Tire/Wheel Physics

[Superman]

Tyson is a pretty smart guy (read: brilliant), and he said that rim of a wheel should be about the same width as the tire's tread. In other words, if the tread is 9" wide, then the wheel should not be a 16x7. Unfortunately, SCs came this way from the factory (16x7 rear wheels with tires that are 225 mm wide). So, even at a glance you can see that the tire pooches out from the wheel rim. ("pooch," is a technical term).

I'm not likely to disagree with Tyson, particularly in the area of automotive suspension physics (right, Jack?). He says this allows the tire to lay over sideways. Lessens effective sidewall rigidity. Makes sense. So what is my question? It's this:

Why then are police cars and NASCAR cars also set up this way?

[magilla]

Easy question- So that NASTYCAR will continue to suck and so the Police can't catch us.... DUH!

[dd74]

Originally posted by Superman
Tyson is a pretty smart guy (read: brilliant), and he said that rim of a wheel should be about the same width as the tire's tread. In other words, if the tread is 9" wide, then the wheel should not be a 16x7. Unfortunately, SCs came this way from the factory (16x7 rear wheels with tires that are 225 mm wide). So, even at a glance you can see that the tire pooches out from the wheel rim. ("pooch," is a technical term).

I'm not likely to disagree with Tyson, particularly in the area of automotive suspension physics (right, Jack?). He says this allows the tire to lay over sideways. Lessens effective sidewall rigidity. Makes sense. So what is my question? It's this:

Why then are police cars and NASCAR cars also set up this way?

So would this mean that a 205 (as opposed to a 225) would not pooch over on a 7-inch rim?

As for NASCAR and police cars - police cars have suspension "packages" that give them better roadholding, right? Rim and tire size may be relative to whatever's available and cheapest. NASCAR I don't know about. In F1, it looks like some of those tires barely fit the wheel width, which leads me to believe sidewall rigidity is of utmost importance in those cars. Hence, more turns in their course, not just oval racing.

Could it be that when a tire "pooches" it also creates understeer, which for an unseasoned Porsche driver, adds in a measure of safety to they don't spin the car? I'm taking this from the thought less sidewall rigidity bogs/slows down cornering, keeping everything a little bit safer.

[twin plugged targa]

Because IF the side wall on cop cars was more rigid, then the suspension etc would be more rigid and every small bump would spill mr policemans dunkin doughnut's coffee and make him not so friendly! .
Nascars- don't -even go there

[dd74]

Originally posted by twin plugged targa
Because IF the side wall on cop cars was more rigid, then the suspension etc would be more rigid and every small bump would spill mr policemans dunkin doughnut's coffee and make him not so friendly! .
Nascars- don't -even go there

Hilarious!

[arcsine]

In the context of NASCAR, one reason could be the centrifugal forces the tires experience at 150 mph +. As in drag racing, the tire "grows" in height as centifugal forces increase. The NASCAR boys may design in some tire "pooch" to allow for that growth. If the growth did not come from the sidewall, it may come from a distortion (bowing) at the top of the tire which in turn may lead to some pretty complicated wave dynamics as that bow flattens out when back in contact with the ground.
Just a thought.

[Superman]

Excellent use of technical terminology, dd74! So, how are F1 cars set up? Are the tirew wider, narrower or same width as wheel rim? BTW, in my motorcycle days I worked at the place where the cops bought and maintained their motorcycles, and those suspensions and engines were stock. Tyson thought that perhaps wheels winder than tire tread would be better than vice versa.

[dd74]

...because it certainly looks that way with F1 cars. However, I would not think that this setup is limited to just F1. In several high-level Porsche racing classes, I've seen similar looking set ups where the rim appears wider than the tire.

Now, how that translates on the street is a matter of how much rigidity the driver can take over less-than-smooth tarmac. I venture to guess a wheel/tire combo where the tire retains shallow sidewalls and smaller widths in comparison to the wheel, would make for a ride worthy of only a dentist's smile (as he replaces the one that has dropped away after someone with this wheel/tire combo has lost all their teeth)

[Superman]

It may be that cop car wheels are narrow because narrow is cheap. It could be that NASCAR vehicles are limited by rule to certain wheel sizes but not tire size. This would explain. I know I am limited in wheel size if I want to stay in my stock autocross class.

[Eric Coffey]

I think that everything is a trade-off. If you are only concerned with straight-line grip, than a huge tire on a narrow rim might be beneficial. If better cornering performance is the goal, than Tyson is right on the Money. The tires are a key (often overlooked) component in the suspension system. A large tire on a narrow rim will (generally) be more compliant, and offer less (cornering) performance due to more side-wall flex, tire "roll-over", and be less responsive due to extreme slip angles (tire pointing in different direction than rim in a turn). The same tire on a larger rim will result in a stiffer "spring", less side-wall flex, and more responsiveness. For any skeptics, I offer this test: Take your SC or Carrera on a "spirited" drive (or hot lap at your favorite track/AX course) with the stock 16x6&7's on 205's and 225's. Now swap the rims out with 16x7&8's using the same tires. You will be surprised at the difference.

[dd74]

...195 50 15s on a 15x7 rim. Is this possible? Or 205 50 15s? I was looking at putting 225 50 15s on the rear. Yokohama or Kuhmos, but now this debate has me wondering as to maybe using a smaller diameter 50-series tire...

