DIY Carbon Aerobars

I recently got into the world of triathlon, and a very common upgrade for bikes is to add on a pair of clip-on aerobars. Given that most off-the-shelf options have price tags well north of the triple digits, I thought I'd try my hand at making my own.

I opted for carbon since it's light, moderately easy to form into different shapes, and most importantly, it's got that highly coveted cool factor. I also got my hands on some scrap materials, and I'm a huge fan of recycling and reusing. So with that all out of the way, let's get started!

Materials

1. Rigid Foam
2. Chopped Carbon Fiber Tow
3. Epoxy Resin & Hardener
4. Mold Release
5. Clear Coat or Lacquer
6. 1/4" Wooden Dowel
7. Disposable Paint Brushes
8. Glue
9. 10-24 x 1/2" Screws

Tools

1. Bandsaw or Jigsaw
2. Scissors
3. Box Cutter
4. Scale
5. Sandpaper
6. Clamps
7. 3D Printer
8. Hammer
9. Chisel
10. Small Pry Bar
11. Tin Snips
12. Sandpaper and Scouring Pad

Equipment

1. Nitrile Gloves
2. Respirator
3. Safety Glasses

I'll be following the tried and true carbon manufacturing method of molding carbon around a foam core.

I was lucky enough to score some free insulating foam offcuts from a friend who was doing some renovations. This foam was labelled as 13mm (.512") thick, but it measured closer to 14.7mm (.578"). No big deal, I'll be cutting and sanding everything to size later anyway.

I wanted my extensions to be somewhere around 1 inch (25.4mm) in diameter, so I glued two layers of foam together to thicken it up. If your foam is already thick enough you can skip this step.

Once the foam blank was all prepped, it needs to be cut to shape. I cut a few templates out of Bristol board and traced them onto the foam blank. I was able to cut my templates on a laser cutter so they were a single piece, but there is a PDF template attached that can be printed on standard US letter sized paper. Just cut out the two halves and line up the alignment marks.

These are fairly generic profiles that worked well for me, but they can also be modified to be any shape or size you like. The great part about building things yourself is that you can make them into exactly what you want! Just keep in mind that if you do decide to change the design, you'll also have to model your own mold for this project.

The foam core is dimensioned so that there will be about .100" (2.54mm) of carbon once the bar is at the 1" diameter. This is all approximate; I am sanding the foam cores to shape by hand after all. I could probably make this a little more accurate, but for my purposes this isn't really that critical.

I used a bandsaw to cut out the rough profile, but a jigsaw or coping saw would work just as well. The corners were then cut off with a box cutter and rounded with sandpaper.

Again, I was lucky enough to have a friend who works with composites at their day job and I picked up a bag of offcuts from them. Carbon is expensive, so it may be worth visiting some local composite manufacturers to see if you could get some freebies. Otherwise, a local or online composites supplier would probably have a wide variety of carbon to choose from.

Since I'm using offcuts and scraps for this project, I opted for the chopped short strand "forged" carbon approach rather than the traditional lamination or infusion methods. Short strand carbon also has the added benefit of being a little bit easier to mold into various shapes, so that's pretty useful.

I pulled the fabric apart into the fibers and then cut them into short sections with a pair of scissors. These were collected in a small takeaway container.

Remember to wear a respirator, gloves, and long sleeves when working with carbon. It's quite hazardous to our health.

I designed and printed a set of molds to go around the foam cores. These were designed to fit on a standard Prusa print bed and locate together with a pair of 1/4" wooden dowel pins.

My molds were printed on a Prusa Mk4 with the 0.15mm structural settings and 15% infill. Nothing fancy, just something to define the overall shape and be strong enough to compress the carbon a little bit later on. I settled on a slightly taller bottom half and a top half with "lips" on each edge that would catch and form the extra carbon I'll be adding in the bottom. Caps on each end prevent everything from squeezing out when clamping to ensure effective compression. Of course this is not going to get anywhere close to the pressures and temperatures used in professional carbon fabrication, but we're going for the hobbyist special of "good enough" here.

Once the parts were done, I sanded the insides smooth and assembled them with the dowel pins and super glue. The end caps were fastened with some 10-24 x 1/2" screws. The printed threads in the mold aren't the best so it might need some chasing with a tap to get the screws to fit well, but they'll hold just fine for keeping the carbon and epoxy inside.

A few coats of mold release will help the part come out easier once the resin is cured. I used Meguiar's Ultimate Paste Car Wax. Not technically recommended, but it's what I had and it worked pretty well.

I used the West System 105 epoxy resin with the 207 hardener for no reason other than that it was the only stuff my local supplier had in the small quantities that I wanted. Any dedicated composite resin should work fine for this project.

