Tire size vs acceleration/speed

[jsmithcds]

Backround:
I bought the car to play with on the track with 6 others who have basically the same cars but stock asthetically. I put 12" kenisis rims on the rear with 9"s up front. Will they be able to kill me on the track because the the extra rubber/weight? Anyone with first hand knowledge of acceleration and/or top speed differences?


79 3.0 sc with flares added
ssi's
turbo ties
pop off valve
new shocks all around
good year g force t/a kd tires
engine otherwise stock
Kinesis k27 rims


here is the car with the old rims and tires

[porschenut]

I think you'll find that, with similar cars among you, it's the driver that will make the difference rather than the tires.

I also think those wheels/tires are huge overkill for an SC. There just isn't enough horsepower available to make use of all that rubber.

[moneymanager]

Extra rubber or weight will add unsprung weight, reduce handling. Should have very little to do with top speed or acceleration. More to the point, what is the diameter of the new wheel and tire versus the old? If you've gone from 15's to 17's for example, you will lose acceleration roughly in proportion to the amount of the change. It's like putting higher gears in the tranny.

[IROC]

I went from 225/50-15s to 275/40-17s when I flared my car. This was with the stock 2.7. Overall, lap times came down due to the incredible increase in stick, but I did notice that the car was slightly more sluggish coming out of low speed turns due to the flywheel effect of the larger tires and wheels. Not only were they larger in diameter, but the weight was generally located farther out, so it takes more energy to accelerate that mass-at-a-distance thing...

Having said that, I wouldn't worry about it. It's not a huge difference.

[jsmithcds]

I realize theses tires are over kill. $1250 for the set with rubber. Rims are in really nice shape and they bolt right up. They look incredible on the car. I want them for the looks. I need a set of track rims and tires because the tires on the car now, while new, will not hold up to Road America in May. That being said the kinesis rims already have perfect track rubber on them. I am looking for input to see if this is a good idea to buy these. I will spend close to that amount putting new rubber on my existing rims. Hope this explains the advise I am looking for.
If I put anything skinny on the car I will loose the desired asthetics. Would I be better off finding a skinny tire and spacers for track day applications?

they were 17's on the car before
they measured 8 inches wide in the front and 10 inches in the back. New ones measure 9.5" and 12" respectivly.
I have never driven a stock sc on the track so I have nothing to compare it to until i get behind one of my buddies. I have however race with them in a hopped up Subaru Legacy 2.5 gt and we were all pretty much dead even. I could dive into corners much faster than they could and they could beat me in accelaration out in the straights.

[Bill Verburg]

They difference in weight is not likely to be perceptable unless it's greater than ~10#/corner

the difference in rolling radius is very noticeable.

As an example here's a comparison of thrust curves for tires that are fairly close in rolling radius.



The absolute value of available thrust goes down, more so in lower gears than upper and the speed where max available thrust occurs moves to the right(higher speed)

It's easy to do these charts if you have the torque #s from a dyno and know your gearing and tire loaded rolling radius.

[Paulporsche]

Aside from the above, don't forget that an overall smaller diameter tire will yield a quicker acceleration time, while an overall larger diameter will yield a higher top speed. This is w/ the same gearing.

[Jack Olsen]

The diameter of the rear tire is what's going to affect your effective gearing, and it's a common misconception that there's a direct relationship between rim diameter and tire diameter. Some 18-inch and 17-inch tires will have smaller overall diameter than 15's, even in the same width. Some are bigger.

But if you and your friends are new to tracking the cars, these kinds of differences won't matter at all. The fastest guy will be the one who's the best student, since track driving has much more to do with learning the skills involved in track driving than which car you're driving. I've passed a GT3 in a rental Mini Cooper, which of course doesn't mean the Mini has anything on the Porsche. That guy just had less experience and coaching than I did.

[jsmithcds]

Thanks for all the info fellas. Any others that actually have track experience with this size of tire/rim combo?