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Weighing a Car with Tire Pressures

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Re: Weighing a Car with Tire Pressures

#61

Post by frank2 »

tmoney1818 wrote: post #18
guygurari wrote: post #16

The 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.

He mentions the flat tire, but he ignores the fact the pressure of a flat tire is 0, leading his measuring method concludes that the weight of the car = F = PA = 0A = 0. His measuring method flunks his sanity check, but he never notices.

Let P_m be the pressure recommended by the manufacturer. A better sanity test would be to fill the tires to .75P_m, then to 1.5P_m, and see if the area in contact with the ground has shrunk in half.

Re: Weighing a Car with Tire Pressures

#62

Post by mauvehaus »

zten wrote: post #54
mauvehaus wrote: post #48

Earlier quoted context omitted.

I guess I'm way out of date. I remember hearing that people who ride e.g. PBP run way wider tires than normal road racers because of the road conditions, but I didn't realize that the mainstream road racers were running that wide now. Also: hookless? Like they got rid of in, like, the '70s or '80s? My beater road bike (that's older than I am) has 630mm hookless rims, and I got a discount because of it. Are the new on…

Well, a lot of this is driven by the gravel racing gang... the people partying at gravel events are already happy running 40+mm width, or MTB tires, on very wide rims. But there's also a growing segment of people who have figured out that you can have a bike that's a little bit more comfortable off pavement by going wider, without totally compromising speed on the pavement, and it also makes tubeless less likely to b…

That was informative; thank you. I’ve been pretty out of touch with the cutting edge for a while.

> I am unsure if this hookless is the same hookless that is from way back in the day. With the way things go in the bike industry (cyclically, no pun intended), I wouldn't be surprised if it were.

I’m sure they’re holding way tighter tolerances in manufacturing now than they were back then at the very least. Probably more on the tire side than the rim side.

You used to have to get matched sets of V-belts for multi-belt applications, table saws being a common one.

Nowadays, people mostly accept that the tolerances are tight enough that you don’t need to do that, at least for the lighter-duty uses.

Re: Weighing a Car with Tire Pressures

#64

Post by angry_octet »

Maybe use a lever instead? If you have a decent length of steel pipe or 2x4 and a trolley jack. Lever one end of the trolley jack up on a pivot, you should easily be able to get 20:1, within range of a cheap luggage scale or convenient heavy objects. Have to do front and back of course.

Re: Weighing a Car with Tire Pressures

#65

Post by frank2 »

If you drive the tire onto a rail half as wide as the width of the tire (specifically, the width of the area in contact with flat ground), then according to his logic, the length of rail in contact with the tire will be twice the length of flat ground in contact with the tire (assuming the contact area to be roughly rectangular in each case). I have no doubt that the length would be more, but severely doubt it would be anything close to twice as much because I think a large percentage of the "normal force" (the weight of the car) goes into keeping the tire bent out of shape in ways that do not contribute linearly to the area in contact with the rail.

My guess is that the tire's resistance to being bent out of shape is also a significant factor in the usual case of the tire's resting on flat ground and that he got as close as he did through a combination of coincidence and cherry picking (deciding after he knew the right answer how much the added gas weighs, changing his estimate of how much of tire's surface consists of grooves after he knew the right answer).

Re: Weighing a Car with Tire Pressures

#66

Post by beervirus »

mauvehaus wrote: post #34

Relatedly: if you and your bicycle weigh 200 pounds, and your tires are inflated to 90 psi (a very reasonable pressure for a road bike; racers run much skinnier higher pressure tires), your total contact patch is a little over 2 square inches. Think about that the next time you’re watching somebody fly down a descent going 45mph or more. That’s not a lot of rubber keeping you on course. As an added bonus, you can bas…

With ideal friction assumptions, the contact area doesn't matter. The friction force is determined by the normal force (weight) and the coefficient of static friction.

Re: Weighing a Car with Tire Pressures

#67

Post by jordanthoms »

cryptoz wrote: post #14

You 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…

That's amazing - the accuracy of phone barometers is great, I found they could reliably detect the difference in pressure from raising or lowering the phone a meter or so.

The compass is still terrible though!

Re: Weighing a Car with Tire Pressures

#68

Post by crazyjncsu »

jedberg wrote: post #58

> The first thing I wanted to do with my Saab 96 was to establish a baseline weight. The cheapest scales used for racing are over $700, You can do this for (almost) free if you go to the local dump. They usually charge you based on weight of what you dump by weighing your car on the way in and on the way out. Just ask them what the reading is on the way out. If you bring a couple of pounds of garbage, it will cost a…

Or go to the metal scrapyard instead, and leave with a few dollars more than you came with! (and get weighed) .. and if you have a pickup truck, you get to see them use this cool magnet to suck your junk out of the bed ;)

Re: Weighing a Car with Tire Pressures

#69

Post by kube-system »

throwaway0a5e wrote: post #56
function_seven wrote: post #9

How 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.

>How much does tire sidewall stiffness contribute to the force holding the car up? A car? Not much. At any reasonable driving air pressure the air is doing most of the work. A pickup/van with E or better rated tires? Eh, some but not much in proportion to the weight said tire is expected to carry, especially if the tires are anywhere near the top of their pressure range. A skid steer or some other off highway machine…

I'm sure the sidewall of a quality passenger car tire could hold 50 lbs just fine. A decent quality passenger car tire will hold my body weight if I sit on one tilted upright. It could be a significant percentage.

Re: Weighing a Car with Tire Pressures

#70

Post by jessaustin »

frank2 wrote: post #61
tmoney1818 wrote: post #18

Earlier 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.

He mentions the flat tire, but he ignores the fact the pressure of a flat tire is 0, leading his measuring method concludes that the weight of the car = F = PA = 0A = 0. His measuring method flunks his sanity check, but he never notices. Let P_m be the pressure recommended by the manufacturer. A better sanity test would be to fill the tires to .75P_m, then to 1.5P_m, and see if the area in contact with the ground has…

With a completely flat tire, the rubber sandwiched under the rim will transfer weight directly from the rim to the floor, so the pressure equation won't apply.
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