In Workshop #1, we constructed front fairings out of Coroplast for a short wheelbase and a long wheelbase recumbent. In Workshop #2, we will construct Coroplast tailboxes.
What’s a tailbox, and why add one? A tailbox is a trunk for your bike. Just as a fairing helps split the air in front, so an aerodynamically designed tailbox smooths the airflow behind. The low pressure area behind a bike, often called the wind shadow, or draft, represents a sizeable drag on the rider. Tailboxes try to minimize or eliminate the phenomenon by shaping the air back together. As in front fairings, the rider must ride over about 15 mph to gain the aero advantage of a tailbox; but at higher speeds, benefits still accrue at a non-linear rate!
In addition to aerodynamics, a tailbox can add more storage space than most cyclists could dream of. It’s a great place to carry a water reservoir system. Weight weenies may gasp at this, but some riders find it advantageous at times to be riding a pack horse, especially when it can still run like a thoroughbred. Tailbox designs can be as simple as a plain box or as elaborate as fitting it to the rider’s body.
Tailboxes are a logical step in becoming fully-faired. A component setup consisting of front fairing, tailbox, and lycra mid sock can be nearly as fast as a full hard fairing, but easier to transport and store. Plus, if the rider wants to open things up, the mid sock can be removed in mere seconds, and stowed in the tailbox.
Who benefits most? Tailboxes seem to be more advantageous than front fairings for riders who are highly reclined, whereas front fairings tend to help more for those who ride with a more closed, upright riding position. Tailbox benefits are noticeable for either riding position, though.
What is Coroplast? For the uninitiated, Coroplast is a trademark name of Coroplast Inc. It is short for corrugated plastic, and Coroplast’s web site describes it as a polypropylene copolymer. It’s very similar to the material in milk cartons, with a corrugated structure somewhat like a cardboard box. Chemically, it is inert. At regular temperatures most oils, solvents and water have no effect on it, allowing it to perform under adverse weather conditions or when exposed to harsh chemicals. Coroplast is available in what I refer to as the basic “Crayola colors,” plus translucent. Due to its structure, it is very strong when bent across the flutes, but tends to collapse and fold when bent along the flutes. Industrially, it is usually bonded via induction or ultrasonic spot welding. We will use high-tech zip ties.
Coroplast is used to construct ‘semi-rigid’ shells; that is, somewhere between the lycra of a soft fairing and the carbon fiber/fiberglass of a hard shell. Coroplast will ‘give’ when hit or pushed, but is rigid enough that the airflow won’t cause it to ‘cup,’ as lycra is prone to do. Large panels will probably need some support. A 4x8 foot sheet of standard 4mm Coroplast weighs about 4 pounds. Thicknesses can range from 2mm to 10mm, but 3 or 4mm is ideal for our purposes. Wholesale cost is about $7-8 per 4x8 sheet, plus shipping, which can raise the price to the $8-10 range. Sign shops also carry it; although their prices will be significantly higher, and selection will likely be severely limited.
Tailbox Advantages:
1. Speed advantage above 15 mph increases at a non-linear rate
2. Unlike a front fairing, a tailbox does not impede forward
vision
3. SWB, LWB, upright, doesn’t matter; basic construction is the
same for all
4. Less sensitive to sidewinds than front fairings
5. Adds cargo capacity
6. Riding in rain: pseudo fender; no skunk stripe
7. Can discourage clingons(tm)
8. Anything goes with design – even tail fins
Tailbox Disadvantages:
1. Extra weight will affect hill climbing and handling
2. May affect weight distribution
3. Transporting, design for removability
4. Crosswinds may still affect you more than unfaired
5. Increases visibility by dogs
6. Geeky/Dorky looking? You decide...
Tailbox design 101: At HPV speeds, a tailbox’s angle of closure must be small to get the best performance. This would normally necessitate extremely long tailboxes, 6 feet or more. So, for normal riding situations, engineering compromises are in order.
