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Weighing a Car with Tire Pressures

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Re: Weighing a Car with Tire Pressures

#101

Post by mannykannot »

mrfredward wrote: post #75
mannykannot wrote: post #49

Earlier quoted context omitted.

When the tire is inflated, the sidewalls (and the tread area) are under tension, right? (They would just fold up under any appreciable compression.) If so, does that mean there's no downward force from the sidewalls? Perhaps we should be asking if there needs to be extra pressure in the tire to keep the sidewalls under tension?

The pressure of the tires pushes horizontally outward on the sidewalls and helps them hold their shape, so they can absolutely be in compression and not fold up.

I am not convinced by this. It would seem that, if it works the way you say, a bubble's surface could be under compression.

Consider a section of tire wall, with the same pressure on either side. It is curved outwards, and if you put compression on it, it will buckle outwards. If there is higher pressure on the inside of the curve, that would help with the buckling, not prevent it.

In an inflated tire, as in a bubble, it is the tension along the curved surface that resists the internal pressure. What keeps the sidewall of an inflated tire from buckling is that the buckling would decrease the radius of the curve locally, increasing the net inwards force of the tension over the area that is buckling.

Re: Weighing a Car with Tire Pressures

#102

Post by gambiting »

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…

I mean, public weighbridges are a thing pretty much everywhere, whenever you have trucks bringing in any kind of goods they usually have to be weighted before and after. There's at least 3-4 public ones that you can turn up to and use without an appointment around me, it's £10-20 depending on the vehicle you're weighing.

Re: Weighing a Car with Tire Pressures

#103

Post by kd5bjo »

spuz wrote: post #95
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...

It’s possible (but laborious) to use a bicycle pump to fill a car tire: they use the same kind of valve and bicycles use a higher pressure than cars do, there’s just a lot more air to move.

Re: Weighing a Car with Tire Pressures

#104

Post by retsibsi »

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...

I wonder how often people overdo this, and face some awkward questions when their weight is higher on the way out.

Re: Weighing a Car with Tire Pressures

#105

Post by metaphor »

YarickR2 wrote: post #90
metaphor wrote: post #78

> I have a decent pressure gauge that I got for $25, so I feel I can be reasonably accurate. Such a false sense of confidence for a product[1] whose actual performance specs aren't properly published. And it isn't just Longacre playing this game. If you're in the recreational motorsports scene, you've probably noticed what I'm referring to. If you're an engineer in the same, it probably has driven you nuts. I'm not e…

Any recommendations for decently precise tire gauges ?

Fluke[1] 700G06 or 60psig span Ashcroft[2] 2089 with choice of option(s). These are just gauges with standard 1/4" NPT male interface, so you'll have to retrofit your own tire pressure adapter. To be sure, these are not your typical consumer-friendly price points.

[1] https://www.fluke.com/en-us/product/calibration-tools/pressu...

[2] https://ashcroft.com/products/pressure_gauges/digital_gauges...

Re: Weighing a Car with Tire Pressures

#106

Post by barrkel »

spuz wrote: post #95
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...

A 12v pump which plugs in to a cigarette lighter socket costs about $15 / 12 GBP. They're not the fastest things in the world but they work and there's no labour other than screwing them on and off.

Re: Weighing a Car with Tire Pressures

#107

Post by mannykannot »

There's a period, after it rains, when, as cars that have been standing in it drive off, they leave behind a wet print of their tire contact patches. With a jug of water and some patience, perhaps you could use this to get a more accurate measure of the contact area?

Re: Weighing a Car with Tire Pressures

#108

Post by PoachedSausage »

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?).

Think of it as a free muscle toning workout for your ass. I miss the days of 8 miles a day bike commute on thin road tyres on bumpy roads and paths.

Re: Weighing a Car with Tire Pressures

#109

Post by mannykannot »

pedalpete wrote: post #89

Back when Microsoft was doing those obscure job interview questsions like "how much does a 747 weigh", this was my solution, measure the deflection of the tires and the PSI to calculate weight. Apparently the answers they were looking for were more along the lines of "put the plane on an aircraft carrier in a lake and measure the water displacement"

Ha! Maybe they couldn't figure out whether your answer was valid?
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