autocross/hillclimb/trackday....thoughts on which tire

[CamT]

New to the forum, acquired my 1999 caterham series 3, last summer. Car is an HPC car, Vauxhall 2.0, updated cams, Weber carbs, 6 speed, limited slip differential, has uprated front and rear springs, bigger front bar and adjustable rear bar. Have owned and raced a 1971 Datsun 510 for more than 30 years. I have always used the Hoosier A6 or A7 tires. Previous owner did autocoss the car(10 years ago)and used bogart wheels, 13x10 with Hoosier R25 slicks. Going over various posts the common tire appears to be the Hoosier bias ply slick in the softest compound. What are people's thoughts on the A7 in the 255/40-13 on a 9" wheels vs either 20.9x9/13 or the 22.5x10/13 bias slicks on 10" wheels? Is there a reason the dot radials don't appear to be the tire of choice?

thanks

[yellowss7]

I autoxed for 8 years and used the R25B 10 inch Hoosiers at about 12psi. I think the reason the big Hoosiers are used is that you really don't have any time to get heat in to the tires with the short Autox runs. I've also used those same size hoosiers on the track, but moved to a slightly taller size for better top end gearing and they work great too.

[CamT]

Thanks Tom for the quick reply. Your name came up last week as I was reaching out to MichaelD. He mentioned you had a vauxhall powered 7 and a big autocrossing background. Apologies for being a little daft, but I always believed the A7 Hoosiers were the go to tire for autocrossing? One season I used Hoosier cantilevered bias slicks on the Datsun at the Knox Mountain hillclimb and never used them again. That said the main reason for discontinuing using was clearance issues. Since then we have been using the A7's. Most of the competitors at the Knox Mountain hillclimb are using A7's or some use the R7's.....very few are using slicks?. You mentioned the challenge of getting the tire up to temp......do the bias tires heat up quicker? I don't want to reinvent the wheel here, I just want to be clear so I can order the correct wheel from Kodiak.

In previous posts you also mentioned back spacing of 4.5 to 4.25 depending on sidewall stiffness.....previous owner had 10" wide wheels made with a 5.0" backspacing and claimed no clearance issues. His wheels were not Kodiak.

Any further advise is much appreciated....

thanks...Cam

[yellowss7]

Hi Cam, Don't really know how fast the tires heat up, but most of my Autoxing courses were 65 seconds or less, so I doubt I could get much if any heat into the tires. I never hillclimbed so can't opine there. The ten Inch hoosier slicks did not get as much heat on track as the narrower tired cars but they stuck like glue in my opinion. I have the Kodiak wheels as speced by Michael D. They are on the car and the car is up on the lift so I can't readily measure the back spacing but 4.5 inches sounds about right from memory. If you have Clams you'll be fine up front. Cycle wings Not so much. I'm still trying to figure out how to fit them on my orange car. Tom PS my yellow car has a 1.8L Rover K series, not that Vauxhall boat anchor. :jester:

Edited December 6, 2017 by yellowss7

[Davemk1]

A bias ply tire has a number of advantages compared to the DOT A6 or A7. It is much lighter (nearly 1/2 the weight) and this has obvious advantages but also a few that are less obvious. First is that the unsprung weight goes way down and this helps car control and keeps the tire more firmly on the pavement. The lighter tire spins up and slows down more quickly. The softer and more supple (no DOT steel belts that are found in a radial) tread area conforms much better to the road surface making grip much better......a light car with a stiff tire can see the tire lifted off the surface but a good sized piece of gravel. And a lighter tire has less mass and is easier to get heat into during a short run like a solo or hillclimb. This of course has a flip-side in that the lighter tire will also shed heat more quickly between runs but overall the light bias ply tire will run warmer.

 

Stay away from the cantilever tires (heavy and odd sidewall stiffness) that were meant to allow cars running in classes with wheel width restrictions to run a wide tire. Instead run a wide rim and wide tire and life will be good.

 

dave

[James A]

Cam, I'm with you A7s for autocross because they stick from launch and don't need a lot of heat in them, and R7s for track although on cold days I track the A7s. The only issue you may need to look in to is the camber on the car. Sounds like the car may be set up for bias ply tires with very low camber. For A7/R7s you will want the 1.5 or 2.0 degree camber ears in the rear and set the front close to the same. As an alternative to the R7s for track take a look at the Hoosier radial race tire, we run Hoosier 43549 in S80 compound on the E Production race car. These are somewhat narrow because we are restricted on wheel size and this is the biggest tire we can use on the rims SCCA allows us to run. If you have larger wheels you can find wider tires, the S80 compound serves us well in Road racing.