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Old 04-02-09, 07:37 AM   #1
mrgardner
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Attaches to valve stem, keeps tire inflated?

I can't remember what this device is called. I saw it at a bike shop out of town and am looking to buy one. It was a small, weighted device that you could keep attached to your valve stem and as your wheel rotated, gravity and the device would pump small amounts of air into your tire, thus keeping it inflated.

I want this for a wheelchair tire that has a presta valve. I also have an adapter on the stem for standard pumps. Really I just need to know what it is called so I can find them at some etailers. I've searched high and low and I can't seem to find the right terms.

Thanks!
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Old 04-02-09, 08:20 AM   #2
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gravity? perhaps centrifugal forces?


there's this thing: http://pump-hub.com/Pump_Hub.html
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Old 04-02-09, 09:07 AM   #3
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Originally Posted by mrgardner View Post
I can't remember what this device is called. I saw it at a bike shop out of town and am looking to buy one. It was a small, weighted device that you could keep attached to your valve stem and as your wheel rotated, gravity and the device would pump small amounts of air into your tire, thus keeping it inflated.
Are you sure this was a serious device, not an April Fools joke?
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Old 04-02-09, 09:33 AM   #4
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LOL yeah it was a real device. The guy showed it to me but it was a year or two ago. I saw it with my own eyes. He even showed me how it worked, which I'll explain along with the image attachment.

In the pic (I did it in paint, I know its a rather pathetic schematic!), the tire is on the left and I tried to show how the device sits on the stem and its relative size. On the right is a close up. Note the 'weight' on it near the top of the device. Its the T section on the device running perpendicular to the vertical valve stem.

Basically what would happen is that as your tire rotated and the stem was at the top (in a hanging position), the weight would pull the device and expand the chamber (this is why I said gravity in my original post). Then, as the tire rotated the stem to the ground (in a standing position), the weight would push the device and contract the chamber. There must be a stopper inside like a regular tire pump because the air that was drawn in when it expanded would then be trapped and could only go into the tube as the device contracted.

Typing this up I realize how impossible this sounds. How could such a small weight be enough to contract this device with enough force to push the air into the tube? I don't know the answer to any of this. I will try to contact the bike shop and see if they remember what it is called because I'm sure I'm not the only one that would be curious to find out what exactly this thing is.

Thanks for the replies. If anything else comes to mind or if my description and crappy drawing help jar any memories, let me know.

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Old 04-02-09, 10:05 AM   #5
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Please let us know if you find out what it is and who makes it. I can't believe that if it really worked it would be so obscure.

Somehow it reminds me of those gadgets sold on late night TV "infomercials"
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Old 04-02-09, 12:28 PM   #6
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Seems like it would only work on very low RPM wheels. And would need a substantial weight to allow it enough force to pump air...
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Old 04-02-09, 01:01 PM   #7
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OK, from an engineer's perspective.

Assume the weight is about 1.125" diameter and 1" tall; that would weigh about .28 lbs (4 1/2 ozs) in steel.

If that weight was attached to a small enough piston it could produce much more than 1/4 lb of pressure. The pressure is proportional to the ratio of piston diameter to 1 square inch. For instance, a 1/8" piston is 1/80 of a square inch, so .28 lbs X 80 = 22.4 PSI. Max pressure available with this size piston and weight.
This is not much pressure, but lets use this as a starting point for calculations.

If this piston had a 1 inch stroke it would generate .01227 cubic inches of air per cycle. It takes about 50 cubic inches of air to fill a road bike tire to ambient pressure so 50 / .01227 = 4075 revolutions.

That tire is about 7 feet in circumference. 7 X 4075 = 28,525 ft; or about 5 1/2 miles traveled to pump up the tire.

Better be in no hurry though, because at much more than 2 MPH centrifugal force will hold the weight out against the wheel and no pumping will take place.

