951 Tire Sizes

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951 Tire Sizes

Postby PShipman on Thu Aug 23, 2007 1:27 pm

Since we are starting to count posts :lol: I decided it must be time for me to start writing!

I am trying to better understand how/why tire sizes for front/rear seem generally to be the same on Spec 944, -S and -P cars but seem to be quite different smaller-front/larger-rear sized on all out 951 track racers.

In my reading I understand that oversteer increases with speed and that a 951 GT class racer will be putting out nearly 600 hp. Is this the reason for the big difference in front to rear sizes?

Or am I misinformed or ???

Inquiring minds want to know :)
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Postby Kim Crosser on Thu Aug 23, 2007 8:24 pm

In the simplest terms, tire size relates to two factors - weight distribution (front/rear) and power delivery (which actually affects weight distribution under acceleration).

Lower horsepower cars with neutral weight distribution (or more weight in front - e.g., front-engined cars) tend to have even front/rear sizes. Although as the power goes way up (think 928's), you need more rubber in the rear to avoid excess wheelspin under acceleration.

911's, with lots of rear weight bias and lots of power, need more rubber at the rear, or you would have huge wheelspin and power-on oversteer problems.

Boxsters have very neutral weight distribution (slightly toward the rear), but have enough power that the rear tires are a bit larger than the fronts.

If someone built a front-engined, front-wheel-drive car with a heavy front end, you would probably see them running larger front tires than rear tires.

There is a cute physics problem which illustrates some of weight distribution effects - determine the weight of a car using just a tire pressure gauge, a piece of paper, a ruler, and a pencil.

Solution: roll a front tire onto the paper, mark the tire patch on the paper with the pencil, compute the area using the ruler (resulting in sq. inches), take the tire pressure (psi or lbs./sq. inch), multiply the tire pressure times the tire patch area to get the weight of the car on that wheel (lbs.). Multiplying that weight by 4 gives a pretty reasonable estimate of the car weight. If the front/rear sizes are significantly different (or have significantly different pressures), then repeat with the rear tires and multiply the front by 2 and the rear by 2 and add the results to get a better estimate.

Now picture a 2400 lb. car with even weight distribution on identical front/rear tires - each tire is holding 600 lbs. If the tire pressures are 30 psi, then each tire has a contact patch of 20 sq. inches.
If that same car has a front/rear weight distribution of 40/60 (40% of the weight in the front, 60% in the rear [just like a 993]), then the front tires are only supporting about 480 lbs. each, and the rear tires are each supporting about 720 lbs. The rears are now supporting 50% more weight than the fronts. If the tires are still the same sizes and same pressures (30 psi), then somehow the contact patch of the rears has to increase from 20 sq. inches to 24 sq. inches, while the fronts decrease from 20 to 16 sq. inches. This would produce much more wear on the rears as they are now deforming much more than the fronts as they roll along (in order to maintain that greater contact patch).
Increasing the width of the rear tires in this case allows increasing the contact patch without forcing the tire deformation. If the original 20 sq. inch patch was 6" wide by 3.333" front-to-rear, then increasing the tire width to 7.2" would allow the same front-to-rear patch depth at the same pressure.
In practice, as the rear tires increase in size the pressures are usually increased, so in this case you might use a 6.5" wide tire with a tire pressure of 33 psi to achieve the same rolling profile. (You also typically see a drop in pressure on the fronts, to increase that contact patch.)

On the 951, the weight distribution is nearly a perfect 50/50, but if it is boosted to nearly 600 HP (a Las Vegas one claimed 564 HP and 560 ft-lbs of torque!) the standard tires just aren't going to grip the road when you mash the accelerator.
(Remember your PCA-SDR Driving School classes - when you accelerate, weight transfers to the rear.)
With that much punch, you probably get something close to 100% of the weight at the rear under full acceleration, so unless the tire width is nearly 2x stock, it is either going to deform a lot or (more likely) just spin and smoke.
(This is one reason why dragsters use rear tires that are 17 inches wide and have 10 inch front-to-rear contact depth, for a contact patch of 170 sq. inches. Plus, there is a lot of science in how those tires change chemically and physically as they burnout and then race.)

Gosh, I knew that Physics degree would come in handy some day. :lol:
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Postby PShipman on Fri Aug 24, 2007 8:23 am

Kim,

What a great explaination ... 'course ... as in any explaination from a physicist ... I had to read it a couple of times for the explaination to sink in (or at least begin to :roflmao: )

So is it the deformity of the tire which makes it grip less or ??? ... this seems to make sense to me on an intellectual basis but some how I don't understand it yet at a visceral/gut level.

Perhaps I should read your note one more time :)

One last note ... I wasn't sure if I should take offense at the implication that a non-951/944 was a "lower horsepower car" ... if I look at the HP specs of your 2000 986 vs my 1990 944, I see 9 hp difference ... of course that MUST be the ONLY explaination of why your times are always better than mine, eh? :lol:

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Postby Kim Crosser on Fri Aug 24, 2007 9:25 am

Hi Perry.

You are correct on the power - my 2000 986 Boxster is rated at 217 HP. By the way, I consider the Boxster (at least the 986 base model) to be a "lower horsepower car" - certainly in comparison to the similar year 911s, etc. (No doubt this will produce some comments from the drivers of older 911s running 150 or less HP. :wink: )

I should have checked the Boxster weight distribution first - it is 46/54, versus the 944's 50/50. Ergo, bigger tires in the rear for the Boxster vs. same size tires for a 944.

