Reinforced Plastics, July/August 2003, R.H. Nelson. An article I wrote in Reinforced Plastics on the Trek OCLV bicycle frame development. An article I wrote for Reinforced Plastics on the development of the Trek line of OCLV Bicycles, which received huge mass market TV exposure when ridden to eight victories in the world's most viewed athletic event, the Tour de France. Carbon fibre composite bicycle frames have developed into a high volume consumerapplication over the past decade, and are now dominate on high-end racing bikes.Ron Nelson of ClosedMold Composites, primary inventor of the Trek OCLV bicycleframe, explains how the integrated development of a manufacturing process andframe design lead to a successful commercial product line.Lance Armstrong has ridden Trek’s composite road bike to victory in four Tour de France events.

1. 36 REINFORCEDplastics July/August 2003
C
arbon fibre composites are nearly
an ideal material for racing bike
frames. They have exceptionally
high strength to weight and stiffness to
weight ratios. They also have better
vibratory damping characteristics than
metals, contributing to a smoother feel
when riding.
Carbon fibre composites
are nearly an ideal
material for racing bike
frames.
Traditionally, bicycle frames are con-
structed of metal tubes joined at their
ends by welding, or are brazed or sol-
dered onto metal lugs, forming the
frame. Composite materials have a
lower density, higher specific strength
and stiffness, and better damping qual-
ities than traditional metals, and there-
by provide an increase in frame
strength and stiffness with a reduction
in weight, as compared to earlier metal-
lic frames.
Design requirements
There are many design requirements for
road racing bike frames, but the fore-
most are light weight and high lateral
stiffness. Light weight is essential to
minimize energy consumption up hills
or during accelerations, and provide a
more responsive feel to the rider’s
movements. The stiffness characteristics
of the frame are important because they
contribute to how the bicycle rides.
Resistance to lateral deflections and
frame twisting under pedalling loads
minimizes energy loss that would go
into flexing the frame rather than into
forward propulsion. Lateral stiffness
also provides a stable feel, for example
when descending or cornering, and it
provides confidence in the frame’s
response to rider actions.
Lateral twisting of the frame around
the steering column can cause self rein-
forcing vibrations at high speeds, i.e.
wobbling of the frame and steering col-
umn. In its worst case this can cause a
loss of control of the bicycle. In motor-
cycles this vibratory mode is called a
‘tank slapping vibration’, referring to the
handlebars oscillating back and forth so
much they hit the gas tank.
The ability of the frame to damp road
vibrations and provide vertical compli-
ance to absorb shock are also important.
0034-3617/03 ©2003 Elsevier Ltd. All rights reserved.
Bike frame races carbon
consumer goods forward
Carbon fibre composite bicycle frames have developed into a high volume consumer
application over the past decade, and are now dominate on high-end racing bikes.
Ron Nelson of ClosedMold Composites, primary inventor of the Trek OCLV bicycle
frame, explains how the integrated development of a manufacturing process and
frame design lead to a successful commercial product line.
Lance Armstrong has ridden Trek’s composite road bike to victory in four Tour de France events.

2. 37July/August 2003 REINFORCEDplastics
This contributes to a smoother and less
fatiguing ride. Carbon fibre is noted for
its ability to damp road vibrations rela-
tive to metal frames.
Strength requirements
Frame strength characteristics are
something which the rider hopefully
never has to experience since the frame
should never break.
Reliably producing
high static and
fatigue strengths is
essential to mini-
mize in service fail-
ures which affect
profitability, product
image, and can pro-
duce legal liabilities.
A bicycle frame experiences several
types of loads in its lifetime. The event
which produces the highest loads in a
bike frame occurs when the bike runs
into a fixed object and the kinetic ener-
gy is transferred into the front area of
the frame. This load case typically
occurs only once, if at all, during the
frame lifetime.
Geometry and interface requirements
Even though it is much more, it has been
said that the frame is just something to
hang all the equipment on. The frame
provides the key interface for all the
other componentry which comprise the
bicycle. The geometry of tube centerlines
affect rider position and handling dra-
matically and must be chosen with care.
Equipment which must be interfaced
includes the wheels, front fork, steerer
tube and bearing assembly, seat and seat
post, seat clamp, handlebars, derailleurs,
brakes, cable routing features, pedals,
cranks, bottom bracket, and water bottle
mounts. There are clearances and opera-
tional dimensional constraints for all the
equipment.
The frame material selection affects
the design of the frame considerably.
