performance decrease with 18" rim

About to buy a set of 06-07 spec b rims. My buddy keeps saying i am making my car slower. Right now i have the stock 05 Gt wheels and a stock sized tire. I cant see a having a huge decrease in performance with an 18" rim on 215/40/18. The spec b rims weight the same as the gt rims, and the 0-60 times for the spec b was the same as the gt.

 

Any thoughts

Your friend needs to stop being involved in your automotive decisions, because he isn't very good at it.

I think he has a point. Larger wheels (18" vs 17") will have most of its mass farther from center (.5" to be exact ;-) causing rotational moment of inertia of larger wheels bigger than smaller wheels even when keeping the total weight the same.

Whether you will be able to tell the difference or even measure the performance difference are separate story.

 

Krzys

 

PS Do not forget that GT and spec B differed not only in wheel size but tire type too.

GT had 215/45R17 Potenza RE92 all season (mediocre except rolling resistance). SpecB had 215/45R18 Potenza RE050 (I hope I remember it right) summer performance.

I think he has a point. Larger wheels (18" vs 17") will have most of its mass farther from center (.5" to be exact ;-) causing rotational moment of inertia of larger wheels bigger than smaller wheels even when keeping the total weight the same.

Whether you will be able to tell the difference or even measure the performance difference are separate story.

 

I was going to say something to that effect, however considering the GT wheels are larger, but weigh the same, that means they aren't the same design (obviously) which means the mass might not be distributed the same. Couple that with the fact that lower profile tires would be (I imagine) lighter, and I'm not sure the overall moment of inertia would be any higher. It might in fact be lower!

 

The point is, unless his friend has these kinds of mathematical specs worked out, he needs to shut up!

 

To the lab!

Yea i was not to sure. When I had my 95 Impreza and I went from 14" to 16" for autocross it felt a lot slower. But I figure the jump from 17" to 18" is not as extreme.

 

What tire would you recommend for spec B rims...high performance all season tire

Wait a minute. You ARE matching the tire size and rim size to get the same OVERALL diameter... right? The only reason you would have noticed a significant decrease in speed in your Impreza was if you physically increased the overall diameter of the tire by, say, mounting a tire with the same sidewall on your larger rim as the smaller rim. That would be the equivalent of making all your gears taller... and that's not the right way to go about what you are doing.

Stang, think for a moment.

The most wheel designs have the most of the mass not in the hub or spokes but the cylinder (I wish I knew what is the proper name of this part) where the tire sits.

18" wheel has this further from the center than 17" just because it has 1" larger diameter.

 

At least this is what I think.

 

Krzys

 

PS I am making wild assumption that the wheel width is identical (as it is 7" for both GT and SpecB).

I'm just saying that the tire, which sits even further out might be lighter. However that might also be a lie because I would imagine the sidewalls on low profile tires are thicker. I think at the end of the day we are both speaking based on what we think, lol. What we need is for someone who actually KNOWS to chime in and point out that one of us is being a moron, lol.

I personally think 17 and 18 are the best sizes you can go with with 17 just edging out 18. Car and driver did a test that is probably more comprehensive than any of us could do. Draw your own conclusions. Check it out:

 

http://www.caranddriver.com/features/effects-of-upsized-wheels-and-tires-tested

The most wheel designs have the most of the mass not in the hub or spokes but the cylinder (I wish I knew what is the proper name of this part) where the tire sits.

I've heard it called the "barrel." I'm willing to bet this buddy of yours is talking strictly from the perspective of tire circumference, which would still make him wrong, provided you're running the correct tire sizes.

 

Saying "you're making your car slower" without any supporting information isn't a good use of air.

This all seems like a moot point. With tire diameter the same and the rim being of equal weight, any loss of performance could probably only be measured throughout several laps of a road course running the vehicle at the absolute edge. Even then, the changes may be negligible.

 

However, last time I checked, the '06-'07 Spec B rims are a few pounds heavier than the oem 17" you are running now so you are mistaken and they are not the same weight Sources seem to indicate 22 pounds whereas the stockers are in the 18-19 pound range. More rotating mass can make a difference and maybe that's where your buddy is coming from.

The Impreza may have felt slower becasue of the weight difference from 14" to 16"

 

I saw that the GT tims were 22lbs on nasioc, could have been wrong. So does that mean the spec b is slower? Maybe by tenths of a second

The bigger rims will also throw the speedometer off by 1% or so. Which at 60 MPH looking at ~1 mile/hr diff. As someone said with the car and driver tests, the smaller rim will get you faster 0 - 60 and better MPG, but again only talking fractions. I have 18" rims on my other car, with same tires, and even though the car self will handle different, I can tell the size of rims make a difference in how it corners and road feel.

The bigger rims will also throw the speedometer off by 1% or so. Which at 60 MPH looking at ~1 mile/hr diff. As someone said with the car and driver tests, the smaller rim will get you faster 0 - 60 and better MPG, but again only talking fractions. I have 18" rims on my other car, with same tires, and even though the car self will handle different, I can tell the size of rims make a difference in how it corners and road feel.

Uhhh... that depends almost entirely on the tires. Any change in actual acceleration due to a change from 17"-18" tires is negligible.

As I said it's barely enough to make a diff. But larger rim, he'd prob change tires as well. That was more my point, since there was some discussion on speed change with different rim sizes. So yes there is, but very slight unless your change in size is dramatic. Handling will be different and might be more noticeable.

As I said it's barely enough to make a diff. But larger rim, he'd prob change tires as well. That was more my point, since there was some discussion on speed change with different rim sizes. So yes there is, but very slight unless your change in size is dramatic. Handling will be different and might be more noticeable.

 

You HAVE to change tires if you put a different size rim on the car. The whole point of putting larger wheels on the car is SUPPOSED to be to improve handling, which is achieved by fitting a lower profile tire. If you're doing it explicitly for the looks, then you are still going to fit a low-profile tire, so either way you get better performance.

 

You should always try to end up with exactly the same diameter. Not always possible, but I came pretty damned close on my car. I went from 15" rims to 17" rims and the overall diameter is the same... I think the circumference is technically 0.1 inches longer. So when you do the math, you end up with exactly the same speed on the speedometer.

 

As far as the change in handling and how it "might be more noticeable," I can tell you that unless you absolutely suck at driving, you should be able to tell a difference even with a 1" rim size change.