[HN Gopher] Weighing a Car with Tire Pressures
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Weighing a Car with Tire Pressures
Author : as89
Score : 34 points
Date : 2020-06-29 21:22 UTC (1 hours ago)
(HTM) web link (surjan.substack.com)
(TXT) w3m dump (surjan.substack.com)
| cryptoz wrote:
| 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 the weight of your first
| object and the change in pressure observed, you can calibrate the
| scale to know how much pressure change correlates to weight
| placed on top.
|
| Then you can weigh arbitrary objects using your phone! Be
| careful, don't break your phone (don't try to stand on it to
| weigh yourself!)
| kyleblarson wrote:
| Wasn't there a Feynman story where he did this exact same thing?
| jaxx75 wrote:
| Assuming all tires are the same, I think this would be most
| applicable for weight distribution alone, but not really for the
| weight. For performance applications, distribution is of course
|
| The author also did not ensure the ground was level, which of
| course would have an effect on the weight distribution.
| guygurari wrote:
| 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, by changing P we can set F to whatever we want, which
| shows that we are not actually measuring the weight of the car.
| The basic issue here is that the air pressure is not the only
| thing that is supporting the car.
|
| I don't know how important the stiffness effect is for real
| tires, but I suspect they are sufficiently stiff that it matters
| a lot.
| tmoney1818 wrote:
| 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.
| guygurari wrote:
| I agree that higher tire pressure results in smaller surface
| area, but it's not clear to me that the tire itself does not
| support the car, skewing the result. So I'm not convinced
| this sanity check is sufficient. For example, as another
| commenter mentioned, run-flat tires can support the car on
| stiffness alone.
| abstrakraft wrote:
| I agree, the sanity check is not sufficient. It confirms
| the concept that as pressure decreases, area increases, but
| is not sufficient to confirm a linear relationship. In the
| limit, the area clearly does not become infinite when
| pressure drops to 0.
|
| For the case you suggested above, this is essentially how
| run-flat tires work. Of course, the sidewall of the tire
| isn't as stiff as the support ring, but they do provide
| some support.
| Gunax wrote:
| > I don't know how important the stiffness effect is for real
| tires, but I suspect they are sufficiently stiff that it
| matters a lot.
|
| It's odd, because as I was reading your comment, I was thinking
| the exact opposite. I am considering the force it takes to
| squish a flat tire. A person can squish a (regular car's) flat
| tire with his/her hands.
|
| My gut-check tells me that the stiffness of each tire can lift
| less than 10kg/20lbs... multiplied by 4, that still is not a
| huge contributor to the overall weight.
| guygurari wrote:
| It's certainly possible my intuition is wrong. Run-flat tires
| can support the whole car, but it is possible this support
| only kicks in at very low pressure.
| sm4rk0 wrote:
| A genuine hacker!
| swsh wrote:
| Pretty difficult to get any form of accuracy with this because
| you can't easily account for tyre sidewall stiffness
| nickff wrote:
| How does sidewall stiffness matter? The ground is holding the
| car up; the air is not.
| JoshuaDavid wrote:
| In the case of a perfectly rigid sidewall, the internal air
| pressure could be 0 and the car would still be held up.
| phkahler wrote:
| The quickest way to realize the contribution: Imagine the
| tire is made of steel so it does not deform when the air is
| let out. This would be terrible to drive on, but it would not
| require inflation even though it's hollow like a rubber tire.
| A rubber tire will usually not "go flat" even with the
| rims/wheel installed even without pressure (aside from
| atmospheric which is present when it's put on the rim). So
| some load is supported by the structure of the tire itself
| through the side walls. How much will vary with the tire.
| dognotdog wrote:
| Kudos, for My first inclination would've been to jack up the car
| and look at pressure differences of unloaded vs. loaded tyres...
| then look at loaded wheel center height above ground vs. nominal
| radius, compute the the reduction in volume, and assume contact
| patch to be the area of the flattened section of the tyre
| cylinder... giving me a whole lot of parameters with wild
| inaccuracies and probably equally wild results :)
| function_seven wrote:
| 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.
| surjansingh wrote:
| Oops that's a great point. I completely forgot about sidewall
| stiffness. My messing around with treads was probably just me
| adjusting things to make myself look good then!
| danbr wrote:
| Upvoted for the Saab
| JoshuaDavid wrote:
| I wonder if you could do even better by putting paint or ink on
| the outside of the tire, driving over a piece of paper, and
| seeing what fraction of the paper touched your tire (and thus
| what fraction of the rectangle boundary actually touches the
| ground).
| dmurray wrote:
| And to measure how much of the grooves are in contact with the
| ground, you could add a known weight to the car (like 100kg of
| people), measure it with and without that weight and see what
| proportion of the grooves you need to account for to get 100kg
| difference.
|
| You'd get better accuracy from a bigger added weight, but the
| bigger the added weight the more you risk changing what you're
| measuring by forcing more tire rubber into contact with the
| ground.
| nickff wrote:
| You'd basically just be measuring the maximum width of the
| contact patch, not its area. The best solution would be to lift
| up each tire, put a piece of paper under it, let the car down,
| then remove it, measuring the darkened area.
| aj7 wrote:
| Remarkably inaccurate.
|
| This is a negative result.
| YarickR2 wrote:
| Well, the idea is nice, but, as others have said, we need to
| account for sidewall stiffness. But we can do better with law of
| large numbers and employing metering tape to measure difference
| in car height with different pressure in tires. First we need to
| collect baseline data - tire brand, model and size, tire
| pressure, and car height difference , for known car weights, for
| two different pressures (say, 20 and 35 psi), this way we could
| deduce tire sidewall stiffness for given make/model/size of the
| tire. After that we can do the same for all cars with same tires,
| by measuring car height difference between two pressure points.
| Measuring height changes on both axles (wheel center to lowest
| point of the wheel arch) will get us weight distribution ; but
| we'll need to account for staggered tire setup .
| ordx wrote:
| Can moderators remove utm codes from the url?
| dang wrote:
| Ok. Submitted URL was
| https://surjan.substack.com/p/16-weighing-a-car-with-tire-
| pr....
| serf wrote:
| 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!
| abstrakraft wrote:
| 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.
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