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  1. #1
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    Pumping up your Car's tires with a bike pump

    Okay guys, I'm having a huge debate with a guy at autozone...Here it is:

    In passing conversation I told him that I was planning on adding air to my car's tires with my bike pump. He immediately told me that was stupid and dangerous.

    Me: Why dangerous. I could see you saying it'll take a while, but it shouldn't be dangerous.

    Him: Your car weighs over a ton. Each tire holds up a quarter of that. There is no way your little bike pump will be able to withstand that weight.

    Me: The weight should be fully represented by the PSI. My bike pump can go to 150+ PSI I think. My car's tires are at 35 PSI.

    Him: You're forgetting about the weight of the car.

    Me: Nope, it's what's driving the PSI.

    Him: That's just the pressure, what about the weight?

    Me: Arghhhh

    Okay, I'm 90% sure I'm right here, but given how adamant he was that I'd hurt myself and/or my car, I thought I'd check with the gang here. Can you spot any flaws in my logic or did I miss something?

    P.S. In terms of how many more pumps, I'm thinking that given equal PSI, volume differences should drive # of pumps (effort). I estimate my tire has say 5-10x the volume of my bike tire, so adding air should take me ~3-5 min of pumping. Does anyone do this occasionally?

  2. #2
    bicycle rider
    Reputation: morganfletcher's Avatar
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    He's an idiot. PSI is psi.

    Morgan

  3. #3
    Dropshot Champ!
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    lol arguing with someone at autozone is dumb.

    ps, did you buy the lucas oil treatment and the brake squeal stop paste at the counter too?

    psi is psi, I air up slicks with a bike pump when I don't have a compressor handy. It does take noticeably longer though.

  4. #4
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    I did it for a while before I finally plugged a hole in my tire.

    Autozone employees work there for a reason.

  5. #5
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    "What about the weight?" LOL.

    If you want to return to help him, the weight is spread out over the contact patch of the tire with the ground. The lower the pressure, the larger the contact patch. A 4000lb car with 40psi in the tires will have exactly 100 square inches of contact patch distributed over the four tires.

    If you want to drive that car on the beach, you can lower the pressure in the tires to 20psi and get double the contact patch so it won't sink so far down into the sand. We regularly run 10psi in our tires on Pismo. It's almost impossible to get stuck. You're floating.

    Math is beautiful.

  6. #6
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    Good one

    On other side it took me forever to just "add" some air with floor pump when I found slow leak in remote location.
    I used to run tubes like you are, but then I got thorn in my wheel.

  7. #7
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    Quote Originally Posted by Neela
    P.S. In terms of how many more pumps, I'm thinking that given equal PSI, volume differences should drive # of pumps (effort). I estimate my tire has say 5-10x the volume of my bike tire, so adding air should take me ~3-5 min of pumping. Does anyone do this occasionally?
    Bwahahahahah. Good luck there. Unless it's a little 14" skinny skinny snow tire, it has way way more than 5-10x the volume.

    I pumped up a 16" tire from ~10psi up to ~30psi one time with my old Silca Pista pump and it took forever (had a slow leak from a nail and needed to get the car to the shop). There was a bit of danger ... the pump barrel got too hot to touch after a couple hundred pumps.

    I pump up our wheelbarrows tire with the bike pump these days, and anything bigger gets the compressor.

  8. #8
    ballbuster
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    The only danger....

    Quote Originally Posted by Neela
    Okay guys, I'm having a huge debate with a guy at autozone...Here it is:

    In passing conversation I told him that I was planning on adding air to my car's tires with my bike pump. He immediately told me that was stupid and dangerous.

    Me: Why dangerous. I could see you saying it'll take a while, but it shouldn't be dangerous.

    Him: Your car weighs over a ton. Each tire holds up a quarter of that. There is no way your little bike pump will be able to withstand that weight.

    Me: The weight should be fully represented by the PSI. My bike pump can go to 150+ PSI I think. My car's tires are at 35 PSI.

    Him: You're forgetting about the weight of the car.

    Me: Nope, it's what's driving the PSI.

    Him: That's just the pressure, what about the weight?

    Me: Arghhhh

    Okay, I'm 90% sure I'm right here, but given how adamant he was that I'd hurt myself and/or my car, I thought I'd check with the gang here. Can you spot any flaws in my logic or did I miss something?

