THSCC Forums

OK, I am assuming that when your tire has cord showing the size of the contact patch and 360degrees around the outside it means that it is time to replace them The question for me now is: What size? I have a 90 miata in Estock so I need to keep the same 14" wheels. My last set were 195-50s or one size up from the stock 185-60s I have two questions

1. What is the largest size that will fit without clearance problems?
and
2. Is there any benefit is going larger for Autocross?

Just mulling over my options.

Thanks

Rob
90 Red Miata ES13 (Who will be running on Azenis for the next event)

rob, I'm guessing here but from what I've seen, as long as the tires are streetable, DOT approved tires that you can run them. I don't think the size matters. (at least here. before one of you says something ) Go with Azenies to keep up with the cheater tire crowd though. cheap wally-world tires don't last long at an auto-x. mine are dead after 6 months of driving and 1 event.

_________________
beware of EV powered Ford EXP's they are quiet and come for you when you least expect it... hide behind an aluminum light pole...... ya that will help

Well, you can easily run 205's without any problem. I've heard people say they ran up to 225's without a problem, at least as far as fender clearance goes. However, increasing the width of the tires does not increase the size of the contact patch, it just changes the shape, and really wide tires give you extra unsprung weight, and sloppy sidewalls to deal with.

You can use the miata.net tire size calculator to play with different tire sizes: http://www.miata.net/garage/tirecalc.html

You can play it safe and stick with OEM tire diameters, or go down in diameter and capture a little more torque, at the expense of lower mph shift points, if you desire.

Of course, there's also the issue of whether the tire you want to use is available in the size you've decided on - ain't life grand?

_________________
Marvin McBride
Spec Miata #62

Rob,

Go with the 205/50/14's in either a Victoracer or if they have them, the V710's. Since you can rotate them, I think either set will get you through a season. I know the Victoracers will. No one has been running the V710's long enough.

If you are taking an EVO school, use your street tires

_________________
Jim Pastorius
2008 Silverado VortecMax
1992 Camaro CMC#92
2002 BMW R1150R

2009 3rd Place CMC Mid-Atlantic Championship
2009 CMC Hyperfest Winner

205/55-14 and 225/50-14 are the two "bigger" 14" sizes. I'm not sure how those effect the gearing on a Miata.

Scott

Really? Am I the only one that didn't know this?

_________________
Vincent Keene
'06 Ford Mustang GT (track rat)
'15 Dodge Charger R/T (yeah, it's got a HEMI!)
'07 Ford Fusion SE (205,000 miles and counting)
'98 Chevy Z-24 (retired)
'93 Acura Integra (Team SWB 24HOL Car)

I'm sure you aren't the only one! However, I do agree with Marvin.

The key is the *shape* of the contact patch. A wider tire will be narrower front to rear, but wider side to side (helping lateral grip, probably not helping acceleration or braking that much).

If a different size tire allows you to run a lower pressure, then it is possible you can increase the area of the contact patch, however, it will have a lower loading per square inch (generating less grip per square inch).

All of these variables are why the top autocrossers will go through several combinations in a test and tune. I think Rob would be served well by going to Miata.net and seeking out the advice of a person like Andy Hollis. No need to rediscover the wheel here.

Scott

The size of the contact patch *is* pretty much determined by the weight of the vehicle. The *shape* of the contact patch does change and gets wider as the tire gets wider. This should give you more lateral g pulling capability.
I'm sure the CSP guys don't run 235 series tires on 13x8 rims for nothing ....

To Rob's question - Stock limits you to factory rim width, say 6" for the 15" wheels on a Miata . The rules allow you to go to wider than factory tires but I've read at the Tirerack site that your tire width should be no wider than 80% of your rim width for Autocross. You need to do a metric/English conversion, but 6" is about 150 mm, hence your tire should be no wider than about a 190 with that rim width. So while the tire size is not restricted the rim requirement effectively does that. Not to mention the additional unsprung rotational weight of bigger tires ...

Frank

I used to run the old school BFG R1 (not G-Force) on my 14x6 rim. They only had clearance problems at full lock, and it wasn't a big clearance problem. Didn't have the sloppy sidewall issue... but they had some stiff-a*s sidewalls.

When I ran Kuhmos, I ran the 205s without issue.

For the track, I run the 195 Toyo RA-1. They don't make a 205 for the 14" wheel. For the 15" wheels I need in SM (SM = Spec Miata, not Street Mod)this year, I'm having the 205s mounted up (today in fact).

My suggestion for your money... 205s, if you can get them in the tire type you want.

YMMV...

_________________
Diane (Hall) Bundas
1992 Spec Miata #48 - 1997 Chevy Tahoe - 2007 Honda Civic Coupe

Dude, that's just wrong.

_________________
Has no responsibility whatsoever.

My comment about the "sloppy side walls" was presuming the stock wheel width. Obviously, if you go with wider wheels, that takes care of that issue.

I never had the luxury of having several sets of tires in different sizes mounted on stock size wheels at a test-n-tune to try to optimize - and I suspect different courses might have different optimums - long flowing sweepers could make good use of the wider contact patch, and the weight penalty wouldn't matter as much, whereas tight courses with lots of heavy braking would do better with lighter tire/wheel combos, and make better use of the narrower contact patch. (Wider contact patch - better lateral g-holding ... narrower contact patch - better fore/aft g-holding.)

That being said, I agree with Jim and Diane - go with the 205's - if you're not at optimum, at least you'll have plenty of company among experienced Miata autocrossers.

_________________
Marvin McBride
Spec Miata #62

While the theory that you'll find on sites like the tire rack is nice and all I'd highly reccomend finding out what the top Stock class people are doing and just copy that. Things can be way different in practice from what appears correct. For a glaring expample of this just look at Carpenter's CRX. He won nats with 225 tires on 13 x 5 (or 5.5) rims. By the "standard" formula he should have run 175's or something like that.

From memory I think the big dogs are running 205's like everyone above me is saying.

--Kevin H.

_________________
2003 WRX (again!)

Rob:
Your car has 14 X 5.5" wheels.
Max tire size you should use is 205. Only 'R' compound tire from Kumho is the V700 in 205-55-14.

They also make the 195-55-14, which might be the best choice, since the rolling radius is a little (not much) shorter, and the 1.6L is, of course, a bit low on torque. The 195 will also be lighter. Contact area difference? Minimal.

My $.02 worth.

_________________
Brad

Well,

Interestingly enough, I find myself in a similar dilema. I spent a good amount of time searching Miata.net and could find no such info.

Rob, what size, tire model did you choose? Now that you have completed a season on them, how did you like your solution? Are you going to stick with the same formula, or go up or down in width?

I will be buying tires for 14x6 rims.

_________________
“I feel safer on a racetrack than I do on Houston's freeways.” - A.J. Foyt

Kevin Butler
Mobetta Autosport Spec E30 #612
2003 C5 Z06 Corvette
AFR Miata, SM2 61, '93 w/200 SC'd RWHP - soon to be resurrected
Waaaay too many other projects....

I have the 205 Victoracers, and yes I think they were the right decision. I am going to stick with them with the exception that I have 3 195's with some tread left so I may put them on the front and play with the suspension for fun to see if I am smart enough to adjust for the understeer

I'd go with the 205's