Jimbert

I found this, believe it or not, in the comic section of today's newspaper. It is the answer to a "Dear Beakman" letter. It's a thing for kids, but hey, we are all just kids with expensive toys!

What you need:
Tire pressure gauge
measuring tape or ruler
2 pieces of cardboard with straight edges

What to do:
The car must be parked on a smooth surface like a garage floor. Put one piece of cardboard on the floor against the front "edge" and one against the rear "edge" of a tire. Push them tight against the tire, and make sure they are parallel to the tire edges and each other. Measure the distance between the 2 pieces of cardboard, in inches. Call this distance L.

Move one of the cardboards to the inside edge of the tire and the other one to the outside edge of the tire, again keeping them parallel. Measure the distance between the cardboards in inches. Call this distance W.

Measure the tire pressure in the tire, in pounds per square inch. Call it P.

Now multiply the 3 values, LxWxP. This will be the weight that the one tire is holding up. Repeat for the other 3 tires, and add all of them up to get the total weight of the car.

You can do this with metric units too, as long as you use metric for the distance and the pressure measurements.

Theory:
LxW gives you the area of a tire's contact patch. In english units, tire pressure gauges give you pounds per square inch of the air that is supporting the car. So when you multiply the area by the pressure, you get how many pounds that tire is supporting.

I don't know how accurate this technique is, and I haven't tried it yet.

Bryce

We took the air pressure and measured the wheel base, then measured up the makings on the graph paper the tires made for the area. The accuracy of Beakman's method is not that good, unless you have slicks on your car when you measure things. It would be interesting to see what error snows compared to slicks and other summer tires could create with his method though.

huachele

distribution on my Avant. I'll post the results ASAP

Pelican Pete

The air in the tires is not actually supporting the car's weight. Instead, the air is supporting the cords of the tires, which is what actually supports the car's weight.

While it is a nice experiement for kids to try, I do not agree with the idea that we should lie to children, just to give them something to do. They don't want to be patronized -- they want to learn. And we should not feed them misinformation.

-- Peter

Damon (Jeepin 4 now)

The Beakman experiment is flawed. I was about to post the same thing. Technically, the air alone is not supporting the weight of the vehicle. If the car were sitting on four pistons, THEN the air (minus x figure for stiction) under the pistons WOULD be supporting the car.

You may also be able to estimate the weight of a vehicle by putting it onto lift, filling the tires to a prescribed PSI, and then lowering the car back to the ground. You could measure the change in tire pressure to determine vehicle weight, but the formula would be relatively complex and would have to account for the air volume in the 4 tires, as well.