Construction
Techniques and
Materials
Fairings can be constructed of anything from cardboard to
carbon fiber. What you build yours out of depends on your budget, time constraints, and
how fast you want to go. As in the previous page, I'll break down the methods into three
groups, practical vehicle, club racer, and speedbike. The cheaper and less
aero materials
and techniques will be used on the practical bikes ('cause they don't need to go as fast!)
and the more expensive materials will be used on the two zootier vehicle types.
Practical Vehicle
Suggested materials for building a practical vehicle start with your basic Coroplast
fairing. Even though Coroplast (Coroflute in England) is a flat material, it can be bent in a direction
perpendicular to the flutes. The 2mm thick variety bends much easier than the 4mm thick
stuff. The 4mm Coro is much stronger. Also Coroplast can be bent at an angle to the flutes
by evenly heating the area to be bent with a heat gun and then bending it. You cannot bend
Coroplast parallel to the grain without causing serious wrinkling, but you can cut thin
strips and cut them to approximate curved areas. Another technique to create a
1 dimensional curved
surface is to slit just the inside of the flutes every inch or so. It has
been noted that a fairing made from Coroplast is faster with the flutes parallel to the
direction of travel. A practical vehicle will have a much higher incidence of surprise
stops, so the chances of crashing, usually at very low speeds, are much higher than for a
racing fairing. Coroplast is a great material for a practical vehicle because it is very
crash resistant. Coroplast sections are usually fastened together with plastic wire ties
but can be stitched with fishing line or glued with silicone glue. A supporting structure
is normally made out a combination of 3/4" x 1/8" aluminum strap
and aluminum or plastic tubing. ABS plastic water tubing and elbows work
nicely but be aware that they become brittle in cold weather. Extra
bracing is recommended. Aluminum 1/8" pop rivets
do a good job of fastening the Coroplast to the support structure. Use steel pop rivets to
attach the support structure to the mounting brackets, as aluminum pop rivets fatigue
easily in these areas.
Other Materials
Other materials are Lycra, Zote Foam, or plastic sheeting. The Lycra used in practical
fairings "body socks" is generally much thicker than obtainable at the local
fabric store, and is sometimes rubberized on the inside to provide better weather
protection. Lycra works well to provide a decent boost in aerodynamics, and is very
practical in the fact that it can be removed and packed away into a small area. It is most
effectively used in combination with a hard fairing to fill in the foot well and
head/shoulder area. It can easily be attached with Velcro and adjusted with zipper to
allow variations in ventilation. Lycra does not survive a crash well. As anyone who has
ever crashed in bike shorts can attest, Lycra melts down pretty quickly when sliding on
pavement. Zote foam has been used by quite successfully to build fairings. It is quite
crash resistant, and much quieter than other materials. It is reasonable
easy to work with, and can be blown or heat formed into aerodynamic shapes. Once it is shaped it is much
stiffer and self-supporting than in its sheet form. It is fastened together quite well
with rubber cement. Thin plastic sheeting such as Lexan or Vivak (PETG) can be used for the flat
areas of the fairings, or the areas with 1 dimensional curves. Materials such as Cintra
and Plexiglass are not recommended as they crack easily and may be dangerous in a crash.
It's a good idea to paint or otherwise cover any clear plastic that you don't have to look
through, as sunlight will cause the plastic to degrade. Also clear fairings make great
solar cookers. A good way to test plastic is to hit a sheet held under
stress with a hammer a couple times. If it cracks then it will probably
not last through a crash.
Nosecones
The smoother you can get the fairing, and especially the nose area, the faster it will go.
It's very hard to make a fast nose cone from Coroplast because it will be a conglomeration
of flat surfaces. One way to get that smooth nose cone is to make or buy a blown bubble
nose cone. Mueller Human Power
makes a nice one that is based on the WISIL blown
nosecone. They can easily
be attached to a wide variety of recumbents. Properly executed, this in conjunction with a Coroplast body and tail section will result
in a vehicle capable of being ridden at 25MPH for extended periods, and in most weather
conditions. Another option is to build or purchase a fiberglass nose cone. Lightning makes
one for their F-40.
