wanna911

It would yes, but those wheels have proven not to fit the 6 piston bigger brake rotors/calipers. And 17's and 16;s look like crap on cars that are getting bigger and bigger. They look like hubcaps. Some what of a sacrifice is worth making. 20's, no.

Don't see any F1 cars rollin on dubs or mini dubs.

Bigger wheels are heavier compared to the same wheel in a smaller size. Ditto for the tires, especially if you keep the sidewall height constant. The difference is huge per corner for 20's vs 18's. They will probably ride smoother, like I said, but not better overall, especially on track.

Petevb

Like I said, I'm running 17s over 6 pistons calipers and 350mm rotors on mine...
Let's take looks completely out and simply discuss performance.

I think we can agree that you can't put a car on skate-board diameter tires and expect it to perform well. And the rolling diameter wouldn't let you simply drive over potholes in the road, let alone ride well.

We might also agree that given complete freedom, powerful race cars tend to have larger OD tires to help put power down- look at any dragster. While an F1 car does have rules limited wheels and tires now, the taller rear tires are not an accident. Similarly a 911 RSR has rules limited wheels but a large OD rear tire, despite the weight penalty. Pretty much any time in the history of powerful race cars, from Can-Am to Top Fuel dragsters, that designers have had the freedom to put on a larger rear tire, they do.

So I will argue that a tire wants to be bigger OD to put power down as cars get more powerful- the longer contact patch simply helps. Obviously when you're not traction limited it doesn't help, however- more rotating mass, more unsprung weight, etc. These things argue for keeping the wheel small. However with a big tire and small wheel the tire sidewall height grows, and the tire's "relaxation length", or the distance it needs to travel down the road before it builds lateral Gs and reacts to a turn, increases. A dragster driver doesn't care, but a sports-car driver sees this as a delay in turn-in or transient response: how quickly a car reacts. Transient response is critical- it's what makes a car feel sporty. And when you double the weight of a car you also double the amount of force that needs to build up before it turns, and hence you roughly double this "relaxation length". So a tall tire combined with a small wheel and a heavy car is a recipe for wallowy handling.

So you want a tall tire to put power down, but you don't want wallowy handling. Two ways to address this issue:

Option A) Small wheel and stiff sidewalls, ie hoosier. Unfortunately due to beam theory when you double the height of the sidewall you need to make it 8 times stiffer in order to have the same response rate. Thus this is a losing game unless you make the tire very stiff, transmitting tons of road harshness.

Option B) Big wheel, softer sidewall. Quick response, less road harshness.

Having 17s, 18s and 19s on one of my cars, 16s on another and 18s and 19s on a third, I've had a little experience with this myself. Clearly lighter cars can get away with smaller wheels- no surprise as less mass means the relaxation length is shorter. What's surprised me is that on my heavy car (3400 lbs) 19s (street tires) now outperform 18s- and I'm pretty sure this wasn't the case just 5 years ago. Yes the 18s break away less abruptly, but they are significantly slower in transient response and ultimately have less grip at the limit (tire distortion?).

In any case for lighter, lower powered vehicles smaller wheels and tires are clearly best. As power and weight grow, the benefits of going larger outweigh the costs, and the ideal wheel size increases. On a stock 170 hp VW golf 17" to 18" may well be the optimum size for lap times:
http://www.caranddriver.com/features...d-tires-tested
However on a heavier car with 3x the power the optimum will almost certainly be bigger. Especially when taking the need for quick transient response coupled with the ride issues into account.

Do you think a Nissan GTR is that fast with a wheel size 2"-4" too big? Imagine how fast it would be with the right size... Or it's a big heavy, powerful car that needs a bigger wheel.

wanna911

6 Pistons under 17" wheels is not common. Maybe not impossible, but surely impractical. Most wheels will not fit in most applications, especially that now many of the 6 pot kits are pushing much bigger rotors. We are talking about the norm here. Not one offs.


