Weighing a Car with Tire Pressures • Hacker News Board

Re: Weighing a Car with Tire Pressures

#91

Post by lmm » Tue, Jun 30, 2020, 7:43 AM UTC

rlpb wrote: post #71

> We want to find the force at each tire. So by rearranging, we get F = PA. The pressure is gotten by a tire pressure gauge. Can someone explain why the air pressure inside the tire is the same as the pressure that the tire applies to the ground? Ignoring sidewall stiffness as pointed out to others, why must this be true?

The force must be the same (because the net force must be zero, otherwise the tire surface would move up or down), and the area is the same on both sides so the pressure must be the same.

Re: Weighing a Car with Tire Pressures

#92

Post by lmm » Tue, Jun 30, 2020, 7:44 AM UTC

mauvehaus wrote: post #47

supernova87a wrote: post #42

Earlier quoted context omitted.

Wow, I just road ride for leisure on my own, don't know any racers/university types, and my tires are 5 years old 19mm on an old carbon frame. I'm way out of current trends.

My ass hurts just reading that. I bike toured about 7000 miles on 622x28ish tires, and I can't even imagine riding 19mm tires on anything other than completely immaculate pavement (or maybe a velodrome? What do they run?).

I think the velodrome gang have stuck to 23mm as road bikes move to 28?

Re: Weighing a Car with Tire Pressures

#93

Post by [deleted] » Tue, Jun 30, 2020, 7:51 AM UTC

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,…

[deleted]

Re: Weighing a Car with Tire Pressures

#94

Post by Fezzik » Tue, Jun 30, 2020, 7:54 AM UTC

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…

Not as convenient as a dump, generally, but there are also regularly unattended weigh stations for trucks all over the place. The scales are always on. You can drive across those and you don’t even have to stop, just slow down. I have even driven over them while the weigh station was attended and the dude manning the booth did not care at all.

Re: Weighing a Car with Tire Pressures

#95

Post by spuz » Tue, Jun 30, 2020, 8:13 AM UTC

joosters wrote: post #44

Another way to possibly get better accuracy is to pump the tyres to several different pressures, and for each pressure, re-measure the tyre contact surface. From there, you could then simply average out the results - or better still, the data would help you reason about the pressure area relationship. Just going by the P=F/A formula, pressure should be inversely proportional to area, but people have pointed out thing…

I was thinking the same but I imagine you'd have to buy a tyre pump as well in order to re-inflate the tyres. Not sure how much they would cost...

Re: Weighing a Car with Tire Pressures

#96

Post by roelschroeven » Tue, Jun 30, 2020, 9:20 AM UTC

mhh__ wrote: post #31

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 :(

I don't follow F1 very closely these days, but I think I've read that they recently changed, are will change in the near future, to lower profile tires. Is that right? And that the teams have made, or will made, the suspension softer because low-profile tires contribute less?

Re: Weighing a Car with Tire Pressures

#97

Post by supernova87a » Tue, Jun 30, 2020, 9:24 AM UTC

mauvehaus wrote: post #47

supernova87a wrote: post #42

Earlier quoted context omitted.

Wow, I just road ride for leisure on my own, don't know any racers/university types, and my tires are 5 years old 19mm on an old carbon frame. I'm way out of current trends.

My ass hurts just reading that. I bike toured about 7000 miles on 622x28ish tires, and I can't even imagine riding 19mm tires on anything other than completely immaculate pavement (or maybe a velodrome? What do they run?).

You prompted me to look at new tires. $50-60 per tire! I am really getting old...

Re: Weighing a Car with Tire Pressures

#98

Post by fouc » Tue, Jun 30, 2020, 9:28 AM UTC

I've been assuming that public buses would be able to detect how many passengers might be on board by detecting the weight changes from an internal sensor of the tire pressures. Count the jumps up and jumps down in weight as people step on and step off, etc.

And that they'd be reporting that back to the central database along with their gps etc.

Re: Weighing a Car with Tire Pressures

#99

Post by ummonk » Tue, Jun 30, 2020, 9:54 AM UTC

The edges of the footprint won't be bearing full pressure because they will be in tension (in fact, load distribution profiles of tires tend to look like a rounded trapezoid).

Additionally, the grooves are rather stiff and will help hold the treads - the air pressure on the inside of the grooves will be helping push the treads onto the road. Likewise, tire sidewalls have some stiffness and will also be helping hold the car up beyond just the air pressure, as you've noted in your addendum.

This is a good heuristic for approximating the weight of a car but you won't be getting particularly accurate weight measurements out of it.

Re: Weighing a Car with Tire Pressures

#100

Post by HPsquared » Tue, Jun 30, 2020, 10:33 AM UTC

jedberg wrote: post #79

crazyjncsu wrote: post #68

Earlier quoted context omitted.

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 ;)

When I would bring stuff to the dump, I would see people pick up all the metal items from the pile after they dump their trash. It would add weight to their car, so they were charged less, and then they could recycle the metals for even more money. I found it to be a brilliant system. Not entirely sure if it's fraud or not...

Theft followed by fraud I think. Edit: or maybe the fraud occurs at the weigh scales and the theft occurs at the exit gate? IANAL