EASY TO BUILD DIY MOUNTAIN BIKE TANDEM
If you have a pair of similar mountain bike frames lying around your scrap pile, then why not whip up a simple mountain bike tandem? You can complete this project in an afternoon using a single mountain bike for parts, along with a second frame of similar or equal size. If you can find a pair of identical parts bikes, then the geometry will look perfect and all of the transmission components will match, but this is not absolutely essential as any pair of similar frames can be welded together. You will need two frames, two sets of cranks, a front and rear wheel, front forks, two seats, and two pairs of handlebars for this project.
The plan is very simple - cut join the front triangle from the first frame to the rear triangle of the second frame using the leftover frame tubes as shown in this photo. If the leftover frame tubing is of differing diameter, then use the larger tube to join the lower part of the frames between the bottom brackets, and use the smaller diameter tube to join the top part of the frames between the seat tube stems. Cut your frames and lay out the tubing so you can see how your final frame will look.
Once you have worked out your frame tubing, the top and bottom joining booms will need to be fish mouthed at each end in order to make a joint that can be welded properly. The top boom tube will have to conform to the seat tubes on each frame section and the bottom boom tube will conform to the two bottom bracket shells. Both seat tubes on each frame should end up parallel to each other; this is how you will know that your frame geometry is correct. The distance between the two seat tubes will be about the same as the length of the front frame top tube (the tube between the head tube and seat tube).
Weld your frame together, making sure that the alignment is correct as viewed along the length of the frame. Weld all the way around the joints, being careful not to strike the inside threads on the bottom brackets. You can leave the bottom bracket cups screwed in while you weld in order to protect them from accidental strike and welding spatter. Your tandem frame is almost complete.
If you look at the geometry of a bicycle frame, then you will see that it is made up of two triangles - the front and the rear. Triangles are the strongest possible shapes you can make from tubing, so we will triangulate the tandem frame by adding a truss diagonally across the two sections. You can use any 1 inch or similar diameter tubing to make the truss, even scrap bicycle frame tubing or electrical conduit. Cut the tube to form a triangle between the two frame sections starting at the rear bottom bracket joint and extending up to the front frame seat tube joint. This tube also needs to be fish mouthed in order to form a decent joint for welding.
This photo shows the truss tube installed into the frame in order to split it into a pair of triangles, making it ultra strong. Also shown is a chain tensioner made by welding a scrap rear derailleur to the center of the lower boom. This chain tensioner will pick up the slack in the return chain, stopping the chain from falling off the chain rings when the bike travels over rough terrain. The reason this tensioner is needed is because it will be almost impossible to join together a chain that is perfectly tight between each chain ring since the bottom brackets are now in a fixed position.
The front handlebars (captain's handlebars) are easy since you just install them as they were on the original bicycle, but the rear handler bars (stoker's handlebars) need to be affixed to the front seat post as shown here. An easy method of installing the stoker's handlebars is to cut off a steel gooseneck and weld the clamp section to the captain's seat post. This configuration allows the stoker to adjust the handlebars up and down for optimal comfort.
The last thing to do before you can test ride the tandem is to install the brakes and transmission. The rear chain is easy since it is the same as it was on the original bicycle, running from the larger rear chain ring through the rear derailleur system. The joining chain is set on the smaller chain ring on both crank sets so that they are linked in sync, having the crank arms at the same angle to each other. Cut the smallest possible chain that will fit on the chain rings and then join it with a chain link tool (see our main page tutorials on how to join a chain). The derailleur that is being used as a tensioner can now be pulled under the chain to pick up the slack.
Once you have taken your newly completed tandem mountain bike out for a test run to ensure that everything works properly, you can take it all apart for painting. One can of spray primer and one can of paint will be enough to get the job done, following the instructions on the cans. Remember to allow the primer a day to cure and the paint two days to cure before assembly or you may scratch the pain. Add your dual brakes and cables, as well as the rear shifter and your new mountain tandem is ready to ride.
Riding a tandem bike is almost the same as a regular upright bicycle, the only difference being a slightly wider turning circle and the weight of the stoker behind you. Instruct your stoker to simply act naturally, and not try to lean accessibly around corners or he/she may force you to counter steer. Since both cranks are linked and in sync, the captain can decide when to start or stop pedaling, taking control over both the transmission and steering.
Have fun on your new tandem bike! You will soon find out that you can really get some serious speed on a tandem since you have two engines powering one lightweight cycle, and the wind resistance of only a single bike. If you want to increase your top speed even more or tackle some serious hills, try adding a front hubmotor, which will multiply your power by four under clean and silent electric power.
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