Many of you who have read about the
first composite road frame I made have asked me what new projects I am
working on. In addition to my regular busy schedule, I am
trying to squeeze in a new carbon fiber time trial frame this
year. Unlike the last carbon fiber frame I made, this time I
plan to use a plug to make a mold, from which I will mold the
frame in two halves.
The benefits of using an external mold
compared to wrapping wet cloth over an internal foam core
are:
smoother exterior surface (less
hand finishing)
more even wall thickness (due to
less sanding)
ability to make more than one
frame by using the mold over again
However, I still think wrapping wet
cloth over a foam core is easier for a beginner. Here are the
benefits:
less time, money and work required
(don't have to make a plug and mold before you can
make the frame)
less waste if you don't like the
design (no bad mold hanging around)
Obviously, making a bike frame in a
mold requires that you have a mold. The way I've chosen to
make my mold is to cast it off a plug. A plug is an exact
model of the desired shape. So I have to make the plug first,
then make the mold from the plug. Of course, I want the frame
to be straight, which means the mold should be straight,
which will happen only if the plug is straight. So I made a
simple jig to make sure the plug will be straight.
The Jig
Here is the plug
shown on the jig I am using. The jig is made out of a a solid-core door from Home Depot. I laid it on a card table in my
garage to form a rigid base. I drew the side view of the
frame in AutoCAD, then paid $20 to have Tiger Reprographics
plot it full scale on cheap bond paper. I laid the paper plot
on the door, and made various fixtures to serve as standoffs
to hold the plug's foam core in alignment. At the front of
the frame, on the right side in the photo, you can see I've
left the head tube quite long. I use the extra length to rest
on the jig's lag screws. You can also see a bit of foam lying
on the table. When I bond it in place, it will form the head
tube's leading edge.
Here is a view of
the frame jig, looking from the bottom bracket area up toward
the head tube. The bottom bracket cavity in the frame wil be
formed on the tube mandrel in the foreground. It is 1.5"
diameter aluminum, with both ends cut off in a lathe. I rely
on the square ends to align the rest of the frame. The top
edge of this bb mandrel is the datum plane from which I
reference all other points on the frame. This plane is 34 mm
from the central plane of the frame. To check centerline on
any point on the frame, I rest a straightedge across the
face of the bb mandrel and extend the length of the straight
edge toward the point I want to check. Since the bb shell is
68mm wide, the centerline anywhere around the frame should be
half that distance, or 34mm, below the edge of the straightedge.
I keep the head tube in alignment by using
adjustable metal parts mounted on the table. At either
end of the head tube, I use one long lag bolt and an angle
bracket screwed into the table top. I adjust the height of
the head tube by screwing the lag bolt farther in or out of
the table. I adjust the head angle by sliding the metal
bracket to tilt the head tube. You can see the rubber band
that holds the head tube in place. I check alignment on both
ends of the head tube using the straightedge as described
above.
The Plug
I
used Yoram Leshinski's Airfoil Plotter shareware program (see
source list below) to choose and plot aero cross sections
to scale. Airfoil Plotter lets me use my windows printer to
plot them, so I didn't have to take them to a print shop. I
used two airfoils: GOE460 and EP478, both symmetrical. I
don't know enough about aerodynamics to know which one might
be better for use in a bike frame, so I decided where to use
each airfoil based solely on where their proportions fit most
conveniently.
I carved
the plug out of two-pound-per-cubic foot rigid polyurethane
foam. (see the source
list below). In the future I
will use heavier foam, because I think it will be stronger
and more rigid. That way I may be able to get away from the
need for so much support while glassing the plug, since the
foam will be rigid enough and strong enough to support more
of its own weight. The light foam sags a bit.
At this point the plug has two layers
of 8 oz. fiberglass on it. I used a vacuum pump (see the source list below) to draw a vacuum in a clear garbage bag
to provide compaction, but it didn't work very well: I did
not use the breather/bleeder plies as directed, and my set up
provided virtually no pressure. The pump does the job as
advertised when I include the recommended breather/bleeder
plies! I have a little body work to do to fix the wrinkles it
left behind.
Metal Parts
The plug's head tube is 0.049" wall
1.25" diameter steel. It will remain several inches
longer than the frame's final head tube will be. The extra
length on the plug will form a similar cavity in the mold.
That cavity will hold a head tube mandrel that will in turn
form a cavity in the frame that accepts a secondarily bonded
aluminum head tube. That means the leading edge of the frame
must be at least wide enough to surround the 1 1/4" head
tube plus the composite wall thickness. That makes it about
the same width as most Kestrels and Trek OCLVs, that is,
slightly wider than most steel frames. I'd like to mold the
head tube in an hourglass shape, narrower in the middle and
wider only at each end where the head cups press in. But
using two separate inserts in the top and bottom is a lot
more complicated than simply using a single full-length piece
of tubing, so for now I'll suffer with the minimal increase
in width!