Lower density materials such as compos-
ites typically utilize larger tube diameters
to increase structural efficiency. In fact,
aluminium frames, which are generally
lighter than steel, would not be any
lighter if they used the same tube dia-
meters as steel and the same frame stiff-
ness was desired. The stiffness to weight
ratio of aluminium is actually lower than
steel. However, its much lower density
allows the larger more structurally effi-
cient tube diameters to be used. Denser
materials such as steel are limited to
smaller diameters because the tube wall
thicknesses become too thin at larger
diameters, and bifurcation buckling of
the tube walls occurs.
Inefficiencies of metal lugs
Frequently, carbon fibre tubes used in
bike frames are adhesively joined to
metal lugs. The disadvantage of using
metallic lugs is their weight relative to
composites. The weight of the metallic
lugs significantly exceeds the weight of
the composite frame tubes, thereby
greatly limiting potential weight reduc-
tions. Another problem with using
metal lugs at the joints is that the
designers must use smaller than optimal
lugs to reduce the lug weight, since
metal is denser. The smaller diameter
lugs use smaller diameter tubes to
reduce the lug weight. The carbon fibre
tube diameter is therefore much smaller
and less structurally efficient. The metal
lugs cannot really exploit the benefits of
the lower density carbon fibre which
requires larger tube diameters to be real-
ized. The metal lugs would also not
have the superior damping qualities of
the carbon composite. The material
density characteristics of metallic lugs
have also prevented the development of
structurally efficient large gusseted
aerodynamic shapes for the lugs on
account of the weight increase inherent
with such shapes. Smoothly gusseting
transitions between the main tube
members reduces stress concentra-
tions, allows thinner walls, and is
more structurally efficient. The entire
frame needs to be constructed of carbon
fibre composite to really obtain the ben-
efits of the composite material.
Previous carbon frames
Several all-composite carbon fibre
frames have been produced, with the
first more than 40 years ago. The design
and manufacturing were not both fully
optimized in these previous attempts
and that is why they were not large com-
mercial successes.
The first all-composite bike frame was
the Spacelander invented by Benjamin
Bowden in 1960, which consisted of a
futuristic monoque fibreglass framed
bicycle. Another notable example is the
Kestrel frame invented by Brent Trimble
and produced by Cycle Composites Inc
Bike frame races carbon consumer goods forward
Then and now: the Bowden Spacelander
(above), the first composite bicycle frame
(picture: Menotomy Vintage Bicycles Inc at
http://oldroads.com), and the Trek 5900 (below).
Trek road bike frame and section.

3. 38 REINFORCEDplastics July/August 2003
from 1987. The Kestrel was moulded in
one piece in a single-step cure. Carbon
frames inventor Craig Calfee made an
all-composite frame which was marketed
under the LeMond brand name in 1991.
The Calfee frames used an elegantly sim-
ple design roughly analogous to steel
frame construction. Small carbon lugs
are cured directly onto closely mitered
carbon tubes. Small flat gussets are left
between most of the tubes.
One-piece frames
In a bicycle frame, stress loads are
the greatest at joints, and therefore joint
construction is a strong influence on
frame design and construction. To avoid
inherent problems of material discontinu-
ity at frame joints, numerous designers
have attempted to reduce or eliminate the
number of joints in a frame.
The manufacture of high quality, reli-
able one-piece, jointless frames has
proven difficult and expensive. One
large impediment involves the difficulty
of reliably producing uniform high com-
paction pressures in the composite lami-
nate during cure, due in part to the
failure to develop reliable internal pres-
sure bladders to operate satisfactorily
throughout the frame.
Relatively inelastic bladder materials
such as polyamide were used. These
bladders could not stretch and conform
to the interior surfaces of the frame.
They were not shaped to mimic the inte-
rior surface of the frame either. The blad-
der would frequently bridge over some
detail areas reducing compaction pres-
sures dramatically. Sometimes foaming
epoxy resin was used in these detail areas
in an attempt to provide some com-
paction pressure. However, this material
is basically parasitic and tends to deaden
or reduce the liveliness of the frame.
Lower than optimum compaction pres-
sures in actual practice reduce material
strength. This results in lower structural
performance and an outer surface finish
which requires a large amount of manual
labour to repair.
In essence, the complexity of manu-
facturing one-piece carbon frames pro-
duces poor laminate quality.
Trek’s OCLV
One example of a successful commercial
carbon fibre frame is Trek Bicycle Co’s
Optimum Compaction Low Void (OCLV)
frame. Built in Waterloo, Wisconsin, the
frame was developed with Trek in the
early 1990s by a team at Salt Lake City-
based Radius Engineering led by Ron
Nelson, then president and co-founder of
the company.
