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# Self-Inflating Bicycle Tube

Tube with integral pumping mechanism
 (+9) [vote for, against]

I've been thinking about the inflated bicycle innertube. My first go at this didn't go down that well <link>. Basically, I'm getting bored of topping off the air in my tyres*. So, a solution.

Imagine a standard bicycle innertube. Now, make a compartment in that tube around where the valve would normally be, say around 4 inches X the full diameter. Now, fill that compartment with a VERY firm (in the 60 psi range), completely porous foam.

Next, install three valves in the compartment.The first leads from the compartment to the rest of the inner tube and is opened by a large pressure difference**, say 5 psi. It's one way, and lets 60 psi air from the compartment to the rest of the tube. The second, is a simple one way valve that allows air from the atmosphere into the compartment. The last, is an over pressure valve, going from the compartment to the atmosphere. Set at say... 70 psi. This is a safety blow off valve.

How it works: When weight is applied to the tire, and the compartment happens to be on the bottom, the tire deforms, compressing the compartment and foam within. This forces air out from the compartment into the general innertube via valve #1. Next, when the weight is relieved, (when the wheel rotates for example) the foam re- expands, creating a partial vacuum. The vacuum is filled by air from the atmosphere via valve #2.

This process repeats until the pressure difference between the compartment and the general innertube is no longer enough to open valve #1. Instead, if there is enough weight applied, it may trip the safety valve #3 and the excess is vented.

*there's no puncture, they just seem to slowly leak, I blame the frankly ridiculous heat out here in the colonies, it's likely softening up the rubber.

** such valves exist, I have these clothes storage bags which have a valve that opens when I sit on the bag, squishing the air out, and then close, preventing the air getting back in. Made, apparently, out of some arrangement of the general polythene structure.

 — bs0u0155, Aug 19 2014

Compartmentalized Innertube Compartment_20Innertubes
[bs0u0155, Aug 19 2014]

[doctorremulac3, Aug 19 2014]

inflaTUBE http://www.halfbakery.com/idea/inflaTUBE
[mitxela, Aug 19 2014]

Not shown but proper pressure would fold the levers back. Low pressure would allow them to pop out. [doctorremulac3, Aug 20 2014]

 Sounds doable. How about something you screw onto your valve stem that has a little centrifugal force powered pump build in?

A heavy little piston pushes air into the tire while you're pedaling then resets with a spring and reloads more air when you stop.
 — doctorremulac3, Aug 19 2014

In general it sounds good, but some issues may occur. The device would create an imbalance noticable at higher speeds. It would probably only work at low speeds due to time required for action. The valves would be vulnerable to dirt and create more possibilities of slow leaks (+)
 — afinehowdoyoudo, Aug 19 2014

 a 5 gram mass will get you about 14 Newtons if the bike is kicking along at about 5 metres per second (10-12 mph ish).

 That would have to overcome say 60 psi. Which is about 410000 Newtons per metre square. So, all you have to do is design the piston small enough so that 14 will do it. Er... 0.41 Newtons per square mm. That means you can have a a 34 square milimetre piston, that's like 5mm per side.

Doesn't seem right... where have I gone wrong?
 — bs0u0155, Aug 19 2014

 //The device would create an imbalance noticeable at higher speeds//

 I have a 1966 Raleigh 3 speed. High speeds were carefully designed out of this machine, particularly in the steel hub, steel spokes (40 of, about half a pound each) and steel rim combination.

Also, just put a weight on the other side.
 — bs0u0155, Aug 19 2014

 I know, how about simply having a pendulum at the hub turning a pump? Basically put a weight on the handle of a rotary pump and turn the pump (with the wheel) while the handle is held in place by the weight.

Just bought a bike pump myself and it seems really stupid to have to pump your tires up regularly by hand.
 — doctorremulac3, Aug 19 2014

The link is not obvious about the mechanism, just mentions a second tube. This could be pre- pressurized, and finite. OR they could have a similar thing to my idea. hmm.
 — bs0u0155, Aug 19 2014

 It does look like it's another version of your approach maybe.

 How about a little pump you screw onto the stem whose pump handle extends beyond the tire (hard to describe) so it hits the pavement and gives a pump each time the tire turns around? So you snap it on and ride a few feet till your tire's pumped up.

 Not worth the hassle?

Naa, something that's always on that you don't have to think about is the ticket.
 — doctorremulac3, Aug 19 2014

 I like this idea very much.

It's going to be tricky to make everything work right so that you don't feel like there's a bump on the wheel, but it's a great idea.
 — MaxwellBuchanan, Aug 19 2014

I'm struggling with the concept of a completely porous foam.
 — EnochLives, Aug 19 2014

there are plenty of tough open-celled foams. You just make the cell-walls thicker until it's tough enough. In fact, to take a reductionist approach, have one cell... like a squash ball with a hole in it.
 — bs0u0155, Aug 19 2014

Why not just use the front shocks as the air pump? Big diameter, does plenty of work. If US army HMMVW's use through-hub air pressure adjustment, surely it could be built into a bike wheel somehow.
 — Custardguts, Aug 19 2014

 /where have I gone wrong?/

 where you went wrong is the force. F=ma, a=v^2/r, v=5 m/s as you say, and lets assume 20 inch wheels I like, so r=0.25 or so. Thus a= 100 m/s^2 or about 10 G's, and the force is 100 * 0.005 = 0.5 N.

 So you would need a 140 gram weight to get your 14N. For a really small amount of pumping.

So i think there are better ways to operate a pump than a weighted piston using centripetal acceleration.
 — afinehowdoyoudo, Aug 20 2014

[+] but at any given time, if used exclusively, there will always be a little bump. Also, and this is not necessarily a bad thing, the heavier the rider, the more inflated the tire.
 — FlyingToaster, Aug 20 2014

If I'm understanding this correctly then in its relaxed state the foam-filled compartment contains air at one atmosphere of pressure, having just received a fresh charge of atmospheric air. Let's say that the remainder of the tube is semi-inflated and you want to top it up to 45psig, which is typical for a mountain bike. Now to raise the pressure in the foam compartment from 0psig to 45psig under ideal conditions requires that the air is volumetrically compressed to 35% of its original size. That sounds like a very uncomfortable flat spot to me and incompatible with the very stiff foam as specified in the idea description.
 — EnochLives, Aug 20 2014

 //i think there are better ways to operate a pump than a weighted piston using centripetal acceleration//

 How about something you screw onto the stem that has a little lever that hits the forks every time the tire spins around? Tap into the same energy you use when you put a card on the forks to slap against the spokes to make a motorcycle sound.

 The lever's position could be extended automatically when it senses that the air is low. The lever would hit the forks every time the tire went around operating a little pump. It wouldn't have to be very large because it would pump a little every time the wheel went around. You'd be riding along and suddenly hear a little clicking sound for a half block or two while the pump mechanism engaged, no big deal. You could have the lever covered in rubber to minimize noise when engaged. When proper pressure was achieved it would withdraw the lever.

 Not sure that makes any sense, maybe I'll draw it if it passes the "have a first cup of coffee and think about it" stink test. I do really like the idea of something you screw onto your existing tire. Pretty sure you could sell those. I'd buy a couple. Might be a good Kickstarter campaign.

 For that matter, you could just take one of those old bike light generators, stick a pump in it and flip it onto the tire when it was low.

Kind of bulky though. Naa, something you can just stick onto that valve stem and forget is the ticket.
 — doctorremulac3, Aug 20 2014

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