Introduction: Method for Making 3D Shapes (used to Make a Bike Fairing)

About: I'm an applied physicist/mathematician with an interest in checking global warming.

This Instructable shows you how to make a fairing (enclosure) for your bicycle/trike/etc from a combination of sheet plastic and some plumbing items (PVC and some pipe for supports). It goes hand in hand with the wind powered bicycle project that I am working on (to be posted a little later), and represents a continuation of my work on Thinkcycle. The technique I explain here can be applied to a wide variety of different items: greenhouses, fairings for other objects, sculpture, boats, etc. For vehicular applications this also forms a perfect backplane for the attachment of solar cells, etc. I'll cover how to do that in subsequent instructables.

There are lots of reasons you might like to have an enclosure on your bike: they can improve your aerodynamics, help with thermal regulation, serve as structural attachment points (for hanging lights, cycle computers, etc), but in my opinion the biggest thing is that it can provide a solution to the thing I hate most about winter biking, which is rain. At least part of the reason I have pursued this work is that there were no good options for getting my hands on a nice fully faired HPV, and nothing spells freedom like being able to get where you want to go at any time.

The basic idea for how to build this thing is to decompose your ideal shape (for the example I use an elongated ellipsoid as per some of the photos in step 2) into crescent segments using a little math (don't worry, the attached spreadsheet does all that for you, at least for the simple shape used in this tutorial). Then you can cut sheet plastic (0.080" PETG or polycarbonate) into correctly-sized pieces and then tape the outsides of them together. If you're very careful about the outline of the segments you cut, you can generate a very good approximation to the large shape you want (similar to the way the almond-shaped segments of a football are stitched together to make that three dimensional shape). Once the segments are taped together, you can fill a turkey baster up with pipe glue and run a bead of ;pipe glue along the joint. You let it harden, clean it up with a Dremel tool, and use it! For the shape I'm building, I also show you how to attach it to your bike. This is accomplished by cutting PVC T-joints in half and then using pipe clamps to attach them to the bike. The mounts have proven to be kind of tricky to get right, however, and there's some room for improvement in this department.

The motivation for this work was that I wanted to be able to make big three dimensional shapes for a number of projects but was unhappy with the existing techniques. Either they required a big oven (necessary for most vacuum and blow forming techniques) or were messy and generated lots of extra stuff (ex: cutting foam molds for fiberglassing). If you still want to go those routes, the technique I show you here can be useful in building supporting structures for use in jigs and whatnot.

The one note that I should add is that for reasons I will detail in the wind powered bike instructable (I may also call it a "Side wind safe" bike) you have to be careful about the design of your fairing if you are using a front-steered only bike (whether upright or recumbent). Basically, large fairings are unstable in high winds (so you want to keep the cross section from the side low by adding openings or making it very compact) on a regular bike. In the wind powered bicycle project I show you how to build a bicycle that will get around these shortcomings (you have to steer the front and the back wheels), but you can also attach this to a tricycle (just keep the width of the vehicle as wide as possible and the center of mass low). The fairing I build in this example is a test piece and probably too large for everyday use (although you could try cutting holes in it).

Step 1: Things You Will Need (and Some You'll Just Want)


PETG or Polycarbonate Sheets (you can order them from the following places: MSC, Mcmaster-Carr, Tap Plastics, Aircraft Spruce & Specialty, wick's, and probably a lot of local suppliers as well). You want the 1/16" thick stuff, and I went with 4'x4' sheets ($~20/a piece). This is the only somewhat obscure material, you should be able to get everything else from your local hardware store.
Packing Tape
Some long strips of cardboard suitable for transferring patterns
Spray paint if you're planning on painting it (optional)

Aircraft Tin Snips (or DREMEL, etc)
Sharpie or other means of marking cardboard
Ruler and/or measuring tape (nice to have both, however).
Protractor for making baffles
Hacksaw or bandsaw for cutting PVC pipe for the supports.
Drill plus some small bits for drilling mounting holes in fairing.
A small wood rasp for doing cleanup (unnecessary if you have a Dremel)
C clamps for holding parts (at least a couple little ones) together.
Vise for holding PVC pipe.

Two options for bonding: (you have to pick one and get everything mentioned for that case):
I). ABS Glue, a turkey baster, masking tape, small bottle of MEK (methyethlykeytone), lots of newspaper, a glass jar, gloves and googles for cleanup, I would also recommend a mask: be very sure you have good ventilation. This option is essentially "chemical welding".
II). plastic welder (hot air, conduction or ultrasonic, you may need an air compressor if you go the hot air route) plus welding rod plus skill. This option can be a lot more dicey with transparent materials because any sort of heat distortion will ruin the transparency, but it is possible with a fed-rod design to get nice joints with it.

