Archive-URL: http://search.bikelist.org/getmsg.asp?Filename=tandem.10310.0538.eml
Date: Wed, 22 Oct 2003 19:19:46 -0700 (PDT)
From: "Craig Calfee" <Craig(AT)calfeedesign.com>
Subject: [T(AT)H] Re:CARBON Demystified (Part 1)

I would like to respond to some of the technical points that Bill raises:

> First, in the process of being pressure-cured, carbon fiber tubing gets a
> controlled (smooth) side formed against an impermeable mandrel or mold,
and an
> uncontrolled (uneven) side that results when gasses and voids are flushed
out.
> With the standard method, the carbon is cured while sandwiched between a
solid
> mandrel and heat-shrink tape. When the tape comes off a rough exterior
> remains.

The "rough exterior" in a tape wrapped carbon tube is simply the ridges of
epoxy formed from the overlapping tape.  The tape wrapping process is an
excellent way to allow the outgassing that is normal to the curing process.
The dimensional accuracy is impressively accurate and the surface needs only
a light sanding to make a smooth, paint ready surface.

A more expensive way to produce tubes is "inside out" or "molded"
> construction where carbon is wrapped around an inflatable mandrel and
locked in a solid
> cavity. In this case the carbon is cured within a high-pressure and
> high-temperature mold---very similar to the method used by Reynolds to
build our 500g
> Ouzo-Pro carbon tandem fork.

This is called bladder molding.

Because the rough outer plies of a cheaper
> mandrel-wrapped tube must be partially destroyed (ground away) in order to
produce a
> uniform exterior, a molded tube with the same weight and diameter can test
20%
> stronger.

I would be very interested to see this test data because I have seen other
data showing roll wrapped and taped tubes to be stronger than molded tubes.
The reason being that it is harder to control the fiber placement and
overlapping layers of a bladder molded tube.  Once that mold is closed and
the air pressure is turned on, you can only cross your fingers.  With tape
wrapped tubes, you can see the layers as they are rolled onto the steel
mandrel and check for wrinkles.

While many of the best framesets now include molded carbon forks and
> stays, Santana and Titus are, as yet, the only builders using molded frame
> tubes.

All the Italian frames (mostly made in Taiwan or China) with their fancy
shaped carbon tubes are made with bladder molded tubes.

> Second, a molded tube allows advanced geometric shapes. While Ben Serotta,
> for instance, clearly understands the importance of an externally-tapered
> downtube (a concept he pioneered 15 years ago), his carbon composite
frames can't
> incorporate this advantage.

The carbon down tube on the Serotta Ottrott is tapered.

Because of proprietary molds, Santana's Beyond not
> only includes a Serotta-inspired tapered downtube, it's the first carbon
tandem
> with an oval bottom tube.

Oval and aero shaped bladder molded tubes seem to have the most problems
because the carbon material tends to bridge across the tighter radius of the
oval.  It's hard to check once the mold is closed.  Vyatec may have solved
this problem, but I haven't heard about it.

> Third, the inflatable mandrel allows a carbon tube to be internally
> double-butted. Frames built from double butted tubing (no matter what the
material) can
> be 25% lighter without increased risk of failure.

Unless the transition from thin wall to thicker wall is done without a
taper, then there could be disastrous results in a carbon tube.

In addition to being
> lighter and/or stronger, a double butted frame is also more comfortable.
Double
> butted carbon tubes are another Santana/Titus exclusive.

Our frame construction method results in externally double butted tubes.
Something we have done since 1988.

> Fourth, in addition to internal butting near the ends of each tube, the
> inflatable mandrel also allows added internal layers of unidirectional
"stripes" of
> carbon at the right and left edges of our tubes. These thickened edges
> provide greater lateral stiffness while damping out vertical harshness. No
other
> bicycle frame tubes (in any material) share this feature that is exclusive
to
> Santana.

An excellent feature of bladder molding.  Kestrel pioneered the use of
selective reinforcement within their bladder molded frames in 1986.

To be continued in Part 2