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Discussion Starter #1
So just how do you tune tire sizes on a car?

I'm having fun playing with these tire & rear end calculators:

http://gs.tolan-hoechst.com/tirecalc.htm

http://powerdog.com/tiresize.cgi

http://www.csgnetwork.com/gearratcalc.html

Playing with these calculators, I began wondering about tires on performance cars. The Sky will come with 245/45-18s.

Now, why is it that so many performance cars have wider tires in the rear with a smaller diameter tire? Is this due to weight distribution, horsepower demand or what?

Here are some examples...


Model ........................ Front ............... Rear

BMW M3 ................ 225/45-18 ...... 255/40-18

Porsche 911 ........... 205/50-17 ...... 255/40-17

Aston Martin DB9 ... 235/40-19 ...... 275/35-19

Honda S2000 ......... 215/45-17 ...... 245/40-17

Corvette ................ 265/40-17 ...... 295/35-18

Viper ..................... 275/35-18 ...... 345/30-19

All the cars have in common wider rear tires than front, probably to induce understeer. I can understand that. But why do all the foreign cars have smaller diameter tires in the rear than the front? The 911 even steps down a full inch in aspect ratio front to rear. That is going to lower the rear end a lot and spin that rear much faster than the front tire.

The American cars step down in aspect ratio and step up in wheel diameter to yeild the same overall diameter front and rear. The foreign cars, even the Honda S2000, all step down in aspect ratio while maintaining the same wheel diameter front and rear.

Why would the engineers want the rear wheel spinning faster than the front? In addition, this change drops the ground clearance and centroid of the rear end by a half inch.

Not all foreign cars tune their tires this way as I found the Acura NSX has identical wheels and aspect ratio front and rear, merely widening the rear tires. While the Audi S8 and Subaru WRX STi both carry identical wheels and tires on all 4 corners, like the Sky.

Any hints would be appreciated.

I guess my point is, would it help handling to reduce the Sky's tires from 245s to 225/45-18s up front and drop the aspect ratio of the rears from 45 to 245/40-18s? And what affect would this have on handling. I'm a bit confused. I guess I figure if it is good enough for an all-out racing sports car like the Honda S2000, then it's good enough for the Sky.

Does anybody understand tire tuning? Can someone refer me to a technical website to try to understand these details? It is looking like black-magic at this point.
 

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jimbo said:
All the cars have in common wider rear tires than front, probably to induce understeer. I can understand that. But why do all the foreign cars have smaller diameter tires in the rear than the front? The 911 even steps down a full inch in aspect ratio front to rear. That is going to lower the rear end a lot and spin that rear much faster than the front tire.

The American cars step down in aspect ratio and step up in wheel diameter to yeild the same overall diameter front and rear. The foreign cars, even the Honda S2000, all step down in aspect ratio while maintaining the same wheel diameter front and rear.

Why would the engineers want the rear wheel spinning faster than the front? In addition, this change drops the ground clearance and centroid of the rear end by a half inch.

Not all foreign cars tune their tires this way as I found the Acura NSX has identical wheels and aspect ratio front and rear, merely widening the rear tires. While the Audi S8 and Subaru WRX STi both carry identical wheels and tires on all 4 corners, like the Sky.
first, the rear tires having a wider track, a lower ratio, and a bigger diameter all make the circumference of the tire the same if not longer than the front ones. this actually means that the rear tires will be spinning slower than the front ones.

as far as why would they have the different size, those are the drive tires... most of those cars have more than ample power to spin the rear tires effortlessly. so they have more rubber to help them grab the road in acceleration, not handling as you originally thought. the audi s8 and the subaru wrx sti are both AWD cars. so when they accelerate, they need the rubber on all the tires, not just the rear ones. and, when you see some tuners taking their cars to the strip to do their quarter miles, some will even use dougnuts for the rear tires if they have front wheel drive. again, because it is the drive wheels that need the rubber.
 

