Comments on: Air powered trials bike

By: eradler

eradler — Sat, 28 Jun 2008 18:50:00 +0000

Well my 2cents would be: steam up the frame with solar power (evacuated top tube as absorber)and then steam away- should be much more efficient and store more energy….

By: B. Nicholson

B. Nicholson — Fri, 27 Jun 2008 22:10:00 +0000

I just don’t understand. We are not on the moon. We don’t have to carry all the energy we use on a trip with us! Can’t we get some on the way? Take for instance, electricity. Streets are lined with power poles. Why can’t we wire the streets and get a small battery recharged as we roll along? Batteries are so heavy and electricity is so light weight! Tesla and now some MIT guys (finally) manage power transfers short distances through the air. Surely it is cheaper to send electricity someplace on power poles than it is to charge a battery and CARRY the stuff that way? Cyclists can eat on the way, drink rain, not to mention breathing–refueling, right?

By: Peter Eland

Peter Eland — Fri, 27 Jun 2008 21:39:00 +0000

"I chose to use air power because it is a very lightweight power source when compared to electric drive."

Is this really the case? How did you come to that conclusion? I may be missing something but e.g.:

http://en.wikipedia.org/wiki/Energy_density

suggests that the energy density (i.e how much oomph you get per unit weight) of a compressed air system is around 1/5 that of Li-Ion batteries. Their figure is at 200 bar in a carbon fibre tank. We could double that to be generous, but even then you're still getting less than half the energy out of the air system than you would out of the same weight of batteries.

I guess the air motor and associated control gear might possibly be lighter than an electric motor and associated circuits, but I doubt there's a lot in it.

I'd expect to see compressed air more widely used already in performance applications if it did have a performance benefit...

OTOH the performance might well still be sufficient...

Also not sure why high revs are a good thing for a bicycle. Wheels turn at relatively low revs/high torque compared to most applications.

By: bikesgonewild

bikesgonewild — Fri, 27 Jun 2008 20:37:00 +0000

…a more simple & immediate application if a ‘braking recovery system’ were light enough to work, would be for tire inflation on an mtb or town bike in case of flats while riding…

…just a thought…

By: Anonymous

Anonymous — Fri, 27 Jun 2008 19:36:00 +0000

I remember an article in MBAction from a couple of years back where I believe the senior editor was riding his bike on a trail and someone was keeping pace with him, even though the guy was out of shape, and after talking to the guy, the editor found out that the bike used air power to propel the guy uphill and I believe put air back in on downhill when braking.

Mike

By: Peter

Peter — Fri, 27 Jun 2008 18:42:00 +0000

As a follow up, I based the engine concept around this Australian design that you can see here http://www.youtube.com/watch?v=Dq8aZVLpf-c

I intended the storage pressure too be from approximately 300 – 350 bar. That’s 4500 – 5500 psi, operating pressures that are surprisingly common in inexpensive fibre-wrapped tanks.

From what I understand about the air engine design, torque can be increased by simply increasing the output pressure. Obviously at a loss of efficiency.

Safety is a valid concern, but composite tanks have proven to be astonishingly tough in my 10+ years of seeing paintball players abuse them.

I’m a designer, not an engineer, so I’m working heavily off of educated guesses, intuition, and ‘back of the envelope’ calculations. My primary concern is to get other people thinking in new ways.

The more feedback I get from informed people who are passionate about design and engineering the better.

By: Art

Art — Fri, 27 Jun 2008 17:53:00 +0000

You could probably fit around 5 liters of air in that frame, which at 200 atmospheres would put out around 80 Watts for an hour through a fairly efficient motor running at constant speed. So it might be functional for short trips where you don’t have to start and stop much. In a variable, high torque application like trials riding it might only last a few minutes. Aside from the efficiency challenges and the risk of detonating the frame in a crash, it’s not a bad design.