Club Racer and Speedbike
Fairings for Club Racers and Speedbikes are constructed using mostly the same techniques.
Generally these bikes are constructed of composite materials. Epoxy, polyester or
vinyl ester resins are the most common "glue" to hold the composite
layers
together. Polyester and vinyl ester are cheaper but are stinkier. Most people agree that
they are also more toxic. Epoxy, while less fragrant, is also toxic to some degree.
Polyester and vinyl ester will eat most common types of foam. If you want to use these
materials with a foam based mold, you will need to use special foams designed to resist
these resins. Epoxy works fine with all types of foam. All these materials require the use
of rubber gloves, protective clothing, and either lots of ventilation, or a respirator
made to protect you from the specific material with which you are working.
Epoxy
I recommend West Systems epoxy. It is readily available, and is easy to mix with the
available measured pumps. Epoxy generally requires a warm place to cure. The epoxy will
cure at temperatures above about 70 degrees, but hardens better at higher temperatures. It
cures quite nicely at around 90 degrees. You can get "fast" or "slow"
hardener. The slow hardener is recommended to give you time to wet the material out
properly and ad additional layers before the epoxy starts to "turn" or
"kick over". It is important to mix the epoxy in small batches. Between 8 and 12
ounces works best. More than that and the thermal mass of the epoxy tends to allow it to
"go exothermic" on you. You know you mixed too much when take a quick bathroom
break on a warm day and return to see your plastic cup of epoxy spitting, smoking and
melting down. It's also important to keep the curing epoxy out of the sun. Even dappled
sunlight can cause bubbling or delamination of curing areas on the fairing. Keep plenty of
popsicle sticks around for stirring the epoxy. Apply the epoxy with 2" natural fiber
disposable brushes. Spread it around and remove all the air bubbles. Small
air bubbles can be removed by poking them with a nail and then stippling
with the brush. Rubber squeegees can
also be used to help spread the epoxy out and remove the excess. You can either remove as
much epoxy as possible without making dry spots, or vacuum bag the parts to make the even
lighter and stronger.
Material Choices
Use layers of fiberglass, Kevlar or Carbon fiber depending on the fairing's strength needs
and your budget. Carbon fiber is the most stiff but can break into shards when highly
stressed. Kevlar resists breakage and abrasion to an amazing degree, which means it's an ideal material to
build a fairing from, but it's very hard to work with. You will need to get special
scissors to cut it, and because of its tendency to fuzz when sanded, it should not be used
as an outer layer (unless you are not planning on any sanding). Use thin layers of foam or
Nomex honeycomb core material at least ¼" thick to strengthen the high stress areas
of the fairing. If you are building a monocoque fairing you'll need several layers of
cloth fiber surrounding the core to provide a rigid tub. If a thin outer skin will be all
you need, then periodic foam or honeycomb ribs can be used to stiffen the fairing in the
critical areas.
Use of Existing Fairings
Designing the actual shape of the fairing is probably the most important single task in
the construction of a club racer or speedbike. Many projects have gotten bogged down in
this step. My recommendation is to either borrow the mold of an already existing and speed
proven streamliner, or to copy the design of a proven streamliner. Note that many
streamliners, such as the Varnas are constructed for individuals under 6 feet tall. If you
are taller that that you will need to use a fairing specifically designed for a taller person.
Currently molds are available for use from the Varna, Moby, Barracuda, Lightning X-5,
WISIL Missile, and probably some others.
Nicholas Cafarelli proposes that a good
fairing can consist of a lathe produced body
of revolution spliced to a wedge. The top and bottom of the fairing
are hot wired as one piece in a hotwire lathe, covered in one piece and
sanded by modifying the lathe with a sanding attachment. Once a high level
of finish is produced, a router is used to produce a joggle and to section
the body of revolution. Now imagine a wedge interposed between the two
parts. The wedge grows in height from nose to tail. This wedge is quickly
produced with a standard hotwire using the two lathe cut parts as
templates.