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Jimbo, a tour around the goodyear site yields the following info on three of the car/tire combos you highlighted. This info is only for certain goodyear tires and the outside diameter from the spec charts is only closely related to the rolling circumference (I sell 4wd tractors and r.c. is extrenely important factor) Most of the tire mfg websites have charts on the spec's of all the tires they have. You just have to snoop around.

The Honda tires, if Eagle F1 and I'm sure they are not the front is .1 inch smaller than rear.

The 'vette tire, is .8 inch smaller in front

The Porsche is .1 inch LARGER in front.

Of course measurements will change from mfg to mfg and tire pattern to pattern. The Acura you mention with a wider rear yet with same aspect ratio will be a taller tire, the ratio being between width and height.

Most of the performance tires in question are assymetrical meaning they are engineered to rotate only in one direction. Too much bite on a nose heavy car like the 'vette or Viper might be a handful to steer.

The Eagle RS-A on the Sol and Sky has an outside diameter of 26.6 inches. For an alternative tire size in the same tire the P225/60R-16 has the same diameter, and 44 lbs greater load capacity per tire but at a .8 inch narrower tread.

Eureka! The tires on the Sky/Sol are IDENTICAL in diameter to the tires (P225/60-16) on my '03 Grand Prix. So for anyone thinking that the kappa tires are huge monsters like offroad trucks get, think again, and again.
 

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jimbo said:
So just how do you tune tire sizes on a car?


All the cars have in common wider rear tires than front, probably to induce understeer. I can understand that. But why do all the foreign cars have smaller diameter tires in the rear than the front? The 911 even steps down a full inch in aspect ratio front to rear. That is going to lower the rear end a lot and spin that rear much faster than the front tire.

The American cars step down in aspect ratio and step up in wheel diameter to yeild the same overall diameter front and rear. The foreign cars, even the Honda S2000, all step down in aspect ratio while maintaining the same wheel diameter front and rear.

Why would the engineers want the rear wheel spinning faster than the front? In addition, this change drops the ground clearance and centroid of the rear end by a half inch.

Not all foreign cars tune their tires this way as I found the Acura NSX has identical wheels and aspect ratio front and rear, merely widening the rear tires. While the Audi S8 and Subaru WRX STi both carry identical wheels and tires on all 4 corners, like the Sky.

Any hints would be appreciated.

I guess my point is, would it help handling to reduce the Sky's tires from 245s to 225/45-18s up front and drop the aspect ratio of the rears from 45 to 245/40-18s? And what affect would this have on handling. I'm a bit confused. I guess I figure if it is good enough for an all-out racing sports car like the Honda S2000, then it's good enough for the Sky.

Does anybody understand tire tuning? Can someone refer me to a technical website to try to understand these details? It is looking like black-magic at this point.

those tires may not actually have a smaller diameter. the aspect of the tire is a percentage of the width. in the case of the porsche
Porsche 911 ........... 205/50-17 ...... 255/40-17
in the front the 205 is the width in millimeters, 17 is the wheel diameter, and the 50 is how tall the tire is, and is expressed as a percentage of the width. therefore, 50% of 205= 102.5. in the rear, 40% of 255= 102. therefore, there is a .5 millimeter difference between the front and rear tire diameter. it is probably this way because that is as close to perfect as they could get. They use thinner tires up front because they don't need as much traction as they do on the rear wheels, and making them as wide as the rear tires CAN (not always) make it slightly harder to steer, lessen the amount you can steer (due to rubbing,) and decrease gas mileage from more friction on the road (which is extra load.)
 

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One of the biggest reasons to have split tire sizes on a car that is close to 50% weight distribution is to make sure there is enough rear lateral grip and cornering stiffness.

In other words, many cases it is to ensure there is enough understeer.

Since the weight characteristics are close to being neutral, you may need a larger rear tire to bring the understeer up to a more stable level (and reducing lateral acceleration response time, better high speed stability, and transient lane change stability).
 

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kappaman said:
One of the biggest reasons to have split tire sizes on a car that is close to 50% weight distribution is to make sure there is enough rear lateral grip and cornering stiffness.

In other words, many cases it is to ensure there is enough understeer.

