Educate me on wheel size please

[Kroggers]

I see a growing trend for people to try and fit 18" wheels on their cars with very low profile rubber. Whilst a load of track cars are running 15" or 16"

I currently have the RUF SpeedLine wheels on my car, and whilst they look nice, they are rather heavy! I am looking to replace the wheels, and trying to understand what is better for a track focused car - is it smaller wheels or larger wheels, and why?

[Bill Verburg]

Originally Posted by Kroggers

I see a growing trend for people to try and fit 18" wheels on their cars with very low profile rubber. Whilst a load of track cars are running 15" or 16"

I currently have the RUF SpeedLine wheels on my car, and whilst they look nice, they are rather heavy! I am looking to replace the wheels, and trying to understand what is better for a track focused car - is it smaller wheels or larger wheels, and why?

There's no one right answer, the interplay of wheels w/ trans & cwp and engine and chassis is complicated and there are many compromises that are going to be made made


For track use and only looking at wheels and tires you generally want the widest wheels w/ the widest lowest profile tires that you can fit with the lowest weight, that will usually mean an 18" wheel/tire it you are talking about an SC/Carrera fendered 911 something like 8 & 9.5 x18 w/ 245/35 and 275/35 x18 is going to be the max

but you could also go w/ 8 & 9.5 x15 w/ 225/45 & 275/35 tires

the latter setup has a much shorter drive wheel height which alters final gearing quite a bit from the 18" option and is somewhat lighter but it has an issue insofar as front tire width being a bit narrow wrt the rear but you are stuck w/ it because there are no 245 x15 that fit the front wheel and wheel well

now this might be an advantage and it might not, it will depend on the trans/cwp gearing and the chassis, particularly weight.

Most of us do a compromise somewhere in the middle

[Jeff Alton]

Much like Bill said, we have equipped a fair number of folks with 18x8.0 front with a 225/40/18 and 18x9.5 rear with a 255/35/18 Hoosier R6. On the light cars we don't like the 275-225 stagger. If you can fit a 245 front then by all means go with the 275 rear...

The 255/35/18 is approx the same hieght/diameter as the 255/40/17 and 225/50/16 so gearing does not get taller compared to stock wheels/tires. We use this 18 inch set up on our own race car, driven by the fact that hoosier does not make a 255/40/17 R6 which is the tire we like to run....

[Kroggers]

Thank you both for the good info. So reading this, I should be looking to go to 18" rather than to 16" is that what you are saying?

[krystar]

it's dependent on alot of factors. some heavier cars run massive brake rotors so it'd be impossible to fit 15's and 16's on. dodge viper rotors are like the size of large pizza's. most ppl try to use the smallest diameter widest width rim that you can fit onto the brake package. and of course that's saying that money is not an issue.

[Bill Verburg]

Originally Posted by Kroggers

Thank you both for the good info. So reading this, I should be looking to go to 18" rather than to 16" is that what you are saying?

It depends, the choice of wheels and tires can get complicated because of the issues related to the car and also due to issues related to availability. Not all tires or wheels are available in all sizes, not all suspensions can deal w/ ultra wide/low tires

Have you decided on a tire manufacturer yet? wheel supplier?

in general the lower profiles used on 18s will be better for track use but it does depend on availability and the effect on gearing

it would help if you could provide some more info as to use, 100% track? 50%? how compromised will you go wrt street use?

and yes, the brake package also influences the decision

[TechnoViking]

Since you are posting this in the racing forum, I can only assume you are talking about racing your SC?

If so, and you plan to race with PCA, keep an eye on the rule changes for 2011. There has been a trend towards the 15" setup with Hoosier R6 tires that results in a lower drive ratio as Bill points out. There is a proposal to change the PCA rules penalizing that wheel/tire combo.

Wheels and tires are funny. As I understand it, 16" rims were a popular "upgrade" when 911SCs were new. When I started racing in 2006, all the fast cars were going with 17" rims, the theory being a lower side wall height resulted in less tire flex. Those that could afford high-dollar Fiske 17" forged wheels were the envy of everyone at the track.

But ow 15" is the trick setup. Unless the rules change, and then it won't be anymore.

[Bill Verburg]

Originally Posted by TechnoViking

Since you are posting this in the racing forum, I can only assume you are talking about racing your SC?

If so, and you plan to race with PCA, keep an eye on the rule changes for 2011. There has been a trend towards the 15" setup with Hoosier R6 tires that results in a lower drive ratio as Bill points out. There is a proposal to change the PCA rules penalizing that wheel/tire combo.

Wheels and tires are funny. As I understand it, 16" rims were a popular "upgrade" when 911SCs were new. When I started racing in 2006, all the fast cars were going with 17" rims, the theory being a lower side wall height resulted in less tire flex. Those that could afford high-dollar Fiske 17" forged wheels were the envy of everyone at the track.

But ow 15" is the trick setup. Unless the rules change, and then it won't be anymore.

