Weighing a Car with Tire Pressures • Hacker News Board

Re: Weighing a Car with Tire Pressures

#31

Post by mhh__ » Mon, Jun 29, 2020, 11:10 PM UTC

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.

Depends a lot on the car and tyre configuration. In F1 cars the suspension is so stiff that the tyre stiffness plays a large role in VD.

I have some data on this for a few different tyres but I don't think I can legally quote it here :(

Re: Weighing a Car with Tire Pressures

#32

Post by mulmen » Mon, Jun 29, 2020, 11:11 PM UTC

nickff wrote: post #12

swsh wrote: post #8

Pretty 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.

I can drive my car safely at 55mph with 0psi in the tires. I believe this methodology would suggest my car is weightless in such a situation.

Re: Weighing a Car with Tire Pressures

#33

Post by mips_avatar » Mon, Jun 29, 2020, 11:25 PM UTC

I got this question asked in an interview for a mechanical engineering internship. I was asked "you are given a ruler and a parked car, how can you measure the weight", took me about 10 panicked minutes and 4 hints before I figured it out.

Re: Weighing a Car with Tire Pressures

#34

Post by mauvehaus » Mon, Jun 29, 2020, 11:26 PM UTC

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 basically ignore sidewall stiffness for bicycle tires.

Re: Weighing a Car with Tire Pressures

#35

Post by as89 » Mon, Jun 29, 2020, 11:28 PM UTC

kyleblarson wrote: post #6

Wasn't there a Feynman story where he did this exact same thing?

That would be cool. Do you remember where you heard this? Would love to see a source.

Re: Weighing a Car with Tire Pressures

#36

Post by as89 » Mon, Jun 29, 2020, 11:30 PM UTC

serf wrote: post #26

a 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!

Interesting - similar meaning they measured the contact patch of the tires? Or something else?

Re: Weighing a Car with Tire Pressures

#37

Post by jcrawfordor » Mon, Jun 29, 2020, 11:31 PM UTC

A related method is used commercially for self-weighing trucks. Typically it's done by measuring change in the pressure in the suspension airbags and needs to be calibrated for the specific suspension and airbags being used. One major vendor lists the accuracy as +/- 300 lbs which is pretty good for use in the 30-40 ton range but not so good for a passenger vehicle.

As a bit of cool technology trickery, the Air-Weigh system encodes the weight data onto the existing tractor-trailer electrical connection so that the weight can be displayed in-cab without any extra wiring to connect. They don't say much about how but I'd guess RF modulation on the electrical for the lights or something.

Re: Weighing a Car with Tire Pressures

#38

Post by phibz » Mon, Jun 29, 2020, 11:34 PM UTC

Can you do this purely with pressure?. Lift the car off all 4 tires then add a known pressure to all tires, just enough to seal the beads. Then lower the car and measure the pressures again. I'm sure you could get to the ratio of weight distribution for your car. And with the right coefficient for your tire you could probably accurately approximate weight.

Re: Weighing a Car with Tire Pressures

#39

Post by [deleted] » Mon, Jun 29, 2020, 11:37 PM UTC

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.

[deleted]

Re: Weighing a Car with Tire Pressures

#40

Post by perl4ever » Mon, Jun 29, 2020, 11:43 PM UTC

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…

On measuring things - I prefer going in the opposite direction and weighing even where it's unconventional.

I have a Melitta funnel for making single servings of coffee, but when I'm using it I can't see how full my mug is without lifting up, and they are mostly opaque, and the funnel doesn't hold a full mug's worth of water. So I started putting my mug, funnel, filter, and coffee on a digital scale with tare, and remembering how much each size holds in dry ounces. It avoids the situation in which I'm waiting for the water to run out to make sure it won't overflow; I just pour until I hit the right weight and then go do something else until it's done.