15" or 16" at the track- is bigger really better?
 

I have been looking for a set of 7" & 9 (or 8)" wheels for track tires for my '88 Carrera. I have been looking for 16". Most of my track time is at Road Atlanta and I never really run out of 5th gear and I could always use a little more grunt coming out of turns 7 & 10. So I was thinking that I might be better off with 15" instead of 16". The 15's would open up some more options with selection and $$$ and maybe be less of a clearance concern. I have been thinking Fuchs for the most part but am not tied to them. What are your thoughts and opinions.

While we are at it anybody have a set for sale:)

Thanks,
Andrew

thats the first time i've heard that here!;) i believe there are more tyre options with 16's.

15's would definitely offer shorter gearing - they are quite sort after in the uk for that reason.

15s offer the advantage of lower weight and less expensive tires. They can also be smaller diameter which is an advantage in some cases.

But you'll find the track tire selection limited. Most 15 inch DOTR tires are available no wider than 225. The exceptions: Hoosier does offer a 245 and you can get 9 inch wide goodyear slicks.

With 16s you can get 245 in everything and some offer even wider rubber.

One point to add about weight..As your wheel size goes up its weight does too..duhh. But keep in mind that your tires sidewall goes down in size and does not increase in weight like the wheel does. That being said, a 17inch racewheel with hoosiers is bound to be close in weight to a 15inch wheel running dot rubber.
Chucks point about tires in 15s is very true. The only places to get a 245 are hoosier and goodyear. Depending on the $factor, you may price yourself out of the ballpark looking at a race slick.

It's all about tires.

The handling/weight differences are negligible. More wheel in the same diameter wheel/tire means less tire, and vice versa. Sidewall stiffness variations between different types of tires will make more of a handling difference than sidewall height.

So, look at the tire widths and outside diameters you're interested in, and make the decision based on that. If you need wider than 225's, then 16's tend to make more sense. In some widths, some 15-inch tires can give you a smaller outside diameter, that allows you reduce your effective gearing. But some 15-inch tires are just as tall as 16's.

Your car must be an early 88, because later that year the car came standard with 16 inch wheels. That being said, as manufacturers go to larger wheel diameters, it will be increasingly difficult to find a good selection of tires in 15 inch. I would recommend moving up to the 16 inch (7s in the front and 9s in the rear).

Try and separate the gearing issue from the tire-grip-handling issue. 911 transmissions are designed to be re-geared to suit the application. All street gearing is very tall to accommodate perceived land speed record attempts and keep the revs down for noise, fuel economy, and emissions reasons.

Going fast on the track is all about handling and grip. For the best track performance there is no substitute for new real Goodyear race tires.

That said there are issues of cost, conforming to class rules, and what you want to do with your Porsche. For pure fun there is nothing like a new set of race tires on a well set-up 911.

Best,
Grady

I had 15" track wheels for one year. As stated above, the tire selection in the problem. 225s are the widest available with the exception of hoosiers. Driving to the track on hoosiers wasn't an option for me.
I currently have 16" Fuchs for the track but plan to go to 17" after these tires are gone. You might consider just going to 17s.

Another critical reason to use larger diameter wheels is the the sidewalls are shorter, very important from a performance point of view.

I've also heard that wheel weight/size is almost negligible as Jack states, and can see why if there is less sidewall area when compared to tire width area - at any event, it'd be, I think, splitting hairs.

More critically would be - at least for me - gearing issues. And as Bill states, sidewall flex is an important issue as well.

This link can help in determining the size/gearing differential when comparing larger wheel/tire combinations.

http://miata.net/garage/tirecalc.html

Good luck!

SmileWavy

Like Grady said, get the tires you want , then gear the trans for the use you want;)

Andrew, I've been pondering the same question. With the prices of 16" fuchs these days, 17s may be an option. I'd love to find some 7s and 9s in 16 fuchs but i can't help but notice cup wheel sets for well under $1000. I know you have to figure in spacers, more expensive tires, etc. Thoughts??. BTW, you going to the DE next wknd? Ray

Thank you all for your replies. I just can't say enough about the wealth of info and advice on this forum.

After digesting all the input and weighing the options I bit the bullet and closed the deal on a set of 7 & 9 x 16 Fuchs. I think the tire selection issue was a big part of it. The only bad news (beside once again spending $$ I don't have) is I don't think I'll have them in time for the DE next week. So its one more weekend on the "skinny" 6 & 7 x 16's. I guess the bright side is, the skinny tires will make me fine tune my driving skills more...and that's always a good thing.

Grady - The gearing option is very interesting. I have always felt the gearing was a little too high & wide for the power and weight of these cars. But opening up a transmission is one DIY frontier I have not yet attempted...someday!

Ray B - I looked at the "take-off" 17" options but most needed the spacers and I didn't want them on the track. And new 17" wheels that were track worthy and had the correct off-set were more $$ than the Fuchs. Maybe I was looking in the wrong places. And yes, I will be at the DE, see you there!

Thanks Again
Andrew

One more thing to consider about going to larger wheels is the rotational center of mass of your wheels. Let's say that you go to larger wheels and thinner tires. If the total weight of your previous tire setup is the same, but the new tire weighs less, then the wheel should accelerate more quickly (at least theoretically) because the center of mass is closer to the axle. While this is likely to be a very small contribution, any tiny bit of leftover torque not used by the wheel and tire will now be translated to the pavement, and there will be less drag from the non-drive wheels. The distance from the center of the drive axle to the pavement is probably much more significant, as a 1/2 inch difference can make a real difference on torque to the pavement. But, when you're racing, every little bit helps!

For the best in sidewall stiffnes you need some Tweels...

click here

Just a thought...I remember reading it somewhere. I know it can depend on tire size. But wouldn't the 16in wheel with tire have a little bigger "footprint" /contact patch than the 15 inch? Because of it's radius being larger.

I love those Tweels! Thanks for the post.

I just had this vision of 32 inch Tweels on a Hummer...need another glass of wine.