Not an easy one to answer. What's the exact size of the 18" tire? The "series" number alone isn't enough to know the rolling circumference of the tire...the "series" is an aspect ratio: sidewall height expressed as a percentage of section width. 215/40R18 isn't too far off...any wider (225/40, 235/40...), and I wouldn't be comfortable with the gearing change.

7 lbs is a pretty significant difference in wheel weight...decreasing unsprung weight is definitely a good thing to do.

I'd keep looking!

As for the width question, the width of the wheel doesn't matter (assuming equal weights) so much as the width of the tire. Compare a 225/45R15 to a 205/50R15 (sizes I'm more familiar with from autox)...they have essentially identical rolling circumference. The 225/45 is probably better for launch and braking...the wider contact patch lets you distribute the force so that the overall threshold is higher (more grip). Technically, I guess you'd suffer a little from increased rolling resistance, but doubtful you'd really notice. From a handling perspective, the comparison of wheel width to tire width matters. Jamming a 225 on a 6" rim isn't necessarily a good idea...the sidewalls may go too far from vertical to perform well, and the contact patch may become somewhat rounded.

Hope that helps some

Bryan

 

well inertia wise the inertia of the rotating tire increases exponentially with increased radius size and proportionally with mass increase... so you're better off doubling the weight rather than the radius.

 

well inertia wise the inertia of the rotating tire increases exponentially with increased radius size and proportionally with mass increase... so you're better off doubling the weight rather than the radius.

Si senor. Basically you could think of the heaviest part of both the wheels and tires as the very edge, since they have most of the weight at the edge. Tires are usually significantly heavier than rims at the edge, so when you put a tire on a larger diameter wheel, its going to affected by radius squared.

A good hypothetical way to look at it is as though its a ring (since thats what most low pro tires represent in easy terms). If the tire is a ring, its moment of inertia is MR^2 where M is the mass and R is the radius. Therefore, if you doubled the radius at which the tire was its moment of intertia would increase by 4 times (2^2 = 4). If you simply doubled the weight of the tire in the same position, it would only be twice as much of a difference. This is part of the reason why upsizing can be harmful when done drastically. Since most of the weight of a Rim/Tire combo is on the outside, when you plus size, it has more of an effect than simply what the new weight is.

 

yup... a 16" wheel has an 14% higher moment of inertia than the same weight in 15" wheel. a 17" wheel that weighs the same as a 15" wheel will have 28% more rotating inertia. If you're actually concerned about keeping your rotational inertia the same you have to find a 17" wheel that weighs 13lbs... Strong arguement agaist upsizing ... not to mention of course that if you're driving on a 13 lb 17" rim *if you find one* you'll have serious structural deficiencies. when autocrossing though you have to remember that when upsizing you decrease your profile height, which means more of the wheel/tire combo is composed of a harder material.. which is good (on the course). just one more thing that proves how everything in auto-x life is a compromise.

 

wow good answers guys
thanks
I had always wondered about that stuff