brendon

I went from stock WS6 rims and tires to an 18x9.5" Z06 rims in the back with a 285/35. I know that bigger rims usually rob a little power but my question IS, the overall tire diameter is the same with the 18" as the 17", and the tires normally attributes to the majority of the weight right, so does it really hurt the performance that much?

Ragtop 99

iTrader: (7)

The maximum (outside) diameter is the same, but the minimum (inside) diameter is increased, so the average distance from center is increased, thus requiring more torque to spin. Also since the rim has a greater circumference and thus requires more metal. An increase in tire width to the 285 from a 275 tire can add more mass too.

WeatherGuy

There are two issues with bigger rims:

1) Angular inertia. The extra mass you have in your tires/wheels represents mass that must undergo angular acceleration. That can rob a significant amount of power (can produce physics equations here) - mass on your car that must be spun will slow the acceleration of your car more than mass that merely has to be accelerated in a straight line.

2) Sidewall flex - larger rims decrease the amount of available sidewall flex in your tires. Under hard acceleration, sidewall flex both allows you to maintain better traction and also acts like a minature torque converter. You lose this significantly as you go to larger rims.

Having said all of this, larger rims on the street give you improved cornering/braking/handling, and also allow you to get a larger tread contact area (mostly since wider widths are available at larger rim sizes). It is just not optimal for the strip.

Jacobi

Here's one way to look at it...

Get a yard stick and put a big c-clamp on the end of it, now swing it around (be careful)!

Now put the clamp near the base of the stick and swing around again.

When it's near the outside it's harder to get started, but keeps moving easier. Same goes with rims...further the weight is out, harder to get it spinning but its easier to keep it spinning (this means, harder to accelerate, harder to brake)!!!

DCPunkSteve

In terms of overcomming initial wieght, you could simply choose a higher quality lighter rim, and thats the end of that issue. Now a taller tire will take from your low end, and increase your top speed. A larger tire on a larger rim, will probably wiegh more, adding to the wieght problem (even if you use a rim that is lighter than the original) ect. The bottem line is use some commen sense with it. If the Zo6 can get way with 18" rims, so can our cars.

CMNTMXR57

Actually, there is still a slight size difference.

275/40-17;
Sidewall = 4.330"
Radius = 12.830"
Diameter = 25.661"
Circumference = 80.617"
Revolutions/Mile = 785.931

285/35-18;
Sidewall = 3.927"
Radius = 12.927"
Diameter = 25.854"
Circumference = 81.223"
Revolutions/mile = 780.067

This will lead your speedo to be .7517643448910745% to slow.

fastlif

iTrader: (1)

If you are talking only POWER and not ET, the answer is NO. You will never notice the difference in POWER between stock wheels and z06 wheels. The only way you will see, feel a difference is if you go to a heavier(way heavier) wheel. z06 wheels should be lighter or damn near. You can argue about tire weight etc but not enough to matter.

IF you are talking ET or drag racing the traction is the issue. less sidewall means less traction usually. Of course if you are going from 16" goodyears to 17" drag radials you are still goint to improve ET.

brendon

Good info, thanks everyone, I got rid of my stock wheels so long ago that I have forgotten how much of a SOTP difference there is with bigger rims. The rear set up I have now is much lighter than the 17x11 Zr1's w/315's I had on the rear before, I could tell the difference just by lifting them up, BUT the 18's don't really feel any different as far as power. I am under the impression that it is not the weight so much as the diameter of the tires (not for E.T. but for street driving feel). The last thing I want to do is spend $1000 on headers just to compansate for nicer looking wheels <img src="graemlins/gr_barf.gif" border="0" alt="[barf]" />