think I can tweak the cornering a little if it will not do what it needs to.

What about this combo so I can keep the balance similar to my street tires and still be able to use them for DE days:

Tec-R 205/50/15 Front and 225/45/15 Rear.

 

Up untill this year I drove with Kuhmos or Hoosiers to our local AXs.....about 5 miles, not to mention some exercise jaunts around town that were somewhat longer. Never had a problem.

To the longevity aspect. All these R spec tires like a fair amount on negative camber....around -2 deg. Without it, you'll not get anything near the potential of the tire when autocrossing(and they will wear more quickly). With it, the longevity of your street tires will suffer.

I have no experience with the 710 Kuhmos, but a season on Vracers & one each on Hoosier's R3s & A3. I seriously doubt the 710 Kumhos will wear any better than the A3 Hoosiers. The 710s were a disaster last season and not available in my size. They are a force to be reconned with NOW, but I've gone to slicks.....

In what part of Orygun are you located?

 

Thanks for the info. I'm down in Salem.

I got the feeling from things I read that the Hoosier A3 tires had much less life than the other R Coumpound tires. Some people even corded the tires after only 5 autocross events. Given that and the price I rulled out the A4s that are now on the market.

I am also not finding 710s in the size I want which is why I was looking at the Avon Tec-R tires now.

 

I ran the A3SO4s all season.......2 drivers, prolly 150-180 passes. They still had life when I sold em'.
My car is fairly light, bout 2200 lbs and the alignment was pretty close to what they wanted...per the temps. I'm sure the lousy grip at our AX venue (PIR) helped some.

Do you guys in Salem have a good venue?......I gotta get out more

 

That's pretty good life then. Mine 911 is pretty light as well around 2300 i think.

The Salem Venue is not that interesting. The course is just too small and they keeps speeds very low. Nothing like PIR autocross. I have heard Eugene has a better Venue though, so I might head down there or up to PIR.

What size tires did you go with in the A3S04s?

 

225 X 45 X 15 at both ends on 7 X 15 Cookies. Our first AX of the season is 17 April, AM, in the South Pits....come on down.
Ughfortunately, my car won't be ready by then, but I will be. Looks like Mr.2 gets the nod.

I too have heard on a lot of guys going thru the SO4s rather quickly.....40 or fewer passes. I can't account for the discrepancy except for the info I gave before.
The SO5s are out.......the 710s got Mr. Hoosier's attention. Maybe it was the line ups at the Kuhmo trailer They are selling SO4s cheep.

 

JPIII,

I just purchased 4 of the 225 x 45 x 15 I hope these will fit on my SC and the grearing is not too far off!

 

So, are we going to see you on the 17th?

 

Maybe So!

Do I need to pre-register for that event?

Can you fit four extra wheels/tires in a 911 that has fixed sparco seats? I don't have a trailer yet and this is not a high interest activity for my wife on a sunday morning...

ALSO: LOOKING TO PURCHASE --- 15x7-8 set of fuchs asap!

 

No need to preregister. Just show up. If you can't bring your new tires, just run whatchagot.

 

Hi Leeds,

In the way of hijacking this thread for a spell, perhaps you can help me out.

My (tired, old) 911 developed a tremendous push in the those long slow sweepers at Autocross last year. This winter I replaced the shocks, put new bushes in the rear and had car lowered, aligned and corner balanced. While I am sure this will help a bit, if figure I can mess with tire pressures and sway bar settings (I have adjustable front and rear bars) if I still have the problem. What approach would you recommend? Any advice is appreciated.