Contour Enthusiasts Group Archives Forums Modifications Duratec Performance Acceleration with 18's

It is still possible for a wheel/tire combo to have this combination while still possessing a greater moment of inertia. The moment of inertia is a measure of the mass distribution of an object about the center of a rotational axis. Since most wheels are symmetric about their rotational axis, the moment of inertia is proportionate to the mass of the wheel/tire and the square of the radius that defines the average distribution of the mass from the wheel center. So if this radius from the lighter/smaller-diameter tire/wheel combo was greater than that of the heavier/larger-diameter wheel, there is a possibility that that factor could outweigh the mass difference between the wheels, and produce a greater inertia.

There we go! I knew one of you knowledgeable guys would cut in with the REAL explanation. Thanks alot, my physics classes are way too far... didn't do so good back then either

Ok, but what about the tire. We are talking about the mass of the wheel, but even further out is the tire.

Ah, I think I've answered my own question. Doesn't a lower profile tire have to have a thicker sidewall to support the same amount of weight?

With the lighter wheel 18" (or any 18" wheel for that matter), you get a bit heavier tire. Even if you keep the width the same. Or at the very least, the distribution of mass is even more skewed away from the center of the wheel.

So what we are looking for is the point on the wheel where we reach the average mass of the wheel.

So given two wheels of the same mass, the one where the average mass point is further from the center of the wheel will have a greater moment of inertia.

I've got to stop, my head hurts now.

But I think I understand where you are coming from.

TB

Well, to make this a fair argument we have to assume that we're using the same model of tire from the same company with the same type of material properties and tread patterns. Given this ideal situation, I'm not sure! lol
I would think it would have to have an even thinner sidewall because it's not as tall (thinking of the stability of a tall skyscraper versus a short building having the same width). I don't know if you can put it as easily into those terms, though.


Assuming the same conditions as above, not necessarily. Depending on the height of the sidewall, a smaller diameter tire can have a greater radius of mass distribution. Let's take 2 tires for arguments sake, say a 215/60-R16 and a 215/35-R17. Without knowing all of attributes of the tires accept that they have the exact same tread pattern, material density, etc., let's assume that the average center of mass distribution is half-way up the sidewall. The outside diameter and sidewall height for the 16" tire is 21.08" and 5.08", respectively. Those same numbers for the 17" tire are 19.96" and 2.96". That would mean that the average center of mass distribution for the 16" and 17" tire are 18.54" and 18.48", respectively. So it's possible, but I agree not likely.


Yes!


So does mine.

I thought a lower profile tire has a thicker sidewall because there is less air to support the weight of the car.

Let's take this physics a bit further to the tire.

At rest, all forces balance. So the force the mass of the car exerts on the tire must be equal to the force the tire exerts on the car.

If I am not mistaken, this force, or at least some of it, is measured as the air pressure in the tire.

If we have a tire with less interior volume (our lower profile 18 in this case) I think it would have to operate at a higher pressure to support the same car mass.

So I'm thinking the sidewalls must be thicker in a lower profile car.

But I could be wrong here

TB

Hey! Who told you I also had a low profile car!?!

I believe a tire's main function, when speaking in terms of supporting the weight of the vehicle, is to act as a container for the air pressure, not to use the rubber/belts to support the mass itself. For example, an unmounted tire can hardly keep its circular shape without any weight applied to it. One attribute a lower profile tire has is a smaller amount of surface to keep the air contained within itself (which leads to a higher pressure distribution on the rubber). The only aspect I'm not sure about is whether the ability to contain the air pressure or the ability to support the longitudinal/lateral forces created at the contact patch is the limiting factor when determining the sidewall thickness.

But as for needing a higher pressure to support the mass of the car, this also depends. Forgive me for using another example Let's say we have a vehicle that weighs 3000 lbs. Dividing this weight by 4 (750 lbs) will give us the amount of static load each tire must support. Let's assume the contact patch at each tire is 4"x6" (24 sq in.). This means the road is creating a pressure of 31.25 psi on the tire (and vice versa). Now if your tire presssure decreases to 20 psi, the contact patch must grow in area (to 37.5 sq. in) in order to support the 750 lb load from the vehicle weight. Obviously there is a limit to how much it can deflate before the wheel starts supporting the weight, and the lower profile tire has a higher minimum pressure for this due to the smaller volume of air it contains.

But to support a higher pressure, you need a stronger container.

So if we do need a higher pressure on the lower profile tire, you do need a stronger container to hold that pressure. Steel belts, etc not withstanding.

Cheers,

TB

Mmmkay, i'll put my $.02 in here. Pascal's car shouldn't be losing to the cars it did, and i highly doubt that it's due to something as small as a wheel/tire package. I also doubt that it's heat soak, because it wasn't *hot* outside that evening.

Trust me, I KNOW it wasn't that hot! And I've ran with Jeff before, even on a cold rainiy night and he's always kept right up with me. I've always had the 18's when I ran him, but from the input you guys are giving me, it seems like the problem would be somewhere else. Granted his mystique is really strong and he's a good driver, but I'm not too bad either, I honestly don't think it's driver related. Micah drove my SVT against Jeff and also lost big time to him. There's not much you can screw up when pulling through 3rd gear anyways... I would take Jeff on the launch because of my grippy tires but as soon as he'd hook up, he'd creep up until he'd downright pass me.

Here are the things I'm going to look into:

1. Change plugs & wires
2. Clean upper intake manifold & TB
3. Replace cat converter (plugged?)

Anything else?? Could the timing be off on my car, like retarded all the time?? Plugged fuel filter? Bad ignition coil? What do I need to check?