h a l f b a k e r yOn the one hand, true. On the other hand, bollocks.
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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.
Compartmentalized Innertube
Compartment_20Innertubes
[bs0u0155, Aug 19 2014]
An interesting approach
https://www.youtube...watch?v=yJP6HKcvH00
[doctorremulac3, Aug 19 2014]
inflaTUBE
http://www.halfbakery.com/idea/inflaTUBE
[mitxela, Aug 19 2014]
How about this?
https://dl.dropboxu...tire%20pump%202.png
Not shown but proper pressure would fold the levers back. Low pressure would allow them to pop out. [doctorremulac3, Aug 20 2014]
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Annotation:
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Sounds doable. How about something you screw onto your valve stem that has a little centrifugal force powered pump build in? |
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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. |
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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 (+) |
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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). |
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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. |
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Doesn't seem right... where have I gone wrong? |
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//The device would create an imbalance noticeable
at higher speeds// |
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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. |
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Also, just put a weight on the other side. |
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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. |
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Just bought a bike pump myself and it seems really stupid to have to pump your tires up regularly by hand. |
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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. |
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It does look like it's another version of your approach maybe. |
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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. |
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Naa, something that's always on that you don't have to think about is the ticket. |
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I like this idea very much. |
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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. |
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I'm struggling with the concept of a completely porous foam. |
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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. |
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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. |
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/where have I gone wrong?/ |
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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. |
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So you would need a 140 gram weight to get your 14N. For a really small amount of pumping. |
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So i think there are better ways to operate a pump than a weighted piston using centripetal acceleration. |
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[+] 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. |
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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. |
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//i think there are better ways to operate a pump
than a weighted piston using centripetal
acceleration// |
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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. |
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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. |
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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. |
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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. |
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Kind of bulky though. Naa, something you can just
stick onto that valve stem and forget is the ticket. |
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