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I've seen in many movies and programs the event where a car plummets off the road and in to a body of water, sinking rapidly. The car slowly fills with water as it sinks and the occupant(s) don't break free until well under the water (50m say).

At this point, when outside of the sinking vehicle, they then breathe air from the tyres and swim to the surface. Obviously it's not that easy to breath air from a tyre due to the valve set up (having to push the pin in to draw air under pressure out). But what I really want to know is, if a tyre is at 30psi and there is no pressure regulator used, and your under 50m of water pressure... Could you actually do this? Surely there'd be issues with the pressure at that depth, or even at atmospheric!

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  • $\begingroup$ Just for the record .. what film was this in ?! never seen anything like that. $\endgroup$ – Fattie Aug 17 '14 at 8:45
  • $\begingroup$ Also, note that 50m is a huge depth. Don't you mean more like, say 10 feet or so?? $\endgroup$ – Fattie Aug 17 '14 at 8:45
  • $\begingroup$ Why don't they breathe the air from car interior, before it fills with water? This seems to be more practical: you don't lose time to horse around with tire valves, just swim up. $\endgroup$ – Andrey Regentov Jun 17 '15 at 10:11
  • $\begingroup$ @AndreyRegentov "don't break free until well under the water" $\endgroup$ – user2875404 Sep 10 '15 at 20:40
  • $\begingroup$ @user2875404 well, car fills with water very slowly and there should be big air bubble in upper part, under the roof anyway. $\endgroup$ – Andrey Regentov Sep 11 '15 at 4:48
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breathing: Since the tyre is a about 2 atm at the surface, at 50m it will have a pressure of 6 atm if the tyre is thin (membrane) or less if is "partially rigid" (down to 2 atm if the tyre is totally rigid). If it behaves like a membrane, air will come out if you push the valve (with the tongue, I guess it would require practising a lot :D) because the density of air is less than that of water. I can't tell you if it is technically possible to breathe but you could calculate an order of magnitude of the velocity with Bernoulli (even though it creates bubbles) and thus answer this question.

swimming to the surface: assuming you managed to breathe, you should be careful:

  • your lungs will gain in volume as the pressure diminish, so you have to breathe out!
  • the increase of pressure causes the dissolution of azote in the blood, so you shouldn't stay deep too long.
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  • $\begingroup$ Why does the tyre maintain the additional 2 atm pressure when it is compressed? $\endgroup$ – LDC3 Aug 17 '14 at 13:49
  • $\begingroup$ @LDC3: I corrected my answer accordingly. $\endgroup$ – anderstood Aug 17 '14 at 16:50
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The other answers have gone quite sideways with this. Firstly let's cover the easier parts of this answer (and some of the comments) before we get into the technical stuff. So you're probably pulling this from "The Transporter" 3 or something similar movie. Why you wouldn't want to breathe from the car's interior is the petroleum products are all lighter than water. As the water displaces the air surrounding components -including the engine- it is going to strip these products off and extract them from the car making the air also contain more and more concentrated fumes. Since I rarely see people wash their engines as a mechanic, that is the reality.

However, the air in the tires isnt going to be much better. It has been sitting inside a petroleum product that experiences heat cycles. Anytime you let air out of a tire it carries that distinct smell.

Air is more buoyant than water. It is also compressible. All air-filled tires flex, no matter how rigid. If this wasn't the case, pressurizing them with air would be unnecessary to help give them strength to support a vehicle. So no matter the depth, as long as air pressurized before being submerged remained in the tire you could draw air out with your biggest problem in tripping the valve.

Now onto the bead popping. I like your idea, but you're terribly, terribly wrong. That device used to break the bead is done on tires that have had their pressure equalized and can allow additional air to escape. Also the hand lever models always include at least two rotational pivots, there by multiplying their force much higher than you think. Let's use the Audi A8D3 from the aforementioned movie. The math will be simplified to save me keystrokes. First you take the tire size 235/50zr18 and translate that into a rough cylinder. So you have a cylinder about 9.25 inches tall and a diameter of 22.25 inches. Then you punch out the rim to be left roughly with the tire. Figure the outside surface area of the tire, knock off 50 sq in for the change due to tires thickness and subtract the outside area of the rim's surface that the tire covers. You're left with roughly 1500 square inches. To seat a tire you need about 65-80 psi on average to get the job done. Since this is the closest that you could get to equal pressure exertion to unseat the bead -in theory- you're looking at roughly 105,000 pounds of force exerted at 65 psi. So that's about how much pressure you would expect to have to use to unseat a bead uniformly if it only contained ambient air at sea level right after it was seated. Remember, using said tires is going to introduce variables that will effect how much it resists. One part- no more tire lube. Yeah, that's a thing. Heat cycles will cement it to the rim too. Youre not looking at this being any easier to do once it hits the road. So with that external pressure requirement on an unpressurized tire, my educated guess is that it will have to descend much more substantially to make that happen. Also dont forget the temperature change as it travels through the water. As it cools, the tire will constrict even more- making it that much more resolute to keep its interference fit on that rim.

