The frame truss tube installed.
Imagine compressing an empty picture frame by two diagonal comers. It would of course, fold with very little effort. If we added another piece between those two corners, it would not bend at all since it forms a triangle, which is an extremely strong shape.
To make our trike frame strong, we will add this extra tube (truss) between the two corners of our square. The truss tube will run between the joint at the front seat tube and main tube to the joint at the rear seat tube and boom. This tube can be lighter than the main tube (I used ¾-inch conduit), but any tube that you have will also work fine. Remember to add a little extra to the measured distance to compensate for fish mouth cuts before welding.
Installing the synchronizing chain.
Now that your frame is complete, you only need to add the components and you will soon be able to try out your new machine. Begin by adding the two crank sets to the trike. Although both crank sets do not have to be identical, there are certain requirements that have to be met for everything to work properly.
First, the smallest chain ring on each crank set needs to have the same number of teeth. Since these are the chain rings that will join the captain’s crank set to the stoker’s crank set, they must have the same number of teeth in order to keep the pedals in sync. If one of the chain rings had even one extra tooth, eventually the pedals would fall out of sync, and a collision of feet would occur.
Second, at least one of crank sets must have two chain rings. The rear set needs to have two chain rings since it will deliver power to the rear wheel.
The dual chain ring at the back is necessary because there are two separate chains. The front chain transfers power from the captain’s crank set to the stoker’s crank set and keeps both cranks in sync. The rear chain then transfers all the power from both crank sets to the back wheel. Even a long chain poses a problem.
One chain would only catch a few teeth on the top and bottom of the stoker’s chain ring as it passed by. The chain will jump right off the chain ring if any pressure was applied to the stoker’s pedals because a chain needs to cover at least half of a sprocket in order to function properly. I did try it once, but don’t bother — it doesn’t work!
Now that you understand what we are trying to achieve with the chain, make a chain long enough to ride on both smaller chain rings. This chain shouldn’t be so tight that it can’t be put on by hand without force. The chain should also not be any longer than the minimum length that will fit. In the end, you will have a chain that has a little slack, as it will be slightly too long, as shown here.
The return chain pickup system.
Once your connecting chain is in place, you need a way to pick up the slack to stop the chain from falling off. If you spin one of the cranks right now, it won’t take long for the chain to bounce of one of the chain rings, and this would not make for fun riding. Chain slack is not a new problem, nor is it a hard problem to fix. Look at any bike with more than one speed and you will see a device designed to deal with that problem — the rear derailleur. The two small plastic wheels and spring mechanism on the rear derailleur do just that by pulling at the return chain in to keep it tight at all times.
There is no reason to re-invent the wheel, so we will just use an existing rear derailleur to pick up the chain slack. The derailleur is welded to the boom so that the two plastic wheels are placed in line with the chain and in the center of the boom. Notice how the front side of the derailleur (the side that you normally see when looking at the gear side of a bike) is facing the opposite way in our configuration.
To get the chain into the derailleur, remove the bolt that holds one of the plastic wheels on, and put the chain in the middle of the two. Once the derailleur is welded to the frame, you can turn one of the two small adjusting screws in order to further tweak its alignment between the two chain rings.
After welding the derailleur in place, you should be able to spin either crank set wildly without the chain hopping off either chain ring. If there is any rubbing between the chain and the metal frame of the derailleur, try adjusting the position screws. If this fails, you may need to remove the derailleur and try welding it in a different position. When everything is set up properly, there will be very little friction and noise, and no rubbing of the chain against any part of the derailleur.
The final drive chain installed.
The rear chain is no mystery. In fact, it is no different than on an ordinary bike. The chain will be placed on the larger rear chain ring, then through a standard rear derailleur to the back wheel, as shown here. If your rear crank set has the chain rings really close together, the connecting chain and rear chain my rub together. This can be solved in one of two ways.
First, most crank sets have three chain rings — a really small one (the one we choose to connect stoker to captain) and two larger ones. If your crank set has three rings, use the smallest and largest, leaving one empty in the middle. This will allow plenty of distance between both chains. The second option is to take apart the chain rings by removing the bolts or rivets, and add a washer in between both chain rings to keep both chains from rubbing; however, the first method is much easier.
The only other concern with the rear derailleur is cable length. Now that your trike is 2 feet longer than a normal bike, a standard shifter cable will be too short. You can either go to your local bike shop and have one made, or mount the shifter at the rear so the stoker can operate it. The first solution is the better one since you may want to ride solo, and reaching back to shift is not cool.
Parts to make the Stoker’s handlebars.
You have almost a fully functional two headed, all-weather tandem mountain trike now, and all you need to do is throw on the rear seat and go for a test ride. Of course, your stoker has nothing to hold on to, so we need to make some type of rear handlebar. You could cut a pair of handlebars in half and weld them right to the front seat post, but this would not allow for any adjustment. A simple but effective solution is to cut the end of an old gooseneck, and then weld it to the front seat post. This will allow you to change different handlebars, as well as front and back adjusting.
When choosing a pair of stoker handlebars, choose a wide and tall set similar to ape hangers, or BMX style (with the crossbar cut out), as the captain will have to sit in between them. If your stoker bars are to low, the stoker will have to bend over to reach, or they may interfere with the knees while pedaling. If they are too narrow, the captain will not be able to sit on the seat because the bars will be in the way. The easiest way to find a good set is to weld the neck piece to the seat post, sit on the bike and try them out until you find the perfect set.