bigpete
Veteran Member
Tire pressure measurements state the amount of outward air pressure placed on the inside surface of the tire. Changing the weight of the tractor can dynamically change tire shape, but changing tire pressure does not change the weight of the tractor. There is a minor connection to ground pressure through changing tire surface contact area as follows:
Internal tire air pressure contributes to ground pressure only in the effect that it has on the size of the contact area of the tire on the ground. If you reduce air pressure, you may increase the total surface contact area of the tire and distribute the weight for that tire over a slightly greater area by changing the shape of the tire. Conversely, if you increase tire air pressure you can reduce the surface contact area of the tire.
This stuff really isn't too deep. As a simple example, let's put a 190-pound man on a 20-pound unicycle. Let's also say the tire on the unicycle has a surface contact area of 7 square inches under a 210 pound load (unicycle and rider). The total weight of the man and the unicycle (210 pounds) is assumed to be evenly distributed over 7 square inches. 210 divided by 7 equals 30 pounds per square inch. Put more air in the tire and you might be dividing by a smaller number. Put a fatter guy on the unicycle and you have more weight to distribute. Of course, adding the exra weight may slightly increase the surface contact area, but you get the (ugly) picture (of a fat man on a unicycle)...
It's tougher with multiple-wheeled vehicles because you must measure the weight per axle (or wheel), because it is very unlikely that the weight is distributed perfectly evenly on all wheels.
The bottom line is that the more tire surface area in contact with the ground, the less the ground weight--per square inch-- will be for a vehicle of a given weight. The post above (soundguy) describing a need to ban trailers with more than one axle--regardless of the size, seems to answer the original question. Stating a rule like that is a lot easier to enforce than weighing trailers and measuring the weight distribution and tire surface contact areas of trailers.
I just want to make sure that we keep fat men on unicycles out of my neighborhood. /forums/images/graemlins/crazy.gif
Internal tire air pressure contributes to ground pressure only in the effect that it has on the size of the contact area of the tire on the ground. If you reduce air pressure, you may increase the total surface contact area of the tire and distribute the weight for that tire over a slightly greater area by changing the shape of the tire. Conversely, if you increase tire air pressure you can reduce the surface contact area of the tire.
This stuff really isn't too deep. As a simple example, let's put a 190-pound man on a 20-pound unicycle. Let's also say the tire on the unicycle has a surface contact area of 7 square inches under a 210 pound load (unicycle and rider). The total weight of the man and the unicycle (210 pounds) is assumed to be evenly distributed over 7 square inches. 210 divided by 7 equals 30 pounds per square inch. Put more air in the tire and you might be dividing by a smaller number. Put a fatter guy on the unicycle and you have more weight to distribute. Of course, adding the exra weight may slightly increase the surface contact area, but you get the (ugly) picture (of a fat man on a unicycle)...
It's tougher with multiple-wheeled vehicles because you must measure the weight per axle (or wheel), because it is very unlikely that the weight is distributed perfectly evenly on all wheels.
The bottom line is that the more tire surface area in contact with the ground, the less the ground weight--per square inch-- will be for a vehicle of a given weight. The post above (soundguy) describing a need to ban trailers with more than one axle--regardless of the size, seems to answer the original question. Stating a rule like that is a lot easier to enforce than weighing trailers and measuring the weight distribution and tire surface contact areas of trailers.
I just want to make sure that we keep fat men on unicycles out of my neighborhood. /forums/images/graemlins/crazy.gif
