prodracing.com

Curious as to those that have switched to radial slicks how much change did it take from bias?

Do you feel you have gotten back to the same speed as the bias?

Spring rates?

Did anyone decamber a live axle?

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Joe Harlan Member #175515

This request is a little behind the curve as I have already converted back to bias ply, but since your question is in regard to a live axel rear presumably for a RWD car, I have as much info as anyone. We increased front camber by about 2 points, increased spring rates by 10 to 15 percent, and plugged in an additional 0.5 degree of camber on the rear. It is all about getting heat into the tires so they work. People that complain that the radials didn't work for them usually didn't find a way to get heat into the tires. They thrive on abuse which is why they are a great choice for FWD cars. We did feel that our lap times were comparable to the bias ply with increases in top speed due to reduced rolling resistance and similar grip in corners, but ultimately I believe that the inability to plug in enough rear camber on the radials is the difference on a live axel RWD car. This and other personal considerations are the reasons that we converted back to bias ply.

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#4 HP Midget
HP National Champion

I never got around to changing spring rates, but will echo the remainder of what Ron said with the exception of the abuse thing..

I have ~3deg camber up front and around 1 in the rear (solid axle RX7 on the 245s). In order to get the tires to last more than a single race on my car, I'm running the S80 up front and S100 in the rear. I can still cook the rears before the end of a race if I'm pushing it. Still fiddling with tire pressures, but I've found that by the time I get them high enough in the rear to get the 'crisp' handling, I burn the center section off the tire and still have >1/2 the wear dots on sides while the centers are corded. Dropping pressures to even out the tire wear causes the rear end to wash out and the car is unstable. Have been trying to find the proper compromise between tire wear and staying on the track.. otherwise, these guys are 1-race-wonders on my car.

As compared to bias plys, I tend to like the bias better. this RX7 was 'built by Tom' and has much of his driving influence in it.. you have to drive it hard to get speed out of it, and the radials go off by the end of the race. I didn't seem to have that problem on the bias ply. that's maybe car setup as well, but do know the car felt better on bias. I was much more confident throwing it in a high speed corner and expecting it to stick, whereas the radials I wasn't sure if it was going to turn, go straight, or come around on me.

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Matt Blehm
Houston Region
EP 1st Gen RX7
STU Nissan 240SX

What tire pressures? I have heard that the radials use higher pressures than Bias. Asking because I am planning to run the Bugeye in vintage where you need a treaded tire.

Rod Hahnemann

Rod,
Yes, pressures for the radial slicks are similar to the DOT slicks. *roughly* 30psi hot, depending on car, weight, tire size, etc etc.

However note these are radial slicks with zero tread. If you need treaded tires, you will need to look elsewhere. Hoosier has a 'Vintage' treaded racing tire, but various organizations have various rules on which ones can be used. Many of the local guys run Toyo R888, but check with the organization to ensure which is their local favorite.

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Matt Blehm
Houston Region
EP 1st Gen RX7
STU Nissan 240SX

This is a surprisingly negative set of responses given the very high % of Prod cars now running radials...

I thought that there was a general consensus that the radials are not only faster but that they last longer. Maybe the issue is that Joe asked about live axle cars and the fact that it is hard to get any/much negative camber on a live axle?

I've been feeling like a dinosaur for not having switched to radials on my VW Scirocco...!!

Al Seim
HP VW Scirocco 1.6

It would be interesting to quantify the rolling resistance issue.

In my garage sit my HP Scirocco on bias slicks and a GTI Rabbit street/autocross car on radial (200TW DOT) tires. It's amazing how much easier it is to push the GTI which is probably 400lbs heavier!

Not to mention the locked front diff. Pushing it in a straight line, OK. Turning it? forget it

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Tim Linerud
#95 GTL VW Rabbit GTI
2014 GTL Sunoco Hard Charger
2016 Western Conference GTL Champion

GTI is open diff, Scirocco is a Peloquin ie ATB. I don't like locked diffs on fwd, personal preference. My old GP Datsun had a locked diff, drove OK but hell to push around if turning.

My car turned about as good as my Suburban on bias tires, radials brought it to life.

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99 HP Toyota Yaris
2005-2008 SCCA Solo National Champion
2017 SCCA Runoffs Champion