autoX tire size

As I was changing the calendar on my early 911 Registry calendar today I noticed that the long hood had wider tires on the rear. My question relates to optimal 15" tire sizes for autoX. Do you guys run 225 in the rear & 205 in the front to tame oversteer?


I have 205s all around now on 7 inch rims. Last year I added an adjustable rear sway bar which transformed the cars handling. I used to have low speed push and virtually no power oversteer. The rear bar has given me much better throttle steer control. However, I havn't used this new setup on the cone course yet and wonder how careful I will need to be throttling off coming into corners.

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Originally Posted by joetiii

As I was changing the calendar on my early 911 Registry calendar today I noticed that the long hood had wider tires on the rear. My question relates to optimal 15" tire sizes for autoX. Do you guys run 225 in the rear & 205 in the front to tame oversteer?

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Joe-

I don't think it's necessary on the early cars for autox, as they can be made to handle well with same-size tires all around, and some oversteer on a short, tight course is not a "bug" it's a "feature," as they say in the software biz. It helps get the car rotated quicker and on the gas sooner when exiting corners.

You may also have problems fitting 225 tires on the rear with stock fenders. Even if they clear the fender lip, they may touch on one side or the other in hard corners and bubble/burn the paint on the fender a couple of inches above the lip. Tire widths and profiles vary quite a bit between manufacturers even in then same purported size--stack 4 Hoosier R6s up sometime next to four Toyo RA1s of the same size and you will see the difference in width clearly. A quarter inch in tread width or a squarer profile to the shoulder can make all the difference in rubbing or not. Almost every early car I've seen has different clearances on the left vs. right fenders, too, so even if they fit on one side, they might rub on the other. Just be careful in experimenting. Put some layers of white duct tape on the inside of the fender well above the lip and then build up cornering speed slowly in a session and check for black tire marks on the tape between runs, if you value your paint job at all.

You may also be constrained by rule sets. Here in Zone 8 PCA autox, you get a 2-point penalty for running anything over 205 on an early 911, which can bump you up a class. If you can successfully fit 225s on the rear, I would also go with 225 in the front, as that is usually an easy fitment.

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I have 205s all around now on 7 inch rims. Last year I added an adjustable rear sway bar which transformed the cars handling. I used to have low speed push and virtually no power oversteer. The rear bar has given me much better throttle steer control. However, I havn't used this new setup on the cone course yet and wonder how careful I will need to be throttling off coming into corners.

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I have run the same setup, 205s on 7" rims, on SWB and LWB early cars with great success. Unless you're running a modified engine in your '69, power-on oversteer will not be a big problem. There just isn't enough torque to cause huge tire spin even when stomping the gas instead of feeding it in smoothly. If you have no LSD, a bigger problem will be losing traction on the inside, unloaded tire and not being able to get the power down effectively. This may be less of a problem with a Targa, as chassis flex will help keep the inside tire on the ground. They can make surprisingly good autox cars (witness Terry Zaccone's success with "Marcel", his '68 Targa).

If you can tune your rear bar to give you a nice, controllable, trailing-throttle oversteer for tightening arcs mid-corner, you will be in great shape. As long as you're braking hard in a straight line and trailing off them on turn-in, you shouldn't have a problem with excessive oversteer on corner entry either.

Good luck and have fun--that's the main point.
TT

TT,

My car is lowered a bit and I did have rubbing last year when I was running without the rear bar. I had the rear lips rolled, then added the rear bar. Havn't had issues rubbing 205 50s since.

I picked up some 7 x 15 inch Revolution wheels this winter that have an offset to the inside that is half way tween stock fuchs and 7Rs. They fit with no rubbing in the rears suspension. I think these will fit great with 225s.

I do have a 3.0 Transplant that makes judicial use of the go pedal necessary. I will be heading out to an EVO school this weekend to get more seat time. After I scrub my Azenis down to the cords, I plan to mount some V710s. I'm thinking 225s all around will be the ticket.:D

BTW, because of my displacement bump I ran in Improved NERegion. This year they changed things and lumped all normally aspirated 911s in one categrory. I may get protested so we'll see what happens.

Joe,

I'm running 205/50-15 Hoosiers, which fill out my wheel wells. The Hoosiers seem to run a bit wider than 205/50-15 street tires. I'm running on 6" rims. I don't think that wider tires are going to make much of a difference, unless you get to really wide tires with flared fenders.

We have a lot of competition with early 911s, now. Hope to see you at the next event.

Tom

On mine, I use Hoosier 205.50's all the way around, on 6" Fuchs. They really do the job.

Vic

Tom,

I decided to sign up for the EVO school running this weekend so last weeks NER event did not fit the schedule.

I've had some conversation with Ron after they changed the classifications. Are you running in normally aspirated 911s?

Anybody protest a 3.o in a long hood?

No - but I should probably be in Class 9.

I wouldn't use any wheel other than a 15x6 fuchs as everything else is heavier and takes longer to spin up/spin down. Fast response is key to AX.

Also, most courses on the East Coast are run in second gear- first is only used for a launch. If the rules allow, go for a smaller tire diameter to lower the effective gear ratio and get the revs higher up on the tach during the run.

ok JC, point taken. I weighed these Revolution wheels and they are 16 lbs each which seems to be 4 lbs more than fuchs and 3 lbs more than cookie cutters. That is 12 lbs total of unsprung weght. Fairly significant I think. I also see that hoosier has a 225 45 size making the overall height, 23.9, the same as a 205/50. I'm thinking this might be the ticket!

I've been autoxing using 205/50/15 Hoosiers on 7x15 cookies for years on my 72. Like you... I'd really like to try 225's on the rear but I'm afraid of the rubbing that MIGHT happen. I certainly don't want to tear up a $225 Hoosier tire testing this. With almost 3* neg camber it should'nt but why take the chance. BTW my wheel combo weighs in at 33.5lbs each.

Kevin

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Originally Posted by joetiii

ok JC, point taken. I weighed these Revolution wheels and they are 16 lbs each which seems to be 4 lbs more than fuchs and 3 lbs more than cookie cutters. That is 12 lbs total of unsprung weght. Fairly significant I think. I also see that hoosier has a 225 45 size making the overall height, 23.9, the same as a 205/50. I'm thinking this might be the ticket!

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Yes, that will be the ticket!

Over the past ten years I've used pretty much every wheel possible for autocrossing with a narrow bodied 911, and the 7x15 Revolution wheel is a good, inexpensive solution for using 225s. No need for spacers or longer studs like a 7R wheel. Yes, they are a bit heavier than 6x15 Fuchs - but the wider rim will more than make up for it. Have fun!