Physics question - and stealing air

[christian]

I was reading a blog post about needing to seat a tubeless mtb tire and not having a compressor... and I thought about something.

If I rigged up a schrader valve and a presta valve, separated by 12" of hose, could I use such an assembly to seat tires?

I think that physics would tell us that the overall pressure across the two tires would try to equalize, so if I had 36 psi in the car tire and 0 psi in the mtb tire, and I connected the hose first to the mtb tire and then the car tire, would the air then seat the mtb tire and equalize both of them to, say, 32 psi? (Assuming here that the volume is obviously a lot smaller in the bike tire.)

Thoughts?

[45K10]

I have been able to seat a tubeless MTB tire several times just using a floor pump.

As far as your idea, the air will flow until the pressure equalizes but I do not think it is going to help you seat the MTB tire. The advantage of using the compressor is the shear flow-rate it can achieve much higher than the flow-rate of the air escaping the car tire.

But give it a try and report the results

[tiretrax]

I don't think pressure would equalize because the volume is unequal (stay higher in the car tire). That might also limit the amount that you can transfer from the car to the bike. Also, you'd probably need to seal the connections very well and figure out how to create a flow pattern. Last, I always hear that a compressor is needed to seat a tubeless bike tire because a pump doesn't apply pressure rapidly enough. I think that would be another issue. But, I think you win a guest spot on McGyver.

[malcolm]

I have been able to seat a tubeless MTB tire several times just using a floor pump.

As far as your idea, the air will flow until the pressure equalizes but I do not think it is going to help you seat the MTB tire. The advantage of using the compressor is the shear flow-rate it can achieve much higher than the flow-rate of the air escaping the car tire.

But give it a try and report the results

I agree. I've never had a problem with a floor pump. The trick is to use just thick enough tape so you can get the tire on but not have too much of a gap.

[AngryScientist]

get yourself a cheap pony air tank from harbor freight or similar, not a lot of dough. it'll come with a valve on it. hook that thing up to your tire, flip the valve and boom, lots of air fast. what is this nonsense about hooking up to a car tire all about?

http://www.etoolcart.com/ProductImages/omega/OME-W1005.jpg

[goonster]

if I had 36 psi in the car tire and 0 psi in the mtb tire, and I connected the hose first to the mtb tire and then the car tire, would the air then seat the mtb tire and equalize both of them to, say, 32 psi?
It depends. ;)

( . . . on whether the initial 36 psig is enough for your particular setup.)

[christian]

I have a compressor and I don't run tubeless.

This is about whether it's possible to steal air from car tires in a pinch. Not about how I seat my own tires.

Focus, guys, focus.

[Joachim]

I have a compressor and I don't run tubeless.

This is about whether it's possible to steal air from car tires in a pinch. Not about how I seat my own tires.

Focus, guys, focus.

So you are out in the middle of nowhere, with a flat tire and only one car around??? Does the car belong to you? Otherwise you will be stealing air. You know, that stuff that costs 75c at the gas station....

[MattTuck]

The question is whether 32 psi (or whatever your car tire is at) is enough to seat the tubeless tire. My hunch is that it isn't. And that is based on letting air out of car tires when I inflate them too much. You just don't get much force.

In my opinion, you'd be better off with two presta valves and a tube, as I bet the higher pressure in a bike tire would provide better seating force than a car tire.

[MattTuck]

So you are out in the middle of nowhere, with a flat tire and only one car around??? Does the car belong to you? Otherwise you will be stealing air. You know, that stuff that costs 75c at the gas station....

Haha, is this really a hypothetical from property rights 101? and just masked as a cycling question?

[dogdriver]

I like the way you think, but I don't think that 32 (or 33, or 34) psi is enough to seat the bead. On my setup (Stan's Arch/ Racing Ralph) it takes about 50 or so psi to get the bead to stop "popping" and seat.

Safety tip: wear a face shield (or at least glasses) as the bead has been known to blow off the rim when seating. Have never had it happen to me, but know many who have had a face full of high-velocity Stan's or Cafe Latex goo...

[christian]

face full of high-velocity Stan's or Cafe Latex goo...And with that visual, you win the internet today.

[Mark McM]

I think there are two questions here: Whether a car tire can be used like the reservoir on an air compressor; and whether there is suitable pressure/flow to seat a tubeless tire.