The 15" option is only attractive to Hoosier users, no one else makes a comparable tire which is why I asked if a decision about tires has been made

here are the permutations for Hoosier;

rear 275/35x15 on a 9 or 9.5" rear, 23" tall
a 9 is at the extreme low end of the design range, this is bad for performance as the sidewall will flex much more than w/ an 11 which doesn't fit in an SC/Carrera wheel well

225/45 x15 front on an 8 - 22.9" height is a good match but the width is too narrow to dial out understeer in most 911s
245/50 x15 front on an 8 - 24.8" height makes it more difficult to set the chassis up when a 23" rear is used, side wall is tall leading to more flex, heat and under steer

rear 275/50x15 on a 9 or 9.5" rear, 25.5" tall - big change in gearing that can be good or bad, it depend on the engine , chassis and track
245/50 x15 front on an 8 - 24.8", still a bit of a height mismatch f/r, side wall is tall leading to more flex, heat and under steer

rear 275/45x16 on a 9 or 9.5" rear, 25.6" tall - almost no change in gearing compared to a 275/50x15
245/45x16 on an 8" front - 24.7" tall which is a mismatch to a 25.6" rear, better sidewall profile
though
245/40x17 on an 8" front - 24.5" tall, still the height mismatch but more brake clearance and better sidewall

rear 275/40x17 on a 9 or 9.5" rear, 25.5" tall - virtually no change in gearing and lighter than the 275x16 version
245/40x17 on an 8" front - 24.5" tall, still the height mismatch but more brake clearance and better sidewall
245/35 x18 on an 8" front - 24.7" tall - less height mismatch, more brake clearance, better sidewall profile lighter than the 17" version

rear 275/35x18 on a 9 or 9.5" rear, 25.5" tall - virtually no change in gearing and lighter than the 275x16 versions, better sidewall profile
245/35 x18 on an 8" front - 24.7" tall - less height mismatch, more brake clearance, better sidewall profile lighter than the 17" version

you need to go through these kinds of permutations for the tires and wheels you want to use

[Tom W]

I run 16" wheels because my race class requires a specified Goodyear bias ply slick in the 16" size. Generally people pick the wheel size to run the tires they want to run.

[M758]

Bill covered alot, but it all comes down to weight, diameter, sidewall and width.

Overall diameter can have a big impact as it changes the final drive usally short is better.

Width - Wider is better in most cases, but if you run a diffenent tire in front vs rear the stagger between these is imporant not just width.

Sidewall - Shorter sidewalls mean less flex and the means more responsive car.

weight - lighter is better.


So when you consider ll 4 factors it can be complex since some are some what exclusive of each other and some are not . However all must be balance by availble tire sizes, wheel sizes and what fits on your car.


Also that thing call budget.

[Kroggers]

This is all good stuff and I am starting to get a better picture regarding the options now...

Yes, we are talking about my SC, but I am based in Sweden so do not have to worry about the regulations that you have over there. Also Hoosier are not that easy to find over here, so at the moment I am running Toyo R888.

The car is a street legal / registered race car - mainly so that I can (if I want to) run it on the road for a quick spin on a sunny weekend afternoon, and also as the regulations within the Swedish Porsche Club dictates that all cars that are driven must be road registered...

Like I said, I like the RUF wheels that are on the car, but they are heavy. I will either get a second track set-up or replace them for a new set to use all the time. Just looking to understand if I would benefit from 18" or 15". The main factor is that to stay road legal I need to maintain the factory overall size - so the indicated speed does not go out of spec.

[Jeff Alton]

Here is some quick pics of the 18x9.5 rear with 255/35 Hoosier R6 and 18x8.0 front with 225/40 R6... The Hoosiers are light tires to boot.





Cheers

[Kroggers]

OK, so it looks like the best option will be to go 18x9.5 rear with 255/35 and 18x8.0 front with 225/40. If this will also give me as close as possible to the factory supplied original size in gearing etc?

Jeff, what wheels are those?

[Bill Verburg]

Originally Posted by Kroggers

This is all good stuff and I am starting to get a better picture regarding the options now...

Yes, we are talking about my SC, but I am based in Sweden so do not have to worry about the regulations that you have over there. Also Hoosier are not that easy to find over here, so at the moment I am running Toyo R888.

The car is a street legal / registered race car - mainly so that I can (if I want to) run it on the road for a quick spin on a sunny weekend afternoon, and also as the regulations within the Swedish Porsche Club dictates that all cars that are driven must be road registered...

Like I said, I like the RUF wheels that are on the car, but they are heavy. I will either get a second track set-up or replace them for a new set to use all the time. Just looking to understand if I would benefit from 18" or 15". The main factor is that to stay road legal I need to maintain the factory overall size - so the indicated speed does not go out of spec.

What do the authorities consider to be the overall size originally? 225/50x16? which is ~24.7" How much wiggle room?

