Fwd: Re: Folding liquid fiberglass bicycles

[Kragen Javier Sitaker]

Forwarding comments on the bicycle idea:

Date: Sun, 17 Sep 2006 22:10:37 -0500
From: "Michael Leonhard" <michael206 at gmail.com>
To: "Kragen Javier Sitaker" <kragen at pobox.com>
Subject: Re: Folding liquid fiberglass bicycles
Message-ID: <21f8b6e20609172010s648de273j7ee32cc60a60aa99 at mail.gmail.com>

Hi Kragen,
This is an interesting idea.  What kind of material did you have in
mind?  Glass fibers can be embedded in many kinds of materials, some
flexible and others stiff.  I've used very light weight fiberglass in
radio controlled models.  First, I must cut the fiberglass cloth and
lay it over a form.  Then I apply epoxy, which soaks into the cloth.
For added strength, several layers of cloth may be applied
successively and soaked in epoxy.  After the epoxy hardens, I destroy
the form, leaving a thin fiberglass-epoxy composite shell.  The
flexibility of this shell depends on the kind of epoxy and the number
of layers of fiberglass cloth.  A single layer of very light weight
cloth can yield a part that is about as flexible as the cardboard from
a cereal box.  It's not the kind of thing you would roll up and stick
in your pocket.  I suppose there must be some very flexible plastics
that could be mixed with fiberglass to make very strong and flexible
cloth.  Have you heard of anything in particular?

If the frame will be filled with water, perhaps osmosis create the
necessary pressure?  According to Wikipedia, ocean water has 27 atm of
osmotic pressure.  That is about 400 pascals, three orders of
magnitude less than you estimate is needed.  But I suppose it would be
difficult to make a semipermiable membrane that is strong enough to
contain the 300-650 kpsi necessary for the structure.

How about a scooter?  The wheels could be shaped like shallow bowls
that fit together, holding the deflated frame and axels inside.

Have you read Neal Stephenson's book The_Diamond_Age?  He describes a
future where nanotechnology facilitates cheap and perfectly recyclable
machines.  An automated manufacturing machine is called a 'matter
compiler'.  While the automated recycler is a 'decompiler'.  Recycling
is then 'deking'.  A common mode of transportation is the 'velociped'.
 It's a pair of robotic stilts or bionic leg extensions.  They allow
the person to walk comfortably while taking enormous strides to travel
very fast.  One character in the book uses a new velociped for every
commute.  His home compiler produces a them every morning for him to
take to work.  When he gets to work, he dekes them.  At the end of the
day, he compiles a new pair for the trip home.  The velociped is
created by a computer so each one is identical.  It is very enticing
to always have perfectly new machines.  Dropped your PDA?  Just deke
it and compile a new one!  I dunno about fiberglass though...

-Michael

Michael Leonhard
uic at tamale.net
michael206 at gmail.com
http://tamale.net/

On 9/11/06, Kragen Javier Sitaker <kragen at pobox.com> wrote:
> How small can a folding bicycle be?  How light?  I'd like to carry one
> in my shirt pocket for emergencies, or for travel by bus.
>
> Perhaps the frame could consist of waterproofed fiberglass fabric tubes
> filled with water, pressurized with a thumbscrew, maybe with a little
> bit of air to support elasticity.  Any frame mass over 20kg is probably
> unreasonable, and let's say 80% of that is the water.
>
> That's about 16kg of water.  If the water-filled frame comprises 7
> segments, each around half a meter, for 3.5 meters total, that's 4.6
> liters per meter, a cross-sectional area of 46 cm^2.  That makes the
> tubes about 7.7 cm across.
>
> If one of the tubes is under bending stress, the tensioned part of the
> tube is about 4 cm wide, and we just have to keep the water pressure
> high enough to keep both walls in tension (so the "compression" wall
> won't buckle).  The tensile strength of glass fibers is around
> 300-650kpsi (see
> http://www.roymech.co.uk/Useful_Tables/Matter/Glass.html for details.)
> Only half of that (or so) is available to resist the bending stress,
> since the other half of the fiber runs around the circumference to keep
> the tube from splitting lengthwise.  So how much force do we need to
> apply bending a tube to rip the fabric?
>
> We have 4kg of glass over 3.5m of tube.  I think that's somewhere around
> 1000cc of glass, or 285 cc per meter.  7.7cm diameter means 24cm
> circumference, or 2400 cm^2 per meter, for a thickness of 285/2400 cm,
> about 1.2 mm.  1.2 mm over 4 cm of the circumference gives a cross
> section for the tensile stress of 0.48 cm^2, or about .074 square
> inches --- with a tensile strength of, say, 150kpsi, that gives us 11000
> pounds of strength.  That's working on a lever arm of 7 cm against a
> force potentially applied in the middle of the strut, with a lever arm
> of 25 cm, so that force has to be 3000 lbs. to break the fabric.
>
> So we could probably get by with one tenth of that strength, and thus
> one tenth that amount of glass: 0.4kg, occupying about 100cc.  There are
> still practical problems: how do you make wheels that fold, or do you
> just use tiny scooter-like wheels?  Do you make legs with some kind of
> mechanical linkage that produces a smooth gait instead?  How do you keep
> the fabric from creasing and breaking glass fibers when you put it in
> your pocket?  (Maybe you unzip the tubes lengthwise and roll them up
> around a dowel?)  Wouldn't even an 0.12-mm-thick layer of fiberglass be
> pretty stiff?  Maybe it would be better to make water-filled fiberglass
> stilts instead.