Index Home About Blog
Subject: Re: A physicist's question about tensioning a wheel
Message-ID: <yDPJb.6483$>
Date: Sun, 04 Jan 2004 07:53:02 GMT

Jim Beam writes:

>> It probably has carbon-steel spokes, as did most bikes back then.
>> Although carbon-steel spokes are not as strong as stainless (and
>> prone to rusting) they are less subject to fatigue than stainless
>> spokes are.

> The carbon alloy steel spokes were galvanized most likely.  Flashy
> [literally] ones were chrome plated.

> Technically, carbon alloy steel wire is usually stronger than
> stainless.  It's still used in steel rope where strength [&/or
> price] outweighs weather considerations.

If exposed to weather, such cables are galvanized and often painted to
protect them from rusting.  This is mostly a matter of cost, stainless
cables being far more expensive than carbon steel.

> Carbon steel spokes /were/ less prone to fatigue.  Modern stainless
> steels are significantly better than they used to be and now the two
> are roughly comparable.  High strength carbon alloy steels do not
> typically have a fatigue endurance limit - it's just that it's been
> possible to make "cleaner" carbon alloy steels [cheaper] longer than
> has been possible for stainless.

> If ever you travel to San Francisco, there are two pieces of
> interesting wire that you must see:

> One is at the golden gate bridge.  There is a section of the main
> span rope cut open for inspection at the visitor center.
> Interesting un-twisted strand design.  High strength carbon alloy
> steel.

You will notice that these "cables" are completely encased in a
waterproof steel jacket and are in real life pumped solidly full of
grease like preservative... as they are on most suspension bridges.
The suspenders on the GG Bridge are exposed strands but are helically
wound and are galvanized and painted with the traditional red paint of
that bridge.  What is less evident to the casual observer is that
because the bridge lives in fog and salt spray, it does not have
exposed riveted lattice-work as other bridges around the bay.  It's
towers are enclosed so the structural elements within the towers can
be kept dry and clean.

> The other is the crowd barrier rope at the cable car turnaround on
> Powell & Market.  It's a piece of actual cable car traction rope.
> Unlike all the [wire] rope you normally see that is twisted to make
> the exterior strands run axially, this one is twisted to make the
> exterior stranding as close to perpendicular as possible.  This is
> to give maximum possible friction for the cable car's rope clutching
> mechanism as it goes up the hills.  Again, carbon alloy steel.

Since these cables bend around pulleys and curves, they must be
helically wound or they would break immediately.  They are made this
way for the same reason control cables on bicycles are helically
wound... so they will have uniform stress in all strands when bending
around curves.  Cable car cable has a special layer of external
friction strands that are smooth and rectangular, like the ones for
aerial trams where cables are used as rails on which the tram rides.
This is done to reduce wear that would occur if the outer strands had
a round cross section or even themselves made of strands, as are old
Campagnolo bar-end shift cables.

> /utterly/ useless trivia, but thrillingly geeky!

Don't pass along myth and lore.

Jobst Brandt

Index Home About Blog