I am considering purchasing a set of wheels and tires for autocross for my car which is a 2006 C4S. I am currently running PS2's in 19 235/35 front and 305/30 in the rear (stock). I have been told that I should consider purchasing a set of 18 inch wheels because of tire selection and cost. I realize I've got a bunch of noob questions but I'd really appreciate any help I can get!

I've read a lot of the threads here about tire selection and it seems like V710's tend to get a lot of favor for performance, wear and cost. It seems like Hoosiers are a close second?

Could you all help me with some specific questions?

1. What is your opinion about moving to 18's? (I did read a thread here which discussed the resulting changes in gearing which is a concern). Would I be looking for 235/40 and 295/35 18's if I went that way (the stock size for a C4) or should I look for a similar width of tire in 18's as in my street tires? I assume that a narrower tire would significantly change the handling.

2. Would the V710's or other tires recommended for autocross also be suitable for an occasional track outing like a DE or high speed touring event or would it be better to stick with stock tires?

3. What is a realistic expectation for tire life for tires like the V710's?

4. I've read some other threads that discuss alignment for autocross and finding a balance between street and track in terms of alignment. Where can I find out what a good compromise would be?

5. If moving to 18's makes sense should I be looking for someone's cast off lobsters or do I need a purpose built set of wheels?

6. What did I forget to ask, or am too ignorant to ask, that is important to know about this?

THANKS for any help you can give me!

Tony

 

http://www.kumhousa.com/tires/passenger/competition.do

The link will take you to the Kuhmo website and you can compare the 700 and the 710. 18" 700s are not available. Available sizes are also on the link. Click on "printable spec sheets".

I'm not a tire expert but I do AX and use the Kuhmo V-700. I chose the 700 over the 710 because of longer tread life (FWIW) and the ease of driving the car to the event and not having to change out tires on-site.

You might want to get in touch with Jason (a Rennlist member) @ Tire Rack (a Rennlist sponsor).

 

NC: I drive to and from autox and track events on Michelin Cups. I used the Victoracers until I went to 18" rims. Do you know how the Ecsta V700As compare to either tire? The Ecsta V700s used to have to be shaved to 4/32 for any dry use, or the tire would show localized excessive wear. Have the V700As overcome that problem? For autox, the Cups don't get as warm and sticky as the Victoracers did.

 

It was suggested I get my 700s heat cycled and shaved prior to delivery. I opted to heat cycle and wear the tread down myself. The tires were mounted 6 months ago and have well over 1000 miles on them.

My 944 is not a DD. Typically when I drive the car it is a spirited drive on some local twisties, AXing, or just out to the store every two weeks to keep them from flat spotting.

My 700s are not driven in wet conditions. There is a PCA meeting tonight and I'll be driving the beater pick-up because of the inch of rain expected (we desperately need the rain).

Kuhmo is releasing a W-710 this spring. The W stands for "wet".

 

Answers to some of your questions:

Without knowing the specifics of your particular car, I can offer some general tips.

Firstly, gearing changes could be advantageous. A lot of Porsches are geared too high for typical SCCA courses, which top out around 60 mph -- for example, second gear in my 986S is good for 70 mph, which means I fall off cam when rounding slower corners in second. A lot of people actually deliberately change tire diameters to get second gear to top out in the 60 - 65 mph range, but this depends on how fast your local courses are.

A very rough rule of thumb is that wider tires add more stick to that end of the car. For this reason, almost all serious autocrossers run wider tires than their cars came with from the factory. If your car understeers or oversteers, you may want to consider running different width tires to compensate -- for example, it's popular to run equally sized tires on all four corners of a Boxster even though the factory rear tires are something like 30 mm wider than the fronts.

V710s do work well on the track, although if they get too hot they can get a bit slippery. I don't have any firsthand experience with Hoosier A6s at the track; some people recommend against using them on heavy cars for fear of overheating them, but this is a bit strange to me since they're more tolerant of heat in autocross applications than V710s are.

V710s last at least 70+ autocross runs before wearing out; lighter cars (e.g. not yours ) can get 100+ runs. They tend to get significantly slower after 30 or 40 runs, though; how much this matters to you depends on how serious you are. I'm not sure how the above translates into track days.

