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Tires and sizes for Autocross with 90 GTU

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Tires and sizes for Autocross with 90 GTU

Old 05-25-05, 10:32 AM
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Tires and sizes for Autocross with 90 GTU

Hey guys,

I'm getting into SCCA autocrossing. I did it 4 or 5 times last year with a different car but this year I've taken out my 90 GTU N/A to 2 events. I think I want to stay in E Stock class because I don't have the money, time or facilities to do extreme or expensive modifications.

The first event I went to, I got some bad oversteer, the back-end wanted to come around bad. I looped it and actually spunout once, did a 360. "Hang on, Gomer!!!" This was with a set of street tires from my other car. I swapped another set of street tires for the other event last weekend and cranked up the pressure all around (44 rear, 34 front) and it seemed to do better. At least I didn't loop it and the rearend seemed to be more stable... or maybe I wasn't driving it as hard because I was "gun-shy" about the oversteer. Anyway, I didn't do so hot with the times.

I know R-compound tires make a BIG difference. I'd like your advice and input from your experiences and knowledge, please.

I think I can pick up a set (4) of used Hoosier R3S04 tires for about $200. I may be able to get either 205x50x15 or 225x45x15. According to SCCA rules in Stock classes, I need to run stock rims. I believe my stock rims are 6", is that correct? If so, then are the 225x45 tires going to be too "pinched"? These being wider might be better but if they don't fit my rims well, I may be better with the 205x50. If I can get either size, what do you think?

Of course I'd rather have new tires but not really in my budget. If I had the money, I think I'd go with Kumho V710 or Avon Tech R instead of Hoosiers, but I really don't want to spend $600 - $700 for new. As I said, the used Hoosiers are R3S04, not the A3S03 or 4. I'm sure the A's would be better than the R's, since they're autocrossing tires instead of roadracing tires and supposedly come up to heat faster. But I'm limited on my selection of used and don't really have a choice so I this may be what I have to go with... or am I wasting my money?

Ok, guys... jump in with your comments, suggestions, etc. on this, please. I appreciate any input you can offer. I'd also appreciate any other suggestions you may have as far as other things I can do to the car and still remain in E Stock class. Also, is my 90 GTU actually competitive in this class? And, yes, my driving skill and talent needs to be "refined", I just want to be as competitive as I can be and kick some ricer butt!!!

Thanks!!!
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Old 05-25-05, 12:28 PM
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I'm new to autocross and I started a thread similar to this the other day. I got a lot of good advice if you want to check it out. The biggest thing is to get more seat time. Try to remember that the shortest race line around the course and the fastest race line around the course are two different lines.

https://www.rx7club.com/race-car-tech-103/tire-pressure-autox-tried-searching-427871/
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Old 05-25-05, 01:07 PM
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Get a good race oriented alignment done, thats the most important. I don't know what settings people are using for FCs since I haven't autocrossed one but by changing from the factory alignment to -1.2 camber front and -1.8 camber rear and 1/16 toe IN front and rear I cut over 3 seconds off my time with everything else the same. I was having bad oversteer problems before this. The toe in on the rear will help your oversteer and this should wear pretty good on the street. Only thing I plan on doing different is increasing the camber on the front and going a tad more toe in the back but I am going to replace my shocks, springs, larger matched sway bars, toe links and sway bar brackets first and get a tad more agressive alignment but keep the toe the same and see how it does.
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Old 05-25-05, 01:10 PM
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Icemastr, do you have the rear steer eliminator bushings installed on your car?
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Old 05-25-05, 01:28 PM
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The only cars I race are my 99 Miata and a 93 RX-7. I have not tried racing my 88 RX-7 so no I do not have rear steer eliminator bushings, however I have heard they are recommended as they can cause problems with unpredictable cornering.
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Old 05-25-05, 02:50 PM
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Icemastr is right. Get an alignment. Unfortunatly you will not be able to achieve the numbers he told you to dial in. Especially with a stock car in a stock class.

Get as much camber and caster as you can get.

Mods you can do in a Stock class.

Cat back - every little bit helps
K&N Air Cleaner - again every little bit
Front Swaybar - I would go with the racing beat one.
Rims / Tires - Look for a set of used Vert rims. Then go find some Spec Miata takeoffs. You can usually score a set of 4 Toyo R-compounds for 100-200 bucks.