My initial overall setup would be 205 50 15s front, 225 50 15s rear, with 15x7s all around. This is for a narrow body 911.

Anyone have any ideas?

Thanks.

[surflvr911sc]

Originally posted by Eric Coffey
For any skeptics, I offer this test: Take your SC or Carrera on a "spirited" drive (or hot lap at your favorite track/AX course) with the stock 16x6&7's on 205's and 225's. Now swap the rims out with 16x7&8's using the same tires. You will be surprised at the difference.

I'll second that. I noticed a difference right away after I went to 7 & 8X16s w/ 205 & 225s (same size tires).

I don't quite understand the other side though. Why would you want more sidewall flex in a car going in a straight line? It still seems to me that keeping the tire on the ground as consistently as possible would yield the better traction.

But then again, I really know nothing about tires other than what I've tried on my own car.

[Superman]

Big difference you say, Eric? Tyson thought it would be better but I'm not sure he had done a scientific test. Anybody else want to report experiences/wisdom regarding tread width versus rim width?

Surflvr, 205 is about the perfect size for 7" Fuchs, I am told. Or at least, the rim width and tread width will equal, or nearly so. For a 225 tire, 8" is probably nearest. This is if you want the rim and tread width to be the same.

I see quite a few souped-up 911s at autocross with wheels at least as wide as the tread, or wider.

Edit: Nope, I just ran the numbers and 205 is about 8", and 225 is nearly 9".

[mskala]

Originally posted by Superman
Edit: Nope, I just ran the numbers and 205 is about 8", and 225 is nearly 9".

Guys, these numbers are the section width or where the bulge is supposed to be in the sidewall. The actual width of the tread is usually significantly less.
Mark S.
'70 914-6

[Superman]

Okay. thanks.

[masraum]

I tend to agree, sidewalls with as little pooch as possible, or even the tire smaller than the rim thing is the best for handling. I don't have any hard data other than looking at lots of race (sports, F1, etc...) cars and noticing that they are that way. I have also hypothesized the same as Tyson that if there is more "pooch" then there can be more sidewall flex which is pretty obviously bad. Notice the Turbo uses 205's on 7's. I also remember a thread on the Rennlist where someone who had a racecar said they went to a larger rim but kept the tire size the same and that solved some handling problems that they had. Also I think in certain situations due to the short stiff sidewalls in the tires that we use I think you would run into more tread deformation than sidewall pooching like the drawing below.



When I bought my car the PO had been talked into putting 245 45's on the stock 7" Fuchs. It didn't look bad, but they definitely wore faster in the middle of the tread, and when I put the 225's on that I have on now the rear end is much more planted than it was (possibly due to tire make model change, tho')

The only problem with going too small like a 195 50 on the front of a car with 16x6's is that the load carrying capability of the tire will be much reduced which I believe could be a serious problem, overheating, blow outs, problems when you are pushing the car really hard which is the last time to have problems. Unfortunately for our wallets I think the best answer is wider rims for the same size tires.

[dd74]

What interests me is how a lightened 911 might react to 195 50 15 tires with seven-inch rims. If lightened, I figure there would be less propensity for "catastrophic" failure.

Also, what about braking? How would running a smaller side-wall smaller width tire effect braking? I figure braking distances would be longer, right with a higher tendency to "lock up?"

[masraum]

I would think the braking may be affected, but I think you'd have to be able to hit 9 or 10 10ths to really tell.

The difference in size may be anywhere from .2" to .5". On the bridgestone website they list the measured rim width for the 205 as 6.5 and the and for the 195 as 6 which I believe is the optimum wheel size for that tire, but I could be wrong. For the bridgestone RE 730 they list the tread width as 6.9 and 7.2 for the two sizes while for the Potenza S03 they list the tread widths as 7.2 and 7.6. I don't know how much stock to put into all of that, but there it is.

[Adam]

David,

I have 15x7 Fuchs all round on my '76. They currently run Pirelli P5000s in 205/60-15, which in my mind is way too big. I bought the wrong tyres. You will have problems with 225/50 on skinny-bodied 911s. My 205s are quite chunky and the right rear rubs on bumps at speed (it's lowered a bit too much... )



Check out that rear-wheel to fender clearance!

I want to change them for 195/60-15 fronts and 205/55-15 rears, or possibly 205/55-15 all round on some sticky Yokohamas or perhaps Bridgestones. I think it's a good compromise.

[dd74]

Originally posted by Adam Chaplin
David,

I have 15x7 Fuchs all round on my '76. They currently run Pirelli P5000s in 205/60-15, which in my mind is way too big. I bought the wrong tyres. You will have problems with 225/50 on skinny-bodied 911s. My 205s are quite chunky and the right rear rubs on bumps at speed (it's lowered a bit too much... )



Check out that rear-wheel to fender clearance!

I want to change them for 195/60-15 fronts and 205/55-15 rears, or possibly 205/55-15 all round on some sticky Yokohamas or perhaps Bridgestones. I think it's a good compromise.

Yes Adam:

I'm getting some chafing too. I'm either going 55 series or 50. During the TRE run, I saw a narrow body 911 with 225s in the rear on 15x7s. Evidentially a lot of camber was dialed in to get the tires to fit.

Why would you use different diameters between your front and rear rims?

BTW: gorgeous car. Reminds me of another that's near and dear to my heart.