earlyapex

I'm at the point in my race car build that I have to decide on what wheels to use. The car is a 78 SC that is being built to PCA GT3 rules. Given the same tire and wheel manufacturer, what would be the preferred wheel size? I already know that many of racers are successfully running 18 inch wheels, but is it because they are faster than 17 inch wheels or is it because that's what everyone else in the paddock are running? Obviously, the 18 inch tires will have shorter and presumable stiffer sidewalls. This I assume translates into less deformation during the turn and better transitional response and feel, making for a faster combination.

Comments?

Experience?

Lap time comparisons?

Or have I answered my own question?

38D

I have seen many people run 16s/17s and 18s on their G class SCs. The main reason to go with the 17s or 18s is a wider variety of tire choices. That being said, I don't know of any quantifiable data that says that 18s are faster. I did once talk with Keith Peare about the wheels he ran on his GT4R 914. He ran 10x16 and 11x16 with slicks. He had actually tried 18s and found them to be slower (and Keith is very, very good).

Btw, are you really going to run a SC in PCA's GT3? Your car is going to be seriously outclassed by high horsepower 951s and race 3.4s.

KRA993tt

Eric,

I have been wrestling with the same question. I have not spoken to any wheel mfg. yet but I would think 18's are heavier than 17's of the same wheel. That would point me toward 17's, but I don't know if that is a valid reason.

jaydubya

I posted a similar question about a month ago - there is a thread here somewhere...

There are several F and E class cars locally that have recently switched from 17" to 18". These guys are driving SCs, Carreras and 951s in the PCA stock/prepared classes.

Their argument for switching was primarily based on tire sizes and their effects.

In 17" on an SC or 951, in Hoosiers, you can run either a 245-40-17 with a 24.5" diameter, or a 275-40-17 with a 25.2" diameter on a 9" wheel. So you have a tradeoff between tire width and diameter. What does a bigger diameter mean?

Well a bigger diameter effects the car performance in two ways. First is gearing, and most everyone understands that. The second is thrust. With a smaller diameter tire, you actually get more thrust force at the wheel-track interface. It's like adding hp.

So you have tradeoffs between tire width availability, gearing changes and thrust gain/loss.

A lot of peole I know say that a SC in PCA stock classes really doesn't need anything wider than a 275 given the HP. Others say 245 is sufficient. In a GT class car that's not going to be true.

The folks who have gone to 18" wheels are running 285-30-18 on the rear (or all around on the 951). This gives you a really wide tire (especially for a SC or Carrera) and, relatively speaking, a really small diameter (24.6"). Assuming that the change in gearing doesn't screw up where you shift or where you have power on the tracks that you drive, you can have your cake and eat it too. The local guys running this setup have been very happy with the change and swear it is good for 1/2-1 second on a ~3 minute lap time.

The issue of weight - well you reallly have two issues there. One is sprung weight, the other is rotational inertia - i.e. the distribution of weight from the center of the wheel to the outside edge. It really depends on the tires and wheels that you choose, but you can do those calcs for a particular tire and wheel.

All this said, I have a set of 16" wheels and a set of 17" wheels and that's were I'm going to be until I can afford to buy those 18" tires - a 285-35-18 Hoosier R6 costs $285! I'll just have to find the time somewhere else. So let me know if you have any 17s for sale

Hope that helps,

Jeff

Edit - about the shorter/stiffer sidewalls on 18" versus 17", I asked about that and none of them really felt that this was contributing to the time difference.

Gary R.

Jeff - Put a set of Hoosier Wets on the 16" wheels 225/255 and R6's on the 17" wheels and your good to go! (I run 16" wets and 18" dry but WTH..)

KRA993tt

285/30/18's require 10" rims.?? This is out of bounds for pca G class AFAIK.

It looks like a 275/35/17 or a 255/40/17 would be nice

jaydubya

No wets for me - I'm scared of rain

jaydubya

They are using a 9" wheel with the 285-30-18. Fits fine.

If you are in prepared, you could run a 10" wheel.

A 275-35-17 would be GREAT!

Gary R.

That's because you never ran on Hoosier Wets!

jaydubya

I see you are on the east coast so things are different out there. I had over 30 days at MMP this season. It rained once in qualifying (scared the **** out of me) and drizzled during one practice session

Gary R.

Yes they are... and we have a lot more to hit when we off.... wish ALL tracks were like MMP! So new plan. Sell the 16" wheels and get another set of 17" and you have interchangeability!

jaydubya

Yeah, I've done so much dirt farming at MMP that I should plan crops

jaydubya

That's where I'm headed.

My lap times are nearly identical between the 16s and 17s, but I can feel the sidewall flex more on the 16s. On the 16s I run 225-50-16 / 245-45-16, and on the 17s 225-45-17 / 245-40-17. Yes I'd like to run wider tires, but most 911s in G out here are running the same sizes as me.

earlyapex

Colin, you're right. This season I'm going to get stomped by the big boys. The car will have all the suspension and brake goodies to make in competitive. It will be under powered for a couple of seasons while I save money for another engine rebuild. The current 3.0L is far from stock, but unfortunately puts me in no man's land with regards to PCA classification. I can either go up to 3.4L or down to 2.8L. I'll decide after a couple of seasons.

Thanks for everyones' insight on wheels. I've done some calculations on 17" versus 18" tires and there is no significant weight difference. There is allot of difference in side wall height and construction. Thus the differences in the tires cannot be attributed solely to the lower side walls in the 18" tires. I'll post pictures when it's done, which could take a few months.

DrJupeman

285 on 9? Sounds like trouble, particularly on the rear of a 911.