Mark Dalen

I am happy to talk theory, but let's move on to going faster. What does this model of contact patch vs. pressure have to do with the real world? What does contact patch have to do with the speed at which you go around the track?

How does STATIC contact patch even relate to what the tire is doing at speed. Over bumps, camber changes, load shifts, slip angles and temperature changes?

Is there any point to this discussion other than trying to understand what the tire is doing?

BTW - If you or anyone else wants good info that is practical, read Carroll Smith rather than Rowley or Milliken/Milliken.

Mark Dalen

I just don't agree. Even if you could get two different width tires with EXACTLY the same characteristics, compound, etc. they are not going to follow your model. Think about this. Start with the car in the air. As you lower it, the tire contact patch is a long rectangle (very narrow). The force per square inch on the narrower tire is higher than the wider tire at the same total weight. Hence the rubber of narrower tire is going to squish more than the wider tire. At that point, we have changed the characteristics of the tires. Both are responding to air pressure but the narrow tire now has a different mechanical property and will behave differently.

The tire contact patch model has many variables. You are trying to simplify it to one or two variables and that does not match the real world behavior of a tire.

ryoung

Solely because the wider tire has a more uniform pressure distribution under the footprint, the wider tire has a higher cf and therefore more total grip. As the contact patch of the narrow tire wraps around the radius of the tire, the cf improvement (grip/load of the lower left quadrant) does not make up for the losses directly under the tire, where the cf falls off significantly.

BTW: This same behaviour of cf with load is the sole explanation of how a car's balance can be tuned with spring and/or sway bar changes.

<img src="http://pictureposter.audiworld.com/17157/tiregrip.jpg">

ryoung

The following is much condensed summary of a 2001 conversation I had with the race tire engineer responsible for the construction, compound design, and track support for the tires of six cars at the 24 Hours of LeMans. Two of "his" cars finished 2nd and 4th. Anyway, among the many things we discussed was the effect of sidewall stiffness on handling.

Our engineer reminded me that there's stiffness in three different axis, and they all need to be varied individually to obtain the optimum compromise. He emphasized that maximizing the size of the contact patch is critically important, and that he'd examine the patch by watching various tires from underneath thru a glass plate under various cornering and downforce loads.

1. Longitudinal axis, the "easy" one: Since high power is not a consideration with our cars, stiffer is generally better to improve braking response.

2. Lateral axis: Softer makes the tires more forgiving; but too soft can result in the inside of the tread lifting during corning, reducing the size of the contact patch, and requiring more static camber. Stiffer improves feedback to the driver.

3. Vertical axis: Since the tire is also a spring, too soft results in a spring rate that can't be controlled by the shock. Too stiff and under heavy cornering the tire will tend to tip like a solid barrel, rather than the outside sidewall collapsing before the inside sidewall lifts, and again reducing the size of the contact patch. Soft in the vertical axis requires less static camber, which is the opposite of what's required for a tire soft in the lateral axis.

Our helpful engineer said there's no easy way to predict the results in advance. I can just barely imagine trying to optimize a F1 tire under various conditions, with downforce varying by a ton or more depending on cornering speed. In any case, you can be sure that no two tire manufacturers have chosen the same set of design criteria - with the result that each tire requires experimenting with camber, tire pressure, springs, and shock adjustment to achieve the best performance.

Simple huh?

Tomasz@Startup

So far you have provided use with excellent arguments why my assumptions may not be applicable. Let me refresh what I am saying:
"Wider tire DOES NOT equal larger contact patch." I have learned from your posts. I have learned that you like Shelbys book more than Rowleys, I have learned and agree that a tire is not like a ballon, that sidewall stiffness adds to the equation. But you have failed to provide us with comments on the statement above. Is wider better? What happens when we install a wider tire? I think that there is a general belief that we must go wider to achieve more grip. But I am starting to believe that wider may not always be better, and that I should be happy with my 245's and focus on other parts of the car. I have seen Hondas pull high skidpad numbers with pretty narrow tires. What did the NSX run? 205s in front?

So, lets try the question again. Do you believe that a wider tire will give us a LARGER tire patch? Or, is your answer - "depends, and does not matter"? (I am afraid it is the latter, which may be correct, just not satisfying.)

FixedWing

Or pressures? The pressure of a tyre on the ground is not going to be even at all points.

FixedWing

This isn't the only factor but it is a factor. Even in straight line braking, wider can provide more traction. On cornering it is a lot clearer that wider does provide more traction.

Of course, there are lots of other characteristics that the tyre designer is looking at when he determines what tyre is ideal.

Stephen

ryoung

The pressure distribution under the contact patch of an individual tire is the key!!!

Lets assume we have 700 lb on the corner of a car with a narrow tires, and that the result is a long narrow 7" x 1" contact patch and an average pressure on the patch of 100 lb/in². Take a look at Figure 7 on page 15 in the pdf <A HREF="http://www.control.lth.se/articles/article.pike?action=fulltext&artkey=gaf%2B03">Tire Model for Braking and Cornering</a>. The example shows 7 different vertical loads in one patch, and they vary by nearly 50% from max to min. The average pressure may be 100 lb/in², but it varies from maybe 75 to 150 lb/in².

Now lets assume we have a wide tire with a short wide 1" x 7" contact patch. In this simplistic case, the pressure distribution across the wide patch is a uniform 100 lb/in². Obviously the pressure distribution is not uniform in one of those square inches, but the variation is less because a one inch long patch is smaller relative to the diameter of the tire than a 7" long patch.

And this brings us back to that chart of lateral grip versus vertical load. The total grip available from those two 7 in² patchs is significantly different, and the wide patch with it's more uniform load distribution will provide the most grip.

ryoung

The blue line represents your theoretically ideal world. Grip is proportional to load, and the coefficient of friction is constant (and based on the materials in question).

The black line represents the available grip of a real tire (whether cornering or braking) which is almost, but not quite, proportional to the load on the tire. As a result, the coefficient of friction between tire and road is not constant. If you double the load on a tire, the available grip does not double. My reply to Fixed Wing goes into detail about why narrow and wide tires behave differently.

<img src="http://pictureposter.audiworld.com/17157/tiregrip.jpg">

Imolavirus@BuehnEngineering

as load increases the corresponding increase in grip grows exponentially smaller.