DIY DETACHABLE TANDEM TRAILER
Tandem cycling is great fun for big kids and adults, but if the smaller ones want to come a long for a ride they will not be able to reach the pedals due to the size of the frame on an adult tandem bike. You could have the kids tag along on their own pint-sized bicycles, but this also may be a problem because of their lack of speed or ability to make the long journey.
The detachable Tandem solves the problem for the smaller bicycle enthusiast in the family by allowing them to "tag" along, yet still participate by doing some of the work. This simple "half-bike" attaches to a special X/Y joint on the main bicycle's seat post allowing the unit to move up and down, left and right, but not allowing the unit to tilt side-to-side and fall over. Because of this, the rider on the detachable can feel like he or she is riding a regular bicycle, yet there is stability.
With a 20 inch wheel on the rear of the Detachable Tandem, it would be quite easy for a kid to pull his or her own weight as part of the riding team, although you can't expect the young ones to keep up the pace when you are blasting around at 30 km/hr. If you do happen to "out run" the usable gear range of the Detachable, the rider only has to stop pedaling and glide until the speed returns to normal, since the pedals are fully independent of yours.
This Detachable Tandem can be built in a day or two from common parts that you likely already have in your "scrap pile". The main part of the project will require a kid's bicycle frame and rear wheel that fits the frame like the one shown here. There are many different sizes of kid's bikes with wheels from 12 inches to 24 inches, but for best results use a wheel no smaller than 20 inches. If you choose a wheel smaller than 20 inches for the Detachable Tandem, then the child riding it would never be able to contribute any pedaling power due to the incredible speed he or she would have to rotate the cranks in order to match your pedaling speed.
Once you have your donor frame and rear wheel, put it away for now because we will first create the most important part of this project, the swivel joint. This swivel is the key to this project, and has to allow the detachable to move up and down as well as left and right but not sideways. This may seem a little confusing at first, but think of the reasons behind this design. If the Detachable could not move up and down, it would either be lifted off the ground if you rode over a large steep hill or snap right off at the joint. You could not have all three wheels on the ground at the same time unless one of them could move up and down to conform to the slope in the grade. The Detachable also has to move left and right so the main bike can steer. This system works the same way as a truck pulling a trailer, no mystery there.
Ok, now you may be wondering - why not just use a ball joint or something similar to a trailer hitch then? Well, at first this would seem logical because it would allow any type of motion at the joint including up, down, left and right. The problem is that it would also allow the Detachable to move side-to-side, or roll which would allow it to fall over, even if the main bike was standing straight. Since the Detachable has no front wheel or steering, it cannot balance independently of the main bike and would fall over and be dragged along. Imagine a truck pulling a trailer with only one wheel - not good.
Now that you understand the purpose and mechanics behind the XY swivel joint, let's start making it work. You will need a decent front hub from any sized wheel. This can be removed the nice way by undoing all the spokes, or the fast way by cutting them all away using a set of heavy duty wire cutters or even your zip disc. This hub will form the part of the swivel joint that allows the Detachable to move up and down. Since there will be parts welded to this hub, make sure it is made of steel, not aluminum.
The other two pieces that make up the swivel joint can be cut from a short piece of 1-inch thin walled electrical conduit or similar diameter tubing such as bicycle frame tubing. You will need two pieces that are two inches in length. The tube chosen must be able to fit over the seat post on the bike you plan to pull the Detachable with. The tube should be fairly snug but not so tight that is has to be forced on. It should be tight enough that there is not a lot of slop or gap as well. One inch electrical conduit fits just right over a seat post, but if you do not have any there are many other types of tubing including the seat tubing cut from an old frame that would also fit.
Grind a groove (fishmouth) out of the end of one of the pieces of tubing so that it can be welded to the shaft between the two hub flanges. The groove should be about as deep as half the diameter of the hub shaft as shown here so the two can be welded together easily. When you are welding the two parts together, watch your heat and take care not to burn a hole into the hub or you may end up with a seized axel. Also, this tube should be connected so that it forms a 90-degree angle with the hub shaft.
Once the tube is welded to the hub, you can then groove out the other end of the tube so the second piece of tube can be welded to it at 90 degrees. Since this is the tube that must slide overtop of the main bike's set post, take care not to burn a hole through it or warp it badly from too much welding heat. If you do burn a hole through this tube, you will have to use a hand file to remove any imperfections on the inside.