I'd say that for Toyo R888 or RA1 you'd want 255/40x17(25.1") on a 9.5x17ET19 wheel and in front 235/40x17 on an 8ET25-31 wheel,


in R888 you could also go 225/45 x16 on 8ET25-31 and 245/45x16 on 9ET12-15. These are 23.8/24.4" respectively. for stock brakes this is fine but w/ most bbks you'd want 17s

The other street legal tire that might work for you is the Michelin Pilot Sport Cup but there are fewer choices there and you'd have to go to 18s
225/40x18 on 8 and 265/35 x18 on 9.5. these tires are 25.1 /25.2" tall respectively

[Kroggers]

Thank you Bill.

I am not sure what is considered the original overall size, the basic is that the spedo must still measure within spec for the wheel and tire combination...

I am not running with stock brakes if that makes any difference.

What I am really trying to find out is what would be the best wheel and tire combination for track use on my car. And if it is the Ruf SpeedLines that I have now, then I might as well keep them...

[Bill Verburg]

Originally Posted by Kroggers

Thank you Bill.

I am not sure what is considered the original overall size, the basic is that the spedo must still measure within spec for the wheel and tire combination...

I am not running with stock brakes if that makes any difference.

What I am really trying to find out is what would be the best wheel and tire combination for track use on my car. And if it is the Ruf SpeedLines that I have now, then I might as well keep them...

hmm, thats a pretty vague def wrt to speedo. Stock is only accurate to ~5% of true


any way I have 2 sets of wheels and tires, same sizes. The Speedlines are fine but heavy, on my 993 I use Speedlines for street and BBS for track, the BBS w/ MPSC are 42# lighter and this is much appreciated on track


If you track the car a lot, a separate set of light track wheels makes sense, if you only do an occasional track day it doesn't

just as a side note on my C3 I have 8 & 9.5 x17 Kinesis w/ Dunlop Star specs 225/45 & 255/40, total wt 180# compared to BBS/MPSC 225/40 and 265/35 on 8 &10 x18 with a total wt of 162#

[Kroggers]

Having two sets of wheels is worth considering. I would define my car more as a road registered track car than a road car that some times is used on a track. The car is set-up for track use not road comfort, and I would say it would be uncomfortable to use on long road trips - I have the family Grand Voyager for that

I know the regulations are vague. They basically say that you can put any wheel and tire on the car that will not alter the factory set overall dimensions, which I can understand on a road car - they do not want the speed reading to be wrong!

Ideally I would have two sets, but I will probably dump the Speedlines and replace them with a set of BBS wheels - all I need to know is if they should be 16", 17" or 18" ones to give best performance on a track?

[Tom W]

- all I need to know is if they should be 16", 17" or 18" ones to give best performance on a track?

You ask the impossible. You could put crap tires on any wheel and go slower or great tires on any wheel and go faster. Bill gives you some good options with respect to tires and the wheels you should use with those tires.

You realize it is the tire size and not the wheel size that determines what your speedometer says and impacts acceleration given the transmission gearing? As Bill says, it's likely that your car expects a 25" diameter tire. Deviating from that is going to be what causes changes in the speedometer reading. Smaller means the speedometer reads fast and it is slightly easier to accelerate.

I reiterate: you need to decide what tires you want to run and buy wheels that are appropriate. Most choose MPSC, RA1 or R888 on 17" or 18" rims.

For a non-pro driver, I don't believe that the sidewall size/flex is going to have a significant impact on your lap times. I also believe that most intermediate level drivers will not see a profound difference in lap time with the tire options mentioned. Decide what you can afford in terms of the tire and light weight wheels and go for it.

[Kroggers]

Thank you Tom, as I said at the start I was looking to be educated and to better understand why people are making the selection they do - what makes a person select the 18" and another the 15"...

But as you say, I am asking the impossible - so I will consider myself educated (but still a bit confused - or is that ignorant ) and leave it at that...

[Bill Verburg]

Originally Posted by Kroggers

Thank you Tom, as I said at the start I was looking to be educated and to better understand why people are making the selection they do - what makes a person select the 18" and another the 15"...

But as you say, I am asking the impossible - so I will consider myself educated (but still a bit confused - or is that ignorant ) and leave it at that...

It isn't really that complicated
15 would only be used by a very narrow and specialized group of drivers that want to use the Hoosier tires and are willing to put up w/ the compromises associated w/ them. The short gearing can be good or bad, when ever you shorten gearing you narrow the usable rev and speed range of a given gear this is moot for you as you need to maintain a tire height of ~25" stock

16 would be used mostly by those that want to stay w/ Fuchs 8ET23 front and 9 ET15 rear 225/45 & 245/45 tires,

17 or 18 is pretty much a toss up, MPSC isn't available in 17 any more but that is of no concern to you Toyo currently offers good choices in both 17 & 18 for you

pick a set of wheels that are available in the ET and width you want and the rest is going too be easy