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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?
> 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?
Commercial trucks with air suspension do this, there are various "on-board" scales that tap into the air lines and try and give a weight. It's obviously much more of an issue when you have regulations involved in how much you can carry, so weighing is a big part of trucking. Here are a few: https://www.air-weigh.com https://rwls.com https://www.appweigh.com There are many.
Most of the trucks I've driven have either an analog "rear suspension pressure" gauge, or a digital calculation of approximately how much weight is there for the given pressure. With an analog gauge, you as a driver start to get a better idea of how much weight is on your drives, possibly saving yourself from shelling out $20 to scale up on a regular basis. As I sit in my truck without a trailer, I currently have a reading of ~10psi; this truck for instance, every additional ~5k lbs over the drives is an additional ~10 psi in the bags.
This method doesn't account for the support of the tire itself through the sidewalls, as opposed to the pressure contained within it. You should be able to estimate this contribution by fitting to the curve of area vs pressure at lower pressures. For example, the linear F=PA relationship clearly breaks down at 0 pressure, as F>0 and A>0 when P=0.
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.
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?
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.
> 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…
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 even talking about full NIST traceability (although if a serious customer wants that and is willing to pay, it shouldn't be an issue...good luck with that) but just a really basic warm and fuzzy.
Intercomp[2] pulls the "Certified Accuracy" bullshit marketing wank, but doesn't even specify the overarching standard that their products are certified against.
Joes Racing[3] labels their analog "Pro" model as "ANSI grade B40.1", but if you've actually read ASME B40.1 proper (currently published in ASME B40.100-2005, which contains 5 related standards including B40.7 for digital gauges), you'd know that's bullshit marketing wank...for all anyone knows, they're selling pseudo-Grade D chinesium trash at a premium price point.
Pick any consumer-grade product and you'll discover the same pattern.
The only markets I've seen any semblance of meaningful published specifications with respect to pressure gauges are industrial and scientific/metrology. Of course, the typical players in those markets aren't nearly as naive.
[1] http://www.longacreracing.com/products.aspx?itemid=1716&prod...
[2] https://www.intercompracing.com/documents/Literature/7005110...
[3] https://www.joesracing.com/product/joes-pro-tire-gauges/
> 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 ;)
> 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…