    P.S. In terms of how many more pumps, I'm thinking that given equal PSI, volume differences should drive # of pumps (effort). I estimate my tire has say 5-10x the volume of my bike tire, so adding air should take me ~3-5 min of pumping. Does anyone do this occasionally?
    ... is throwing out your back. You can do it, it just takes a while and a metric buttload of pumpstrokes. I had to do it with my mini spare once. I actually keep a floor pump in my car at all times for riding.


    Quote Originally Posted by redmr2_man

    ps, did you buy the lucas oil treatment and the brake squeal stop paste at the counter too?

    I still can't believe they sell something to intentionally make your motor oil foam up. Oil is a way better lubricant than air.

  9. #9
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    Reputation: Buzzaro's Avatar
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    I think the weight is factored right into the P in PSI. The real danger in pumping up your car tire with a bicycle pump is some sort of repetitve motion injury like carpal tunnel...

    a 30" car tire thats 8" wide with a 16" rim has a little over 4k in^3 and (assuming your not on a niner) mtb has a 21 inch rim and 26" tires roughly 2.25 wide so should have a volume around 400 in^3. Around 10 times the volume, you'd be pumping forever. The MTB tires are probably even a little less, I just figured them with square shoulders more like a car tire....
    All out of S**** and down to my last F***
    I think I strained a pucker muscle

  10. #10
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    Quote Originally Posted by Fast Eddy
    "What about the weight?" LOL.

    If you want to return to help him, the weight is spread out over the contact patch of the tire with the ground. The lower the pressure, the larger the contact patch. A 4000lb car with 40psi in the tires will have exactly 100 square inches of contact patch distributed over the four tires.
    It's probably more complicated than that. A tire with 0psi in it doesn't have an infinite contact patch (or a 1000sq in at 1psi), and a run flat tire can maintain a usable contact patch without air.

  11. #11
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    Quote Originally Posted by anotherbrian
    It's probably more complicated than that. A tire with 0psi in it doesn't have an infinite contact patch (or a 1000sq in at 1psi), and a run flat tire can maintain a usable contact patch without air.
    Can you ever really have 0 psi?
    All out of S**** and down to my last F***
    I think I strained a pucker muscle

  12. #12
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    Quote Originally Posted by anotherbrian
    It's probably more complicated than that. A tire with 0psi in it doesn't have an infinite contact patch (or a 1000sq in at 1psi), and a run flat tire can maintain a usable contact patch without air.
    Kidding, of course. I thought PSI was the force exerted on the area of the tire by the gas and not the weight of the item being supported. You know, your tire pressure is the same whether the car (or bike) is airborne or on a workstand or lift or whatever.
    All out of S**** and down to my last F***
    I think I strained a pucker muscle

  13. #13
    Sweat is just fat crying.
    Reputation: Finch Platte's Avatar
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    The handle of the pump will blast out the top, killing you instantly.
    Mountain Biking Is Not A Crime stickers, free! (You pay postage. PM me for details.)

  14. #14
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    Don't forget that most car tires have schraeder valves

  15. #15
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    Quote Originally Posted by Finch Platte
    The handle of the pump will blast out the top, killing you instantly.
    True, BUT...Air is forced out of the tire through the now busted pump so fast that it creates a tear in the Time Space Continuum that sucks your now deceased body into an alternate, but perfectly identical dimension. There you will no longer be deceased and unable to inflate a car tire with a floor pump. It will, however, take you a really really really long time to inflate the tire. By the way you will have no knowledge of your alleged death in this dimension and no one here will have any recollection of you, so....now you have been warned.
    Since I don't like writing, I don't have a blog to pimp. This space for rent.

  16. #16
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    Quote Originally Posted by EBrider
    Don't forget that most car tires have schraeder valves
    So does my Rockhopper.
    Since I don't like writing, I don't have a blog to pimp. This space for rent.

  17. #17
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    Done it! 30psi from dead flat on a Corolla; full size tire, 1980 Medai floor pump. Enough to get to a gas station.

    Works great, just exactly like you know it will. Also a helluva lot o' pump strokes, just exactly like you expect. More of a wrist workout than I would care to experience again, especially post bumpy ride.