Since the weight characteristics are close to being neutral, you may need a larger rear tire to bring the understeer up to a more stable level (and reducing lateral acceleration response time, better high speed stability, and transient lane change stability).
or to reduce the amount of oversteer ;-)
 

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jimbo said:
So just how do you tune tire sizes on a car?

I guess my point is, would it help handling to reduce the Sky's tires from 245s to 225/45-18s up front and drop the aspect ratio of the rears from 45 to 245/40-18s? And what affect would this have on handling. I'm a bit confused. I guess I figure if it is good enough for an all-out racing sports car like the Honda S2000, then it's good enough for the Sky.

Does anybody understand tire tuning? Can someone refer me to a technical website to try to understand these details? It is looking like black-magic at this point.
sorry, i wanted to give my 2 cents on this before but i forgot to. I think it would be more beneficial to increase the width of the rear tires than it would be to decrease the width of the front tires. since they are 18x7 (i think, i could be wrong on that) you should be able to put anything from 225- 255 wide on there (I could be wrong there too.) so i'd say keep the 225's up front, and put some 255's in the rear, or even wider if you replace the wheels, ESPECIALLY if you decide to make more power ;-) basically, adding is better than subtracting. 225's are decent, but they aren't huge by any means. 255+ in the rear would be NICE just for the extra grip during acceleration, and less wheel spin through the turns
 

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Discussion Starter #8
I had quite forgot that aspect ratio modifies tread width and therefore varies.

Gentlemen, can you ever forgive me! :eek:
 

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Another thought on the tires:

We had a vigorous debate going at solsticeforum on wheel weight and something to certainly look at is the alternative tire that has the exact diameter, the P225/60-16's. Without weights of the tires themselves I can only speculate that the wheel/tire combo would be less. A way to check would be to select a combo at tirerack or elsewhere in each size and see if they have weights. The narrower tire would quicken the steering response but would have more sidewall flex at the same time giving up some lateral traction. Staying the same d all around though does two things, keeps the car level and keeps the speedo calibrated. Narrower would in my guess offer slight improvement in wet (rain or snow) due to increased psi against the road due to smaller footprint.
 

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Discussion Starter #10
Let me rephrase my question.

The Honda S2000 weighs 2840 pounds. The Sky, 2860 pounds.

Weight distribution for the Honda is 50/50 and the Saturn is nearly so (51/49?).

While the Honda has more power (240HP), the 170 HP Sky Redline will likely match that.

Yet Honda engineering tuned their tires with 215/45s up front and 245/40s in back to the Sky's 245/45s all around.

So what is up? Is the Honda fine tuned from years of development the Sky lacks? Does Saturn think it is unimportant to balance front-rear traction. If we assume the Honda is balanced, then the Sky is going to be subject to oversteer, won't it? Or is the power and weight too low to bother with the fine tuning the Honda displays, so it is more for show?

My biggest reason for asking is to reduce unsprung weight. I think I can save quite a few pounds moving from 245s on 8.5-inch wheels to 225s on 7.5 inch lightweight wheels. But I don't understand the affect if will have on the car's handling.

It will be very intersting to see if the Redline comes with 245/45s all around or a set-up similar to the S2000 or Corvette.
 

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My guess is the Honda, with a driver, may be below 50% weight distribution - forcing them to split their tires to get enough understeer.

BTW, there was a FWD pontiac (grand prix?) that acutally reverse split to accomplish the opposite - a smaller rear tire to REDUCE understeer.

There is a distinct advantage for cost - both in tire rotation/wearout and replacement - in having the same size tires all around. So a bit of front distribution is not a bad thing at all, and helps in overall "stability".

1% change in weight distribution for the Solstice may have been enough they didn't need to split tire sizes - assuming they were able to achieve everything else they wanted to do with geometry.
 

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All my FWD Grand Prix's and Bonnevilles as well as 6000STE all had same tire at all four corners. The interesting thing though is when putting on winter skins which are slightly larger even in same listed size, it is better to put snows as well on the back non-powered wheels. Got to be alittle bit of tuning involved that keeps rear from breaking loose that balances overall handling.
 
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