What is the theory/technical relationship on/between tire size and handling?

[BLS]

We all know the standard reasons for putting plus sizes on our cars... filling the wheel wells, looks and so on.

Does anyone have a solid and scientific explanation of when a larger tire size benefits handling?

This would include a discussion of why the stock size might be the best and how to know that. When a larger tire produces better handling, and how to know that.

Obviously wider results in a larger contact patch and better grip which is beneficial, but what about going from say 17" to 18", 19" or even 20"?

I started thinking (again) about why I ordered 18" for my M coupe and I automatically assumed they would help the car handle better, and I'm wondering why this is so.

[SleepRM3]

Larger wheels/tires aren't automatically going to increase handling. There are many negatives with running larger rolling stock--extra unsprung weight, extra rolling resistance, decreased steering feel are three that come to mind. One assumes that these three negatives are outweighed by the extra stick through the turns (decreased laptimes). One could counter the negatives by choosing lightweight wheels/tires to go larger (wider and bigger), but cost becomes prohibitive. It comes down to what you want out of your car. If it's looks, then you should be aware of the negatives, and enjoy. If it's for performance, then perhaps staying with stock diameters, and going wider in rim and tire width will be a good compromise between weight and stick. For me width is more important than wheel diameter. The only reason to upsize in diameter is if my brakes require the wheels to be upsized.
We all know the standard reasons for putting plus sizes on our cars... filling the wheel wells, looks and so on.

Does anyone have a solid and scientific explanation of when a larger tire size benefits handling?

This would include a discussion of why the stock size might be the best and how to know that. When a larger tire produces better handling, and how to know that.

Obviously wider results in a larger contact patch and better grip which is beneficial, but what about going from say 17" to 18", 19" or even 20"?

I started thinking (again) about why I ordered 18" for my M coupe and I automatically assumed they would help the car handle better, and I'm wondering why this is so.

[apollo322]

Larger wheels/tires aren't automatically going to increase handling. There are many negatives with running larger rolling stock--extra unsprung weight, extra rolling resistance, decreased steering feel are three that come to mind. One assumes that these three negatives are outweighed by the extra stick through the turns (decreased laptimes). One could counter the negatives by choosing lightweight wheels/tires to go larger (wider and bigger), but cost becomes prohibitive. It comes down to what you want out of your car. If it's looks, then you should be aware of the negatives, and enjoy. If it's for performance, then perhaps staying with stock diameters, and going wider in rim and tire width will be a good compromise between weight and stick. For me width is more important than wheel diameter. The only reason to upsize in diameter is if my brakes require the wheels to be upsized.

Exactly. For all the reasons mentioned above, you are usually better off with a set of lightweight rims in your stock tire size. Most of these cars now come with pretty decent sized rubber even in 17's, so changing to 19's doesn't really benefit the cars handling.

[BLS]

Very interesting, I hadn't thought of your first three. I am loosing around 15 pounds per wheel with the new wheels and I'm only going an inch wider and taller, so overall I'll still be loosing weight at each corner.

Loosing steering feel is not desirable to me at all.

With plans of a super charger, upgraded brakes will be installed and they will bring some weight loss at each corner.

I'm trying to understand the whole picture, so on the reverse side what are the positives and/or negatives of downsizing? I wouldn't do it, but I'm still curious.

I thought Tirerack.com might have a good article on this but I didn't see one.

[apollo322]

We all know the standard reasons for putting plus sizes on our cars... filling the wheel wells, looks and so on.

Does anyone have a solid and scientific explanation of when a larger tire size benefits handling?

This would include a discussion of why the stock size might be the best and how to know that. When a larger tire produces better handling, and how to know that.

Obviously wider results in a larger contact patch and better grip which is beneficial, but what about going from say 17" to 18", 19" or even 20"?

I started thinking (again) about why I ordered 18" for my M coupe and I automatically assumed they would help the car handle better, and I'm wondering why this is so.

You want to make that coupe REALLY handle better, get a set of these. I've driven the coupe with 19's and 18's, and nothing made the car handle better than these rims with some michelin Ps2's on them:

[BLS]

Those look great, and extremely easy to clean!

The style looks familiar, but I was thinking they were TC Kline. I see the BMW logo in the middle though, so which style are they?

Also, what is the size and weight?

The RS-GT that I ordered are 20.7lbs front and 21lbs rear. I love the look of these rims but I like the clean style on this Estoril Blue M Coupe.

The stock suspension leaves a lot to be desired and I'm hoping all the parts to the TC Kline True Match kit will be in this week. I placed the order what seems like over two months ago!