I decided to go with option (I) for this instructable because of the heat distortion issues (which are probably addressable with a fed-rod welder, I just don't own one). Some additional notes: ABS is not your only option for the glue base: a better option would be to use a material designed for use with PETG: I just used it because everyone has access to it and I believe it will hold up pretty well. If you want to make your own, I have elsewhere detailed some experiments using a coffee grinder to grind up your PETG cast offs and some MEK to dissolve them. I do not recommend this route unless you are really concerned with having a clear fairing on 100% of the surface (not a bad goal at all, just has a cost to it).

You are going to have to be (or get! you can do this!) comfortable working with PVC pipe: both cutting and gluing it. There are a ton of tutorials online (for instance, here), so I'm not going to repeat them here, but always remember that you can do test pieces on a smaller scate to get a feeling for things. I highly recommend doing so whenever possible, you will see that for my first build I did not mask things properly and lots of the resultant joints were messy.

Optional: Dremel tool or equivalent (any small high speed hand grinder) plus tungsten carbide cutting bits (there's a photo below of the kind I like) for doing cleanup, plumb bob (you can make your own), drill press, V blocks, PVC pipe cutter. All of these tools are worth having and will make the workflow go a lot faster, but it is possible to get by without them if you're motivated to save money.

Step 2: Things I Learned (and Wish I Knew Before I Began!!)

A). The main thing (and you will see this in the photos!) is that the dream of not having drips is a pipe dream. The real solution to glue management with these things is to use masking to prevent the pipe glue from coming in contact with the non-joint areas of the fairing. You can use the packing tape I used to secure the crescents while they were being glued or any stiff plastic tape, but regular masking tape is a no-no because you won't be able to remove it if there's glue on top of it. All you have to do is put two strips on either side of your joint and the messy joints you see here will be a thing of the past (you can always clean things up with the dremel). Having said that, if you're planning on painting the fairing, you can dispense with the masking, and the job will definitely go faster. Ditto for if you're planning to build solar panels in, even with the drips I had going you still have plenty of transparent surface. Having said all that, if you want to go for show, dripping black glue on one side of the clear plastic gives you some really interesting visual effects.
B). The second thing is to always start any sort of attachment at the tip (whether the curvature is highest) and tape your way back to the back edge.
C). Third, if you can do this with someone it's more fun and a little bit easier (extra hands are welcome when you're dealing with big sheets).
D). Do not underestimate the amount of monkeying you will have to devote to the brackets that hold it. I'm going to be updating it with some improved designs over the next few weeks, this ended up being tricker than any other part of the project, even with access as I have to good CAD tools.

Step 3: Cut the Master Pattern and the Crescents

In order to cut the crescent shapes properly, you need to make a ruler out of cardboard that has just the right shape on it. I've developed an excel spreadsheet that helps you calculate what you need: you just use the Chord Length and Chord Height values along with a ruler to go ahead and plot the curve shape you need on your cardboard.
Once you've made it, you can use this cardboard cutout to transfer the shape you've drawn to the plastic sheet. You will find that most plastic comes with a protective film coating both sides: I recommend leaving this on as long as possible, which usually means stripping off the inside for gluing and the outside when you are done with the finished shape.
Transfer the shape to the plastic, then get a stout pair of aircraft tin snips. You will find that this piece of the project is by far the worst, it is very annoying to have to cut the shapes out (but hang in there!). You can also try using power shears, scroll saws, etc. Cut the shapes out, one (or two) for each segment on the fairing.

At this point, you should also cut out the baffles that will brace the shapes while you're gluing. The shape that I decided on for my fairing is a wide shape that will allow me to test out my side wind safe vehicle (detailed in another instructable). So I decided on a shape that if you cut it in half along the longest axis looks like an elongated stop sign (octagon) with 10.5" sides on the smaller sides. The total length of the fairing was to be 100".

Step 4: Tape and Glue Three Crescents Together

The subject says it all. I use regular packing tape, you can probably use a lot of stuff, you just need it to contain the pipe glue and stoutly hold the segments together. Be very careful with the glue and sure to use lots of newspapers to protect your floor from the drips, they will eat through most floor coatings.
The procedure I used was to pour the glue out if it's can and into a large glass juice container. Then you can use a turkey baster to transfer the pipe glue into the joints, making sure that you have a nice smooth fillet (tapering at the edges) rather than a ball shaped bead (which would be weak where the ball intersected the plastic sheet). I included some pics to give you the idea.
You will find that it's easiest if the joint is substantially horizontal/below the area you're gluing (as per the pictures). This can be accomplished by working the joints up in sections: I glued three crescent segments together at a time (so that there are two joints that are substantially horizontal most of the time).
If you start with the non-elongated top/bottom of the fairing, it's a little easier.