Fairing Design Tips
If you really must make your own or think you have a "better idea", be sure to
follow some guidelines: No sharp transitions, no "planes", and no flat spots.
Make all curves flow into each other. Don't make a knife edge nose. Keep the shape as
monolithic as possible. Use "lofting" techniques to help make the curves. Build
a scale template of the rider and a mock up of the pedal configuration to determine the
foot clearances, knee clearances and shoulder clearances. You can model the fairing
however you want, on paper, in CAD, or out of clay. Just remember that you will be
converting it to full sized templates eventually to create a male mold. If
you are a CAD guru and want to have the shape
CNC milled, that's great, just remember CNC milling for such a large object is extremely expensive and
hard to come by. Note that if your fairing is left-right symmetrical, you
only have to make half a plug!
Plug Construction
I recommend building your male mold from foam. Pink or blue 4x8 sheets of 2"
polystyrene insulation foam are readily available from building supply retailers. This foam should be
used with epoxy only. Foam for use with ester-based resins is available from
aircraft
supply retailers. Fairing shaped sections can be cut from these sheets and glued together
to make a rough form. This form can be further shaped using hot wire techniques. Further
smoothing can be done with a large rasp file and then sandpaper. Once the shape has been
approximated, templates can be made to check the critical areas of the plug against the
desired shape. Areas can be built up using drywall compound or an epoxy/micro-balloon
slurry.
Once the desired shape has been attained, the plug can be covered with a couple layers of
8oz fiberglass. After curing, sand and then cover in epoxy based aircraft filler or bondo
to fill in the low areas and pinholes. A long steel straightedge can be used to help find
and fill the low areas. Mount at least a 2 foot long strip of sandpaper (belt sander paper works
well) onto a 2x4 or other flat surface to sand down to the high spots. Alternatively the whole plug can be sprayed with a thick layer of
high bulk primer and then the fairing can be sanded down to the high spots. At this point the
process of smoothing, checking and refilling continues until you are satisfied. Cover your
mold with final layers of primer and finely sand it. Cover the mold with 3 or 4 layers of
Carnuba wax. Apply a thick layer of mold release agent. Now it's time to lay
down some fiberglass! Note that you will want one or more friends to help
with laying up and wetting down the fairing. Drape and wet down each layer one by one as the epoxy won't soak
through multiple layers of fabric at once. Don't allow the previous layer to
cure before adding a new one. The fabric layers after the first one will be
harder to drape and smooth out, as it can't slide around on the wetted out layer below it.
Be sure to have some scissors around that you don't care about to cut slits in the
unsmoothable areas. As much as you will be tempted, don't use patchwork of materials.
Unless you really like to fill in crevices and holes, each layer should be a single sheet
of cloth. If you are going to use a female mold you can only have to make the outer layers
out of a single sheet.
Molds - Good, Better, Best
Now it's time for the decision of what to do with the plug. There are 3 methods.
1) Lost mold technique. - In this scenario you decide you've had enough. To just make the
fairing, just cut the canopy section off and then remove the foam from the plug. This is
easy, but makes it harder to replicate your fairing. Also the fairing is likely to be
heavy because of the filler materials.
2) Male plug molding - Cover the male plug with mold release agent, then drape the fabric
directly over the mold. It's best to do half the mold at a time to make it easier to
remove the part from the mold. Care should be taken to remove the bubbles between layers
and to squeegee out excess resin. This works well but requires filling and finishing the
outside of the fairing after curing to provide a smooth finish. It's not recommended to
vacuum bag on male molds, as the peel ply layers will mar the surface of the fairing.
