Contour Enthusiasts Group Archives Forums Modifications Wheels & Tires Why are larger wheels/tires supposed to make the car handle better?

I can't figure it out. I was thinking about it on the head earlier. Say you have the same car with 14" aluminum wheels and 17" aluminum wheels, same company model etc etc. The 17" wheels weigh more and thus would make the ride worse. They also would have more rotational intertia making the car slower to accelerate and brake. Presuming they were fitted with the same size tires (say 215/50) those should hold as well right? The only disadvantage I can see of having the smaller wheels is looks and possible issue with brake size? Am I missing something?

-Pete

Given the same model of tire and the same overall tire diameter (properly plus sized), the handling is very close. Tirerack has done some testing that I cannot find right now that showed very little performance difference between a 15" combo to a 18" combo. In theory, lower profile tires will have less sidewall flex to provide more traction (flat contact patch instead of rollover) and better steering response. But given a good high performance tire, the performance differen in the real world is small. However, there are key benefits for upgrading the size. The link below gives them, but they only matter if you are using apples to oranges comparisons, larger wheels that are lighter than current ones, higher performance tire than what you currently have.

http://www.tirerack.com/wheels/tech/perform.htm

The biggest probem I see is that there are not too many very high performing tires in 14" size. Most manufacturers are making their best perfroming tires in limited large diameter tires.

IMO, the sweet spot for most mid-sized cars is a light 17" wheel with 40-45 series tires. Great performance, good looks (most cars need to be lowered, though), and little worry about bending the rim on every expansion joint.

Aaron

also, the width of the rim/tire comes into play also....

having 7" compared to 5.5" of rubber on the road makes quite a difference

If I recall my physics correctly, a wider tire doesn't put more rubber on the road.

It does change the shape of the contact patch.

So given the same tire construction and same air pressures, but different width tires on the same car, the wider tire will put a wider, but shallower (from front to back) contact patch on the road.

The narrow tire will put a narrower, but deeper contact patch on the road.

However, both patches will be about the same surface area.

So a wider tire doesn't put more rubber on the road.

TB

You seem to be asking which wheel is better, regardless of tire size.

Holding sidewall height and wheel/tire width constant, and using the same model wheels and tires, but allowing the overall diameter to vary, the best answer (which will be obvious to you the first time you attend an autocross event) is to run the smallest wheels you can that will fit over your brakes. Why? Because unsprung, rotating weight is one of those very critical areas of your car's performance. Of course, there is a gear ratio consideration involved with changing your tire's overall diameter, but we'll save that for another time.

On the street you want to keep your OD as close to stock as possible, which is why we talk about plus sizing. Now, if you run the same width tire (205, for example) in a lower profile, you will actually have a wider, shorter contact patch than you did before, which theoretically means quicker turn-in and better grip. (remember, 205 is the section width, not the tread width...) To get all the advantages, though, of that lower profile, you'll want to replace your apples with oranges. That is, find a larger wheel that is significantly lighter than stock--perhaps even wider--something like the Kosei K-1, for instance )

Grassroots Motorsports did a test a while back on a Civic with progressively larger wheels. Surprisingly, even with the greater weight of the larger wheels, their autox times grew progressively better. Still, you want to go as light as you can.

GRM Plus-sizing Test

In most racing cases, wheel diameter is driven by brake diameter, and brake diameter is driven by heat management concerns. That is, the smallest rotors that are capable of managing the heat of track use are used. Beyond that point, larger rotors just add weight, as would the larger wheels.

Rogerb, you pretty much hit the nail on the head.

When you say section width versus tread width what do you mean? (ie. 205)

How much do you think the contact patch would change given the same tire, proper pressure, and same size (barring the diamater)

-Pete

Section width is the overall width of the tire at its widest point, when mounted on a wheel of certain width (known as the measuring rim width). Actual section width of a given tire varies a bit with wheel width, but the "205" is a nominal figure based on an agreed-upon rim upon which the tire is mounted before taking actual measurements. As you can imagine, a "taller" tire bulges more than a low-profile tire, so there is more difference between section width and tread width in a 70-series tire than there is in a 45-series tire. In fact, you may have to go to a wider rim to fit the same section-width tire in a lower profile, since there just isn't as much room to flex in the sidewall.

I'm not sure how much the contact patch would change. It's pretty much true that for the most part, it's not the size of the patch but the shape that changes. Now, if you're asking about the difference between a 205/45-14 and a 205/45-17, I would guess no difference. But between a 205/70-14 and a 205/45-17 you would notice a significant difference in turn-in and other handling measures. Of course, finding a "performance" tire in 205/70 is pretty much impossible, so you're really talking about tires designed for completely different purposes in that case.