The power determines how much weight transfers back under acceleration, and for us "lower horsepower" cars, it isn't nearly as much as a car with 300-400+ HP and 250-350+ ft-lbs of torque.
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Postby LUCKY DAVE on Fri Aug 24, 2007 9:15 pm

Speaking of 951 tire sizes, finding decent tires in stock sizes (225/50 ZR16 and 245/45 ZR16) with a (kss) class legal tread wear rating greater than 140, but less than ~32,000 is "interesting".
Not much choice between the forbidden "R" compound tires and rock hard Oldsmobile tires in those sizes anymore.
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Postby ttweed on Sat Aug 25, 2007 6:38 am

LUCKY DAVE wrote:... finding decent tires in stock sizes (225/50 ZR16 and 245/45 ZR16) with a (kss) class legal tread wear rating...
You might try the Hankook Z212--it comes in those sizes, has a 200 treadwear rating (0 points), and is fairly inexpensive. I haven't tried them myself, but have heard good things about them from the street-tire crowd.

http://www.hankooktireusa.com/product/Z212.pdf

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Postby LUCKY DAVE on Sat Aug 25, 2007 7:13 am

[/quote] You might try the Hankook Z212--it comes in those sizes, has a 200 treadwear rating (0 points), and is fairly inexpensive. I haven't tried them myself, but have heard good things about them from the street-tire crowd.

http://www.hankooktireusa.com/product/Z212.pdf

TT[/quote]

That's what I ordered. I hope they're decent tires, as they're the ONLY choice, unless I want to mix/match different brands on the front/rear. (I don't)
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Postby Dan Chambers on Sat Aug 25, 2007 1:28 pm

ttweed wrote:
LUCKY DAVE wrote:... finding decent tires in stock sizes (225/50 ZR16 and 245/45 ZR16) with a (kss) class legal tread wear rating...
You might try the Hankook Z212--it comes in those sizes, has a 200 treadwear rating (0 points), and is fairly inexpensive. I haven't tried them myself, but have heard good things about them from the street-tire crowd.

http://www.hankooktireusa.com/product/Z212.pdf

TT


Absolutely!! We have Z212's on both the 912 and the 911 for street/DE tires. GREAT tires. Very inexpensive for the grip. Very predictable at high speed or in a drift. Very durable. I DE on those in the 911SC and have been able to reel in some 993/996's on stickier tires than mine. Awesome tires with lots of confidence-building grip.

Oh, I think they are worth 1 point. Check the Rules carefully. Isn't it 201 and above = 0 points??

You won't regrat them. :wink:
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Postby ttweed on Sat Aug 25, 2007 4:54 pm

Dan Chambers wrote: Oh, I think they are worth 1 point. Check the Rules carefully. Isn't it 201 and above = 0 points??
Oops! You're right, Dan, 200 treadwear is still 1 point. I have always thought that's kind of a silly cutoff. It's the "Falken Azenis" rule, IMHO. In SCCA, 200 treadwear is a Street tire, not an R-compound. I hope they change that in this next year's rules revision. If we allow 140 tires in SS, why penalize a 200?

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Postby LUCKY DAVE on Sat Aug 25, 2007 5:49 pm

Yes, a tire treadwear rating of 200 is one point. But 2 points is class legal, and the car is otherwise box stock.
Treadwear ratings of 140 or less aren't legal in (K)SS.
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Postby Jad on Sat Aug 25, 2007 9:01 pm

Man, I go away for a week and decide to sell my 951 and they get 600 HP???? :shock:

I think you will find 951's generally have about 225-275 hp, unless they are WAY tricked out and unreliable. My tricked' out car which is quite fast has about 250 hp and I prefer the same size tire front and back for optimum balance. Often the factory puts less tire in front to dial in understeer for the typical owner.
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Postby ttweed on Sun Aug 26, 2007 7:42 am

LUCKY DAVE wrote:Treadwear ratings of 140 or less aren't legal in (K)SS.
Actually, treadwear rating of 140 IS legal in SS, it just can't be less than that.

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Postby bibbetson on Mon Aug 27, 2007 12:31 pm

Don't forget the marketing angle. I know, Porsche would never setup a car for the pure purpose of marketing - yeah right.

Look at two cars, one with staggered tires / wheels and one with the same size - which one is a higher performance car? Most people will say the staggered tire car even though they may be functionally equivalent.

You will also find different setups depending on the track use. Short courses / AX require a larger front tire to handle the plowing that slow corners naturally produce. High speed corners are much more rear biased and thus may not need as much front tire.
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Postby rshon on Thu Aug 30, 2007 1:00 am

I just wanted to add one thing to Kim's impressive dissertation.

The grip (which is actually friction between the tire and the road surface) is finite for a given contact patch and weight loading, but you can use it up in either cornering, braking, acceleration, or a combination of cornering and braking or acceleration. When you have a car that produces enough torque to break the drive tires loose, you can use up all the grip and have none left for cornering. That's why higher TORQUE cars will need bigger tires on the drive wheels so that they can accelerate and still corner.

p.s. Grip will go up dynamically (but temporarily) for tires that have weight shifted to them...

p.p.s. Another reason for putting a staggered tire setup is to increase understeer and to make the car more forgiving at higher speeds, for those who haven't attended the PDS.
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