    Made me buy one of those little cigarette-lighter plug-in emergency compressors, which of course I've never needed in the 9 years since I bought it.=P

    I never knew about that Time-Space Continuum anomaly, though. Who could predict these awesome things?
    "I think it's cool how the best line is also usually the most beautiful line" --Kurt F, Tamarancho, Safety Meeting

  18. #18
    Sweat is just fat crying.
    Reputation: Finch Platte's Avatar
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    Quote Originally Posted by GrayBeard Pirate
    True, BUT...Air is forced out of the tire through the now busted pump so fast that it creates a tear in the Time Space Continuum that sucks your now deceased body into an alternate, but perfectly identical dimension. There you will no longer be deceased and unable to inflate a car tire with a floor pump. It will, however, take you a really really really long time to inflate the tire. By the way you will have no knowledge of your alleged death in this dimension and no one here will have any recollection of you, so....now you have been warned.
    Mountain Biking Is Not A Crime stickers, free! (You pay postage. PM me for details.)

  19. #19
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    Should have asked him if he knew how his hydraulic lift/jacks worked.

    Anyways, IF he was right (which he wasn't), you could have just taken the jack out of your trunk, lifted the wheel off the ground and pumped away.

  20. #20
    MTB skillz = NADA
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    Quote Originally Posted by Finch Platte
    Sorry....its not riding induced insanity.
    Since I don't like writing, I don't have a blog to pimp. This space for rent.

  21. #21
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    As long as the car doesn't run over the pump, you'll be fine.

  22. #22
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    Quote Originally Posted by anotherbrian
    It's probably more complicated than that. A tire with 0psi in it doesn't have an infinite contact patch (or a 1000sq in at 1psi), and a run flat tire can maintain a usable contact patch without air.
    As soon as the tire is flat the place where the metal rim contacts the ground begins to have a very high PSI. Since the rim doesn't deform, the equation is no longer applicable..

  23. #23
    Save Jesus
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    you should have bet him for money

  24. #24
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    Quote Originally Posted by morganfletcher
    He's an idiot. PSI is psi.

    Morgan
    I'm going to rule with the AutoZone guy,

    I beg to differ. There are more energy stored in a tire @ 30 psi then a bike tire @ 30 psi.

    A good example, a garden hose flows at 9 gals per minute. That wont move your car or threw your bike accross the street at that volume. Now compare that to a river flowing 9 gal per minute. Your car will wash away in seconds.

    Given the same PSI, more volume means it's more dangerous.

    PV=nRT ... Pressure X Volume = N (constant since it's air) R (constant since it's air) Temperture (constant since it's at the same time)

    Tire Pressure X Tire Volume = Pump Pressre X Pump Volume ...

    I found someone calculated tire volume to be 10 litres. 1 liter = .264 Gallons to 10 litre is 2.64 Gallons

    Floor bike pump = 24 inches tall by 1 inch cylinder: Volume = pi x radius ^ 2 x height = 3.14 x .5 inch x .5 inch x 24 inches = 18.84 cubic inch : 1 gallon = 231 cubic inches : .08 Gallons

    PV = PV ... tire (30 PSI) X (2.64 Gallons of air in a tire) = bike pump (X PSI) X (.08 Gallons of air in a bike pump).

    Bike pump (X PSI) = 30 PSI X 2.64 Gallons / (.08 Gallons) = 990.0 PSI.

    YES, your pump will handle 990 PSI!!! That's safe. Grab the handle tight.

    How does that involve the car's weight .. If your remove the tire off the car, PSI drops down a bit. I know that on my camper, with empty truck bed, tire pressure is at 60 psi, once the 2400 lbs thing is on, it jumps to 80 psi!!!

    So the AutoZone guys is right!!!

  25. #25
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    Quote Originally Posted by Buzzaro
    Kidding, of course. I thought PSI was the force exerted on the area of the tire by the gas and not the weight of the item being supported. You know, your tire pressure is the same whether the car (or bike) is airborne or on a workstand or lift or whatever.
    The weight of an item will cause the tire to deform ... aka flatten the tire. Since the tire can't expand else where (steel belts reenforcement), it will cause the pressure to increase since it now has less volume with the same amount of air molecules. PV = nrt = PV.

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