Kudos, for My first inclination would've been to jack up the car and look at pressure differences of unloaded vs. loaded tyres... then look at loaded wheel center height above ground vs. nominal radius, compute the the reduction in volume, and assume contact patch to be the area of the flattened section of the tyre cylinder... giving me a whole lot of parameters with wild inaccuracies and probably equally wild results :)
Weighing a Car with Tire Pressures
21–30 of 121 posts
Re: Weighing a Car with Tire Pressures
#22The 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.
I agree that higher tire pressure results in smaller surface area, but it's not clear to me that the tire itself does not support the car, skewing the result. So I'm not convinced this sanity check is sufficient. For example, as another commenter mentioned, run-flat tires can support the car on stiffness alone.
Re: Weighing a Car with Tire Pressures
#23Earlier quoted context omitted.
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.
I agree that higher tire pressure results in smaller surface area, but it's not clear to me that the tire itself does not support the car, skewing the result. So I'm not convinced this sanity check is sufficient. For example, as another commenter mentioned, run-flat tires can support the car on stiffness alone.
I agree, the sanity check is not sufficient. It confirms the concept that as pressure decreases, area increases, but is not sufficient to confirm a linear relationship. In the limit, the area clearly does not become infinite when pressure drops to 0.
For the case you suggested above, this is essentially how run-flat tires work. Of course, the sidewall of the tire isn't as stiff as the support ring, but they do provide some support.
Re: Weighing a Car with Tire Pressures
#24The 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 don't know how important the stiffness effect is for real tires, but I suspect they are sufficiently stiff that it matters a lot.
It's odd, because as I was reading your comment, I was thinking the exact opposite. I am considering the force it takes to squish a flat tire. A person can squish a (regular car's) flat tire with his/her hands.
My gut-check tells me that the stiffness of each tire can lift less than 10kg/20lbs... multiplied by 4, that still is not a huge contributor to the overall weight.
Re: Weighing a Car with Tire Pressures
#25Well, the idea is nice, but, as others have said, we need to account for sidewall stiffness. But we can do better with law of large numbers and employing metering tape to measure difference in car height with different pressure in tires. First we need to collect baseline data - tire brand, model and size, tire pressure, and car height difference , for known car weights, for two different pressures (say, 20 and 35 psi), this way we could deduce tire sidewall stiffness for given make/model/size of the tire. After that we can do the same for all cars with same tires, by measuring car height difference between two pressure points. Measuring height changes on both axles (wheel center to lowest point of the wheel arch) will get us weight distribution ; but we'll need to account for staggered tire setup .
Re: Weighing a Car with Tire Pressures
#26a similar was (maybe is) used by the California Highway Patrol to determine whether or not truck drivers are carrying too heavy a load, or too heavy a load down specific streets or highways.
my father was erroneously pulled over by a CHP that used a similar method, taken to a weigh-scale, and was actually under-weight. Wooops!
Re: Weighing a Car with Tire Pressures
#27The 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 don't know how important the stiffness effect is for real tires, but I suspect they are sufficiently stiff that it matters a lot. It's odd, because as I was reading your comment, I was thinking the exact opposite. I am considering the force it takes to squish a flat tire. A person can squish a (regular car's) flat tire with his/her hands. My gut-check tells me that the stiffness of each tire can lift less than 10…
It’s certainly possible my intuition is wrong. Run-flat tires can support the whole car, but it is possible this support only kicks in at very low pressure.
Re: Weighing a Car with Tire Pressures
#28Assuming all tires are the same, I think this would be most applicable for weight distribution alone, but not really for the weight. For performance applications, distribution is most important
The author also did not ensure the ground was level, which of course would have an effect on the weight distribution.
Re: Weighing a Car with Tire Pressures
#29How 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!
Don't worry. Fiddling with your one free parameter to make your model look good is as quintessential to statistics as Student's t-distribution.
Re: Weighing a Car with Tire Pressures
#30How 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!
I imagine you could just inflate the tires to the point of bulging at the treads to reduce the sidewall effects...