17" or 18" Rims

[Mattitude v1]

Which wheel/tire size is better for hard cornering/steering and which is better for speed or does it not make a difference? 205/40/17 or 225/40/18
Currently looking at these wheels: http://www.wheelsnext.com/product.phtml?p=1322
Not sure what brand tires the package comes with yet. Any recommendations are appreciated.

 

[95GSXBLUR]

anytime you increase a wheel diameter, you will add rotational mass to your car. when you do that you will slow yourself down tremendously. if you want to do something like big brakes then get light weight 17"s.. with something like 225/45/17s tires on there.. there are a couple of posts you can search for with info on that.

18's will slow your car down, as will 17's regardless of their weight because they add the rotational mass. if you think about it.. what's harder to get started rolling.. something with smaller diameter or with a larger one.. i can't think of an analigy right now.. unless you're doin' a turbo kit or something, i'd stick with racing 16x7" light weights with some bad ass tires.. because if you don't you're just slowin' your self down dude.

good luck.. and keep racing

 

[wret]

anytime you increase a wheel diameter, you will add rotational mass to your car. when you do that you will slow yourself down tremendously. if you want to do something like big brakes then get light weight 17"s.. with something like 225/45/17s tires on there.. there are a couple of posts you can search for with info on that.

18's will slow your car down, as will 17's regardless of their weight because they add the rotational mass. if you think about it.. what's harder to get started rolling.. something with smaller diameter or with a larger one.

Mostly right.

It is possible to add to the diameter without increasing mass. And remember weight and mass are equivalent (here on earth anyway).

Mattitude, I asssume you are actually questioning three aspects of performance:
1. Acceleration
2. Cornering
3. Speed

This could be a good discussion. I think you'll find that for at least one of these bigger wins.

 

[jtmcinder]

Actually, he was more than "mostly right." Read what he wrote carefully. He wasn't talking about mass; he said "rotational mass" (by which he was referring to the rotational moment of the wheels). Take two tire & wheel set-ups, one 17" and one 18", that weigh the same. Now spin them. The 18" set-up requires more energy to start spinning and more energy to stop spinning. That's the real issue.

Or are you in denial about this? tee hee

- Jtoby

 

[wret]

Yes, I am in denial but that doesn't mean I'm wrong.

1. It is possible to have a set of 18 wheels with less rotational mass than stock 14's.

2. CRF (coefficient of rolling resistance) actually decreases as the size of wheel increases at least on nonplanar surfaces (roads).

These wheels did mach 1.

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[jtmcinder]

CRF (coefficient of rolling resistance) actually decreases as the size of wheel increases at least on nonplanar surfaces (roads).[/IMG]

No. All else being equal, the CRF decreases with increases in the total diameter of the wheel plus the tire. It is not the wheel's diameter that matters.

- Jtoby

 

[wret]

We can continue this but we should probably try to help Mattitude while we are at it. Mat, I notice the 17 inch OR52's don't list a width. And would they actually sell 205/40/17's for an Eclipse? If you are serious about any kind of performance, look for at least 7.5 wheel width, preferably 8 to 8.5. Then look for 235 - 245 width tires.

Jtoby, I wonder if one has two tires of the same diameter, one on an 18 inch wheel and one on 14, same tread width, same pressure, same materials, which would have less deflection, i.e. friction, i.e. rolling resistance?

 

[jtmcinder]

Jtoby, I wonder if one has two tires of the same diameter, one on an 18 inch wheel and one on 14, same tread width, same pressure, same materials, which would have less deflection, i.e. friction, i.e. rolling resistance?

There are two issues here. First, we have the curvature of the tire. This factors argues for the largest possible outside diameter (as I posted an hour ago when I said "all else being equal"). Second, we have lots of tire issues, including tread deflection and the energy lost flexing the sidewalls, which depend on lots of things, many of which are partly controlled by tire pressure. These might end up favoring the smaller wheel, but it would be silly to make a general statement, given all the variables.

But even if most of the variables end up favoring the smaller wheel, I really doubt that they will make up for the loss in HP, increased load on the brakes, and greater gyroscopic issues that a larger wheel brings with it. But since we don't have all the numbers needed to calculate a concrete answer, I'm willing to drop it.

- Jtoby