On the first question, yes, a car tire would essentially act as the air reservoir on a compressor, and you could inflate a bike tire from the car tire - or at least to the point where the bike tire pressure equalized the car tire pressure.

On the second question, seating a tubeless tire is primarily about an adequate air flow. But the air flow throw a small orifice like a presta valve is limited by the pressure behind it. So, while there is certainly enough volume in a car tire for seating the tubeless tire, the pressure isn't high enough to achieve the required airflow.

[Louis]

(P1 x V1) / T1 = (P2 x V2) / T2

Assuming T1 = T2 you then get:

P1 X V1 = P2 x V2

[zmudshark]

Am I really dense, or does anyone think that you can get more than 32psi from a car tire to a bike tire? Pressure equalizes, no matter the volume? Right?

[Mark McM]

Am I really dense, or does anyone think that you can get more than 32psi from a car tire to a bike tire? Pressure equalizes, no matter the volume? Right?

Well, if you used a long enough hose, and spun the bike around by the hose fast enough, the centrifugal force generated would compress the air in the bike tire, and suck it right out of the lower pressure car tire ...

[christian]

Am I really dense, or does anyone think that you can get more than 32psi from a car tire to a bike tire? Pressure equalizes, no matter the volume? Right?I've learned long ago not to speculate on what anyone might think, but the OP (me) is down with Boyle and Charles and though it's been 20 years since high school is pretty gosh darn sure that pressure will equalize across a gradient. Also Louis showed his work.

[christian]

Otherwise you will be stealing air.Did you even read the title of this thread? :)

[67-59]

You lost me at "physics.":help:

[Louis]

Am I really dense, or does anyone think that you can get more than 32psi from a car tire to a bike tire? Pressure equalizes, no matter the volume? Right?

Yes, it will equalize. The only question (answered by the equations above, assuming you ignore changes in volume and elastic deflection of the tubes and tires) is what the final pressure will be.

[jamesau]

This is about whether it's possible to steal air from car tires in a pinch.
The answer to this is: yes.
If the bead seats, the pressures will equalize to something less than the original car tire pressure you started with.
If the bead doesn't seat, you'll end up with 2 flat tires.

Chemical engineer, here. A simple trial is the best way to determine how effective this could be (but maybe conduct it at the filling station :))

[Louis]

If the bead doesn't seat, you'll end up with 2 flat tires.

And one really pissed-off stranger.

[Mikej]

Not going to be possible without a device to actuate each valve from outside of the system. Screwing a hose to a valve won't let the air out. It will also unscrew the other side as you screw the opposite side. Fluid power eng. here. Chem eng guy is correct, however, 32 psi may not be a real world number that transfers through the hose and into the bike tire. You have to have a certain amount of pressure loss and that may cause a lack of volume required to inflate the bike tire. You are just assuming that a pump or compressor is putting out 32 psi and that is enough to seat the bead. It may require substantially more pressure than 32 psi. I've never measured what it takes, but I usually power up to 100 psi to seat a bead.

[Louis]

Not going to be possible without a device to actuate each valve from outside of the system.

You just need the right fitting...

http://ecx.images-amazon.com/images/I/41FA9Jo8srL._SY300_.jpg

[christian]

Not going to be possible without a device to actuate each valve from outside of the system.You only need to actuate the Schrader valve on the car tire. I'm not planning to drive around in my car looking to steal air from road bikes.

[Schmed]

Another ChemEng here.

Yes - the two pressures will equalize. Zero doubt about it.

Yes - I *think* you can seat that bike tire this way. It's more about the FLOW than the PRESSURE. I have my compressor regulator set to 40 psi when I seat my MTB tires. Pops them right on. I'm sure it'd do it at 30 psi, too (but haven't tried.).

What's the volume of a car tire (say a 255/50/17)? About 30 times more than a bike tire? You'd only lose a psi or two out of that car tire if the bike tire seats right away.

So.... yes. Strap a car tire on your back for those times when your bike tire runs out of air and you need to re-seat that tubeless tire. ;)

[Louis]

I can see the headlines now:

"Why Is There Air?"