3) Female mold - Cover the male plug with mold release agent. You can use a layer
of fiberglass then thick layers of chopped fiberglass together with wooden ribs to make a
sturdy female mold. Note that removed objects for molds may require a lot of
force, so build it a bit on the heavy duty side. For asymmetric fairings you'll need to build
separate left and right half molds. For left-right symmetric fairings only one mold is needed. Even with the best wax job and mold
release agents, it may be hard to remove the male plug from the female mold. Copious use
of wooden wedges, Coroplast strips, hot water, and elbow grease may be needed to remove
it. Once the female mold has been freed, it may need some minor touch up. Then it's time
to wax it up and apply more mold release agent. Female molds work nicely because when the
final fairing part is pulled from the mold, the outside surface is already finished. Also
it makes vacuum bagging easier.
Vacuum Bagging
Vacuum bagging involves laying up your fairing, covering it with a layer of "peel
ply" and then another layer of "bleeder ply". The uncured composite
conglomeration is then sealed with plastic, and a vacuum pup is attached. As the air is
evacuated it evenly squishes the composites together. The excess resin is forced through
the peel ply into the absorbent bleeder ply. The vacuum remains on until the epoxy is
partially cured, keeping the layers of composite material compressed together. The entire
mold can be heated to speed this process. You can lay up multiple layers and then vacuum
bag them all to create a well-integrated, strong and lightweight fairing. Once the resin
has cured, the easily removable peel ply allows the peel and bleeder plies to be removed
from the inside of the fairing.
Fairing Integration
Once your fairing shells have been completed, Often the right and left halves need to be
attached together to form the whole fairing. Once the shells have been cut to align
properly, the shells can be fastened together temporarily with tape, or with strips of
fiberglass screwed across the seam. Once that is done the fairing can be cut to separate
the actual canopy or upper and lower sections. This allows access to the inside, which is
where you will do most of the work to fasten the shells together. You can use fiberglass
or carbon fiber tape, along with a slurry of cotton flox and epoxy to fasten the shells
together on the inside, and to fill any gaps in the seam. Be sure to leave enough room to
get the bike or sub-frames in and out, and plan how you attach it so you can easily
install and remove it. If it takes a long time to remove it will be a real pain to change
a tire or perform maintenance. If you are concerned that the fairing may not be aligned
properly, you can make a frame out of particle board for the fairing to sit in that has
the same profile as the fairing at the canopy cut lines. Particle board is the best choice
for this as it very flat and doesn't warp easily. The frame will support the fairing while
you are working on it.
Canopy Considerations
Another important aspect of the fairing is the canopy. In your design be sure to think
about how it will attach and if you will use a head bubble or make sections of the main
fairing clear to see out of. Many streamliners use a piece of flat plastic cut to fit the
curved front and sides of a fiberglass canopy. This makes it much easier to replace when
it gets scratched.
Attaching the Chassis
After the fairing shell has been completed, you'll need to attach it to the bike frame. It
should be a solid attachment, as wind blowing the side of the fairing will push the sides
of the wheel openings into the tires if it's not attached well. Usually a three-point
attachment works well. One attachment in the nose, one under the seat, and one at the rear
wheel usually suffices. The attachment at the nose is usually the most difficult.
Typically it involves a bracket on the boom tube or bottom bracket, supporting a T shaped
bracket, with the ends of the Ts firmly attached to the inner sides of the fairing.
Placing the bracket behind the pedal box is best because the top of the T can be below and
behind the pedal box without interfering with the pedal stroke. Placing the T in front of
the BB can make the bracket subject to high stresses and premature failure. Remember when
making these brackets that they will be subject to a lot of vibration. An aluminum bracket
will need to be pretty beefy or after a couple hours of racing/riding on a bumpy
track/road it will fail due to fatigue. The bracket under the seat will need to be pretty
beefy too. You may need to stand in the bottom of your fairing to get in and out of it.
The rear mount is the easiest, as it normally just needs to prevent the tail of the 'liner
from wiggling around.
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