Radial Race Tires

[jbharrison]

I want to try radial race tires (DOT) before I start altering the suspension geometry for bias-ply slicks.

Here's why: I had the Avon CR500 (175/55-13 and 205/55-13) on the car and they worked really well on road courses. Looking for more traction I found a set of Avon ACB0s (6.0X21-13 and 7.0X2-13) and tried those even though they are bias-ply and my camber was set at 1.2 and 1.7 degrees front & rear respectively. They are horrible - the car slides through turns and I have to be careful with throttle so as to not break the rear end loose.

If I look at section widths as a measure of contact area, the CR500s are 7" and 8" front and rear. The ACB0s are 6" and 7.2". It is interesting that both tires are the same width sidewall to sidewall. The ACB0s bulge out a lot more than the CR500s.

So the fact that the ACBs have less contact area explains part of the problem. I don't know what impact the camber has - I do know that bias ply slicks should run close to zero so that may be part of the problem too. Ultimately, I may change the DeDion ears and front camber to run bias ply, but I thoght I would give radials a shot.

First question: does any one have experience with radial tires for road racing?

Second question: I researched three major radial tires suppliers (the picture shows manuf. data). Does anyone have thoughts on the right tire combinations for a Caterham Superlight? I think there is a limit to front tire width before it begins to rub the front wing support rod.



Sorry for the long post and if it is redundant to what you have seen over at USA7s

[moosetestbestanden]

I don't know much about ACB0s; never used em. The ACB10s I've used are excellent, once up to temperature that is (they need heat) and properly inflated (about 16 - 17psi). They are much better than the CR500s in my experience, at least for the track. They're pretty harsh on the street.

Istr a thread on BC where one guy won on of the CC national championships using the wrong camber for ACB10s.

[jbharrison]

I have a set of ACB10s coming along with a set of aluminum wheels and the Dedion ears for reduced rear camber.

They wont be here until late December, so I think I am going to try the Hoosier 185/60 and 225/50 front and rear. My only concern is the possibility of rubbing.

[Michael Murphy]

I've been down that road. I started with ACB10s - magic tires, highly recommended, probably the best track tire in terms of bang for your buck. I then tried a set of ACB0's, which felt stickier but did not deliver faster lap times. I got three weekends out of them. I am now running Avon ACR0s (the radial equivalent). Here are the sizes:
F: 180/550R13 (Tire #11530)
R: 250/570R13 (Tire #11531)
I had to bend the front wingstays and fit quarter inch spacers to avoid rubbing on the front. I have quarter inch spacers on the rear, with standard radius rods, and there is slight rubbing on the radius rods but not enough to worry about. I believe the watts linkage reduces clearances even more so you need to pay close attention to offsets and the like.

Your camber settings were probably wrong for the bias ply. As for radials, I am now running over 3 deg. negative on the front and the correct radial dedion ears on the back (I think it's 1.25 deg neg but someone will correct me on that). You do not want to mix radial and bias ply camber settings. Take a look at my video of some race laps at Thunderhill in the video section of this website. That was my first outing on the radials and, because the tires were fitted in a rush only minutes before the race, the camber settings were for the old bias ply tires. Understeer was ghastly and the car was pretty slow.

To date, my personal best lap time at Thunderhill was last year on a very hot day on ACB10s. Two seconds a lap faster than my best time since then on slicks. That's probably due to tire temps, as it is very hard to get the tires up to operating temp on a Seven unless it is very hot (or you have a very hot shoe). The ACB10s lasted almost a whole season, vs. two or three weekends for slicks, so I would stay with ACB10s if money is a factor and lap times are not critical.

[Michael Murphy]

I should add that with 3 degrees negative on the front, and zero toe, the radials feel like magic.

[Doug Liedblad]

I have a Watts link setup on my car and identical wheels to Michael's (Kodiak).

No rubbing issues on the front but with the ACB10s the rear tires rub on the front Watts link during cornering. This required a 1/8" rear spacer to correct. A different offset would have removed the need for the spacer.

I also seem to recall reading that shortly after my car was made (late 2001) they made the DeDion tube a bit longer. This could also help any contact issues with the linkage.

Doug