[Quicksilver]

Originally Posted by dd74

Have Formula One tires and wheels increased much in height and width within the last 20 or so years, or have the wheels just become lighter (stronger?) and the tires higher in grip? What about kart wheels and tires?

F1 is a weird little microcosm where the rules have made some anomalies. They had to add a rule that says that you have to have a working suspension. They have suspensions that have joints that bend instead of having some sort of moving bearing like the rest of the world. They have 13" wheels and tires with fairly large sidewalls that provide almost all of the suspension compliance in the car.

What they are doing real doesn't have anything to do with what the rest of the world is dealing with.

Originally Posted by island911

Yes, that is conventional wisdom. But ask your self, why? ...why is wider considered better? If both a skinning and a wide tire each are touching the ground with 30psi, and holding up 600_lbs each, then they should both have 20_in² of contact, each. But wait, conventional wisdom says wider puts more rubber on the road. Well, conventional wisdom is often wrong.(though well propagated) ..."wider" does not give more contact... just makes for a cooler running tire. (less deflection)
. . .

The 30psi x 600lb= 20sq" calculation is correct assuming that you are talking about a tire made of a substance that is flexible but complete resists stretching along the surface and doesn't have a compressible surface.

The fact is that a tire doesn't press into the ground at the uniform pressure equal to the internal pressure. The tire is compliant and will have portions that press into the ground at a higher pressure then the internal pressure. Because of the distortion of the shape of the tread carcass and the compliance of the tread rubber changing the size of the tire does in fact make a noticeable difference in the contact patch.

The most effective increase you can make to maximize the contact patch is increasing the diameter of the tire but this introduces a large number of other problems such as increased CG height, weird suspension angles, tire clearance, etc. Because this means designing a car from scratch we basically go for increased width.

The basic flaw in the "size doesn't matter" argument is it uses the flawed idea that a tires performance can calculated using a "coefficient of friction". A coefficient of friction is a standard physics concept taught in any high school so the repeated misapplication of this idea is understandable.

A tire doesn't have friction. It has grip. This is easy to prove. The highest possible coefficient of friction is 1. Tires have had coefficient of grip higher then 1 for decades so obviously it cannot be described as "friction".

What is the difference?
- Friction is the force that is transfered between the surfaces of 2 objects sliding past each other. It is simple to calculate force because if you double the force pressing the two surfaces together you always double the force required to get the surfaces to slide. Very easy to work with and kind of sounds like what our tire do...
- Grip is the force that is transfered between 2 object by an interlocking effect. With an interlocking effect the force required to get the two objects to slide past each other is not linear. It will in fact be a curve like the curve shown on a tire performance graph. For a blatant example of an interlocking effect look at a cog railway or a rack and pinion. Our tires do the same thing as the rubber interlocks with the rough surface of the road.

We have one more bit to add to our tires besides the mechanical interlocking effect with the road: Molecular Adhesion. The molecules of our tires can actually momentarily bond to the road surface as the tire rolls over it. This bond will be torn apart when the tire continues its rotation but if you are loading the tire at very high levels it can cause the bonded rubber to tear off the tire. This can happen without sliding the tire which is why high effort braking (without ever sliding) will significantly effect tire life. (A new one I learned from a tire engineer in the last year!)

The end result is that increasing the width of the tire will significantly increase the grip of the tire even if no other variables are changed. A lower level of loading will increase the coefficient of grip. Our cars have tires that are no where near the limits where additions to aero and parasitic drag will negatively effect performance (except for gas mileage) so put the rubber down. If you don't believe it, well fine, but you are going to get stomped at your next Auto-x if you have any decent competition.