I'm thinking about getting SSR Comps and Pirelli PZero Nero M+S tires for my WRX. If I go with 17x7.5 wheels, the combination is almost exactly the same weight as the stock 16" wheels and RE92's. If I go with 16x7.5 wheels, I'll save at least 5 lbs per corner in unsprung weight. Which is better...wider or lighter?

They are both the same width? So which is better? I'd say the 16x7.

If youre talking stock 16x6.5 or 17x7, the 17x7 might have the edge, all other things being equal. And it might be close.

It really won't make any real difference with m&s tires...MAX performance summer tires would give the advantage to the 17x7.5

To improve handling, a wider wheel is always better, because it allows a wider tire, which in turn gives you a bigger contact patch, which is your ultimate objective for good grip. What matters is how much rubber ends up touching the road. That's another way of saying "You want the biggest possible contact patch".

As noted above, max performance tires on stock rims will give you more grip than a bigger, wider all-season or even "high performance" tire on a wider rim. Soft, sticky tires grip better.

I'd suggest 17 x 7.5 as the "sweet spot" for the WRX, but the type and quality of the tires has an even bigger effect than the size of the wheels.

The area of the contact patch is a function of the tire pressure withstanding the weight of the auto not the width of the tire.

A wider tire does not create a LARGER contact patch, it creates a WIDER contact patch.

a lousy diagram to explain the concept:

Narrow tire contact patch (front of the car is toward the top of the page)-
----
| |
| |
| |
| |
----

Wider tire contact patch - same area, different shape.


|''''''''''''''''''|
|_______|

You can even measure the weight of a car by knowing the area of the contact patch for each tire and the corresponding tire pressures. (Isn't physics fun!)

If you want better handling, put better (stickier) tires on the auto. No M/S tire will give you max performance.

Less unsprung weight is good > less rotational mass to accelerate and decelerate and less work for the dampers.

regards,

mch.

If you're looking for a larger contact patch, you wouldn't start by using a narrower tire. "Wider" is to "bigger" as "narrower" is to "smaller". Point taken.

Not only will the 16" tires have less unsprung weight, the weight will be closer to the center of rotation, reducing rotational inertia which will help acceleration, braking, and other fun stuff.

I've been told by wise autocrossers that the only reason to go with larger diameter rims is to fit bigger brakes.

Let me try to say this another way. Maximum effective cornering traction, acceleration and braking is a the result of balancing the tire compound (stickyness) with amount of tire contact area (pressure), the shape of the tire contact area (width) along with tire stability (aspect ratio) and optimum operating temperature (speed rating). We are talking track/racing use here - on the street it does not matter as you will never utilize much more than 5/10th of a true HP tire. - (If you do, you are racing on the street and this is not good.) So dv/dt, regarding width of rim and size of wheel - go with what floats your boat.

If the largest contact patch was everything, we would create it a la sand tires for beach racing. All we have to do is to lower tire pressure to the smallest one that will keep the tire on the wheel and live with a wallowing ride. This is not particularly useful for high speed cornering on tarmac. In this latter situation we are balancing tire sidewall rollover, turn in response, sidewall deformation and operating temperature to create the optimal amount of grip. Several of these parameters come into conflict as noted with the sand tire example. To optimize one parameter may very well lead to overall degradation of performance.

You may note that tire pressure recommendations for auto-x ing tend to run in the 40's and sometimes 50 psi range. This is done to keep the tire sidewall rollover within acceptable limits BUT it actually REDUCES the area of tire in contact with the tarmac. These higher pressures are often desirable because they create the sensation of crisp turn in response but they also can let the tire skate rather than stick. (a good thing if you want to create oversteer - just pump up the rear tires more...) However, unless you make objective tests you really do not know. The key principal to remember is you want to get the ENTIRE width of the tire working evenly and at optimum traction. This is not possible if the tire is rolling over onto the sidewall using only the very outside edge of the tread or if the alignment is not set up to take advantage of the extra width of the tyre.

All I'm trying to point out here is that it is simplistic to say wider is always better. Unless you are fully utilizing the performance of a given width, going wider may reduce the amount of traction available.

FWIW - The diagram in my post was hugely simplistic and meant only to make the point that discussing the size of the tire contact area is irrelevant. Two tires of the same pressure will have the same contact area without regard to width. It was not meant to suggest you want a narrow tire.

Have fun!

:eek: Holy sh**. Take a bow.

Edit:

Nobody thinks a tall sidewalled tire that would have to go on a 16" wheel makes for sloppy cornering. ???



Or especially higher speed cornering ie real world performance.

SSR 17 x 7.5 weigh 12~13 pounds...
stockers 16 x 6.5 weigh 15~16 pounds..

now tire weight depend on the manufacturer..

http://www.circlekarting.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=CR&Product_Code=82670

16x7 13.2 pounds.

I use these wheels on my CRX racecar in 13"x8" at 9.4 pounds each :D

jack