i have too much thinking time on my hands

[a4wdhybrid]

stock wheels on the car now are 16", stock tire size is 205/55/16...im running a 205/50/16 so the tire is a little smaller in diameter

i have a set of 15" steelies w/ 195/65/15...stock tire size..stock rolling diameter size

i know youve posted before about wheels/tires making a difference on the dyno

you think one of those would give a better # over the other one? the only thing i think about is the smaller diameter tire changing the gear ratio some

opinions other than im a total nerd..?

[evan@absolute]

The dyno shows a big difference from 17-19". Almost 5% iirc. However, I was reading what bigblue beast(fastest 1/4th mile S4 in the US) was saying on aw and it makes some sense. He said that he likes to run taller tires so he doesn't have to shift as often and therfore keeps the power on more. This makes sense when you look at evos and wrxs that are very strong because the gearing is so low but they have to shift so often that they have a lot of down time. I am thinking about trying some taller tires here soon to see if I am faster. For dyno numbers or say a third/fourth gear race on a closed course using professional drivers and not trying this at home the smaller diameter the better.

[a4wdhybrid]

so i should stick w/ what i got? the smaller ones...will do

[sweaty meat hammer]

Dude, just buy my black 17" reps. They are the fastest.:D

[sflemon]

Now, think about this. Some 17" wheels are actually lighter than same model 16" wheels, same for other sizes sometimes too. Lighter=better, right? For the suspension this is correct. Just depends on how MUCH lighter. Sometimes it's worth it to stick with 16" if it is an ounce. Why? That 17" wheel is lighter but has a larger polar moment of inertia, and thus takes more power to turn. For the stiffer sidewall of a skinnier tire on the 17", just get up-speed rated tires. Much stronger sidewalls. Not necessarily applicable to what you guys were talking about, but neat nonetheless. Might consider a gearing change to get the total # shifts down instead of tires.

[evan@absolute]

I can say from personal experience that light wheels didn't do shit at the dragstrip for me. Same size but over 10lbs lighter per wheel and the results were no better then dropping 40lbs from any other non rotating part of the car. Certainly lighter is better but I don't think the difference is really in the rotating mass thing that would have made the most sense.

[a4wdhybrid]

it would be a lot harder to change a gear or two in a longitudal tranny than in a transverse car

i dont even know if different gears are made for longitudal stuff

most ppl just swap trannies

[sflemon]

Heck, if they don't have one, make it. I'm sure somebody would buy it. HPA has a set for 4cyl and 6cyl MKIV's.

[a4wdhybrid]

its tons easier to disassemble a transverse tranny as opposed to a b5/b6 tranny

just ask evan ;)

same motor...two totally different drivetrains

[evan@absolute]

tires are cheap as hell compared to any gear set installed.

[volthause]

It's not important what the wheels weigh, but where on the wheel the weight is. Most weight is concentrated around the outer rim area for strength.

The farther that weight is from the center (hub), the more difficult it is to accelerate it. So a 17" rim that's 1 pound lighter than a 15" rim might actually take more hp to spin since it's weight is concentrated farther out from the center. (Totally non-scientific, just an example.)

[pinky]

true.. it's all about rotational inertia

[thom337]

In Physics rotational inertia is called moment of inertia where torque = moment of inertia * angular acceleration and power = torque * angular veloctiy...I think...I crammed all that for my final and don't remember half of it :smack: .
To calculate moment of inertia of a hoop (not going to be completely right, but closer to a wheel than just a solid circle is), use Mass * Radius^2...for a solid circle its .5 * mass * radius^2 . Power equals torque * angular velocity.
And then...ahh I don't remember anymore :crying:...enough of equations. Basically your moment of inertia is the resistance of the body to change motion. The further out you position material from your "pivot point", the greater the radius becomes. Because the radius is squared in determining moment of inertia (I = M*R^2), it is much more important than just mass, because it increases the moment of inertia exponentially. So, lets say you have a 15 inch wheel weighing 10kgs and a 17 inch wheel weighing just 8kgs. The moment of inertia of the 15 inch wheel is 10kgs * 7.5^2 = 562.5
For the 17 inch wheel its: 8.5^2*8 = 578....ok my head hurts now, if I can remember more of this crap I'll put it up later.

[evan@absolute]

you will never have enough time to calculate each wheels pros and cons. One is lighter,one is bigger, one is smaller and one is heavier. Then you have to look at the tires that will go on each wheel. Say you run a 60 series on a 15 but a 40 series on a 17. The 17 weighs close but where on the tire is the weight? Then there is the difference of gearing. Then there is the difference in contact patch. Is bigger better? At first yes but after you gain traction no just more fricton and more rolling resistance. I think lighter is better and after that it will all depend on the car you are running it on. It will end up just trial and error.

[thom337]

you will never have enough time to calculate each wheels pros and cons. One is lighter,one is bigger, one is smaller and one is heavier. Then you have to look at the tires that will go on each wheel. Say you run a 60 series on a 15 but a 40 series on a 17. The 17 weighs close but where on the tire is the weight? Then there is the difference of gearing. Then there is the difference in contact patch. Is bigger better? At first yes but after you gain traction no just more fricton and more rolling resistance. I think lighter is better and after that it will all depend on the car you are running it on. It will end up just trial and error.

Yeah, theres definitely an element of trial and error there...way to many factors to actually calculate out. I was just trying to explain how the whole "rotational inertia" thing worked numerically, and how radius is often more important than mass when dealing specifically with the force required to turn something.