A testament to the bike’s superiority,
and the reliability of the manufacturing
process, is that cyclist Lance Armstrong
rode stock OCLV road frames in his four
Tour de France wins from 1999-2002.
The original OCLV road frames
weighed 1.1 kg (2.44 lb), the lightest pro-
duction road bike frames in the world.
These frames were used in the 1999 Tour
de France. The original process used
150 g/m2 fibre areal weight carbon/
epoxy prepreg. Since then 120 g/m2 and
110 g/m2 material has been used to
reduce the weight further.
Trek had previously manufactured
a frame similar to Brent Trimble’s
Kestrel model, called the Trek 5000. The
frame consisted of two pieces (front tri-
angle with rear stays as separate unit)
bonded together, but it was only sold
for about one year before being taken
off the market.
Radius approached Trek after devel-
oping an internal pressure bladder man-
ufacturing technique for forming com-
plex geometric shapes with high
moulding pressures using conformable
bladders in an out-of-autoclave process.
The company had previously produced
tooling and manufacturing equipment
for Cycle Composites Inc for the produc-
tion of the Kestrel bike’s frames and
forks, and believed its process could be
Bike frame races carbon consumer goods forward
Lug lay-up process.
Carbon composite head lug.

4. 39July/August 2003 REINFORCEDplastics
used for the high volume production of a
new, affordable all-carbon composite
frame.
The key to success was optimizing the
combination of manufacturing process
and frame design. Previous all-carbon
frames weren’t designed optimally, and
didn’t have a manufacturing process
capable of reliably producing high per-
formance frames.
Evolution of design
Prior to introducing the OCLV product
line, Trek made aluminum frames which
use cast aluminium lugs bonded to
drawn aluminium tubing. It also made a
carbon tubed frame using aluminium
lugs. This experience with bonding
tubes and lugs together to form frames
played a role in design of the first OCLV
frames.
There were several key factors effect-
ing development of the manufacturing
process and frame design. An important
part of any product and manufacturing
design effort is to avoid infringing on
existing patents. Another requirement in
this case was for the process to be unique
so it could be patented to protect the
product.
Making the frame in
smaller pieces allows a
more specialized, more
reliable manufacturing
process to be used for
each component.
One key feature of the design revolved
around the question of how much of the
frame to manufacture in any one step.
This ranged from moulding the entire
frame in one piece to the other extreme
where all the pieces would be moulded
separately, and then sub-sequently bond-
ed together into a frame. In the end it
was decided to build the frame with a
‘maximum componentization’ design
concept, and this had the advantage of
using Trek’s existing frame bonding pro-
cedures.
Making the frame in smaller pieces
allows a much more specialized, higher
performance, and more reliable manu-
facturing process to be used for each
component. By making the smallest
components possible, the process can be
optimized better, and much better struc-
tural performance is obtained more reli-
ably in smaller parts than with larger
parts.
This is contrary to normal composites
manufacture, where maximum parts
integration is the norm, but similar to
the more traditional manufacture of
metal structures.
Manufacturing and product
development
The two most critical aspects were the
fabrication of the lugs and the design of
the joints between the components.
The fabrication of the hollow lugs
which connect the tubes in the frame
was the heart of the design. The straight
tubes used between the lugs were gener-
ally made via the traditional table
rolled, oven cure process, which uses
hard metal interior mandrels, and exter-
nally applied shrink tape, and free
standing oven cure. In general, the
curved and/or tapered tubes were also
made with the same moulding process
as for the lugs.
To connect the tubes and lugs, a
lightweight and manufacturable joint
was desired that would also integrate
well with the frame bonding assembly. A
male plug extension to the lugs which
fits into female sockets was developed. A
short tapered section at the base of the
plugs reduces out-of-plane shear stresses
and allows the diameter transition to be
made without the addition of any rein-
forcing material. Semicircular radially
spaced ribs or splines along the lug male
member or plug end closely control the
uniform adhesive thickness, producing
thereby a reliable high strength frame
joint.
Frame design
Finite element analysis (FEA) was per-
formed early in the design phase to opti-
mize the structural efficiency of the
frame and size the laminates for strength
requirements.
Bike frame races carbon consumer goods forward
A new moulding process could move the carbon prepreg lay-up from being done on the tool to being
done on internally rigidized bladders.

5. 40 REINFORCEDplastics July/August 2003
The full frame FEA was used mainly
to choose final tube diameters and their
lay-ups. There were numerous locations
where the frame outer mould line geo-
metry affected the carbon stress state
dramatically because there were metal-
lic fittings bonded inside. The stress
states in these areas had to be chosen
carefully.