A Chappaqua man is accused of flattening the tires of five different cars at a local mountain-bike trailhead in an unsuccessful attempt to transfer air to his own bicycle's tire. As he was taken away in handcuffs he was heard to mutter "That damned bead! Why didn't I just stick to tubes? That's what I get for listening to those guys on the bike forum."

http://ecx.images-amazon.com/images/I/51frVdvl43L._SX300_.jpg

[DRZRM]

I bartended on the UWS of Manhattan in the late 80s/early 90s and the restaurant owner's brother fancied himself a bit of a get rich quick inventor and he built and sold exactly what you are talking about. Ran adverts in the back of magazines; length of tube, schrader on each end, "never worry about carrying a pump again". He never got rich, it didn't take off, and they didn't get enough pressure to fill a road tire, but should have worked on MTB and cruisers. He was way ahead of his time.

[pdmtong]

Let PV= nRT be your guide and do not look back.

[Louis]

Yeah, but how many moles of gas are there in a car tire?

[Jaq]

Let PV= nRT be your guide and do not look back.
This.

[11.4]

Jaysus, I better never hear a single one of you complain that you don't get enough time to train.

[11.4]

Yeah, but how many moles of gas are there in a car tire?

Louis .... ..... ..... ..... Bad boy. Go stand in the corner.

22.4 liters at 1 bar per mole. Assume a 27 mm tubular, 25 mm inside diameter, so 4.91 cm^2 cross sectional area. Approximate the volume as cross-sectional area x centerline circumference, or approximately 466 cm^3 or roughly a half liter. At 110 psi, roughly 51.3 liters or 2.3 moles of gas. That's what your tire will take, ignoring losses, heating effects, etc.

Do the same calculation on the auto tire of your choice. And you will then know, to a rough approximation, by how much you will reduce the pressure in the car tire.

And by the way, we just proved scientifically why everyone should only ride tubulars.

Someone, get us off this thread!!!!!

[Louis]

I can't believe you posted that without mentioning Avogadro's Number... ;)

[marciero]

I can see the headlines now:

"Why Is There Air?"

http://ecx.images-amazon.com/images/I/51frVdvl43L._SX300_.jpg

My father had this record. I had the routines memorized when I was a kid.

[oldpotatoe]

likely result..when the stoopid tubeless doesn't seat.

[shovelhd]

The answer to this is: yes.
If the bead seats, the pressures will equalize to something less than the original car tire pressure you started with.
If the bead doesn't seat, you'll end up with 2 flat tires.

Chemical engineer, here. A simple trial is the best way to determine how effective this could be (but maybe conduct it at the filling station :))

This.

[dogdriver]

As much as I've enjoyed this thread (and, 1-yes, the "hose" would work-- just thread to the presta side and push the schraeder end onto the tire valve; 2-where does the air come from to refill the car tire-- the air fairy?-- and why aren't we just using this source to do the bike tire?-- with a $.50 shredder/presta adapter, the world is your bike pump; 3-math written out in longhand is über-geek-chic; 4-I think this started with an mtb tire... someone brought up a road tire late on-- don't care if the tire is the size of the Chrysler Building, if its 36 psi, you're just getting 36 psi max (though it won't go flat for a few years)), just buy one of these:

http://www.lowes.com/pd_553270-70-PCFP02003_0__?productId=50158927&cm_mmc=SCE_PLA-_-ToolsAndHardware-_-CompressorsDiyAndStationary-_-50158927&CAWELAID=320011480002326078&kpid=50158927&CAGPSPN=pla

If you want to cheap out, Harbor Freight has one with a 1/3HP motor for $49.00.

[11.4]

I can't believe you posted that without mentioning Avogadro's Number... ;)

For those who need to know, Avogadro's number is the number of times I've gone out on a training ride and slow-flatted in the first mile.

6.022 x 10^23

[11.4]

likely result..when the stoopid tubeless doesn't seat.

Don't take this photo seriously. This is why Tater rides bikes. His car always looks like this.

[Louis]

My father had this record. I had the routines memorized when I was a kid.

My sister gave me the CD a few years ago. Hilarious.

[bikemoore]

Pressure equalizes, no matter the volume? Right?

Correct. If you open a connection between two volumes of gas, the pressures will equalize regardless of their relative volumes. With open connection, you wind up with essentially one volume with one pressure.

I think folks are missing the mechanical side of this problem: how will you depress the center thingy in the schrader valve stem to release the air from the car tire? Are you rigging up hose with a schrader valve on end and a presto valve on the other end?