Fortunately, there is a phenomenon called the Kamm effect, by which a cutoff shape can ‘fool’ the air into acting as if the full physical length exists. I recommend utilizing this effect when designing your tailbox. A Kamm back will leave a small low pressure area behind the rider, characterized by laminar flow at the boundaries. Extra benefits include more useable storage space, and a nice flat surface to mount a rear flasher unit. You can’t have something for nothing, though, and the disadvantage is that a Clingon(tm) can destroy the nice weightless rearmost three feet of your tailbox.
Here’s a list of considerations for designing a tailbox. As in many design endeavors, some items on the list are at odds with others. The choice of compromises each designer makes creates individuality!
For Speed:
1. Matches your frontal area perfectly
2. Closed top/bottom - don’t allow parachute effect
3. Total closure angle: 6 degrees (3 degrees per side)
4. Transition airflow from seat base, too
5. Cover sprint braces, seat stays, rack legs
For Handling:
1. Light weight, use minimal material possible
2. Avoid anything that resembles a wing from any angle
3. Shorter avoids crosswind susceptibility
4. Design the storage area to keep the load relatively low
Thoughts on trunks:
1. Coroplast is waterproof, design your storage area to be rain-resistant
2. See above, water will get in; allow for interior drainage
Steps involved:
When you’re putting the pieces together, use duct tape for the preliminary
construction, then go back and do the zip ties. For your basic tailbox,
start with a rear rack.
* Clamp a plywood or Masonite base to it using a seatpost quick release
from a mountain bike. Put a layer of Coroplast underneath it for weather
resistance. You may have to make cutouts for the sprint braces.
* Next, put in a front panel, bringing it as close to the seat as practicable.
Again, you may have to make cutouts for the sprint braces. Hold the pieces
together with duct tape until you’re sure they’re in the right places,
then zip tie them together.
* With the front in place, put the back and side panels on , mating
them as closely as possible to the seatback.
* On the inside, partition off the cutouts for the sprint braces if
you made them.
* Finally, put a top on the thing. The easiest top to design and use
is a removable one, like on a storage box; but maybe you have an idea that
uses a piano hinge, or ??? Use Velcro to secure the lid, so it doesn’t
fly open or off at typical faired recumbent speeds.
* Tape the seams and edges and you’re done!
A few notes on using zip-ties:
1. Space zip-ties every 6-8 inches.
2. Make sure you wrap around 1 flute wall at the edge of each
piece.
3. Try to keep the tie’s wrap as narrow as possible so tape will
cover the stitches.
4. Start from the inside, that way the ends won’t show.
5. Keep the ties more than one inch from an edge that will be
taped.
Summary
A tailbox is easier to make than a front fairing, and doesn’t affect
your forward vision. It will increase your speed and add useful storage
space. Coroplast is inexpensive and easy to work with; an obvious material
for a tailbox. From a personal perspective, I think a tailbox 'finishes'
the bike, so you look like you're sitting in a vehicle, not
sitting behind a front fairing. This is one project that's highly recommended!
Material list (quantities needed may vary)
1 - sheet Coroplast, 4' x 8' x 3mm or 4mm
1 - 100 ct. 4" natural zip ties
1 - roll 3/4" or 1" electrical tape. (Cloth tape optional)
1 - seatpost Q/R
1 - ft Velcro (tm) tape
1 - piece of thin Masonite or plywood to use as trunk base (size
varies)
1 - scrap of wood to use for clamp
Tools used:
bike stand, wind trainer, etc
wet-erase marker
razor-blade knife
heavy scissors
duct tape
Internet Resources
WISL HPVers
Chicagoland Recumbent
Riders
Bill Volk’s Coroplast
Craziness Page
Cadillac Plastic (registration
required to browse their catalog - it’s free.)
Coroplast Inc
Also, see how other people have done tailboxes:
http://www.humboldt1.com/~mhp/tailbox.htm
http://home.istar.ca/~marker/tailbox.html
http://personal.riverusers.com/~n4nrk/tailbox.html
http://www.geocities.com/HotSprings/Bath/1913/tailbox.html