[miscreant]

All else being equal (e.g. vehicle weight, air pressure) the contact patch of a narrow tire is the same total area as the contact patch with a wider tire - it's wider, but it's also shorter.

What? shorter????

_______........____________
|..........|......|.................|
|______|......|___________|
patch #1........ patch#2

Which one has a greater contact patch? Both the same? I think not.

-Ron

 

[jag2001]

Whatever tire you want to use still has to be able to produce a contact patch that is big enough to support the car without serious deformation. A bicycle tire would just flatten from the weight of the car until you were sitting on the rim.

Theoretically, a 'slightly' narrower tire will have the same size contact patch as another tire if the psi, and mass it's supporting are equal. It will be narrower and longer.

A real-world tire, however, has other things affecting it than just the mass and psi. It has sidewall stiffness that helps support the mass. It has a curved surface meeting the road at the front and back of the part of the contact patch that is flat, and applying 'constant' force to support the car. It has a tread pattern with gaps that provide no support.

In other words, it's not as simple as mass/psi = contact patch size. And it's not as simple as wider tire=better tire (for handling and/or launching).

Dragsters run tires with basically no sidewall stiffness, very low psi, and sticky tires with no tread pattern. They are basically doing everything they can to increase the contact patch size. But those tires are only good at launching and running straight ahead.

This is really good info!!! I guess the guys running fuel cars and pro stock are after appearance. They ought to put bicycle tires on rear that are made from a sticky rubber (soft) compound similar to the 18" wide tires they are currently using. It should reduce unsprung weight a bunch and give them another 1/10 or maybe more in the 1/4 mile. Contact John Force, Larry Dixon, or Warren Johnson and let them in on your knowledge of physics and coffecient of friction idea and see what they have to say.


Jim

 

[Dubya]

lol

i learned somthing new, the wider the tire the worse the traction (as i roll my eyes)

if you swap out a 245/50/18 for a 285/50/18 of course you'll have worse gearing. you'd put a 285/45/18 so the overall sidewall is the same (thats just a guess, im not sure exactly what aspect to use)

you should tell gm this, cause those big old tires they put on corvettes are expensive, there just wasting money, they should run 185's all the way around if they want to compete with the viper

 

[JEM]

i learned somthing new, the wider the tire the worse the traction (as i roll my eyes)

All else being equal (and, as others have pointed out, all else is not necessarily equal) the size of the contact patch is a function of the vehicle weight and the air pressure in the tire. At street tire pressures, a wider tire will have a wider but shorter contact patch.

Usually, that provides better cornering grip - if the suspension design can keep that wider tread flat to the pavement. This is a traditional problem with Macstrut cars, they need to run a bunch of static negative camber in order to avoid rolling the outside tires over onto the sidewalls in corners.

Whether or not a wider tire produces better straight-line grip is another question, and one that's more dependent on tire construction and air pressure than on tire size. Drag tire construction is a special case that doesn't necessarily apply to street tires.

Corvette comparisons are also of limited value, because the Z06 has a rear-biased weight distribution, greater power-to-weight ratio e.g. more throttle-steerability, and is targeted at a market where ride quality is less important.

The point that should be under discussion is how big a front tire you can put on a CTS-V without clearance problems, and without destroying the steering feel, because in the end, for a road car, particularly for one like the CTS-V that's supposed to be a comfortable daily-driver, and that is at least slightly nose-heavy, you have to maintain balance even at the expense of ultimate grip.

 

[Ralph]

Regarding traction and width of tire contact patch, skinnier tires seem to grip better on snow and slippery conditions! The reason is more weight over a smaller concentrated area. The mags I had on my Charger in HS were fat, and there was NO traction with them. (in snow or wet conditions) Summer was fine. Think of it this way to help understand, in deep snow, you are walking with snowshoes, you DO NOT sink because you are distributing weight over a greater area, right? W/O the snowshoes, you sink up to your knees because there is more weight in a smaller, more concentrated area for the weight to be more effective.

I guess I'm saying that I think in certain situations, thinner tires can be an advantage. They can be just as good as low profile wider ones if they grip better through softer materials, etc.

 

[Dubya]

jem, is what your saying is that you can go too wide? to a point were wider actually hurts? this may be true, but on a fairly heavy 400hp rwd manual car, 245's are far from to wide, for optimal strait line traction, im sure a wider tire like 275's/285's would work much better. i have 235's stock on my 250hp car, which doesn't have much for torque.

 

[JEM]

As you said 'for optimum straight-line traction' you probably can't go too wide, unless the rear suspension has such poor camber control that under acceleration the wide tires won't stay flat on the ground - a common problem, for instance, with old swing-axle and semi-trailing arm setups (something large-diameter wheels and low-profile tires only make worse, because sidewall flex helps thoise suspensions a lot.)

What I'm saying is that if you set the car up for optimal straight-line grip, you're likely to sacrifice a lot of the suspension balance and 'feel' that makes a car like the CTS-V so worth having. In pursuit of that two-tenths off the line you never use, you may sacrifice turn-in and throttle steerability you use all the time.