Dennis,

I'm not sure about all that. For an example, I would offer a skid pad test. I am no expert on this, but I assume that a skid pad is set up with a very even surface. Negating many of the bumps, and pebbles and cracks in the road we experience in day to day driving. However, I would also assume that as a rule, wider tires are going to net higher g numbers on a skid-pad -- no?

 

Hello Tom,
Many years ago, while in college (mechanical engineering) and racing motorcycles, I made it a term project to study the relation between tires and traction. Wow, it is ever so complicated. There are probably a hundred variables. Coef of friction is only one. However, the mechanical interlocking of asphalt and rubber is probably the most significant one. An average tire on average asphalt is about .8 to.9. Nice new sharp edged stones in new asphalt and new soft rubber is much higher. Velcro is about 6.0. (I heard that dragsters are now hitting 5g on takeoff. My previous comment was concerning coef. of friction not bumps or cracks. Sharp edged asphalt is the key word.
Looking through some older engineering texts right now, there are some explanations of different tire contact patch areas and tread stability. Most require finite element analysis to reach a determination of traits. A tire under heavy acceleration loads deforms the casing in front of and behind it as you can see when watching dragsters takeoff. A tire with a wide contact patch is less likely to deform and ripple the contact area than a tire with a long contact patch. This is in opposition to a tire that is cornering that has a side load.
All in all, the reasons that tires are so much better now than yesterday probably owes a lot to computer analysis. I remember that after 12 weeks of study, I was still at the beginning.
Dennis

 

Hello again Tom,
I just read all the posts on this section and I believe that the answer to your original question can be illustrated by tuurning your tire pictures so we are looking at them from the side.
If we start out with an engine of 350 lb-ft of torque and multiply that by 12 for the gearbox and differential, we end up with 4200 lb-ft of torque to pass through he tires. That is a lot of torque. Imagine the twisting forces passing through the axel to the rear of the contact patch and how they would deform, buckle, and mangle the contact patch as it's smoking, heaving and hopping. A 2 x 12 patch will should deform less than a 4 by 6 inch patch since it is shorter in the direction of load. There is also sidewall deformation which will have an affect how stable the contact patch is. When the sidewall and contact patch go all wonkie, friction will be momentarily lost.
I may be wrong, but if you slowly drag a Corvette behind a tractor, the drag will be about the same on tall skinney tires as short wide ones.
I believe that your question is better answered as an engineering/distortion problem than a physics/friction one.
Dennis


[Modified by Dennis Rech, 12:07 AM 8/11/2002]

 

Tom,

There is still a problem with your fundamental assumption that the load on the tire divided by the air pressure will yield contact patch area. That assumption treats the tire wheel system as if it were a simple column of air which it is clearly not. The tire, wheel and pressurized air are a mechanical system working together to carry a load. The correct interpretation of the air pressure is it presses with 30 psi in all directions over the entire area inside the tire and wheel. This is many square inches and results in many tons (20 to 30) of force. The air’s resistance to be compressed, the wheel’s resistance to flex, the tire casing’s elasticity, and the tire tread’s compound ability to deform all contribute to the final size of the contact patch. I can give a couple of examples to illustrate the point. If the tires casing were constructed of a very thin and elastic material (like inner tube material) the contact patch would be very large even at relatively light loads, regardless of the internal pressure of 30 psi. The flexibility of the tire casing would have a great impact upon the size of the contact patch. Low aspect ratio tires, like 35s tend to have less flexible casings by design. Tire designed to with stand high speeds and not over heat also must have less flexible casings. C5 tires by design have to have less flexible casings. Tread compound also plays a roll in contact size. Imagine a tire with the tread band being constructed of 2” thick foam rubber. The foam rubber being easily deformed would give us a large contact patch even when mated to a very inflexible tire casing. Soft tread compounds then would tend to increase contact patch size at the expense of tread wear. C5 tires by design have soft tread compounds. The key to this is understanding that the goal is spread the load over the largest possible area to produce the lowest pounds per square inch loadings. That is why slicks have no tread pattern. Drag slicks, especially those used in top fuel, take all of these tire characteristic to the extreme. Very wide pattern less tread made of an extremely soft compound mated to a very elastic tire casing. These tires deform under load and compress the tire sidewalls to produce a very large contact patch. The casing again shows its flexibility by growing taller as the speeds build. Tires of this design would not withstand any kind of long durations as the heat build up from the flexible casing would soon destroy the tire. The loaded contact patch true area is a function of tread pattern, tread compound, tread width, casing flexibility, and overall tire diameter. Casing flexiblity is tied to aspect ratio and temperature rating. Tread compound is tied to tire mileage wear.




[Modified by Terry Humiston, 1:02 PM 8/11/2002]

 

Good points Terry. I would think that on an top fuel dragster, as the slick balloons during lauch that your contact patch would shrink, therefore you would want to go as wide as possible in relation to your hp/tq and given geometry of you car. I know there is a point where you've gone to wide. You do want to cut down on the weight of the rotating assembly, not to mention the extra drag.

 

They also run 5psi in those tires. Increase the pressure to 30psi and watch the contact patch area shrink.

Not to discount all the other points you made, there are a lot of variables here. But contact patch area is still tied to the air pressure in the tire. Probably more so with a dragster tire than a C5 tire, because it's so flexible and acts more like a simple balloon that our C5's tires.

I think we all agree (or at least I hope we do) that on our C5s, a wider tire is going to have a shorter contact patch (same tire brand, same psi, etc). Although how much shorter is still up for debate.

The question is 'why' is a shorter/wider patch better for handling and/or acceleration?

 

As load on a tire increases, the traction of the tire decreases at a very slight rate. So a tire supporting 800 lbs may support side loads up to 1000 lbs (or the equivalent of 1.25 Gs). But that same tire with only 400 lbs to support may support 520 lbs of side loading (or 1.30 Gs).*

Wide and Thin contact patches are known to have high levels of sideways grip, and abrupt breakaway characteristics (especially when coupled with wide cars with short wheelbases). Wide and thin contact patches were developed on racing cars in the late 1960's and made their way into typical automobiles in the mid 1980's. For racing cars, a low profile into the wind was helpful, so making tall tires was not in the cards. In order to fit in the wheel wells of the target audience, these tires had to expand the contact patch latterally (because a tall wheel that dosen't fit in the wheel well won't sell!) It is easier to widen the wheel well than it is to make it taller (with the ground clearance remaining the same).

Long and narrow contact patches are known to have better acceleration and deceleration characteristics than latteral acceleration characteristics. Thats why dragsters use tall tires with rather square contact patches.

*Although an individual tire can support 1.3 Gs of sidways force, a car with 4 of these tires only achieves 1.0 Gs due to weight transfer, camber curves, bushing deflection, springs, anti-roll bars, and other suspension effects.