Wherever we go on the DateTrike, our vehicle turns heads, gets smiles, and everyone wants one. I’m won’t be making any more of these, but it is no harder than any other mildly complex project, so I’ll present a few of the details here for those who are up to the challenge. If you happen to be inspired enough to build something, please post a comment – even if it is years from now.
I started by doodling with a drawing program and came up with the basic idea shown in the plans above. I was after moderately uniform weight distribution on the three wheels, and a design as compact as possible. The seat spacing is snug, which allows the overall width to be under four feet, with the passengers inside of the rear wheels. I copied the rake of the front wheel from our Sun trike. I wanted the pedals as far forward as possible, but the inside foot has to clear the frame. The front wheel needed to be a small as possible to keep the low profile. For the rear wheels I chose a pair 24″ wheels that are commonly used for garden carts because the hubs could be easily modified to work for the rear axle.
A laminated frame makes possible the graceful front section curve. Lamination also lends itself to building-in only the strength needed using less material. The main frame is made of a box-beam construction. Tight-grained Douglas fir is used for the core and 3/16″ birch plywood is used for the outer skins on the straight beams. For the nose I used 1/8″ birch plywood for the laminations.
The building process begins with cutting out the conical section needed for the inside-most nose lamination. The inside straight-frame laminations are also cut out. Eight-to-one scarf joints are planed into the plywood, and the conical nose section and flat sides of the inside ply are glued together flat on the floor. The basic frame takes shape by bending the pre-assembled inner ply and holding it in place with a couple of sections of 2×6 that space the two sides apart correctly. A second 1/8″ ply is added to the nose section, and then the core spacers are glued onto the sides. More 1/8″ ply sections are cut – but only 2″ wide sections to make up the top and bottom solid sections of the nose. Regions where extra strength requires more reinforcement are filled in with core wood. The nose-to-sides joint area is tricky and can benefit from a bit of filler that can then be planed fair. Plywood likes to only bend in one dimension, so it is critical to keep surfaces “flat” to multi-dimensional bends. I use an epoxy that has good gap filling properties for the lamination. There is lots of room to hide mistakes until the very last lamination. Then much trial and error is required to get the overall length and the scarf just right. Once you add the glue, you are committed.
The cross-piece in the back was made as strong as possible, with gussets to increase glue joint area in the corners and lag screws in the butt joints under the final skin. I was unsure how stiff the frame would end up, and always wanted strengthening options. In the rear axle section, completing the box sections with glued down top and bottom ply decks would help. For the entire frame, a fiberglass skin could be applied. However, the frame proved to have good stiffness without those additions.
Front Wheel Mount
The front wheel mount took some thought because this needs to be strong and needs to hold the front wheel head stock. I used a salvaged head stock that I cut off an old bicycle frame. I took a couple of blocks of wood, clamped them together, and bored a hole along the split between the two boards. I adjusted the hole by hand to fit the irregular shape, testing the fit frequently until the two board halves could be clamped together and trap the head stock. Once epoxied together, the block was shaped to provide the desired rake angle and carefully fit to the contour of the nose lamination Although the part is made of cedar, a relatively soft and not particularly strong wood, I used plenty of long screws and lag bolts to fully reinforce the joint. The block was fastened to the nose before the very last nose veneer was glued in place. All of the screws are hidden under this last ply layer.
The Drive Train
Each person has their own set of pedals, gears, and a drive wheel. Each axle is supported by a pair of pillow blocks. You need to use an old-fashioned “freewheel” type of gear cluster. There are a couple of custom machined parts required for this axle. I built them with the machines I have access to at work. However, the most critical item, the freewheel and break disc hub, can now be purchased from the folks at Atomic Zombie. Serious builders should browse Atomic Zombie for ideas and building suggestions. Reviewing their Kyoto Cruiser was certainly part of the design process I went through. I bought most of the components I needed new, including the freewheels, derailleurs, and disc brakes. The key to using a wood frame is to add reinforcement for the attachment points for these components. Simple blocks of aluminum, or aluminum angle extrusions can be lagged into the wooden frame and provide a very secure mount for brakes and derailleurs. Another spot that needs good strength is the crank hub installation. The threaded “bottom bracket” tube can be purchased (or salvaged). This part of the frame was furnished with a solid core and additional support ply sections were added on each side for added joint area and strength. A hole saw was used to create a fairly tight hole, and the metal part was epoxied in place with a gap filling epoxy designed for adhesion to metal. With the well-supported large-area joint, this will not fail.
Steering and Brakes
It took having the frame and drive train together and staring at the trike for a week or two to finally come up with the cable under-seat steering design. I wanted either passenger to be able to steer, and I liked the under-seat operation because it is a natural position for your hands. But how to do the linkage? Looking for inspiration, I came upon the pedal and crank from my salvage bike and realized that this could be the basis for the steering lever with the chain linkage to the steering cable. Another “bottom bracket” tube is glued into the seat support cross member to hold the steering arm bearing. The cable system works well on the trike because the framework lends itself to the cable circuit. To turn the wheel I had to make a special steering pulley that I could fasten to the front fork through the fender bolt hole. The “half pulley” was built with a groove for the cable and a clamp to fix the cable position. There was a certain amount of trial and error to get the right fit. For one-to-one steering angle motion, the pulley diameter and the crank chain diameter should be the same. Four cable pulleys were fastened to the frame as required to route the cable close to the frame and out of the way of the pedals. A turn-buckle is used to adjust the tension.
The steering is responsive, and allows a tight turning radius. On tight turns the steering lever slips under your legs. The steering tiller also has the brake lever for the dual disk rear brakes. The dual acting lever powers both brakes.
When I came to the seats, once again the project sat on hold while I considered various options, none of which were adequate. I went so far as to purchase a boat seat, but Ellen quickly nixed that idea! It was hard to beat the nice foam seat on Ellen’s single person Sun trike for comfort, so I finally decided that those seat cushions were the place to start. Then, in keeping with the wood work, I decided on simple woven seat backs, similar to classic Shaker tape-back chair weaving. I went for weather resistant polyester webbing on oak frames.
For simplicity, and because this project is mostly for Ellen and myself, I opted for just fixed seat positions. A piece of 3/4″ plywood was attached to the bottom of the seat cushions, with thread inserts so it could be bolted to the frame seat cross member from the bottom. The seat backs were attached to the plywood piece with a pair of hinges so the back angle is adjustable. Back braces are attached to the seat-back frame and fastened to the rear member of the frame. You can give a strong pedal push and securely transfer the force into the seat without concern. It took several stages of adjustment, added holes, and trial and error to get the final seat position. Making the seats adjustable is an improvement that the next builder might want to design in.
If you have read this far, perhaps you should start sharpening your plane blade and get started on building one. Working with wood epoxy construction takes on a pace that is hard to hurry. Each lamination needs to cure before the next is applied. You will become ingenious inventing clamps and jigs to hold things in place while the epoxy cures. You don’t need many special tools. The only additional tools I acquired for the project was a saber saw for cutting the conical sections from the plywood, and the correct size hole saw for mounting the bottom bracket parts. So, its time to get started. Add your creativity to the process, and let us advance the technology and comfort of the bicycle built for two.