Making a strong joint isn’t too hard, but making a good looking one is surprisingly easy. With a little extra work you can have a good looking, calfee-esk bike.
Important: For heavier riders (i.e. 170+ pounds or 77+ kilograms) I would highly recommend using carbon tow, hemp bast fiber, hemp rope or belt for your joints. The additional tensile strength of these braided materials will increase the rigidity of your joints and decrease the likelihood of breaks. One good, and tried and true option is to buy hemp bast fiber from an offshoot of Stalk Bikes. You also might look into using raw hemp bark fiber sold here, I haven’t used it or even touched it, but I might be a good material to use since it has longer strands. Although I think it would require separating it into thinner strands. Might be worth talking to someone at hemp traders first just to make sure you think its a good idea also. Or order the fiber sampler from them first.
2 ) West system epoxy – w. fast hardener (105a resin, 205a hardener)
2a) A reader pointed out that Calfee uses Entropy Super Sap CLR Resin (much more eco-friendly!)
3 ) West systems products like cups and application sticks are useful b/c they are reusable
4 ) Sander
5 ) Dremel – with tungsten cutter, tubular sanding disk (get a multi-speed one, you’ll thank me later)
6 ) Respirator w. organic vapor filter (just use it, if you start getting a headache… its brain damage)
7 ) Spring loaded rubber tipped metal clamps (shown below)
8 ) Heavy duty rubber gloves (disposables get stuck and rip easily)
9 ) Perforated roll of electrical tape (take a small drill bit and drill a bunch of holes into it while its on the cardboard roll)
Stalk Bikes Method
Well before you start reading my methods read through what is in all reality a much more comprehensive and professional description of the correct building method. This was kindly provided by Zack @ Stalk Bikes in Oakland.
Before you start, make sure that everything is as it should be. Of particular interest here is whether your rear wheel fits, how straight it is, etc. I would recommend bringing your tacked frame to a bike shop and have them use their dropout alignment tool. Keep in mind it doesn’t need to be perfect but might as well get it right….
First start by making sure to sand everything down properly. Epoxy does not adhere to the waxy outside. Remove it with 80 grit sandpaper and use some solvent too like acetone (although this stuff is nasty, use a respirator when applying or use something a bit less toxic like this). If you use acetone let it dry for 15 minute before applying epoxy. It should look like the bracing shown in the first picture on this page. Although I forgot to, you should ‘wet out’ the sanded parts with epoxy before starting to wrap the joint. This will ensure proper adhesion, see video.
Now take some hemp fiber carefully make a 1 and 1/2″ wide strip of it. Keep in mind that you want a consistent layer of this stuff, so make it even. Lightly roll it in your hands to make a very loose rope like product. Although making a tight rope seems appealing, it is not. For two reasons, first you want as much overlap between fibers to make a strong bond, and second, with a tighter rope it is difficult to get proper coverage. Wrap one layer of this (about 1/4 inch thick) where you want it and pin it with a clamp. You can also use a glue gun to pin down the end of the rope (very handy). Then mix your epoxy and apply until you think you have just saturated the hemp.
Some loose hemp fibers
(I am now the proud owner of 10 pounds of this stuff)
The metal clamp… very handy.
You can use a hitch to finish up the end of a rope or you can use it to switch the direction of your wrap.
Above I used a piece of pipe insulation to try to keep epoxy out of the treads of the bottom bracket. Not sure if it worked yet. If I don’t come back to complain, assume it worked.
NOTE: the pipe insulation worked Ok, not great. Tips for next time. Try rubbing some of your bike grease on the threads first. Then shove some pencils or something into the pipe insulation (try to over-stuff it, it will expand a bit into the threads, thus keeping the epoxy out).
It is important to not put too much epoxy (reduced tensile strength) but not too little either. I add epoxy up to the point where compression with my applicator stick pushes up a small amount of epoxy from the surface. Make sure that work it into all the nooks etc. As with many other composite materials (such as reinforced concrete), the two materials act together, each overcoming the deficits of the other. Whereas the plastic resins are strong in compressive loading and relatively weak in tensile strength, the hemp fibers are strong in tension but have no strength against compression. Try a practice one to get a feel for it. In the picture below you can see some epoxy being pushed out of the perforated electrical tape as I apply pressure.
