Dustydog said:
Thanks for all that Prof.
I fully understand your point regarding angle of attack. Please will you clarify one more thing for me?
We all agree from a safety point of view the bigger the actual contact foot print the better the road holding will be . Well I think so??
We agree tyre pressures will change both footprint and angle of attack.
In truth is the difference in foot print size from small to large diameter tyres that different?
Hello Dusty.
I'll respond to your points in reverse order.
Footprint area; Pressure is by definition force per unit area, hence being described as lbs per square inch, or Newtons per square meter or as a unified units Pascals or Bar.
For any given load on a pneumatic tyre the contact area with the road will be fundamentally determined by the pressure inside the tyre. There are some other small factors such as caracas strength, but these are small in relation to real world tyre loads and pressures. So broadly Load divided by Pressure = contact area.
This applies regardless of the diameter or circumference of the wheel.
If you are comparing two tyres of different diameters but importantly retaining the same width and load, then the length of the contact area (the chord) will be the same. But the attack angle will be different as the inclusive angle of radii from the transition points to the axle will be smaller on the larger diameter wheel.
In the main wider tyres do tend to offer more grip, but there are limits as to how far this concept can be stretched. Consider the vehicles designed to traverse soft snow. They have big tyres or tracks so the pressure (Load per unit area) they exert on the soft snow doesn't cause the snow to deform much. The same is true of tyre rubber but in this case you need enough pressure to cause the rubber to deform and conform around the irregularities of the road surface to produce a mechanical lock. If you increase the surface area too far and thus reduce the pressure, the tyre won't conform and will slip across the tops of the road irregularities.