Step 5: Tape and Glue on the Sides

You will find that gluing the elongated sections is much more annoying than gluing the smaller segments because they are larger and hence more unwieldy. It's harder to get to the joints if you have to lean over a big section, and a long unsupported top will want to collapse. There are a couple of ways to deal with these problems, but the easiest is to add a cut line midway through the elongate section (the approach I use here) so that you can attach a rectangular piece of sheet stock in a simpler fashion. Having an extra joint will, however, reduce the clarity of your finished piece, so If you are really pushing for clarity wait for a little while and I'll post the technique I used to generate the side-wind safe fairing.
For the meantime, I will show you how to deal with elongated segments on a smaller piece. Be sure that you have your baffles strongly in place before you start, they will help to keep things oriented properly (I have a little note at the bottom about how to tape the baffles in if you're having trouble).
The steps are as follows: first you lay one of the elongated segments on top of the three segment piece (from step 3) and tape the edge you're going to be gluing. Next you tape the tip of the second elongated segment to the tip of the first elongated segment. Finally, you tape the long edge of the second elongated segment to the three segment piece and force the final five segment piece into it's final configuration. I will emphasize that getting the segments to mesh properly at the tip will solve all kinds of ills, but the good news is that a little bit of slop can be taken up by the liquid plastic glue.

Step 6: Make a Little Spreader for the Tip of the Fairing and Attach It.

This step is only required if you're making a partial fairing (one edge open). I made the decision to build this fairing for my conventional recumbent (the intro page has shots of a larger version for the wind powered bike project which is almost done), and noticed when I did it that the partial fairing wanted to curve inward along the unsupported sides (see photo below). To correct for this, I designed my mounts so that it is spread from the bottom (see photos), but the area near the tip/pedals still wanted to curve in a little and I was worried about clearance. So what I did was to take a section of 6" ABS sewer pipe and cut a tiny little crescent off of it, which I could then glue to the bottom of the leading tip while clamping it. This effectively spreads the shape out, and once the glue dries serves as a permanent fix.

Step 7: Clean Up the Joints

This step is pretty straightforward. Basically, you just get a tungsten carbide tip for your Dremel tool and run it along the edges of your part. Be sure to wear a mask and clean up all surfaces afterwards, plastic dust is bad for you! You will find that many of your joints require touch-up, but you'll be able to get everything nice and smooth rather quickly.

This goes a long way toward correcting any areas where the edges separated from the tape, and/or areas where glue was not contained properly.

Step 8: Get the Mounting Brackets Together and Drill Holes in the Fairing

Once you have the fairing done, you are going to have to go back and cut out any cutouts you want. That may include: doors, holes for lights, wheel wells, etc. This step can best be accomplished while the fairing is off the bike. I recommend reinforcing each cut edge with a 1/2" or so wide border of plastic (you can use the same stuff you're using on the fairing) so you maintain your structural integrity.

As far as cutting any holes goes, you can most easily do it by drilling a hole (using a hole saw for large holes) and then slipping the tin snips into the hole and then following new cutlines you've established.

Step 9: Build Up the Supports That Will Attach the Structure to Your Frame

Here you just add cutout pipe T's (as shown) to your frame on an as-needed basis. You can also add additional supports for things like cyclecomputers, etc if you have underseat steering.
The way that you do this is to first determine your frame size, then get PVC pipe that's big enough to fit it (so if you have a 1.5 inch outside diameter bike tube you want to use a 1.5 inch inside diameter tube). I've found 3/4" or 1" tube to be necessary on fairings (the 1/2" stuff just isn't rigid enough), but as always your mileage may vary.

Step 10: Attach the Fairing to the Frame

By temporarily putting the fairing over your frame without gluing the PVC together, you can test to see where the supports end up. Use a dry erase marker to mark out where to add your points of attachment, then build up attachment points as follows: get a piece of strip plastic and glue (again using pipe glue) a pipe coupler (threaded if you want to be able to take it apart later) to it. Attach this to the frame supports from step 8, attach the fairing to the frame, and use a drill to drill a couple of tacking holes. You can either screw, rivet (probably the easiest), glue or weld the supports on at that point.

At this point it is critical to test for heel/toe clearances at the front. If you find you need a little extra curvature, you can attach a cable or string to the front of the vehicle (through a little hole drilled in the tip reinforcement) and tie that onto the frame under tension.

Step 11: Attach the Fairing to Your Frame, and Gloat.

Attach the fairing to the vehicle using the attachment points, screwing it together as needed.

You now have a fully faired vehicle, which is protected from the weather and will protect you somewhat in crashes. Make sure your friends see it!

If you want to pimp it out further, you can spray paint the inside of the fairing (which will give you a water like finish on the outside), add solar cells to it (to power your vehicle), add lights of all kinds (now that you have a secure and waterproof attachment point), add signage, etc. To reduce heating in the summertime, you can add air ducts and a mister to build a miniature swamp cooler to keep you cool. I hope to get to more instructables for them later.