Barracuda tailbox page
Coroplast Tail Fairing Construction
Warren Beauchamp
Tail Box Version 1  Tail Box Version 2     Tail Box Version 3 (MOAT)

Coroplast Tailbox Basics
While a full fairing is great for racing, it's not practical at all on the street. In order to be competitive at all, you need to train on what you are racing. This means I have to ride the 'Cuda on the streets, or on bike trails. I decided to build a tailbox for the 'Cuda, partly because the 'Cuda is so low (7.5" seat height), and is not very visible (flat black paint), partly because it will let me go a little faster, partly because it looks cool, and partly because of peer pressure. Oh, and I can put stuff in it. Being a surfer of HPV sites, I've seen a bunch of tailboxes, and decided I didn't want a "boxy" one. I also didn't want one that was super long, as I didn't want to loose the ability to stuff the bike in the back of my Ford Contour. I've had decent results in the past with bending the Coroplast with a heat gun, and decided that I would use that technique to make a more curvy looking box. I don't know that it is more aero when curvy, but it looks better... For my fist tailbox I started with the requisite yellow Coroplast, measured my butt width, chose an arbitrary total box length, and drew some lines for reference.

Here's a drawing of the sheet of Coroplast, because I really don't want to write 1000 words...
barracuda-tailplans1.gif (7076 bytes)

Heat the area of the bend by moving the heat gun back and forth over the area you want to bend on the inside of the curve, while bending the Coroplast. It helps to use a straight edge (a thin board or something) to hold down one side of the bend and to keep the bend straight. When you see the Coroplast start to wrinkle up, it's hot enough. Also one thing to remember is that the Coroplast shrinks as it cools, so make the angle of the bend slightly greater than what you want to end up with. After you fold it up, and trim the top straight, it should look something like the drawing to the right: barracuda-tailplans2.gif (1796 bytes)

I duplicated the steps above for the top of the box, except made it shoulder width (on the first tailbox). I like the plastic wire-tie and electrical tape method of construction, it looks nice, and you don't have to worry about glue failures. I cut down the sides to match the contour of the seat, and added some filler panels between the seat (which is much narrower than the tailbox), and the tailbox sides. The filler panels prevent air from getting inside the fairing. I had to cut a rather large hole in the bottom to accommodate the chain stays, chain, seat stays, and other bits. This also made it much easier to install/remove. I also added a small compartment above the rear tire, the tape stripe just below the top of the seat in the picture below shows the level of the compartment bottom. To attach the sections of Coroplast, I use a small screwdriver or an awl to poke holes about 1 flutes distance from the edge, then poke the zip tires through from the inside. Use 1 about every foot. Pull them tight and they lay pretty flat. They are an amazingly strong method of attachment. I use electrical tape to cover the seams. There are small bumps where it crossed the wire ties, but it's not too noticeable. Besides being a good place to stash those biking essentials (patches, cold pizza, etc.), it helps to give form and strength to the box. To attach it to the bike, I was originally going to add some aluminum strap, which would bolt to the rear dropout rack mount holes, but upon further inspection, it looked like it would be a real hassle to put on and take off, so I fashioned some clips out of aluminum strap, that grip the top of the seat, and attached the bottom to the seat with those ubiquitous wire ties. It's not the most heavy duty arrangement in the world, but remember, I'm hauling cold pizza in the compartment, not jars of peanut butter.

Tail Box Version 1
 barracuda-tailbox-1.jpg (10902 bytes)

In case you were wondering, that thing that looks like a gun-sight on the front of the bike is a pump. It happened to fit really well in the mount for the racing fairing, which was serendipitous, because I never could find a good place for a pump on the bike before...

In the Right shot below you can see the aluminum attachment tabs on the top of the seat
barracuda-tailbox-2.jpg (4872 bytes)barracuda-tailbox-3.jpg (10786 bytes)
The picture below show the (tiny) compartment in the tailbox
barracuda-tailbox-4.jpg (12282 bytes)

This tailbox ended up being less than ideal in a number of ways. t was a little too tall, too narrow and it's not long enough - Folks on the HPV or Low Racers lists (sorry, I forget who...) have mentioned that ideally it should be 12 or less (see drawing above).

<digression>Also discussed on the list(s) was the "Kamm" effect, which is, if you must make your fairing shorter than ideal, you can use the 12 rule, and just chop it off at the desired length. The flat end you are left with is more aero than using something greater than 12 and having the fairing come to a point. According to Blake Olsen, this effect unfortunately only works at speeds higher than bike speeds. Using my usual rules (what looks fast, probably is fast), I would estimate that having the tail end be 1.5" or less in width is probably close enough to the ideal of "pointed", for our uses, and should shorten the total length of the tailbox somewhat to reduce those dreaded side wind effects.
</digression>

Conclusions:
Overall a worthwhile project. The Version 1 tailbox took me about 6 hours to build, and I probably used less than $10 in materials. It is very light (probably a pound or so...). It was too small to help a whole lot with the speed though.

Tail Box Version 2 - 9/01
Time passes, the need for speed progresses. I decided to build an actual 12 degree tailbox to help me keep up with the other tailbox equipped lowracing yahoos in the Chicago area. This one used the same techniques as the previous tailbox, only it was bigger. It was designed to be the same height and width as my shoulders on top, and butt width on the bottom, and it covered the entire lower drivetrain. The tailbox was 1"wide at the back, and the flat strip in back has holes drilled in it to relieve any excess pressure in the box.

barracuda-tailbox-5.jpg (8434 bytes)

I was hoping that the added depth of the box would be compensated by the fact that the entire bottom of the bike was streamlined. The wheel stuck through a small slot, but besides that the bottom was all closed off. Unfortunately, tested on a grueling 70 mile organized ride which was all large rolling hills proved that this tailbox did not help me out much (though I was still faster than all the roadies)! My theory is that air was forced into the lower section of the tailbox, causing it to have a slight parachute effect. 

