Archive-URL: http://search.bikelist.org/getmsg.asp?Filename=tandem.10310.0539.eml
Date: Wed, 22 Oct 2003 19:42:12 -0700 (PDT)
From: "Craig Calfee" <Craig(AT)calfeedesign.com>
Subject: [T(AT)H] Re: CARBON demystified (Part 2)



Continuing a response to Bill's post:

> Fifth, the largest advantage of our molded tubing is IsoGrid --- a
skeleton
> of Kevlar-wrapped carbon that is affixed to the mandrel before the other
> layers. The resulting tube with its internal lattice is not unlike an
airplane's
> fuselage---where the thin skin is supported by an underlying network of
> supporting ribs. In the case of an IsoGrid tube, however, the application
of heat and
> pressure permanently bonds the Kevlar/carbon ribs to the carbon skin.

Better to take the material in those ribs and spread it within the wall of
the tube at the chosen angle.  That thin skin should be as thick as possible
on a carbon bicycle tube so that minor abrasions do not result in an
immediate catastrophic failure mode.  If the underlying ribs can handle the
stress by themselves, then it would be a nice safety feature.  The fact that
it is Kevlar does not help it because the Kevlar will shear off just as
easily as carbon would in an accident.  Anyone remember Campy's first
generation carbon seatposts?  The Kevlar layer in that seatpost turned out
to be the culprit to their common failures.  Kevlar doesn't play nicely with
carbon and epoxy.  (not bad on boats when separated by a foam or honeycomb
core, though)

> Sixth, just as metals come in different gauges; sheets of carbon come in
> different ply weights. The thinnest plies, made from finer strands, can be
built
> into tougher and lighter tubes.

The strands are not finer, there are just fewer of them.  With
unidirectional material, ply weight does nothing for strength, just
tunability.

Further, additional layers allow a wider range
> of tuneability. Over 95% of the carbon sheeting used by the bicycle
industry
> is "150" or higher FAW (Fiber Arial Weight). A very few items, including
> Reynolds' Ouzo Pro tandem fork, use super-premium 120 FAW carbon. Our
Beyond uses
> ultra-exotic 90 FAW. The downside of thinner plies? Each sheet is more
expensive
> and a higher number of plies means increased labor to produce the tube.
Where
> else can you buy frames built with 90 FAW? Good question!

Our Dragonfly Tandem uses lower FAW material, but that figure alone does not
make it stiffer or stronger.

> Seventh, just as "metal" includes a wide family of materials with various
> properties, "carbon" describes a range of complex hydrocarbons.

The various grades of carbon fiber are described in terms of their modulus
of elasticity.  Standard modulus is considered 33 msi (Million pounds per
Square Inch).  Intermediate is 42 msi.  High is 55msi and higher.  A single
33msi fiber under magnification looks like a Redwood tree, with deep
fissured bark.  42 msi fiber is the same fiber with the bark stripped off.
Higher grades have the outer layers stripped off, leaving the stiffest core
of the fiber.

> (snip) High modulus carbons are exceptionally rare, and are too expensive
for
> bicycle frames. But because the stiffness difference between standard and
high
> modulus is a whopping 48%, (snip)

We've been using 55 msi material in our half bikes since 1991 and in our
Tetra tandem since 1998.  66 msi on the Dragonfly.

> Eighth, just before the numerous layers of Kevlar/carbon IsoGrid ribs,
> unidirectional edge strands, butting plies and 90 FAW high modulus carbon
are placed
> into the mold, a short section of thinwall titanium tubing is slid part
way
> onto each extreme end. During the high temperature and high pressure
molding
> process these titanium ends become permanently fused to the carbon. The
> resulting Bi/Fusion tube is all-carbon except for the short titanium tips.

This can be an excellent way to deal with the very challenging problem of
integrating metal with carbon.  If the titanium is prepared correctly, this
can be an excellent solution.  If not prepped, it's no better than a bad
glue joint.

(snip)  In short, IsoGrid makes a
> carbon tube stronger and more damage resistant; Bi/Fusion makes a carbon
tube
> weldable.

One can do the same with a bonded carbon to ti tube, but this method is more
efficient.  Don't try it with aluminum, though!  It distributes heat too
well, unlike titanium.

> Ninth, to produce similar looking frames Merlin and Serotta glue carbon
tubes
> into pre-welded titanium lugs. At the Santana and Titus workshops, there
are
> no pots of glue. Instead, the Bi/Fusion tubes are mitered, and TIG-welded
> directly to each other. Because TIG joints are both stronger and lighter
than
> glued joints, the process used by Titus and Santana is superior.

I would say it is superior because you need less overlap of the carbon with
the titanium in your direct lamination method.  Adhesive bonding of these
dissimilar materials needs more overlap.  Direct lamination of carbon to
carbon is the optimum, however.

> Summary. While tandem buyers will naturally want to compare our new carbon
> Beyond with previous exotic frames built by Santana, Seven and Calfee, the
> numerous advantages of IsoGrid and Bi/Fusion technologies are undeniable.

See above.

> For instance, when weighed on an accurate scale the Beyond frame is at
least
> 15% lighter than the lightest previous carbon frames and 20% lighter than
our
> titanium frames.

Than would make it weigh 4.675 pounds. (if compared to some of the frames we
have) Very impressive!

> My reply: $9695 will get you a uniquely efficient and comfortable sub-30
> pound tandem with a lifetime frame warranty and proven components. Gram
shavers
> and competitors will be able to wring out an additional five pounds.
Either way,
> you cannot buy a faster and lighter tandem at any price.

An Ultegra equipped Tetra Tandem costs from $7,500 to $8,000.  They average
29 pounds.

> PS: Because tandeming is a reliable indicator of an evolved lifestyle,
> spending more signifies advanced intelligence.

Up to a certain point.  Save some for fine chocolate!

Craig Calfee