MARS projects - Full fairing bomb-bay doors
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Making "Bomb Bay" doors

This page details the construction of "bomb bay" doors for closing off the bottom of a full fairing. The particular project described here refers to John Tetz's foamshell fairing, but many of the ideas are useful for fairings made of other materials. For source information on the foam material used, see the foamshell page

The doors I made for my foamshell are very simple, light and easy to use. They're made of the same foam as the shell, with cloth hinges and Coroplast stiffeners. They're held shut with magnets, and open easily by simply putting your foot down. They close with strings that are positioned so that they can be easily reached by hand... again, light and simple!

The Bomb Bay doors are easily heat formed to the same radius as the shell and trimmed to fit. Total weight for two doors including hinges, mountings etc. is around 100 grams, 3 ounces.

Because the shell is also made from foam the cutout needs a stiffener. I used a length of (.150 inch, 3.8 mm thick) Coroplast along the hinge line. To make a 90-degree bend for increased stiffness, slice one cell along its length. Carefully remove all the material with a sharp razor to allow a 90-degree bend. The short dimension is as wide as the shell foam thickness, the other side is a wide as you want. Mine is 2 inches. A part of this extends beyond the shell cutout by a couple of inches. The joint is glued using GOOP, and tape holds it while the glue dries.

Click on the image for a larger version!!!

The photo above is for the entry door stiffner –the bomb bay door piece is much longer and wider. This can be carefully heated to form a slow curve similar to the shell curve (let it sit overnight to see if the curve shape is stable.

The photo below shows the red Coroplast stiffeners just above the bomb bay doors way down inside the bike. Note how the closing string is passed through a cell in a piece of Coroplast at the door and one up near the handlebars. Use fairly thick string so a gloved hand can grab it.

Click on the image for a larger version!!!

The next photo, below, shows the rider's main entry door hanging open giving a good view of a typical cloth hinge system. Note the width of material that is cemented to the foam to assure good adhesion and stability.

Click on the image for a larger version!!!

A similar piece of cloth material is used as a hinge for the Bomb Bay doors. I use a Vinyl material with a cloth backing (Naugahyde... I shot the Nauga myself!). Fold the material cloth side out and heat it with an iron to get a tight flat hinge line. First cement the hinge to the narrow edge of the Coroplast on the shell such that the hinge crease line is facing out. Then continue the cloth wrap further up the Coroplast. Then cement the door to the hinge, and again along the inside surface of the door as shown below.

Click on the image for a larger version!!!

Hint; when using contact cement on cloth (and Velcro) lay a thin coat and let it dry (even overnight). Then lay a heavier 2nd coat for better adhesion. Also sand the foam and also use two coats of cement.

As shown in the photo above, two narrow strips of thin Coroplast (.1 inch, 2.5 mm thick) are heat formed and contact cemented to the inside surface of the main shell with about an inch protuding, acting as a stops for the door when closed. A small thin piece of steel is glued to the Coroplast. Magnets, one at each end are glued to the door.

The next photo shows how I dealt with wheel interference. An hourglass shaped cut out is needed for wheel clearance when making tight turns. The left side of the cutout is in the nose section (the cut out and the chain interference on the wheel are the limits to turning radius). I could have made a closer hourglass fit to the wheel but I chose to leave a wider opening to accommodate a wooden bike stand. This is what I use daily to park the bike on (and as a work stand). This is a road streamliner not an all out racer so the main objective is reduced rider effort at speeds around 20 to 30 mph rather than all out speed.

Click on the image for a larger version!!!

The front cutout doesn’t have to be right below the forward pedal location. The foot naturally comes back while dropping towards the ground. The rear door edge has to be far enough back that you can stand to get in/out of the bike. My doors are 19 inches long.

You take off from a stop, reach down and give the string a light pull and you hear the magnets click in as a door closes. I generally sit at a stop or traffic light with one foot on a pedal clicked in. Even if you don’t get the doors up they clear the ground except during a tight turn. And then they only drag lightly reminding you they are open. The foam is so light that sometimes the wind blows them shut.

To get in (for my bike) open the canopy and entry door, lift the right foot over the entry door opening and down past the frame and on the ground. Shift your weight to that foot and stabilize yourself by holding on the handlebar and brakes so the bike doesn’t roll out from under you. I hold the center of the handlebar with my left hand, right hand on the brake. The secret to keep from toppling over is by either lifting or pushing down on the handlebar you can use the weight of the bike to balance your-self. Then lift the left leg in and lower yourself in the seat. This normally takes only a few seconds. Close the entry door and the canopy and your snug as a bug in a rug. Click in and off you go. Pull up on the doors and the thrill of streamlining begins.

One thing to be aware of is the limited space between the shell sides and the front wheel axle. See drawing below.

Click on the image for a larger version!!!

Depending on shell design there is about 7.5 to 8 inches from the hinge line to bike centerline. The width of wheel axle from the bike centerline is 3 inches (with skewer). This only leaves 4.5 to 5 inches for the foot to sneak through. Luckily the foot travels above the axle before dropping down through that space. Yes you can move the hinge line up a to widen this gap a bit but then you’re chopping a lot more out of the sides of the shell which can weaken it (and destroy more of the air flow). A large opening could seriously weaken a tub design.

One problem to be aware of is your feet can only spread to about 12 inches rather than the much wider stance normal on an unfaired bike. Luckily the foam bends allowing more leverage in an emergency. In the beginning this will be of some concern to a rider but I feel it is not a big problem. This is annoying only with a heavy crosswind (less of a problem with a low bike).

On a FWD of the type with the derailleur down on the front wheel, the extra width of the axle with cogs plus the derailler parallelogram, eats up so much space that the foot interference is such that I feel it to be in the range from difficult to impractical (but I haven’t seriously tested this). The width of the shell could be increased but then to obtain a proper contraction rate to the tail results in a much longer vehicle (and more wetted surface drag). I’m trying to keep the vehicle length to 80 inches so it will be easy to store and to fit in a car.

Another technique would be to make a flat bottom shell with a sharper radius to increase the width at the hinge line of the doors. This could complicate the airflow between the shell and the ground particularly on a low bike possibly creating more drag. This also decreases lean angle clearance on turns and shell contact to the ground is a serious no no.

Do you absolutely need doors? No, but they make the shell look handsome by covering up an ugly looking cutout. Do they make an aero difference? I have seen an improvement but the test was too crude at the time to tell how much. I haven’t had a chance to make careful enough coasting measurements.