To fix the tailbox, I removed the forward section under the seat, and raised the tailbox up so it is flush with the bottom of the seat. There are now a bunch of ugly bike parts sticking out of the bottom of the fairing, and a bigger hole in the bottom, but it is more in my body's profile now. 

It's very important that your body is as close to the tailbox as possible, even slightly enclosed by it. Because of this, the sides of the fairing wrap tightly around my arms, and the top is close to contacting my shoulders. This tight fit prevents air from sneaking into the tailbox, which improves the streamlining.


Warren at the 2001 Ohio HPRA HPV races

Conclusions:
The Version 2 tailbox was not much heavier than version 1, cost about the same amount, but took more like 15 hours to plan and build (and counting!). The nice thing about the Coroplast and zip tie construction is that it's easy and cheap to reconfigure if you decide you can make it better after the initial construction. This one did help with the speed, but was still not wide enough, and did not cover the drive components.

Tailbox version 3 - updated 5/08/03 The M.O.A.T 
After riding with Alan AriaiI, and seeing what a really good tailbox can do, I decided it was time to build a new one. This one would be the Mother Of All TailCones (MOAT). I decided to approach this one a bit differently than in the past. This one uses a faceted approach to the construction, with no heat bending. It also keeps the grain in line with the airflow, as it was theorized that using the bent Coroplast with the grain perpendicular to the wind may generate some additional surface drag.
 

Start with an aluminum hoop, bent with the 14 faceted flat spots in it. The hoop represents the largest diameter of the tailbox. Size the width by measuring the width of your shoulders, and then adding an inch or two. The height was sized by finding the lowest point of the drivetrain, then measuring to the lowest point of my helmet when seated on the bike. To keep the fairing as rounded as possible, I used 6 facets on each side plus a top and bottom (14 total). The height of the inside of the box needs to be a least as big as your wheel diameter, hence the 27 inches...

This drawing is not to scale, but will give you the basic idea.

Cut the strips of Coroplast. This drawing is a template that can be used to recreate the MOAT. It was created after the tailbox was built by measuring the Coroplast sections. Use a steel straight edge to guide the razor knife when you cut across the grain of the Coroplast.

Note that you will need to trim the leading edge of the tailbox and the wheel opening to fit your bike and body. The leading edge of the tailbox should be right up against your body.

I used 4mm Coroplast for this tailbox, but 2mm should work fine. 

 

You can click on this picture for a bigger image.

tailboxlayout_sm.jpg (18876 bytes)
First, pop-rivet the two side pieces of Coroplast (the two closest strips in this photo), and zip tie them at the back. Working your way to the top from there, cutting the strips of Coroplast to allow a nice shape. First attach the sections at the hoop with pop rivets, then bend them around to the back, zip tying them as you go at about 1 ft intervals. Trim off the bits that stick out.

Note that it's a good idea to use aluminum pop rivets to hold the Coroplast to the frame so you don't smash the Coro, and steel ones to pop rivet the aluminum flame pieces together for strength.

Once the tail section is together, it's time to mount the box. I used 1"x1/8" aluminum strap to form a hoop over the rear axle. It screwed into the rear drop-outs, and slid into an aluminum bracket pop riveted to the top inside of the fairing.

In this picture it's shown without the bike, but there are holes in the bottom of the upside down U structure  to attach the hoop to the rear dropouts. 

Also in the back of the fairing you can see that I added a couple layers of Coroplast with the holes lined up to allow airflow out the back of the tailbox. This is important to relieve any pressure buildup in the box.

Additional mounts were made from aluminum L-channel and aluminum strapping to allow fiberglass rods to be used to stabilize the fairing laterally. This prevents the fairing from rubbing on the wheel in side winds. The rods are held in by the side pressure, so putting it together and taking it apart is easy. The U bracket in the picture above is held into the top support bracket by aluminum clips, so it just snaps in. 

Also visible in the picture above, are the black Velcro mounting points for the blue foam which seals off the front of the box.

The middle of the fairing is supported by a bracket attached to the seat mounts, and to the aluminum hoop at the bottom of the fairing. The front of the fairing is held in place by a strap below the bike frame and a Velcro-ed on chunk of Coroplast on the other side of the frame, just below the front edge of the seat.

Here's the box mounted, before trimming the leading edge.

Here's the fairing a little further along. You can see that the shape is very aerodynamic, and that the drive train is completely covered.  I used blue camp mat foam from Wal-Mart to seal off the front edge of the box. It would be better to have a curved leading edge, but not very practical when working with Coro and Foam. A quick ride around the block proved that the box was not scraping the tires. 

 

Here's a back 3/4 view showing the types of cuts that the Coroplast needs to get an aero shape. It's not hard to do, it just takes a little time.
Visible in this picture are:
  • The shoulder pads which help to make the area above my shoulders more aero.
  • A couple of side strips to block the small gap between my shoulders and the foam.
  • The middle brackets which attach the seat mounts to the aluminum hoop.
  • The Velcro attached Coroplast front attachment. This attachment is not really needed to support the tailbox, it just prevents vibrations.

Conclusions: The MOAT seems to work quite well. I did a 20K Time Trial on a windy and rolling course and ended up with a average speed over 25 MPH. That's the best I have ever done in this type of event. Though I have not conducted a scientific test yet, I'd say it easily adds a couple MPH to my average speed at 20+ MPH speeds.

Racing on the ti-cuda with MOAT at the Indy 2003 races. 
The ti-cuda with MOAT at the Michigan 2003 races.

Testing continues...

Back to the Barracuda main page