When I designed the Semester hextube bamboo bike for HERObike a few years ago, I was frustrated with the costs of components- wheels, tires, chainset, saddle- everything! Trying to keep a simple bike affordable was an incredible challenge. But then someone told me that Walmart sold a pretty decent fixed/ single speed bike for $100 retail. With the help of my students at the University of Kansas, I quickly devised a way to hack this bike, replace the top tube and the down tube with bamboo/carbon fiber hextubes and Voila! the Walmart Hack was born. You're still probably looking at $200 by the time you're finished, but you have a unique single speed/ fixie with a smooth, vibration-free ride!
Start with an inexpensive fixed gear bike. Look for steel frames that are filet brazed with small top and down tubes. The Walmart Thruster 700c is a good candidate.
Step 2: Make Your Hextubes
Check out my other Instructable about making bamboo/carbon fiber hextubes. If you have big bamboo growing nearby, you're in luck! Otherwise, look around for bamboo strips that are about 2' long, minimum 1" wide, and approximately 1/4" thick. If all else fails, get some nice hardwood (maple is a good choice), have it planed down to a 4-5mm thickness. You need to calculate the size of your strips based on the dimensions you need for your hextubes.The inside diameter of the hextube must be slightly larger than the diameter of the tube you are replacing. Let's say you are replacing a 28.6mm tube. The inside diameter of your hextube should be slightly larger- say 30mm.If you are using 4mm thick bamboo, the outside diameter of the hextube (flat to flat) is 38mm.If we use a little math (or an online calculator) we know that each side is 21mm. That is the width of our strips. Length should be a couple inches longer than we need to allow for trimming.
Step 3: Prepare Your Frame
Scary step, but not difficult. Just make sure you measure everything.
First, measure your top and down tube diameters. The down tube is going to be a little bigger so you need to make your tubes different sizes. You will use these measurements to calculate the size of your hextubes. Next, carefully measure the length of the top tube and the down tube. Usually, tube lengths are measured center-to-center. This way, when you install the hextubes you won't change the geometry of the frame. Use a Sharpie to mark your cuts, so your lugs are 80-100mm long, then saw your frame. Rough up the "lugs" that will go inside our hextubes, and tape off the rest of your frame to protect it.
Step 4: Fit Your Tubes
Dry fit your hextubes and carefully shape the hextube to fit tightly to the steel frame pieces. I usually mark with a pencil, cuut with a coping saw, then shape with a rasp. Keep checking the fit to make sure your finished frame will be the same size as the old one. The care you take in this step will determine the overall quality, look and feel of the finished bike. When your tubes fit dry, pull them apart, clean the tubes to make sure there is no grease anywhere, them slather a generous amount of JB Weld or PC7 on the lugs and inside the hextubes. Check to see that your frame is square. Leave overnight. Then take off the tape and reinstall components.
Step 5: Testride!
This is the best part. Be careful at first, in case the frame got misaligned or one of your epoxy joints wasn't set right. Slowly increase speed and distance. Notice how smooth the ride is. Because the hextubes are made by fusing two dissimilar materials (bamboo and carbon fiber) vibration in these tubes is nearly eliminated! Enjoy!