 

The Ecsta V700s are around as fast as the Victoracers for autocross, but are much more fragile, so I'd recommend the Victoracers. Both tires are faster than the Pilot Sport Cups for autocross. All three tires are significantly slower than the V710s (or A6s) for autocross.

 

Yeah, if you're going to buy another set of rims definitely, go for the best set of tires...Hoosier A6's or V710's.
Check out the rules first before you switch the rim sizes...
Alot of folks do it, but if you're in a stock-type class, may not be allowed or with some kind-of width restriction.
Personally, I prefer the Hoosiers, but while they just might last a season, the performance falls off after half dozen events or so...Kumhos seem much more consistant and last longer. Don't believe you'd want to keep a tire much longer anyway.
Different tires like different alignments...but generally something like -2.5 camber with 1/16-1/8 toe out with stock castor for the front and a little less camber like -1.5 and maybe 0 toe for the rear.
Lots of opinions and write-ups on setting up cars and most places that do our cars have found stuff that works they can suggest or start you out at.
Good luck.

 

Tony, it sounds like you've got the budget to buy some wheels and R-compound tires, but I don't think you should do it if you are in fact a noob. As a noob, I can't see what R-comps could possibly do to help bring you along as a good driver. I don't think people should use R tires until they've got a year's worth of experience behind them (say a dozen events of one sort or another). Street tires will teach you more about car control and your car will be capable of incredible performance even on street tires.

This is coming from someone with a yellow 997S who does have 18's with R-comps - but I've been at this hobby for 8 years. As an instructor for more than one club, I strongly urge you to put the wheel budget to better use in buying you more track time and a good helmet. I am sure you will have a great time on street tires. At most just pick up a set of take-off wheels from eBay or the for-sale items on this forum.

If you have a lot of events under your belt, then you're not a noob and this wheel/tire conundrum is a worthy subject for your consideration! And I offer these comments:

Consider that most wheels out there will be made for the 2WD Carrera offset, which is different in the rear. 4S wheels have a 51mm offset, which is a 16mm wider track than the 2S offset of 67. A wheels made for the 2S will look a little funny on your car in the rear, probably.

The Kumho Victoracer is a non-option because it isn't made in 18" or 19" sizes.

There is no one right tire or wheel. What do you want to do? You really need to tell us more about your interests and your budget. Do you have a set of class rules you need to abide by? Can you afford the most expensive lightweight 19" rims made by Champion Motorsports? Are you budget-sensitive? Can you trailer a set of wheels and tires to your event, or do these tires need to get you to and from distant tracks? Do you need a tire that's safe to run in the rain, or can you buy multiple sets or wheels (or commit to staying off the track if it rains)? Do you want something that's a little grippier and more satisfying than the OEM tire, or do you want to get the fastest tire possible? Are you trying to win or just catch up with drivers that were a little beyond your grasp before? We need more information to intelligently advise you. I can tell you that the Hoosier A6 is the best tool for a low lap time, but I wouldn't recommend that tire to many people, not many at all - with its high performance come many compromises.

18" wheels will be lighter and provide lower gearing if you stick with 35/30 series tires f/r. Also the tires will tend to be a little cheaper.

 

Dr.:

Thanks for the reply, and for the questions!

I'm not a complete noob when it comes to driving autocross, just close. I've done about 10-11 events and had some instruction on the autocross track at PDE. I'm planning to attend an autocross driving school later this spring.

I plan to continue to run in my local PCA club races every month and would like to try Solo. In Solo I think my car would be in the super stock class, but I assume that I need to keep wheels on the car in a size the manufacturer spec'd for the car? I'll also run some autocross this year with the local Corvette club. I also plan to attend a couple of DE events this year and perhaps what a local track calls High Speed Touring but my main interest is autocross at this point.

Our club doesn't typically autocross when it's wet so I'm primarily interested in a dry performance tire. I live 7 miles from the track so I can change tires and drive there if the weather is good. I have a trailer which I could haul my car on with extra wheels to the track if I need or want to. I've got a good helmet.

