Is 4.4 seconds vs 5.6 seconds alone worth $10k?

[dsm363]

Goyogi, thank you so much for doing that. Now I know I'm going with the 21s.

What's your reason? I'm probably going with it more for the looks knowing the tire options are limited and ride isn't as nice.

[dadaleus]

What's your reason?.

wider = more tire on the road right? I like that. Before that I was considering getting the aeros since I like the extra range benefit more than cosmetics. But the benefit of more contact with the road tips it the other way for me. And I was annoyed that I'm paying for the 21s as part of the sport anyway.

[jerry33]

wider = more tire on the road right?

Incorrect. The amount of tire on the road is almost entirely determined by inflation pressure (There is a small amount that is determined by casing stiffness).

Wider tires mean a shorter wider contact patch. Narrower means a longer narrower one. Wider tires have more G-force at the expense of easier hydroplaning.

[dadaleus]

Hmm... Confused again. I assumed a 'wider contact patch' would mean better traction / shorter breaking on a dry (I'm in San Diego after all) road. Sounds like you're saying it's the same amount of contact with the road though.

[jerry33]

Hmm... Confused again. I assumed a 'wider contact patch' would mean better traction / shorter breaking on a dry (I'm in San Diego after all) road. Sounds like you're saying it's the same amount of contact with the road though.

Yes. The contact patch area is load per wheel divided by psi. so if the load per wheel is 1000 lbs and the tires are inflated to 50 psi (for ease of math) then the contact patch is 20 square inches. In a tire has 5 inches of tread width then the contact patch is 4 inches long (actually there is a slight ovalness to the contact patch which is dependent on the stiffness of the belts, but for rough estimating 5 x 4 will do). If the tread width is 7 inches then the contact patch length is 2.9 inches.

The trick here is "all things being equal". All things aren't equal because the tread compound and the belt construction are likely to be different between the two tires. So the wide tire may stop shorter, but that won't be due to the wider width. Tire design is actually very complex.

[dadaleus]

A little surfing around and I found this thread. Best bit I found and it also fits your contact patch shape explanation Jerry33:

Probably the best, short explanation is from Paul Haney's book "Racing & High Performance Tire":

"For the same vertical load and internal pressure, a tire with a wider tread has a shorter, wider contact patch than a narrower tire. The area of both contact patches is the same if the internal pressure and the load are the same. . . A shorter contact patch at the same slip angle begins to slip at roughly the same distance from the leading edge as with a long contact patch. But the shorter contact patch has more of its length stuck to the road than the longer, narrower contact patch; and therefore a larger portion of its overall area is gripping." - Why Wider Tires Are Better, p. 101

There is more on the physics in that thread too, especially the fist post. So this sounds like a benefit to me.

[goyogi]

Rim width is measured from inside to inside, not from the outside edges, so the actual rim widths are 8.5 and 9.0.

Thanks for the correction. That makes sense that it should be measured on the inside.

[jerry33]

There is more on the physics in that thread too, especially the fist post. So this sounds like a benefit to me.

Just remember that there is a difference between race track and real-world road conditions (and hopefully driving style ). I'm pretty sure that everyone in this forum has experienced a car slipping sideways on an expansion joint during a high speed freeway exit. A wide tire will slip a lot more than a narrower one. If the contact patch is long enough there won't be noticeable sideways slippage.

[zdre]

Incorrect. The amount of tire on the road is almost entirely determined by inflation pressure (There is a small amount that is determined by casing stiffness).

Wider tires mean a shorter wider contact patch. Narrower means a longer narrower one. Wider tires have more G-force at the expense of easier hydroplaning.

The widths of stock tires for 19" rims and 21" rims are going to be exactly the same, so the only factor for contact patch will be the inflation pressure.

[rogbmw]

I think that the most important factor to me would be mid-range acceleration, not just off the line speed from a dead stop. Of course, I am REALLY spoiled because my daily driver presently is a 1 series M Coupe. Most of us, probably like me, are not a teenager or an early 20's person who wants to light up the rear tires from a stop light. If the performance model has seriously increased mid-range punch, that would be a no brainer.