Inspired by the parasitic bike pump we posted about last week, Simon Jansen decided to try it out for himself. Using his own car tire to avoid the moral issue of borrowing air from an unsuspecting motorist, and a sweet looking motorbike that he is building as the test subject, he found an interesting result– the pressure on his bike tire always ended up lower than that of the car:

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, 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.

He repeated this experiment many times, using various auto tire pressures, and measured the resulting pressure at the end of each experiment. The curious part is that the minibike tire always ended up at a lower pressure than the automobile tire.

My guess is that both tires lose a fixed volume of gas when you remove the valves, and that this affects the pressure of the smaller bike tire more than the car, however we aren’t sure. Anyone know what’s happening?