I took a break from steam powered record players and railway model dining tables and talking John Steed Thunderbird puppets for a bit to play with something I saw on the Make magazine blog.
This is an emergency parasitic bike pump which is basically an air hose that lets you inflate a presumably flat bike tyre from a handy inflated car tyre.
Make in turn got it from the original Instructable which is here: http://www.instructables.com/id/Automatic-bicycle-pump/
If you look at the comments in both of those you’ll see there is a bit of discussion on this. On the Instructable there are some very odd assumptions being made about what car tyres and bike tyres pressures actually are. The maker guys are more concerned about the morality of it all (me included).
One thing that annoyed me was people seem to think that a car tyre inflated to 30PSI would be able to then in turn inflate the bike tyre to 30PSI. That’s obviously wrong as the air has to come from one tyre (at 30PSI) to the other flat tyre (at 0PSI) so you’ll end up with them both at some pressure in between. I assumed they’d both end up at the same pressure.
In the end I decided the easiest way to see if this works was to try it!
So off out to my garage I went. I knew I had one old air hose (red) off a dead foot pump so I dug that out. I also knew the air hose (black) on my air compressors tyre inflator unscrewed. Some rummaging in my junk box produced a brass 90 elbow. I cut the end off the junk pumps hose and pushed that over a hose barb screwed into the elbow. The air compressors inflator hose happened to match the thread on the elbow so, with a bit of teflon tape, that just screwed in. This was handy as I needed to keep unscrewing it between tests to re-inflate my car tyre.
Now I don’t actually own a bicycle. But I have something close. It’s a small mini bike project I started about 10 years ago and haven’t finished (yet). It was to be powered but a small 2 stroke engine and vaguely resemble an old Norton Manx racing bike. The car tyre is on my MGB.
On the car I run at Yokohama C-Drive 185/65-R15s at 32PSI on the front tyres. This is a fairly typical sort of pressure for a normal car tyre to be at. The mini-bike uses small wheels from a kids bicycle. I replaced the tyres that were on it with some Hookworm 16×1.95 tyres since those were what looked most little mini motorcycle tyres to me. A label on the side of the tyre says to inflate them to 110PSI! I don’t actually know what typical bicycle tyre pressures are but I am fairly sure it is more than a car tyre.
To measure the pressure I used a normal pen style tyre pressure gauge. Now this isn’t any kind of rigorous scientific study. I’m just some bloke messing about in his garage on a Sunday afternoon so I did it with bits of junk and what I had on hand. Don’t go worrying that I didn’t use the right kind of gauge or I didn’t have a proper bike or I should have used a red MGB instead of a green one. I just wanted to see if this idea actually works.
So, here is what I did. I inflated the car tyre to 30PSI. Seems like as good a pressure as any and representative of what you’d get on a normal car. I made sure the minibike tyre was fully deflated – no pressure on the gauge. I hooked one end of the hose onto the mini bike then I hooked the other end onto the car tyre.
What happened next is the air could be heard hissing down the hose and the bike tyre would inflate. I would wait a while till the hissing stopped (5-10 seconds) then quickly remove both valves at the same time.
Not that last point is important. You must remove both hoses simultaneously. When you disconnect one tyre first the other tyre is still connected to the hose so all the air will leak out of it. The car and bike tyres both use what is called a Schrader valve. These have a small pin in the middle of them that, when pressed, opens the valve. With the with hose connected the pin is depressed and the valve open. You could probably design the hose with a one way valve in it so air can flow from the car tyre to the bike tyre but not the other way. Then you could detach the car tyre end first without worrying about the bike tyre deflating again. It’s not too hard to detach both at the same time with minimal air loss though.
I did this 10 times and recorded the results. Between each try I had to unscrew the black tyre inflator hose from the manifold and use it to re-inflate the car tyre back to 30PSI again.
I also did one test at 35 PSI and another at 40PSI. I didn’t want to inflate the car tyre more than that so I stopped then.
The results are fairly consistent given my very unscientific method and they are quite interesting. As I thought the bike tyre doesn’t go to 30PSI of course. But the two tyres don’t end up at the same pressure either. I averaged the 10 attempts at 30PSI and here are summarised results below:
Start pressure: 30PSI
- Car tyre end pressure: 27.75PSI
- Bike tyre end pressure: 25.1PSI
- Difference: 2.65PSI
Start pressure: 35PSI
- Car tyre end pressure: 31.5PSI
- Bike tyre end pressure: 28.5PSI
- Difference: 3PSI
Start pressure: 40PSI
- Car tyre end pressure: 37PSI
- Bike tyre end pressure: 32PSI
- Difference: 5PSI
So, what does this all mean? The hose does sort of work. Air will go from the car tyre into the bike tyre re-inflating it somewhat. But the bike tyre will be at a lower pressure than the car tyre and both will be at lower than the starting pressure. If the bike tyre needs to be inflated to 110PSI as it says on the side wall of even the little kids one I have you’d need a car tyre that’s at a pressure greater than that.
Interestingly my tyre pressure gauge only goes to 50PSI so you’re probably out of luck finding a car with pressures that high! I think you’d need to find a handy aircraft to steal air (actually more likely nitrogen) from. Chapter 6 of this Goodyear Tire PDF shows the difference between aircraft and passenger car tyres and indicates aircraft run at 200PSI and cars at 35PSI.
Maybe someone would like to do a more rigorous test with better equipment and method and graph the results to see if there is some kind of trend happening. What I don’t understand is why they end up at different pressures? Is it just leakage when I remove the hoses or is there good scientific reason behind it.
The other thing worth mention is you really shouldn’t go stealing air from other peoples car tyres. One, it could be dangerous and two, it’s just plain rude! I would be very, very unhappy to discover someone had been messing with the tyres on my car without my knowledge.