Argonaut

Hey Norm - thats good stuff. Can you do that calculation for 20" rims where the rim and tire weighs...lets say 10 additional lbs per corner (is that a good number?). I'm curious how big an affect installing 20s has.

Norm Peterson

Tire weights I can find, but wheel weights are anybody's guess depending on the design. The sheet is on the other computer, so it'll have to be tomorrow.


Norm

Norm Peterson

It's almost a wash at 5 lbs per wheel and identical tire weights, believe it or not. In the case of the 20's, you gain just enough on having a shorter rolling radius to pretty much make up for 5#/wheel. Keep in mind that rolling radius affects engine rpm, which affects where on the torque curve you're at. And when the weight differences are so small, the effect of overall gearing and torque curve shape become relatively more important.

[IMG]local://upfiles/62186/D8A5EC5D82FE4560B168E9993597285D.jpg[/IMG]

Here are the summarized differences between Car 1 and Car 2

[IMG]local://upfiles/62186/63517F7B68C74E6281A990B4E98AC8F7.jpg[/IMG]



I did find a couple of very minor things in the sheet that weren't quite right. They affected the 1# per wheel / 1# per tire scenarioby a maximum of about 0.001 seconds and 2".

Norm

Norm Peterson

Here's the actual input area. The torque and HP curves are fit to the HP and torque figures at their rpms plus an estimate of a low rpm data point. It's crude, but if you're comparing different gearing you do need to have some idea how the torque varies with rpm. Whether it's close much past the maximum HP point hasn't been all that important, as I normally set the redline within 500 rpm of where peak HP is developed.

[IMG]local://upfiles/62186/F87AE6FA061C470D92808E54739E1554.jpg[/IMG]

[IMG]local://upfiles/62186/7693B9E96C754E4FB81133AFCDE9B166.jpg[/IMG]

Black Sunshine 07

thanks guys. i don't think i can go wrong going from a 17 x 9 with 235/50's to an 18 x 9 with 255/40's b/c i want the chrome deep dishes and they aren't offered in a 17. but i can't decide if i want to go 18x10's in the rear so i can go wider tires or just stick with 18x9's all the way around. it's bad when you lose sleep over this stuff

xyster

18x10's in the rear!

(typo)

Norm Peterson

Don't you mean 255/45-18? A 255/40 is about an inch smaller in diameter than a 255/45 in the same wheel diameter, so unless you're reworking everything to suit 26" diameter tires . . .


How much wider of a rear tire are you thinking about? Might be that even 10" rims aren't really wide enough either.

Do you need/want to be able to rotate the tires?

Is having a wider rear tire worth the slight increase in understeer? If not, are you willing to retune the suspension?


Norm