Numerous metal components are sec-
ondarily bonded into the lugs, such as
the bottom bracket race seat, the head
set race seats, the seat tube insert, rear
brake boss, rear drop outs, front
derailleur mount, and cable routing and
water bottle mount features. The struc-
tural details for the more highly loaded
of these details had to be addressed in
the lug design and manufacture.
Process design
The use of higher performance matched
metal tooling was combined with better
bladders than had been used before to
allow higher pressures, better surface fin-
ish, better control of product dimen-
sions, faster cycle time and better process
control.
Hard matched metal female cavity
tooling was used in clamping presses.
Previous moulded frames had frequent-
ly relied on bolt together fibreglass shell
tooling which was then placed in an
oven. The fibreglass tooling was rela-
tively flexible, which sometimes limited
the bladder pressure which could be
applied.
A key aspect of the new process was
the use of high performance bladders
capable of high pressures and flexibility
to apply pressure uniformly to the inside
of the part. The conformable formed rub-
ber or thin thermoplastic film bladders
which are removed after cure are a big
improvement over relatively stiff poly-
amide bladders that had been left inside
in previous frame designs. The stiffness
and lack of conformability also limited
the pressures that could be used with
previous polyamide bladders.
A pressure of 1.38 MPa (200 psi) is
applied to the bladder as the closed
mould is heated inside a clamping press.
After cure, when the lug has been
removed from the tool, the bladder is
deflated and removed. The pressure used
is significantly higher than normal auto-
clave processing of high performance
aerospace carbon fibre laminates which
is typically done at 0.86 MPa (125 psi). It
is also much higher than the pressures
used in the previous one piece carbon
frames, which were about 0.35 MPa
(50 si). The high pressure process pro-
duces an exceptionally high fibre vol-
ume, typically 67%, and low void con-
tent. These characteristics produce a
laminate which is much stronger than
laminates produced with lower pressure
processes.
Material lay-up
The parts are made using a standard
aerospace grade carbon fibre in a sport-
ing goods grade of epoxy resin. This
somewhat simplified description of the
process illustrates the main elements of
the forming process. Each lug is basically
formed from two halves of continuous
carbon fibre laminate. Unidirectional
prepreg is preplied into large flat sheets
in a quasi-isotropic orientation, i.e.
0/±45/90.
For the primary preforms, shapes
generally representing each half of each
lug are then die cut out of these eight or
12 ply quasi-isotropic stacks. The need to
minimize waste of carbon fibre material
requires that the die cutting pattern for
these lug shapes be highly nested so the
shapes are rotated at various angles to
nest them together as tightly as possible.
There are also several smaller preforms
used in each lug, typically added for
extra localized reinforcement or material
build-up.
Generally two primary preforms are
used, one for each half of the matched
female mould. All of the lug moulds
have two halves, except for the bottom
bracket mould which also has a small
key piece to form the area between the
rear chain stay protrusions.
The tools are usually heated up to
roughly 50°C (125°F) to assist loading
the preform into the moulds. The
preforms are then carefully pushed by
hand into each mould half. On one
mould half, referred to as the ‘net’ side,
the preform will just come up the edge
of the mould cavity after it is pushed
into the mould. On the other side, the
preform is sized so that about 1 cm (3/8
inch) of laminate rises above the mould
parting plane. This second side is
referred to as the ‘lap’ side, because it
forms the lap which connects the two
sides. The bladder is then placed into
the ‘lap’ side tool inside the preform.
The laps are then folded in over the
bladder. The net side tool half is then
quickly closed onto the lap side tool
half before these lap pieces flop back
out up and potentially get trapped in
the parting plane surface between the
tool halves.
Future developments
The above process has remained relative-
ly unchanged since its implementation
in the early 1990s, but composite frame
manufacture will change and improve in
the future.
Superior moulding technologies are
likely to include moving the carbon
prepreg lay-up from being done on the
tool to being done on internally
rigidized bladders. This has numerous
benefits including eliminating the lap or
seam between halves of the parts. It
reduces fibre wrinkling, greatly increas-
ing strengths and stiffnesses, allows
much more flexibility in fibre prepreg
placement and orientation inside a lug,
reduces material scrap rate dramatically,
and decreases tool cycle time. ■
Bike frame races carbon consumer goods forward
Ron Nelson is president of ClosedMold
Composites and specializes in the develop-
ment of consumer and aerospace carbon
fibre products based on low cost high per-
formance moulding technologies.
Ron Nelson; tel: +1-801-277-0309; fax:
+1-801-277-0298; e-mail: ronnelson@closed
mold.com; website: www.closedmold.com.