[11.4]

So if I have a piece of hose and put a Presta head on one end and attach it to the bike wheel, and on the other end of the hose have a press-on Schrader fitting, am I missing something or if you press the Schrader fitting to the car tire, pressure will flow from the car tire through the hose, open the Presta valve, and fill the bike tire. Let go of the Schrader press-on fitting and you stop flow from the car tire and as soon as that happens, the Presta valve on the bike pops shut. What did I miss?

Of course, I wouldn't think to do this anyway. But since we're sitting around calculating how many moles of air need to be transferred from one tire to the other (and we haven't even gotten into whether the car tire is filled with nitrogen or helium instead of a standard atmospheric mix), it seems we should just work this problem right through to the very bloody end.

PS: I am seriously thinking that nobody on this forum gets on their bikes any longer. Otherwise, how do they have the time to post 9 pages on how to steal air pressure from a car tire? I know I'm locking my gas tank next time I park near any of you.

[MadRocketSci]

Man you guys get nerdy over the weekend...

You can think of this as a simple circuit....the pressure differential is like a voltage, the line the conductor, and the valves as resistors, though the resistance is non linear unlike the electronic counterpart. The valves also have other nonlinearities like stiction (that initial crack to get the presta valve poppet unseated). Assuming you can get the presta valve to pop open with either wiggling or 30-ish psid delta pressure or both, each valve and to a small extent the tubing will excise a pressure drop toll, so that as the pressure in the bike tire starts to build and the pressure differential (total voltage) drops, you'll get less and less flow until at some point it slows down enough that in a practical sense you won't achieve absolute pressure equalization.

Also, I'd recommend using the gas specific version of the ideal gas law,

P = (mass/Volume)*R_air*T_absolute

R for air = R_univ/(avg molecular weight for air = 28.97 g/mol or lbm/lbmol)

then you won't have to deal with mols, rather grams, kg, or lbm

oh, and be careful with T_absolute, in the process of filling (or "charging") the bike tire the air inside will heat up due to the work (P/rho) that the high pressure gas is doing on the interface (or control volume surface). This will result in the bike tire gas being a factor of 1.4 (approx gamma, or Cp/Cv for air) higher than the car tire's air. The car tire's air will cool down similarly by an amount that conserves overall energy, so there are two causes for pressure loss - loss of mass and loss of energy due to the work necessary to drive the air into the bike tire.

nerd on.

[11.4]

Man you guys get nerdy over the weekend...

You can think of this as a simple circuit....the pressure differential is like a voltage, the line the conductor, and the valves as resistors, though the resistance is non linear unlike the electronic counterpart. The valves also have other nonlinearities like stiction (that initial crack to get the presta valve poppet unseated). Assuming you can get the presta valve to pop open with either wiggling or 30-ish psid delta pressure or both, each valve and to a small extent the tubing will excise a pressure drop toll, so that as the pressure in the bike tire starts to build and the pressure differential (total voltage) drops, you'll get less and less flow until at some point it slows down enough that in a practical sense you won't achieve absolute pressure equalization.

Also, I'd recommend using the gas specific version of the ideal gas law,

P = (mass/Volume)*R_air*T_absolute

R for air = R_univ/(avg molecular weight for air = 28.97 g/mol or lbm/lbmol)

then you won't have to deal with mols, rather grams, kg, or lbm

oh, and be careful with T_absolute, in the process of filling (or "charging") the bike tire the air inside will heat up due to the work (P/rho) that the high pressure gas is doing on the interface (or control volume surface). This will result in the bike tire gas being a factor of 1.4 (approx gamma, or Cp/Cv for air) higher than the car tire's air. The car tire's air will cool down similarly by an amount that conserves overall energy, so there are two causes for pressure loss - loss of mass and loss of energy due to the work necessary to drive the air into the bike tire.

nerd on.

Oh, so we need to figure temperature changes as well? Sounds like you really want to see 30 pages of thread here. Nasty.

Why, seriously, are we doing this? This is not how to make sure your tubeless tire seals. You don't need a car tire to begin with, and if you did, you wouldn't seriously seal a bike tire using your left front Pirelli. Right? I guess there are a few of you who would look at the car parked next to you and figure it was fair game, but you get shot for that kind of thing. And again, a pump does it. So ... why this thread? This are valuable minutes in our lives that we will never see again.