The GT-R has Rays forged wheels which are lighter than pretty much all of the wheels Porsche puts on their cars. It actually gains in that area. This is nullified by the heavy runflats, but the heavy dunlops have the grip of R-comps. In fact there are R-Comps made in GT-R sizes (R888), but the stiff lower profile sidewall is more preferable for drivers and the times are equal. So unless you match their grip or have the same application (another GT-R) it's useless comparing across platforms. The GT-R is a HUGE chunk of car as well, 18's look like toys on there if you don't stick a huge tire on it. Looks ridiculous actually.


Even still most of them run to 18's for track duty. 17's are popular among C5 track vettes, in fact, they'll run 18 inch in the front and 17's in the rear.

20's are not for performance gain. They are for looks. And then the tire manufacturer is told to make it work because it just looks good.

aussie jimmy

18's with a wheel designed for bbk clearance are the optimum

Petevb

I've had the privilege of discussing this issue with someone who heads sports car setup for a big three manufacture. Very talented guy, he knew in intimate detail what the fastest options are, what wheels and tires will take what car setup, etc. He was limited, however, not by styling concerns, but by cost. He knew full well that upsizing a wheel gave him performance, but he had a budget and tire replacement costs ruled out many of the options he knew tested better. 18s make the budget more easily, but on heavy cars they don't test better.

Given the lack of tire choices and cost it's pretty obvious why many GTRs run 18s. There is only one R compound available, it's relatively slow, etc- I'd probably swap to 18s as well given those options.

If you take R compounds and limited tire options out of the equation, however, GT-Rs are fastest on 20s: Witness One Lap of America, where GT-Rs took 6 of the top 10 spots last year. These include semi-pro teams with pro drivers and testing budgets, and when given the choice of running any wheel and street tire they want, 20" PSS win, even though the same tire is also available in 19s and 18s in the same width and multiple sidewall heights. Perhaps no one thought to try 18s over the last four years GTRs have been winning? And my experience has born this out- on my heavier car I have more fun on 18s, but the 19s are faster.

We're talking about manufacturers here, with the ability to custom design whatever wheel and tire they want, not a shade tree mechanic that needs to pick and choose from what he can get. There are fundamental reasons why heavier, more powerful cars need bigger wheels and tires, as I outlined, so it shouldn't be a surprise that wheels and tires are getting bigger. I'm sure styling often comes into the equation as well, but the trend is based on physics, not styling. IE the most powerful cars get the biggest wheels and tires, hence they are cool, hence everyone else wants them.

I generally don't agree with the attitude I sometimes see here that when Porsche changes something it's due to to marketing, cost cutting, etc. The initial backlash against rear wheel steering, the new motor, etc. As far as I can see these guys are engineers operating near the top of their game generally doing the things I'd do if I were in their position. While I don't agree with every decision they make, the more I understand the more I respect them.
I was responding to your assertion that you can't run 17" wheels in your application:
You can run 17s over your 350mm rotors and 6 piston calipers if you believe it's quicker. Happy to give you the place to get a 3 piece wheels that'll fit. Of course I think you'll be slower if you go that route, so I wouldn't recommend it.

wanna911

Even better yet, put some 20's on your classic and let's see you go faster. The difference between 18 and 17 would be negligible at best. It would allow me ro lower my car some though without further messing up the geometry. However im not changing two sets of wheels and many more sets of non paid for tires to prove a point thats already being proven. If there were a performance advantage race cars would have them. And they dont.

Of course if you try to stuff a big soft sidewall tire on a small wheel, handling will be compromised. In the case of the GT-R's that would have to be done as smaller OD on already lowered cars results in bottoming out because the car is designed around 20's. But even Nissan's chief engineer has stated the 20's were put there for looks........... NOT performance.

Petevb

You're completely missing the point. I'm not saying bigger is faster. I'm saying the fastest wheel scales with the car and power. 500 lb go kart works with 10-12" wheels. 1500 lb caterhams work with 13-15" wheels. Incidentally F1 cars run lighter and use 13s, indy cars are similar and use 15s. On the other end of the scale is a nearly 4k lbs with driver GT-R at 20" wheels.

My car at ~2300 lb with driver is closer to a the caterham than the GT-R. Even if you ignore the fact that the suspension kinematics were designed around 5.5" wide bias ply tires it's going to want smaller wheels.