Should you do it? That's a personal call. A properly paranoid person would have a SCUBA tank stashed somewhere in the car. One that is well maintained and filled. If you aren't one of those, you still gotta breathe. Just make sure you've always got your knife and never forget your towel.

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  • $\begingroup$ A lot of the stuff in the 3rd paragraph looks pretty tangential. $\endgroup$ – user191954 Sep 6 '18 at 6:18
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The pressure at 50 m is about 6 atm. Since the pressure in the tyre has a pressure of about 2 atm, there are 2 possibilities when the valve is opened. The tyre (which has been compressed by the water pressure) is rigid enough to return to it's previous shape and water enters the tyre (very likely). Or the air stays in the tyre and the water stays out. The compressed air in the tyre is equal to the water pressure.

Only when the depth is less than 20 m would the air exit the tyre when the valve is opened.

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  • $\begingroup$ if the tyre has a pressure of 2 atm out of the water, it will have a pressure of 8 atm at 50 meters deep (if the tyre is thin enough). $\endgroup$ – anderstood Aug 17 '14 at 4:24
  • $\begingroup$ What mechanism would maintain that pressure difference? $\endgroup$ – BowlOfRed Aug 17 '14 at 8:17
  • $\begingroup$ @anderstood Until the pressure outside the tyre equals the pressure inside the tyre, the size changes slightly (the 2 atm of pressure pushes out on the tyre). After the pressures are equal, the tyre decreases in size to maintain the same pressure (inside and out). $\endgroup$ – LDC3 Aug 17 '14 at 15:16
  • $\begingroup$ you are right LDC3. $\endgroup$ – anderstood Aug 17 '14 at 16:49
  • $\begingroup$ The tire has a pressure of 2 atm gauge pressure at the water surface. Water increases pressure at a rate of 1 atm for each 10 m (roughly). This means that the tire would get compressed when it went below 20 m, and the volume of air in the tire would be MUCH less than at the surface when 50 m under water, while the pressure in the tire would equal ambient pressure because the tire had been compressed. Unless the person trying to breathe air in the tire could pull a suction on the tire, no air would come out (in other words, no air will come out). $\endgroup$ – David White Jul 17 '16 at 0:28
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I believe all these answers miss a practical detail - and that we have to be careful about how we use absolute and gage pressures too.

At depth the tire would probably collapse under the squeeze of the increasing external water pressure and the bead would probably pop off the rim and all the air inside would probably escape before it reached that depth. Why?

At full depth of 50 meters (about 165 ft) the external pressure increase on the tire's surfaces due to water depth is about 5 atm. (165ft/32ft) and the total external absolute pressure at depth is therefore about 6 atm. (5 atm. water depth and 1 atm. surface air). Internal tire air pressure at the surface is about 2 atm. gage and about 3 atm. absolute. At depth a totally rigid tire side wall would have to support a differential pressure of 3 atm. (6 atm. abs. external - 3 atm. abs. internal). So what?

Well, today's automotive tires are tubeless and are mounted on the rim by stretching their bead over a hump and into a slight groove on the rim. They are designed to resist a net internal pressure - not an external one. They are installed by stretching their bead over a slightly larger diameter area on the rim located nearer the center of the wheel and then pushing them outward into a groove (a slightly reduced diameter area between the inner and outer area of the rim) where they take a seat and seal to the rim. They are removed from the rim by pushing inward on the outside of the tire side wall at the bead, a relatively easy thing to do when the internal tire pressure has been released. It is normally done in a machine but a manual lever tool does it well too. The lever ratio of the tool is about 4:1 and it can be done with a fraction of a person's body weight acting through the lever ratio. If a worker exerts about half his body weight - or about 75 lbs - on the lever's end of the tool and the tool concentrates this force over maybe 6 inches of bead length, the tool will exert a force of 300 lbs on that length of tire bead and push the tire out of the groove and off of the rim toward the inside of the wheel.

Now, if the tire sidewall area attached to this bead length is 6 inches long and about 5 inches wide over this bead length, the area of the sidewall attached to this length of bead will be about 30 square inches. If the differential pressure is about 3 atm. (45 psid) the force is about 1350 pounds - which should be way more than enough to easily pop the tire off of the bead releasing all the internal air.

So, the movie maker has applied a significant amount of poetic license I think!

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