[MadRocketSci]

dude, it's just for fun. A thought experiment. I didn't see anyone talking about temperature effects, which have to be considered if you're trying to figure out mass consumption from the car tire air as someone thought to consider. We could go into determining the effective orifice diameters in the valves, or Cv values, that vary as a function of delta p, and model it up and do some sims. But not what I consider weekend activity unless forced to do so. Like I said, thought experiments are fun for engineers.

[ShaneAtSilca]

http://www.pinkbike.com/news/Tech-Tuesday-Make-a-Ghetto-Tubeless-Tire-Inflator.html

Been Done for (bike) tire to (bike) tire transfer.

[crownjewelwl]

wouldn't work...need a quick blast to seat the tire

but hey, i got c's in physics (but from a nobel laureate)

[11.4]

dude, it's just for fun. A thought experiment. I didn't see anyone talking about temperature effects, which have to be considered if you're trying to figure out mass consumption from the car tire air as someone thought to consider. We could go into determining the effective orifice diameters in the valves, or Cv values, that vary as a function of delta p, and model it up and do some sims. But not what I consider weekend activity unless forced to do so. Like I said, thought experiments are fun for engineers.

Sir, you earn your forum name.

I started life as a particle physicist. You know the kind, all slide rule and theory and blamed everything in life on not having a tokomak to play with. Did some fun things, and one of the most pleasurable was explaining to engineers how they were at best empiricists and would never be accurate until they really got their math right -- Heisenberg and all that. Particle physicists all suffer from deep-seated insecurities, mostly because we spend our whole lives dealing with things we never get to see, while engineers simply figure out how to change the tire and get going. But we do know we're superior beings, and that has great satisfaction in life. Until we flat.

[MadRocketSci]

Sir, you earn your forum name.

I started life as a particle physicist. You know the kind, all slide rule and theory and blamed everything in life on not having a tokomak to play with. Did some fun things, and one of the most pleasurable was explaining to engineers how they were at best empiricists and would never be accurate until they really got their math right -- Heisenberg and all that. Particle physicists all suffer from deep-seated insecurities, mostly because we spend our whole lives dealing with things we never get to see, while engineers simply figure out how to change the tire and get going. But we do know we're superior beings, and that has great satisfaction in life. Until we flat.

Awesome. I wrote a proposal in grad school to model the plasma within a tokamak using particle in cell (PIC) statistical techniques. It didn't get funded. So I moved into modeling plasma instabilities in high current arcjet thrusters. Then someone asked a friend of mine what i did and she said "he's like a mad rocket scientist or something." So that name kinda stuck.

I now do work in aerospace. Honestly, though, those physicists postulating about space and time and all the other dimensions and subatomic particles and superstrings and even relativity blow my mind. Rockets and stuff are complex systems but the physics are well understood. At least until we get into the exotics like this thing that i just read about:

http://rack.0.mshcdn.com/media/ZgkyMDE0LzA4LzAyLzZmL2VtZHJpdmUuMDRjNTQuanBnCnAJdG h1bWIJMTIwMHg5NjAwPg/47f09438/d78/em-drive.jpg

http://mashable.com/2014/08/02/emdrive-mars-momentum/

So i may be a rocket scientist, but I aint no astrophysicist :)

cheers to finding a fellow techie geek :beer:

[goonster]

So ... why this thread?
Did they make you pawn your empathy when they issued your slide rule? ;)

This is a perfectly plausible scenario. Cyclists stands dejected at trailhead or parking lot. Bike tire is flat. No functional pump, compressor, CO2 widget or other pressurized gas source is available . . . except for automotive tires.

A smart, resourceful, adapt-and-overcome kind of cyclist would think this scenario through, and consider MacGuyvering a transfer line out of available materials, or preparing one for future emergencies.

An auto tire that is supposed to be at 36 psig is still perfectly driveable at 32 psig, if it's not a Firestone Delaminator (tm).

(Looks like I'm ChE No. 3 in this thread.)

[happycampyer]

christian, to get the force needed to seat the bead, perhaps you could rig together a tube with a presta fitting at one end and open at the other, a pocket knife and some duct tape. When you slash the tire, the force of air should be sufficient to seat the bead. The only problem will be waiting until there is no one around their cars...

[Schmed]

Us chemical guys always revert to our comfort zone: explosives. Everclear in the camel back is always handy:

http://i.imgur.com/mL3kNF0.gif

[Mikej]

I still don't think it will work. Not enough volume exiting the car tire.