Re: Review of Sub-8-pound bike? (Customized Litespeed Ghisallo)

Phil, Squid-in-Training wrote:
>
> Chalo wrote:
> >
> > The properties of engineering materials are understood by many of us
> > who post in this group. For us, an 8-pound bike with plastic gears in
> > the chain drive is simply stupid and dangerous, not "scientific".
>
> So what about NASA engineers designing with a factor of safety of 1.1?
> Shouldn't they design at a factor of safety of 2 or 3 or 10?

Funny you should use that example, since I make spacecraft parts for a
living and I get to participate in the design process.

The thing about a spacecraft that makes it categorically different from
a bike vis-a-vis safety factor, is that a spacecraft's entire mission
profile-- everything it is ever going to do, dynamically speaking-- is
determined well before anyone decides how it's going to be constructed.
That includes a reckoning of both the magnitudes of forces involved
and the number of cycles it is expected to undergo. Then it's built to
tolerate just those forces and nothing more.

Typically, a spacecraft gets used once and discarded. All
mission-critical components are tested to failure in a laboratory
setting before they are used in a flight vehicle.

So a bike could be designed to work with a safety factor of 1.1,
assuming:

- The mission profile is known-- e.g. Phil the apprentice squid shall
dock with the grocery retail facility 1.65 km down the street, accept a
payload of exactly 6200g of solid and containerized liquid cargo, and
deliver the payload to the fueling station on base, exerting a thrust
force not to exceed 45 newtons and an engine power level of no more
than 250 watts and maintaining the planned trajectory along Poplar
Street and Central Avenue within 40cm of centerline at all times.

- The cycle life is known-- after Phil has delivered the critical
payload of supplies to base, the mission vehicle will be dismantled,
inspected, and tested to destruction in the hangar/textile processing
facility attached to base.

- Proper range safety is observed-- if either Phil or the mission
vehicle deviates from the planned flight trajectory, they will both be
immediately self-destructed with pyrotechnic charges to prevent
potential downrange damage.

Of course, if you encounter a micrometorite or pothole that exceeds
expected dimensions, causing failure of the mission and complete loss
of the vehicle, well that's a risk you take in a highly constrained
engineering environment-- such as that imposed by orbital physics or,
um, people thinking their bikes should be lighter.

> > What you discuss isn't science in the usual sense, but only in the
> > sense that questing after perpetual motion machines is "science".
> > That is, the underlying principles are well-understood enough that
> > more applied experimentation is of no practical use.
>
> You sound like Bill Gates: "640kB ought to be enough for anybody."

If 640KB was all that anybody could get, or all that anybody could
afford, then for the time being it would have to be enough, wouldn't
it?

When engineering materials and manufacturing techniques improve, bikes
can become significantly lighter. But if you use the same materials
and the same manufacturing techniques that everybody else is already
using, then just cutting the weight in half gives predictable results.


Misusing available materials isn't a good approach either; just because
aluminum is less dense than steel doesn't make it a good material for
lightweight sprockets. Likewise, CFRP is even less dense and even
worse for that job.

Heck, someone should point out to M2Racer that cork bar tape is even
lighter than carbon, so we can see what happens when they make a
cassette out of it. That'd be all scientifical 'n stuff.

> I think it's a good thing that these bikes are out of the reach of the
> typical person, but we gotta start somewhere, you know?

Right. We should start with the prerequisite advancements in materials
and/or processes that will enable us to shave weight without shaving
off necessary strength. With foofoo bikes rapidly approaching the
$XXXX/kg cost of value-oriented space launch vehicles, that shouldn't
be too tall an order.

Chalo Colina

.

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