But stick go-kart wheels, regardless of how wide they are, on a big heavy car and it's not going to have the longitudinal contact patch to do well, regardless of how wide they are. Of course some guys figured out ways around this, back before there were rules, but it's not a very practical solution for most of us:

Please show me a race car that both a) doesn't have rules limited wheel diameters, and b) is over 3300 lbs with driver.

Even with lowering your CG, something the factory can't do as it needs to maintain ground clearance, it wouldn't be enough to matter? Hmm.

Link?

wanna911

There are a ton of race cars over 3300 lbs with no wheel size rules or at least greater than 18's. SCCA mandates a limit of 19" for the ST classes for touring and super touring. And there are PLENTY of cars with minimum weights in the 3200-3500 lb range. Minimum weight requirements are on fumes, they do not even account for running weights either. Also plenty of options for tires as most of these classes require DOT's.

I don't know what formula you are using, but the reason the wheels are getting bigger is not because of the weight of the cars. There are plenty of older heavy performance cars that ran 18's and even smaller. The wheels are getting bigger because they look better, and staying current in the industry means you need to make the car look like dub magazine.

My car is just under 3300 lbs minimum weight, and neither 19's nor 20's would help me go faster. I've driven it on 19's with PS2's and it sucked. Moved to 18 inches with worse bald pirelli tires and improved by several seconds. This was actually when the car was 3700 lbs with me in it.

Now that the cars are being designed from the get go with dubs in mind, reducing the wheel size could be detrimental if the tires are not designed to run with a bigger sidewall and maintaining optimal gear ratios and OD are vital.

Your car and driver link supports that. The smaller wheels came in smaller widths, which reduces lateral contact patch width. Make them the same width and give them the softer compound that was on the 18's and I'll bet you they hold the road better and brake better to go along with the significantly better accel. The 15's were 2 inches narrower than the 19's. That's hardly apples to apples.

Dan39

Correct. You'll recall the 1998 Porsche GT1 ran on 20in rims at Le Mans...

http://www.porsche.com/usa/eventsand...1998-911gt198/

wanna911

And was slower than the toyota and mercedes. It won lemans because they had mechanical failures. But got dominated the rest of the time. In fact the mercedes beat the porsches and toyotas into leaving the series altogether after that season.

Not exactly a ringing endorsement for 20's.

Dan39

The fact they chose to run 20s at Le Mans disproves the point that wheels of that size are only for show. End of discussion.

911rox

20s are for bling to keep up with the Jones's, to house the 410mm PCCBs and possibly to discourage excessive track use thru cost of tyres so people need to pull their heads out of the sand...

In 2010, they had the option to run the GT3RS on 19" slicks at the NBR 24hr, they didn't! They ran 18s! We have way too many chassis engineers here on RL by the looks of it...

Carrera GT

Did Porsche really have 19 inch slicks to fit the RS 3.8? I think they had Cup car engineers who know all there is to know about running that chassis on 18's. I think if they had ready access to race wheels and tires in 19's, it just wouldn't have been an issue. I'm guessing Bentley or Ferrari or someone ran 19" slicks at Le Mans that year, but in the right oddball sizes to extract exactly the driving characteristics that the drivers would be able to "just get in and drive" ... I dunno.

It seems like a story that has aged long enough: Panorama or Excellence could gather the facts and tell a very interesting story. I think Chris Harris was probably close enough to report "insider" knowledge to give us all some insight into the practical reality of racing a, then, showroom fresh unknown quantity 911.

Mike in CA

I've always had a second set of wheels and tires for every car that I've tracked or autox'd. What's wrong with a set of 20's for the street and 18's or 19's for the track? Putting 20's on the GT3 makes sense if they are needed to clear larger brakes, and I can even buy the theory that Porsche are keeping up with the competition. But to suggest that it's a plot to keep owners from tracking due to tire cost seems a little far fetched to me.

Petevb

Exactly.

And the fact that you don't see 20" wheels today is because they banned them, not because they didn't work. Today's Le Mans regulations read:

Rim Diameter, Front and Rear: 18" maximum (no minimum)

And no one uses less than 18" rims. This on cars that weigh about 1000 lbs less than street cars. I think that's about all you need to know.