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I know that shock pumps are meant for high pressure applications so that they can withstand high pressures needed by suspension shocks.

However, can I use a shock pump to inflate my tire in a pinch if I run out of CO2? How would it compare to a standard mini-pump in terms of volume?

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Is it that you're carrying a shock pump while riding? –  WTHarper Feb 27 '13 at 20:40
    
You aren't going to hurt anything, and as Benzo mentions you can use an adapter to do presta. Please try and report back how long it takes to get a decent pressure. –  Ken Hiatt Feb 27 '13 at 22:09
    
Yeah, it would be an interesting experiment to try. Probably no worse than some of the cheap compact pumps. –  Daniel R Hicks Feb 27 '13 at 22:58

2 Answers 2

up vote 12 down vote accepted

Note: this calculation makes many assumptions, so it's only useful in an 'average use case', not some sort of exact measurement. If you find better information, please post it and I'll update the answer.

How many pumps you would need to fill up a tire depends on many variables. First, the volume of your inner tube, which can be approximated as a torus (doughnut-shape) atop the rim size of a given diameter. You can find yours approximated on this graph.

  • 26" Mountain Bike, 2.1" tire width = 4.8L
  • 29" Mountain Bike, 2.1" tire width = 5.2L
  • 700C Road Bike, 35mm tire width = 2L
  • 20" BMX Bike, 1.85" tire width = 3L

I'm finding pump specifications quite difficult to find (I need piston diameter and stroke length), but here's a comparison of some shock pumps on an empty shock cylinder of a 2007 Fox 36 RLC 160mm Fork.

  • Average Accu-Gage Pressure Reading after 100 strokes: 77.2 psi
  • My best guess at RC2 air chamber specifications: 1" (25.4mm) diameter, 160mm length (max travel). If someone can find accurate specs or the true volume, I'd appreciate it.
  • Fox RC2 Air Volume (estimated): Vcylinder = pi*r^2*h = 81 mL

So then, IF pumping air into different volumes is linearly proportional (it's not, but somewhere in the ball park), and you wanted to inflate the tires listed above to ~77.2psi, it'd take about this number of pumps with the 'average' shock pump:

                                           Tire Volume Ratio of Tire to Air Shock  Number of Pumps to ~77.2 psi   
26" Mountain Bike, 2.1" tire width 4.8L             59:1                                   5,900                                          
29" Mountain Bike, 2.1" tire width 5.2L             64:1                                   6,400                                          
700C Road Bike, 35mm tire width 2L                25:1                                   2,500                                          
20" BMX Bike, 1.85" tire width      3L                37:1                                   3,700                                          

So, perhaps if it was a life or death situation, you have a lot of time on your hands, or you just can't afford that gym membership, you might want to pump a few thousand times to fill your tire. You probably don't.

By contrast, if you're just looking for this functionality in one device, you could just use a dual purpose model, such as this Specialized Pump, that can inflate both high pressure, low volume containers (like your shock) and low pressure, high volume containers (like your tire).

Specialized Dual Use Pump

You twist the handle to select between uses, and save yourself a few thousand pumps.

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Raw data. Awesome! –  Benzo Feb 28 '13 at 3:59
    
Nice answer, but I don't think most people run a mountain bike tire at ~77 psi (maybe on pavement, but they'd likely be using smaller slicks in that case). –  Batman Feb 11 at 12:44
    
Well, my city mountain bike tires runt at 65 psi. +1 more for "or you just can't afford that gym membership" –  Vorac Feb 11 at 13:35
    
I have Kenda hybrid tires on my mountain bike, which are 2.1" wide and take 65 psi. If, however, you're referring to most MTB tires that are between 35 and 40 PSI max, just half it: 2,950 pumps for a 26" tire and 3,200 for a 29" (38.6 PSI) –  Ehryk Feb 14 at 2:28

A shock pump is designed to fill a very small volume of space with very high air pressures.

A tire pump is designed to fill a large volume of air, to relatively low pressures, pretty quickly.

You can technically fill a schrader valve tube using a shock pump, but because the volume of air for each repetition of the pump will be so low, it might take a week to fill the tire.

A shock pump will not mate with a presta valve at all.

On a practical level, you should use a shock pump to fill your shocks, and a tire pump to fill your tires.

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1  
Is a week realistic? Maybe 10 mins? I have filled a car tire to 35 psi with a bike floor pump, so I know what it's like to fill a huge tire with a small pump. –  Benzo Feb 27 '13 at 20:02
2  
Couldn't you also just use a presta adapter to fill a presta tube? –  Benzo Feb 27 '13 at 20:02
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@Benzo: It wasn't intended to be realistic. It was deliberate exaggeration. However, it wouldn't be 10 minutes. It took more than an hour to fill a tire with a shock pump, the one time I was desperate enough to try. –  zenbike Feb 28 '13 at 14:32
    
About an hour... yeah, seems like that's better than useless, but far from useful unless you're really stranded. –  Benzo Feb 28 '13 at 17:13

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