A Coroplast Handlebar Bag
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I've used various handlebar bags over the years but I've never found one that I really liked. Being something of a tinkerer, I finally decided to make my own bag out of coroplast and the pictures above show the final result. Clicking on any of the thumbnails will present you with a bigger view.
The rest of the text here is a basic explanation describing how I constructed this particular bag but anyone attempting to construct a similar bag for their own use will most likely have to modify the design to suit their bike. Please consider what I did as a set of guidelines rather than a series of instructions that must be rigidly followed. Coroplast is a very inexpensive and strong material and the best way to figure out how to work with it is to experiment. If you don't know what coroplast is or want to learn some basic coroplast construction techniques, please see my article on coroplast fender construction which can be found by clicking this link .
My basic plan was to create a lightweight, weatherproof bag. I decided that rather than going with a conventional box shape, I'd take advantage of the ribbed structure of coroplast to make a box with a smoothly curved front. While I don't know for certain that this shape really does much to improve the aerodynamics of the bike, I do think it provides a better aerodynamic profile than what you get from a traditional, boxy bag.
I began construction by taking a bright yellow campaign sign that I'd harvested shortly after a local vote on a school bond. The pro-school sign was perfect for my purposes, bright yellow and only printed on one side. With a bit of planning, I could make certain that all the printed surfaces would wind up on the inside of my handlebar bag while the outside would be highly visible. I began by cutting a rectangular strip from the sign that was 16 cm high by 50 cm long. The flutes of the coroplast ran parallel to the long side of this rectangle. Having the flutes run this way are essential to making a box with a smoothly curved front.
Three centimeters from each short end of the rectangle, I scored a crease running the full width of the rectangle. Using these creases, I bent the ends of the rectangle up ninety degrees. I now had 44 cm by 16 cm rectangle with 3 cm wide tabs on either end. Now I had to experiment. I pulled these tabs towards each other, which made the rest of the rectangle form an arc, like a bow. I found that when the ends of the coroplast were 25 cm from each other, the arc had a depth of 20 cm. This smooth shape would be the basis of my bag. The curve would be the front and sides of the handlebar bag.
I now cut another rectangle of coroplast 43 cm long by 25 cm wide. Again I made sure the flutes were running parallel to the long edge of the rectangle. 16 centimeters from one end, I scored a scored a crease running the full width of the rectangle and I made a 90 degree fold at this crease. So this single piece now consisted of a 25 cm by 16 cm rectangle attached to a 25 cm by 23 cm rectangle at a 90 degree fold. The smaller part of this (25 cm * 16 cm) forms the back of handlebar bag while the larger section (25 cm * 23 cm) makes up the bottom of the bag.
I used zip-ties to stitch the 16 cm long tabs of the front/side piece to the back piece, folding the bottom so it was flush against the side of the arc. At this point a good portion of the 25 cm by 23 cm bottom section stuck out past the curve of the arc, but using a marking pen I traced the curve that the bottom should have. I also marked out a second curve which formed a wider arc parallel to the first arc but three centimeters farther out from the center. I cut away any coroplast outside the outer arc and scored a crease along the inner arc. Then I cut away small wedges, shaped like small pizza wedges with their wide part on the outer arc and their points on the inner arc. As I removed these wedges I was left with trapezoidal tabs that I then folded upwards on the inner arc crease. This probably sounds quite confusing, but if you look at the photographs, especially the one on the lower right which shows the final bag from the top with it's lid open, you can get an idea of how this all goes together. The trapezoidal tabs connect to the curved side of the bag and again I used zip-ties to stitch things in place.
Once the sides, back and bottom were all stitched together, the bag had it's basic shape. I constructed a top for the bag using similar techniques to those employed in fitting the bottom. In the case of the top I cut it a bit larger than the bottom and I cut a 4 cm wide arc strip to make an overlapping lip that I zip-tied to the trapezoidal tabs. Thus, the top fit almost like a shoe-box lid onto the main bag. I then cut a trapdoor in the lid and zip-tied the whole assembly to the main bag. I cut another piece of coroplast that was slightly larger than the trapdoor. I zip-tied this larger piece in place over the trapdoor so it overlapped the edges and would keep any rain from seeping into the handlebar bag.
Now that the basic bag was done, I used more zip-ties and bits of coroplast to create some attachment points so I could mount the bag on the handlebars. My handlebars are somewhat unconventional cowhorn-style bars that I made by cutting and inverting a set of standard drop bars. I find these bars to be very comfortable and when I was figuring out how to mount my handlebar bag, I realized that running another piece across the front of the bars would give great support to the front of the bag and give me a terrific place to mount my headlights. I had an old set of the original Profile triangular one-piece aerobars in my junk pile, and I cut down the the back section of those with a hacksaw to make a single curved cross-bar that would span the gap between the tips of my cowhorn bars. The cross-bar didn't dip down quite far enough to touch the top of the handlebar bag, so I made a small spacer from a five cm long piece of PVC water pipe. Using zip-ties, nylon cord and black tape, I attached the cross-bar to the cowhorns and I attached the bag to the cross-bar.
For night safety, I added a white reflector strip to the front of the bag and I attached Princeton Tec Impact white LED lights to the crossbar on either side of the bag. I used big rubber strips cut from old bike innertubes to secure the lights. I've found this method of mounting lights to be very solid and vibration-proof while still allowing me to tweak the angle of the lights as needed.
I added a couple of elastic cord loops to the edge of the trap door. These loops can be stretched over some bits of coroplast at the back of the bag and hold the trapdoor closed. I also added a small zip-tie loop to the trapdoor as a handle. For a final finishing touch, I took a green nylon clear-topped pencil pouch and strapped it to the front bar with a couple of innertube bands. I use this pencil pouch as a holder for my cue-sheets, maps and small snacks. Since the pencil case isn't 100% waterproof, it's wise to keep any maps or cue-sheets in a plastic bag inside the pencil case if there's any chance of rain.
I've been using this handlebar bag for about a month now and it's proven itself to be very handy. It's large enough to hold a rain jacket and some other small clothing items along with some food and other miscellaneous things. It keeps things nice and dry in the rain and I really don't notice that it's effected the handling of the bike at all. The one downside I've noticed is that if the bag is only partially loaded with loose items, it will make quite a drumming sound when I ride on rough roads. But if I keep loose items packed in with softer things like clothing this isn't a problem.
Kent Peterson (kentsbike@fastmail.fm)
Issaquah, WA USA
May 18, 2002
http://www.carsstink.org/peterson/rando.html