I think this myth is busted.
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Old 04-02-09, 04:05 PM   #8
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I agree with your comments crank (well, those that I understood, anyway).

In case I didn't make it clear initially, I just want to say that the intention of the device is not to pump up a tire from zero to full. It's intention is to continuously pump small amounts into the tube to counteract the loss of air that naturally occurs over time. That's all.

I found the bike shops number. I'll give them a ring when they open tomorrow and ask for the crazy salesman with the alien technology
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Old 04-02-09, 05:05 PM   #9
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Gardiner,

Regardless of whether you are inflating from zero psi or keeping a tire inflated the point is that based on weight and gravity it would be impracticable. The issue is that the centrifugal force generated by the moving wheel will compress the piston and hold it in the compressed position. 3mph is a casual walking pace...if the centrifugal force generated at below this threshold is enough to compress the piston and hold it in the compressed position then you would have to move the wheelchair slower than a casual walking pace to keep the device working...you would really only see speeds like that with a wheel chair when the wheel chair bound person is shopping.

I can't believe the cost of a device of this nature would be exceeded by its effectiveness. The speed range for which this device would be theoretically effective would be very impractical. If someone has the ability to push themself in a wheel chair they should, theoretically have the ability to hook up a more practical device that will inflate the tires (e.g. electric pump, air compressor, etc)

-J
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Old 04-02-09, 05:19 PM   #10
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I remember seeing something like that recently on the internets. Except it was definitely not a 'bolt-on' device... it was a hub that powered a self-contained pump as the wheel rotated... again, not to inflate from zero but to counteract gradual loss, and possibly help keep the tire inflated longer with a puncture.

Pumps take energy, though, and we humans have a limited supply... and it would probably be more handy to have a hub dynamo... and pump your tires up at home.
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Old 04-02-09, 06:08 PM   #11
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Here's a short explanation and picture of the hub pump system.

http://www.treehugger.com/files/2005...no_air_hub.php
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Old 04-03-09, 08:42 AM   #12
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Wow, great info everyone. If nothing else, this thread is bringing to light some rather interesting options.

I do need a pump for around the house. Preferably, something portable that I can carry with me. Pocket sized or slightly larger would be ideal.

I'll have to look into those options unless someone knows a few brands/items that would be worth considering.

Back to the main topic here, I just called the number for the bike shop and it is out of service. Perhaps they were shutdown for selling snake oil

Thanks for humoring me, folks. I think the dead end has been reached. If I do hear anything else, I'll let you know. Next time I'm in the area of the shop visiting family, I'll swing by and see if they're still in business.

To quote Greenfieldja, "I can't believe the cost of a device of this nature would be exceeded by its effectiveness". I agree.

Thanks again, everyone. Your insight (from abstract to technical engineering) has all been helpful.
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Old 04-03-09, 09:08 AM   #13
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I do need a pump for around the house. Preferably, something portable that I can carry with me. Pocket sized or slightly larger would be ideal.
The most versatile mini/portable pump I know about is the Topeak "Morphe" series, either Road for high pressures or MTB for lower pressures. They are scaled down floor pumps and have a folding T-handle, a fold out base, a short hose and a pressure gauge.

The hose isolates the valve stem from the pump and eliminates stem breakage which is possible if a reqular frame or mini pump is not used carefully. The base plate and T-handle make operating the pump a lot easier.

These aren't the smallest or lightest portable pumps but the easiest to use.
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Old 04-03-09, 01:34 PM   #14
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HillRider

Thanks for the tip. As I looked over the top rated mini / frame-mounted pumps over at Amazon, I began to realize that perhaps the better option is to obtain a portable 12v cigarette lighter port powered air compressor. I've found a few that would do the job quite well and probably save me some frustration. Could keep it in the trunk for emergency use for the vehicle, and also for pumping up the low tires if I'm not in the mood (or close to) the local bike shop.

So, guess I'll be checking that route instead and go from there.

Thanks again
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