Seat time is important like Project84 said. Look into the Evolution Driving School.

When I was first starting out a guy in a 90GTU would clean up in his stock class and beat me in my modded TII. He is a VERY good driver. A lot of AutoX is about the driver. A well setup car with good tires helps but is wasted if the driver cannot tap the potential of the car.

Sort of like a 1st time golfer using a set of Pings.
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Old 05-26-05, 03:09 AM
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Good advice everyone, thank you for responding.

Icemastr, I thought the alignment (camber, etc) was limited or non-existant without going to something like Mazdatrix has as far as replacements - and I can't use them because they're not "stock" OEM things. I also can't use the rear-steer eliminators. I can't even use polyurethane bushings.

Here's quotes from some of the rules:

"Suspension bushings, including but not limited to those which carry
the weight of the vehicle and determine ride height, may not be
replaced with bushings of a different material or dimension."

"Option package conversions may be performed between specific
vehicles of a particular make and model, but only between
configurations from within a particular model year. Such conversions
must be totally complete and the resultant car must meet all
requirements of this Section. Alternate parts listed in a factory parts
manual are not authorized unless their use is specifically referenced in
the factory service manual or in a service bulletin for the specific
model."

"Any type wheel may be used provided it complies with the following:
it is the same width and diameter as standard, and as installed
(including wheel spacers if applicable) it does not have an offset more
than +/- 0.25 inch from a standard wheel for the car. The resultant
change in track dimensions is allowed."

"Rear anti-roll (sway) bars: May not be removed, replaced, or modified in any way."

"Suspension bushings, including but not limited to those which carry
the weight of the vehicle and determine ride height, may not be
replaced with bushings of a different material or dimension."

Soooo... Jims5543, I can't use vert wheels unless I hack the top off of my couple with a Sawzall and make it a vert because that would fall into the "option package conversion" and it would have to be a complete conversion. Nahhhhh
The rims on the vert are apparently 7" wide and my stock GTU ones are 6", so I'm stuck with them, according to the rules again.

I also can't use a K&N Cone Filter (or any other cone filter, it had to be the stock airbox), although I could use a K&N filter that would fit in the stock airbox.

Project84 is right, I agree. Certainly good advice and a good link. I need seat time and "feel the car" and what it's trying to tell me. Difficult to do sometimes.

Hey, Jim, where might I find a set of those Toyo's from a Spec Miata that you mentioned? I check on Ebay and couldn't find anything. Are they the RA-1? I'd probably pop $100-$200 for a set of those, like you said, if I can find some.

Also, what about putting those used Hoosier 225 x 45 x 15 on my 6" stock wheels? Is that going to "pinch" them too much? Or should I go for the 205 x 50? I assume the 225's would give me an inch or so of rubber on the ground over the 205's.

Good responses, guys, thank you! It sure is limited on what you can do in "stock" class --- gee, I guess they really DO want STOCK! CRAP!!! Go figure!

Keep those opinions coming everyone, I'll read them all and attempt to learn.
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Old 05-26-05, 07:38 AM
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225 on a 6" rim will work.
A set of adjustable shocks (AGX or Illuminas) will help a lot too.
Good luck
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Old 05-26-05, 09:38 AM
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The only modifications you can do are front sway bar, adjustable shocks, R-Compound tires on stock size rim. You can use lighter rims as long as they are the same size as stock. I just mentioned my alignment on my Miata because thats what is working well for me, I said check what FC guys are doing for alignment as it will make a huge difference in your driving. As a beginner I recommend you stay in stock class, it is cheaper and you should get a year of racing in on street tires without having to worry about adjusting suspension and swapping tires every race so you can learn from your mistakes on a car that has less of a threshold for error. If you have crappy street tires, I recommend you upgrade your street tires to something that will fit well on your stock wheels and has great wet and dry capabilities like a Toyo T1-S or something and get a good alignment done(like I said there is a difference between what an alignment shop thinks is a good alignment and a good race alignment, check what other FC autocrossers are doing). Then if your car is oversteering go to a larger front sway bar, understeering go to a smaller or no front sway bar. This will let you drive a good handling car without spending a whole lot of money or doing complicated things and you will learn about what modifications to the suspension will do what. Then if you get real serious go to r-compounds and adjustable shocks or even prepare your car for a prepared of modified class.
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Old 05-26-05, 10:06 AM
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Originally Posted by beacon2
Good advice everyone, thank you for responding.

