Remarkably inaccurate.
This is a negative result.
Weighing a Car with Tire Pressures
11–20 of 121 posts
Re: Weighing a Car with Tire Pressures
#12Pretty difficult to get any form of accuracy with this because you can't easily account for tyre sidewall stiffness
How does sidewall stiffness matter? The ground is holding the car up; the air is not.
Re: Weighing a Car with Tire Pressures
#13This method doesn't account for the support of the tire itself through the sidewalls, as opposed to the pressure contained within it.
You should be able to estimate this contribution by fitting to the curve of area vs pressure at lower pressures. For example, the linear F=PA relationship clearly breaks down at 0 pressure, as F>0 and A>0 when P=0.
Re: Weighing a Car with Tire Pressures
#14You can weigh small objects with most smartphones with decent accuracy these days using the barometer. Put your phone into a ziplock bag, blow full of air, zip closed. Take a small object with known weight to calibrate your scale. Carefully place the object on the ziplock bag with your phone inside. The barometer in your phone can measure the air pressure change by having the weight placed on top of the bag. Knowing the weight of your first object and the change in pressure observed, you can calibrate the scale to know how much pressure change correlates to weight placed on top.
Then you can weigh arbitrary objects using your phone! Be careful, don't break your phone (don't try to stand on it to weigh yourself!)
Re: Weighing a Car with Tire Pressures
#15I wonder if you could do even better by putting paint or ink on the outside of the tire, driving over a piece of paper, and seeing what fraction of the paper touched your tire (and thus what fraction of the rectangle boundary actually touches the ground).
And to measure how much of the grooves are in contact with the ground, you could add a known weight to the car (like 100kg of people), measure it with and without that weight and see what proportion of the grooves you need to account for to get 100kg difference.
You'd get better accuracy from a bigger added weight, but the bigger the added weight the more you risk changing what you're measuring by forcing more tire rubber into contact with the ground.
Re: Weighing a Car with Tire Pressures
#16The suggestion is to use F=PA to measure the car weight, where P is the tire pressure, A is the area the tires in contact with the ground, and F is the unknown: the weight of the car (measured in terms of gravitational force).
I don't think this works. To see why, suppose we make the tires out of a very stiff material. We control the pressure P, but changing the pressure will not change the surface area A. Therefore, by changing P we can set F to whatever we want, which shows that we are not actually measuring the weight of the car. The basic issue here is that the air pressure is not the only thing that is supporting the car.
I don't know how important the stiffness effect is for real tires, but I suspect they are sufficiently stiff that it matters a lot.
Re: Weighing a Car with Tire Pressures
#17Pretty difficult to get any form of accuracy with this because you can't easily account for tyre sidewall stiffness
How does sidewall stiffness matter? The ground is holding the car up; the air is not.
In the case of a perfectly rigid sidewall, the internal air pressure could be 0 and the car would still be held up.
Re: Weighing a Car with Tire Pressures
#18The suggestion is to use F=PA to measure the car weight, where P is the tire pressure, A is the area the tires in contact with the ground, and F is the unknown: the weight of the car (measured in terms of gravitational force). I don't think this works. To see why, suppose we make the tires out of a very stiff material. We control the pressure P, but changing the pressure will not change the surface area A. Therefore,…
I think he nixes your case at the start, when he goes through the sanity check thinking about the flat tire. Also, I think your rigid body case use the walls of the tires to apply force to the care, not the force from the pressure to keep the car static, which is why it fails.
Re: Weighing a Car with Tire Pressures
#19Pretty difficult to get any form of accuracy with this because you can't easily account for tyre sidewall stiffness
How does sidewall stiffness matter? The ground is holding the car up; the air is not.
The quickest way to realize the contribution: Imagine the tire is made of steel so it does not deform when the air is let out. This would be terrible to drive on, but it would not require inflation even though it's hollow like a rubber tire. A rubber tire will usually not "go flat" even with the rims/wheel installed even without pressure (aside from atmospheric which is present when it's put on the rim). So some load is supported by the structure of the tire itself through the side walls. How much will vary with the tire.
Re: Weighing a Car with Tire Pressures
#20How much does tire sidewall stiffness contribute to the force holding the car up? I know that can vary a lot. Some cars have run-flats that won't sag when they lose pressure (or won't sag as much, anyway). Low profile tires probably transfer more of the load through the sidewall than doughnut tires. And of course the contribution from the sidewalls (as a percent) will change as tire pressure increases.
Oops that's a great point. I completely forgot about sidewall stiffness. My messing around with treads was probably just me adjusting things to make myself look good then!