Here
you can see the elliptical aluminum dropouts lying on the
table. I cut them by hand out of 1/4" plate. (See the source list below). The derailleur hanger has Shimano-approved dimensions relative to the rear hub axle, even
though the wheel exits toward the rear of the bike. I decided
to give up the quick wheel changes a forward facing dropout
would give. After all, it is a time trial bike, and if I flat
during a race I would probably abandon anyway. I don't have a
support vehicle following! In exchange for potentially slower
wheel changes, I get to shape the chainstay so it approaches
the cogs more closely. There might be some aero benefit, even
if only imagined, and I like the styling. You can see the
pink foam of the left chainstay in the right side of the
photo. The background is the part of the plot that shows the
dropouts.
The Molding Concept
On the left is a cross sectional sketch
of the mold I will be making. I plan to mold each side with a
small (1/4 inch or so) lip around the perimeter. This lip
will turn inward from the frame's skin to lie flat on the
bonding plane that coincides with the central plane of the
frame. On the right is a sketch showing the two halves that
will come out of the mold. After cure, I'll bond the two
halves together to form the completed frame.
Airfoil plotter shareware program
to plot aero cross sections on my windows printer. May be unavailable on the archived site. An airfoil plotter is online at airfoiltools.com.
Catalog. Cost $5.00 (available online now), but worth it.
Highly recommended for the information alone! Plus
you get $5.00 credit on your first order.
Part number 7781-50 fiberglass,
8.92 oz./sq.yd., crow foot, 0.009" thick. Used
to skin the foam plug to give it strength.
Part number 1080-50 fiberglass,
1.45 oz./sq.yd., plain weave, very thin. I use this
as a surface layer in the laminate whenever a metal
part may contact carbon fiber. The fiberglass acts as
an electrical insulator, preventing galvanic
corrosion.
Part number 01-11900 Polyurethane
foam, 2 lb/cu.ft., 2" thick.
Part number 716-38 Unidirectional
graphite, 4.7 oz./sq.yd., 0.006" thick.
Part number 01-38200 carbon
fabric, 9.1 oz./sq.yd., four harness satin (4HS)
0.011" thick. I use this for the visible layer
since it has that characteristic carbon weave. I also
believe the 90 degree fibers may act to constrain the
zero degree unidirectional fibers underneath from
buckling outward under bending stress.
Here is the plug shown on the jig I am using. The jig is made out of a a solid-core door from Home Depot. I laid it on a card table in my garage to form a rigid base. I drew the side view of the frame in AutoCAD, then paid $20 to have Tiger Reprographics plot it full scale on cheap bond paper. I laid the paper plot on the door, and made various fixtures to serve as standoffs to hold the plug's foam core in alignment. At the front of the frame, on the right side in the photo, you can see I've left the head tube quite long. I use the extra length to rest on the jig's lag screws. You can also see a bit of foam lying on the table. When I bond it in place, it will form the head tube's leading edge.
Here is a view of the frame jig, looking from the bottom bracket area up toward the head tube. The bottom bracket cavity in the frame wil be formed on the tube mandrel in the foreground. It is 1.5" diameter aluminum, with both ends cut off in a lathe. I rely on the square ends to align the rest of the frame. The top edge of this bb mandrel is the datum plane from which I reference all other points on the frame. This plane is 34 mm from the central plane of the frame. To check centerline on any point on the frame, I rest a straightedge across the face of the bb mandrel and extend the length of the straight edge toward the point I want to check. Since the bb shell is 68mm wide, the centerline anywhere around the frame should be half that distance, or 34mm, below the edge of the straightedge.
I keep the head tube in alignment by using adjustable metal parts mounted on the table. At either end of the head tube, I use one long lag bolt and an angle bracket screwed into the table top. I adjust the height of the head tube by screwing the lag bolt farther in or out of the table. I adjust the head angle by sliding the metal bracket to tilt the head tube. You can see the rubber band that holds the head tube in place. I check alignment on both ends of the head tube using the straightedge as described above.
I used Yoram Leshinski's Airfoil Plotter shareware program (see source list below) to choose and plot aero cross sections to scale. Airfoil Plotter lets me use my windows printer to plot them, so I didn't have to take them to a print shop. I used two airfoils: GOE460 and EP478, both symmetrical. I don't know enough about aerodynamics to know which one might be better for use in a bike frame, so I decided where to use each airfoil based solely on where their proportions fit most conveniently.
Here you can see the elliptical aluminum dropouts lying on the table. I cut them by hand out of 1/4" plate. (See the source list below). The derailleur hanger has Shimano-approved dimensions relative to the rear hub axle, even though the wheel exits toward the rear of the bike. I decided to give up the quick wheel changes a forward facing dropout would give. After all, it is a time trial bike, and if I flat during a race I would probably abandon anyway. I don't have a support vehicle following! In exchange for potentially slower wheel changes, I get to shape the chainstay so it approaches the cogs more closely. There might be some aero benefit, even if only imagined, and I like the styling. You can see the pink foam of the left chainstay in the right side of the photo. The background is the part of the plot that shows the dropouts.
On the left is a cross sectional sketch of the mold I will be making. I plan to mold each side with a small (1/4 inch or so) lip around the perimeter. This lip will turn inward from the frame's skin to lie flat on the bonding plane that coincides with the central plane of the frame. On the right is a sketch showing the two halves that will come out of the mold. After cure, I'll bond the two halves together to form the completed frame.