Soooo... Jims5543, I can't use vert wheels unless I hack the top off of my couple with a Sawzall and make it a vert because that would fall into the "option package conversion" and it would have to be a complete conversion. Nahhhhh
The rims on the vert are apparently 7" wide and my stock GTU ones are 6", so I'm stuck with them, according to the rules again.
You can use the vert rims, there is no designation between N/A models of RX-7's only between turbo and non-turbo. The Vert would be in the same class as a GTU and GXL. The huge plus is that the vert rims are very light.

I also can't use a K&N Cone Filter (or any other cone filter, it had to be the stock airbox), although I could use a K&N filter that would fit in the stock airbox.
I was suggesting a K&N Drop in filter.

Hey, Jim, where might I find a set of those Toyo's from a Spec Miata that you mentioned? I check on Ebay and couldn't find anything. Are they the RA-1? I'd probably pop $100-$200 for a set of those, like you said, if I can find some.
Check around town with some speed shops. Find guys that race spec miatas. Go to the local track when an scca race is there and meet some people. Once you get to know some people you'll find them, pollsibly for free.

The Stock classes are drivers classes. When you get into modified classes you start throwing in a lot of who has the best set up car. In the stock classes its more of who has the better car for that class and who is a good driver.

Also I am not sure you can put adjustable shocks in a Stock Class car. I would look that up.
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Old 05-26-05, 02:30 PM
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Adjustable shocks are definitely legal in stock.
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Old 05-26-05, 03:02 PM
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Great ideas, guys! Thank you for the clarifications.

The rules for shocks are very restictive also. "No more than two separate external shock damping adjustment controls. Gas pressure adjustment is not considered a damping adjustment."

I trust a local alignment shop here, so I'll definately get an alignment to do what can be done within the adjustments and restrictions.

As far as rims, good point, Jim. Hmmmmm... there are 6", 6 1/2" and 7" rims for RX-7's, all put in the same class (as long as its N/A). So they shouldn't bump me to a different class. GTU, GXL, vert, it doesn't matter, they're all in the same E Stock class as long as they're N/A and not turbo. I should be able to run any rims (vert or not) as long as they're no wider than 7". So I don't have to hack the top off my coupe to run vert rims afterall! COOL!

The tires on my car are good enough to get me back and forth to events. My thinking is, I'd rather not buy a set of tires that I can use on the street and autocross, I'd rather pick up a set of decent used R-compounds like the Hoosiers that I was talking about for the same or less money than a "dual-purpose" tire. I already have the original 6" rims and also a set of 7" rims (not vert though) that I can mount a set of R-compounds on. I'm not opposed to changing them once I get the the event. Just seems like "more bang for the buck". That's why I was wondering about the 6" and 7" rims for the 225 or 205 tires. Since apparently I can run 7" (if I'm ready the comments correctly), then maybe I should go ahead and go for the 225's, since there's almost an inch more of rubber on the ground over the 205's.

I'm in a relatively small town and there's not even an SCCA chapter here. The closest is 80 miles away in Indianapolis. The speed shops around here are mostly concerned with drag racing and don't know squat about much other than typical ricer stuff. That's why I need YOU guy's help and advice, and I VERY much appreciate it!!!
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Old 05-27-05, 12:06 AM
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You can have double adjustable shocks in stock but I am pretty sure you can't run coil over perch adjusters. Most adjustable shocks are only single adjustment, even very expensive shocks like the Penske shocks ($4000) are only double adjustable and can be run in stock class. I recommend going with good street tires for a year before going with race tires because running street tires will give you a chance to learn what it feels like to push a car at the limit easier and you will notice your mistakes easier. Racing tires hide novice mistakes. Plus your FC might qualify for a street tire class anyways which would be better to compete in than E-stock anyways (IMO).
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Old 06-30-05, 12:44 PM
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just FYI man, i'm pretty sure that the wheels have to be the same size as what came on that exact model of car. so if u have a GXL with 15X6 stock wheels, u cant upgrade to the TII 16X7's. unless the GXL was offered by the dealer with 16X7s.

of course, somebody has to challenge u on that before it makes a difference.
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