13x6 vs 14x6

[lateapex911]

Originally posted by hornerdon:
there is a point in which that contact patch is "good enough" to handle to cornering ability of the car, and any more is at best a waste, and at worst a detriment.

Don, good observation...the cars were early eighties RX-7 GSLs , the drivers were Paul Kozlak (sp?) and myself, and the tire was the legend in it's own time Yokohama A001R.

OK, so lets recap....

If your rules limit your rim width, you must find and use a tire that is the proper section width for that rim.

Between two tires, the one with the smaller outside diameter will always accelaerate faster, as it's another way of changing the final drive ratio.

On tracks with long straights, the narrower tire with less aero and rolling resistance will be superior if the car is horsepower challenged.

And finally, the issue of "traction" or "stiction" .....If the car is light, going to a 30% larger contact patch will lessen the load the tire sees, (in pounds per square inch), and a subtle effect can occur. Much like a narrower tire gets through the snow better, and a wide flat tire is more likely to hydroplane (on the same vehicle), a lightly loaded tire may not actually have enough load to generate it's maximum cornering grip. keep in mind that a switch to a softer compound should be able to overcome that, but, if such a choice isn't available, perversely, a switch to a smaller tire may make the car faster!

And the point of the tire being a rotating mass is well taken, but if the outside diameter is the same, often the larger rim diameter combination will have superior acceleration deltas, due to it's lighter weight. (Generally, an inch (in dia.) of aluminum wheel is lighter than the same inch of a rubber tire.

In the end, there are so many variables, often conflicting, that actual testing is the way to go. Which brings to the original question....sorry Alan, I've got no first hand Golf wheel choice definative answers!

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Jake Gulick
ITA 57 RX-7
New England Region
[email protected]

[hornerdon]

Jake, nice summary. That's the kind of stuff that makes racing so interesting for me (I should repeat for those who don't know it, I have zero driving talent and no eye-hand coordination, so my son does the driving - building the car and thinking about details like this is my race).

This sort of thing is most often the difference between winning and losing, and is what makes a car "fully developed" without spending a lot of money. Testing, which involves buying all the different sizes and paying for the track rental, is one way to confirm it, but a cheaper way is to think it through, ignore conventional wisdom, examine the logic of threads like this one, and apply your best thinking - if the car wins, or if it exceeds all expectations (a Trabant is not supposed to be that fast!), you can be satisfied.

[Eric Parham]

Yes, there are more 13 inch tires than 14s, but the stickiest 13 seems to be the Hoosier, and the size is either too wide or too narrow for the 13x6" rims on an ITB car. The C cars can get away with the 185s, but B cars cook them. The 225s seem to be faster in many cases, but they sure don't look right on a 6" rim. Some ITA cars make them work very well, but they're using 7" wide rims! So, until someone offers a sticky 205/55-13, I don't think any 13" solution is optimal for a B car.

There's a much better SIZE selection offered for sticky 14s. Note that I'm disregarding many perfectly good race tires (Kumhos, Toyos, etc.) b/c they're just not fastest, even if they are better bargains for longevity. I sure miss the old bias-ply Hoosiers (where 225/45-D13 actually worked on a 6" rim)!

[hoser 99]

Joe--Regarding those MOMO wheels, I think that I would be real careful using them. I mounted a new set of tires on a set of them this week,and checked them real carefully for cracks. I mounted them myself so there was no machine damage and I have had these wheels for a long time so I know that they hadn't been beaten up. Starting into the esses today at lime rock, the center came right apart from the outside of the rim..luckily it happened in one of the best places to run off with no control---there's a lot of other places at LRP where it probably would have been below average.. Anyway please really think over use of these as most are at least 15 years old(mine are stamped 84) and they had no visible cracks. There is a picture I took tonite at www.msnusers.com/hoosers

jerry monaghan
itb99

[Bill Miller]

Eric,

I've seen some ITC guys at Summit Point run the 225/50/13 Hoosiers on 13x6 rims. I've also seen them run the 225's up front and the 185/60's in the back. Supposedly it helps the car rotate better, and especially w/ cars like Rabbits/Sciroccos/Golfs, the inside wheel is off the ground in a corner anyway.

For an ITB car, I think the 14" wheels are the way to go. There's still a decent supply of wheels out there that can be had for $75 - $100 / each.

The Hoosier specs confuse me though. Going from a 195/55/14 to a 205/55/14 goes from a 6.8" tread width to an 8.0" tread width (on a 6" vs. a 6.5" rim). Yet a 205/60/14 has a tread width of 7.3" on a 6" rim. What's even stranger is that they list a rim width of 7" - 8" for both of the 205/xx/14 tires, but the 205/60/14 is ~3/4" taller that the other two (which are essentially the same diameter). What's also interesting is that the 195/55's are $18/tire cheaper than the 205/55's, which are only $1/tire cheaper than the 205/60's.

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MARRS #25 ITB Rabbit GTI
SCCA 279608

[Joe Craven]

Jerry, thanks for the warning. I spoke to my friend who I bought the MOMOs from and he tells me that he used them a lot on his Honda with Toyo RA1 tires. I noticed that you had Hoosiers mounted on yours which are stickier and would put a higher load on the wheels. Tire technology continues to improve and perhaps these wheels weren't designed for the potential side loads that tires like Hoosiers can generate. Of course, they are old and aluminum continues to weaken with each stress cycle. There can be quite a few stress cycles in 15 years.

Perhaps I'll just use them for rains with full treads. Are you interested in selling 1 or 2 of yours?

Joe