MadSci Network: Physics
Query:

Re: how does a tire keep a car off the ground?

Date: Wed Feb 23 12:31:36 2000
Posted By: John Link, Physics
Area of science: Physics
ID: 951267433.Ph
Message:

The "30 pounds" that many people speak of is really a contraction of the whole description, which is "30 pounds per square inch", and is a measure of pressure. To obtain the force that this pressure can resist, you have to multiply the pressure times the area of contact of the tire, which would be "pounds per square inch" times "square inch", resulting in "pounds", which is force.

Let's estimate the contact area of a tire on the ground. Let's say the tire tread is 5 inches wide. Now, when I look at the tires on my cars it looks like the contact area of the tread is about 6 inches long (the "length" of the tread that is on the ground). That's 5 inches times 6 inches which is 30 square inches. Multiply that times 30 pounds per square inch and you get 900 pounds. Since my car has 4 tires, and 4 times 900 is 3600, the tires on my car can hold up 3600 pounds of car. Now, my car doesn't actually weigh 3600 pounds, so I have overestimated the contact area.

What actually happens is that the contact area adjusts so that the force is just equal to the exact weight of the vehicle. What happens if you load the car with lots of extra weight is that the tires get squashed down more and therefore there is more contact area on the ground.

John Link, MadSci Physicist


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