Now to reduce the amount of epoxy and to compress the fiber, wrap the area with the perforated electrical tape (sticky side out). I try to pull as tight as I can without breaking the tape. Wipe off the excess resin as you go. After the tape is applied, I massage the area, in an attempt to work the epoxy deeper into the fibers and to remove any excess. Here’s an example. Let it set. Oddly enough the epoxy doesn’t stick to the tape too much. Take it off well after the epoxy is no longer sticky to the touch, after 2 1/2 hours depending on how hot and dry the area you are working in is (epoxy sets much quicker in high temps). It doesn’t matter if you let it set over night either. You will be able to remove it relatively easily. Just make sure to sand it before applying more epoxy.
Here is the first application. The next 2 layers I will concentrate more on the aesthetics. Add more layers progressively until you think you have added too much. Important: Try to apply layers so that fibers criss-cross as much as possible. If all of your fibers lineup in one direction, for instance horizontally, then they will provide no strength to forces pulling at them vertically. You need a matrix of fibers, not a train.
Important: Your joints should be 1/2 to 3/4″ thick and should extend down the tubes about 2 1/2″ to 3 (on all sides on tube) to make sure your joints are adequately stiff.
Additionally, don’t make any assumptions about which joints are the most important. I made this mistake when I decided that I didn’t need to build up the joint between the downtube and the headtube. I gave it one wrap, didn’t build the joint 2″ down the tube, and did minimal prep. In the end a crack formed at this joint and gradually loosened on me. Don’t make this mistake. Keep in mind my bottom bracket (likely the joint taking the most stress) and all other joints held up perfectly, so I am still a firm believer in hemp epoxy joints.
Although I have no scientific evidence, from the diagram above, the most important joints run all along the bottom of the frame (dropout to chainstay, chainstay to bottom bracket, bottom bracket to down tube, and down tube to head tube). Apparently the rigidity of the seat tube is also critical, so make sure you sink a bit of metal in as described here.
Now take your dremel and sander/sandpaper and have at it! The mini tubular sanding disks for the dremel work extraordinarily well, to get a very even surface a normal sander comes in handy too. Try them out you will be surprised. It will save about three years of your life. Get one, you wont regret it. You can shape the joint down to a nice looking calfee-esk thing in a few minutes. Depending on your desires, you might want to add a polish coat of epoxy to clean it up nice (there are also color addatives if you want to go funky, like this one). NOTE: If you build the joint out to the waxy part of your poles (the unsanded part) you can easily cut away excess resin with a razor. This allows you to make a nice clean edge between your joint and the bamboo. This sharp edge can then be shaped using your dremel sander.
Note: If you want to go with the black joint look on some of the calfee bikes I believe he uses an outer layer of thickened epoxy (404 high-density filler) with graphite additive. See below for more info. Also a reader, fox.gao from China just paints them black. It looks surprisingly good.
An example of a good sanding job and an ok mitering…
After applying all the resin I used my dremel to cut the headtube flush.
I used the reinforced cutting disk, it can cut through anything.
I used 404 high density filler to both fill in the bamboo tubes near the dropouts and to build then outwards an inch around the poles. I expect these joints to absorb a fair amount of shock and weight so Im hoping they will last.
Final step of course is to wrap it. Bottom right picture shows me apply more fiber to an area that I over-sanded.
Like your mom said “Always wear protection”
Notes & Comments
Make sure that your resin and hemp matrix does not look like this (below)
Although its not a death sentence of an sort, with a unidirectional matrix (as shown above) there is nothing to keep the epoxy holding individual fibers from tearing apart. Make sure that you criss-cross your fibers and use multiple layers.
Interpreting Calfee Techniques
Surely the best builder in the field is Calfee. Here are my thoughts. First it looks like he is using a graphite additive here from west systems to get that nice dark look. Second I interpret his use of epoxy here as a plug. You build up the bottom bracket arm so that it is level with your tubes using thickened epoxy (additives from west systems) or using your fiber/epoxy mix. This plug will take most of the force off of the bamboo and put it directly onto the metal arms. Then its just a matter of building it up some more and then sanding the hell out of it until you have a pretty looking thing. From the picture below we can see that you need to think about the shortcomings of bamboo also. Since he is using a disk brake here he knows that additional force will be placed on the chain stay on that side. So he builds out some extra support for it.