Originally Posted by godspiral
1. Can you explain how a squared off tire is different than having less air pressure in your tire?
2. doesn't the rear tire have a higher chance of skidding out in a turn (or is that just if there is sand/gravel)?
I am not a physicist, nor do I play one on tv... I didn't even stay at a Holiday Inn Express last night...
The answer to your forst question... sort of...
The contact area is not necessarily related to the flatness of the tire's profile. So, a flat tire is not the same as a flat tire...
An old trick... To weigh a car, inflate all tires to the same pressure and measure the contact patches of all four tires. Multiple the total contact patches (in square inches) by the tire pressure (in PSI) the reult is the weight of the car.
I believe that the same is relatively true for any pneumatic tires.
Disregarding the potential impacts of things like sidewall rigidity etc, a 2" tire inflated to 90 PSI has the same contact patch size as a 20 mm tire at 90 PSI... The patches are different shapes, but they will be the same size.
The size of the contact patch on a bicycle tire is primarily a factor of:
1) Tire pressure
2) Weight of rider plus bike plus anything being carried on the rider or the bike.
Any other aspects impacting contact patch are somewhat irrelevant.
Now if someone comes up with a good argument for the fact that an elongated contact patch is preferable to a square one, then we've got a different discussion. There is also an issue between treaded and slick tires... tread, especially aggressive tread robs power... As an example, that sound that knobbies make on pavement is sound generated by the tires, and sound generation takes energy. But, that is also a different discussion.
Barring differences in the compounds, which impact tires of all sizes, smaller tires tend to have lower rolling resistance than wide ones for only one reason... they can handle higher pressure and
this is what reduces the size of the contact patch...
I am not a physicist, but I believe that smaller tires of similar construction as larger tires can handle higher pressure because they have a lower volume of air at that pressure exerting stress on the tires bead... THe ability to use highwer pressure is also impacted by bead design on the tire and rim... and of course the composition and construction of the tire and rim...
The answer to your second question... I don't know, nor do I have any logical guesses.
EDIT: In my ramblings I never summarized the answer to your first question.
The answer I was trying to get to is that a tire with a flat surface will generally have a smaller contact patch than a round tire with lower pressure.