I mixed up the resin and hardener in the recommended ratio with the help of a small scale. Hot tip: cover the scale with plastic wrap so it doesn't get all messed up with resin if you accidentally make a spill.

Once the resin and hardener is in the mixing cup, the clock is ticking. I used a stir stick to thoroughly mix the resin and hardener before taking a brush and fully coating the inside of the bottom mold. Chopped carbon tow went in next, followed by more resin. Use the brush to press it all against the walls and make sure the carbon is well saturated. Repeat until there is enough carbon/resin mix in the mold. Everything will compress quite a bit when we clamp the mold together, so be generous.

Center the foam core in the mold, press it down, and cover the top with more resin and carbon. Again, coat everything with plenty of resin and carbon. We don't want to find any empty pockets later. Keep everything contained within the pocket area of the bottom mold. Anything on the flat areas can't be picked up by the upper mold and won't end up being pressed into the final part. Applying a thin layer of resin on the upper mold will help prevent pinholes on the part surface.

When the mold is adequately filled, take the upper mold and press it down into the bottom half, making sure all the lips all line up. Clamp the two halves together until the flat faces on the mold meet. Use lots of clamps to apply pressure evenly. Don't worry too much if you feel light resistance and some resin seeps out. That just means there's plenty in the mold and we hopefully won't be left with any empty dry pockets.

Let the resin cure according to the manufacturer's recommendations.

Respirator, gloves, and long sleeves are also recommended for this step.

After the resin has been given adequate time to cure, it's time to pull the molds apart. Remove the two end caps and carefully drive a chisel between the two mold halves, working evenly all the way around. A small pry bar also helps convince the mold to let go. Be patient and go a little at a time. If you're careful, you can save the mold and reuse it for the second part. If you already made a second mold, feel free to obliterate this one to you heart's content.

The extensions will likely have a lot of sharp flashing and pointy bits fresh out of the mold, so be careful and wear some cut resistant work gloves to protect your hands. I trimmed the flashing off pretty much right away with a pair of tin snips and a box cutter so I wouldn't have to worry about it.

If there are large pinholes, voids, or pockets, now is the time to clean them out and fill them. Tiny pinholes can be left alone as the clear coat will help cover them later on. Medium to large pinholes can be filled with a little bit of resin, whereas voids and pockets will probably need the part to be put back into the mold with some more carbon/resin mix. Really big voids and pockets where significant portions of the foam core are visible may require starting over since structural integrity would be compromised.

Use some sandpaper to remove any remaining bits of flashing, blend in patches, and break sharp edges. I started with 150 grit to remove what was left of the flashing and get everything close, then 220 and 320 grit to smooth it all out. Scouring pads or Scotch-Brite works great afterwards to create a nice smooth surface.

Wear a respirator, gloves, and long sleeves for this step! Also try to do this work in a well-ventilated area that isn't health-critical (i.e. not the kitchen or living room) and can be thoroughly cleaned. The dust from sanding this stuff is no good.

Thoroughly clean off all dust and contaminants. Isopropyl alcohol would be a good option here, but simply using a wet microfiber towel or tack cloth should work fine as well. Just be careful and make sure there is no dust, grease, or oils left behind. Let everything dry.

Apply the clear coat or lacquer on the parts according to the manufacturer's recommendations. I put a small hole at the back of the extension and used a stick to hold it up so I could finish the entire part at the same time. The hole was filled with a little bit of resin at the end.

I went through a few different mounting solutions for the aerobars already and I'll probably end up going through a bunch more. Between adjustability, packaging, and manufacturability, I have yet to come up with a design that I'm really happy with. Until then, I'll just have to settle for close enough.

For now, I'm using a set of machined aluminum clamps that use the typical clip-on mounting point on the drops beside the stem. An intermediate riser holds up a pair of threaded bars that the arm cups mount to. A second set of clamps hang below the threaded bars that hold the actual aerobars in place.

It's a little clunky, but it's good enough for now. It'll almost certainly go through at least a few more iterations.

I modeled and 3D printed some simple arm cups. They have a series of mounting holes for fore and aft adjustment. Left and right is taken care of by the mounting bars from the previous step. I attached soft padding to the inside with some hockey tape.

The STL files for two different sets are attached here, but these are just the ones I used. You can model your own to be whatever shape or size you like.

V1 is symmetric for both the left and the right, but it is quite large. V2 has left and right side arm cups and is much sleeker. I like the V2 arm cups.

After using the aerobars out for a little bit, I can say I've been pretty happy with them. They fit well and the carbon looks great in the sun. I also ended up retrofitting a between-the-arms bottle cage, which I am now a huge fan of.

While these don't really make me any better on the bike, they definitely make me look faster and isn't that really the whole point? Anyway, carbon cool and more aero more better or whatever it is that they say.

Thanks for reading, and happy racing!