I'm interested in purchasing a separate set of wheels and tires to 1) avoid tearing up my wheels with the street tires on them 2) I'm not sure it saves any money in the long run driving a lot of events on street tires when you consider the wear but I really haven't figured that out, it just seems to make sense to have the right tool for the purpose 3) go as quickly as I can.

I'm not turning in the fastest times in our club but I'm within a second or two generally. I think that practice is probably the most important thing I can do to bring the times down, but the extra grip wouldn't hurt.

I don't have a budget, or a limit necessarily, on what I can spend. That's part of what I'm trying to figure out. I expected when I wrote my first post that I was probably looking at 3-5,000 for wheels and tires. I don't want or need the very best wheels, it would be nice to find some with the correct offset. It does seem that the choice of tires is a lot greater in 18 in. which is why I was thinking I probably ought to do that.

Hopefully, this gives you a little better idea of where I am. Any additional advice would be greatly appreciated!

Thanks.

Tony

 

Tony: Yes, you'll need to run OE size wheels to remain in your SCCA stock class (Super Stock). Were 18" wheels standard on the '06 C4S, and your 19s an option? If so, you can switch to the standard 18s. Actually, you could run stock with lighter (but more expensive) forged aftermarket wheels, but they'd have to be no wider than the stock wheels. In SCCA (but not in PCA) you can also run wider than OE size tires in stock, but they'd have to work on the stock wheel widths. If you decide to move to 18" wheels for autocross, one can usually find barely used 18" take-offs reasonably priced. Check the wheels classifieds in Panorama, and/or contact Wheel Enhancement. Be sure to get the right offsets for your C4S.

The 18" tire sizes you mentioned are okay; they keep the front to rear diameter and diameter stagger about the same as the OE 19" tires.

Yes, the extra grip of R-compounds won't hurt. If you're already within 2 seconds of where you want to be on street tires, you should be within 1 second on R-comps. If my group didn't run in the rain, and I only had to drive 7 miles to the event, I think I could convince myself to drive to and from on V710s.

 

I checked the order guide for MY 06 on Renntech and 18" wheels were not in the order guide for the C4S in 2006. 18" wheels were standard equipment on the C4 in 2006 with 19 inch wheels an option.

Does that mean that in Solo I would not be able to run in super stock class if I put 18 inch wheels on my C4S? I checked the Solo classifications on the SCCA site and it just lists the 997 chassis for 2005-2007, which it lists as super stock. So can I assume that since 18 inch wheels were standard on some of the 997 variants for my production year that I can use 18's and still be in that class?

It sure looks like there aren't many choices for tires in 19 inch so that would be a bummer...

 

From the 2008 SCCA Solo Rules (page 64), under 13.4 WHEELS. "Any type wheel may be used provided it complies with the following: it is the same width and diameter as standard, ......"

You'd think they wouldn't mind people going smaller in size, but I interpret the above to mean that you can't legally run your 4CS with 18" wheels in Super Stock. Sorry about that, but you could contact the chief tech inspector in the SCCA region you plan to run with to be sure.

Probably 99% of the people at a SCCA autocross (including the tech inspector) wouldn't know what size wheels came on your car, but running 18s might attract another 4CS owner who looks at your wheels and says "Haaaaay, wait a minute ..." On the other hand, you might run on 18s for years without anyone noticing.

 

Jim, you are correct about the SCCA rules, you cannot go down in diameter. It is a gearing and ride height advantage, in addition to lowering unsprung weight and rotational inertia. Generally speaking, if SCCA rules don't say you CAN DO something, then you CAN'T.

If there's one thing SCCA people know about, it's wheels and tires. A plethora of non-Porsche owners would notice the out-of-place nature of 18's on a stock class 997S or 4S.

 

Again, thanks for the help!

So, that leaves me with the option of pursuing 18 inch wheels and tires to run in my local PCA autcross and occasional foray onto the track in DE events. Or trying to find something in 19 inch that I could run in Solo also, I am sure at much greater expense!

 

19's are fractionally more expensive. Right now you have the choice of Hoosier A6, Hoosier R6, Pirelli P Zero Corsa, Michelin Pilot Sport Cup, and Toyo R888. If you want to drive to events, you should get something other than a Hoosier. The Toyo would be a good compromise and bit less expensive than the Michelins or Pirellis.