When this tube is mounted around the main bike's seat post it forms the joint that allows the Detachable to move left and right so the two bikes can steer like a truck and trailer. It is important that this tube is welded into position so that it is 90 degrees from the hub. The hub will form the pivot point for up and down, and the seat post tube will form the pivot point for left and right. If the two joints are not welded at 90 degrees from each other, the Detachable will end up leaning to one side or the other.
Once the swivel joint has been welded together, place it into the dropouts of a pair of front forks that will be used to create the "hitch" for the Detachable Tandem. Any set of forks will fork fine as long as the hub will fit into the dropouts. I used a very small pair of forks from a kid's bike with a 12-inch wheel in my design, but any length will do the job. You should also use a set of "hooked" washers that hold the axle to the dropouts to stop your hub from falling out of the dropouts if the bolts become loose. If your hub comes loose from the forks, your Detachable Tandem will be let go and end up in a nose dive, so make sure this never happens. You may even want to tack weld the nuts right to the dropouts since there will be no need to remove the hub once the project is completed.
The mating seat post must be raised at least as high as the length of the tube that is placed in it so that the seat can be placed back on the tapered end if the seat post. If you built your swivel and hitch correctly, you should be able to move it in any direction left and right as well as up and down but not be able to twist it. Now you can see the true magic in this design; it allows the Detachable Tandem to be pulled along like a trailer yet not fall over no matter what the passenger does. Of course, if the passenger has a lot of energy to burn, he or she may actually push you along for the ride!
If there is excess slop between the seat post and tube, or you are worried about scratching the seat post, then you may want to place a small "bearing" surface in between the two. For this job, a small piece of cardboard or thin copper foil would work nicely. If there is not enough room for anything to fill the gap, then just add a little oil or grease to the joint before you ride. There will be so little movement here, that this will most likely be unnecessary.
To begin work on the actual Detachable Tandem frame, start by cutting off the top tube and town tube ahead of the seat tube as shown here. You can grind off the stubs left over on the seat tube after they are cut from the frame. The goal now will be to mount a length of tube from the joint where the top tube used to connect with the seat tube to the forks used as the hitch. The length of this tube is determined by how far back from the main bike's wheel the cranks on the Detachable Tandem will need to be. Since a typical crank arm is about seven inches long, any distance between 10-12 inches from the back of the tire to the center of the bottom bracket on the Detachable will be safe.
I set my freshly chopped frame up on a block so there would be adequate clearance not only between the cranks and the rear wheel of the main bike, but also the cranks and the ground. Once this placement was determined, the measurement was taken from the top of the seat tube on the Detachable to the top of the forks that make up the hitch. The tube was then cut to this length and placed between the two pieces to make sure it would not rub on the top of the main bike's rear tire.
The tube that makes up the main hitch is made from a length of 1.5-inch muffler pipe or similar sized tubing. You could probably get away with 1-inch electrical conduit, but this would be pushing the limit for a rider over 100 pounds. If you cannot find any 1.5-inch round tubing, a square tube could also be used; this is common material.
When you are satisfied with the placement and length for the main hitch tube, weld it directly to the hitch forks as shown here. Weld the two parts so they are aligned perfectly as if the new tube was just an extension of the original fork stem. Since the original fork stem is not used and would only end up hidden inside the hitch tube, it can be removed and added to your parts bin for later use on some other project.
Your frame is almost done now. To give the Detachable Tandem strength, one of the spare tubes cut from the front end of the frame is added so that it forms a triangle between the bottom bracket, seat tube and hitch tube. The length of this tube is not critical as long as it forms a triangle, so any length between 12 and 18 inches would be just fine. This new tube is cut and grooved out to fit into its new place on the frame. The tube will be welded to the place where the original down tube connected to the bottom bracket and it will extend upwards to the main hitch tube.
Your co-pilot will need a place to hang on, so a steel bicycle gooseneck is cut short, grooved and then welded to the top of the hitch as shown here. The best place to weld the gooseneck is at approximately the same distance from the seat tube that it was originally on the bike, this way the passenger will feel right at home. In my design, this distance was right over top of the extra tube added for strength and this made for a nice looking final design.
Now that all the welding is complete, you can add the accessories - seat, handlebars and crank set. Your final project should look like the one shown here, with adequate clearance between the hitch tube and main bike's rear tire. There is nothing special about any of the accessories, since the back end of the Detachable is pretty much in its original working state. The one modification I did make was to add a larger chain ring (48 teeth) the Detachable Tandem so the rider would be able to keep up with the main bike.
The chain ring that originally came with the kids bike had so few teeth that the rider would have to pedal like crazy just to keep up to the slow pace of the main bike. Kid's bikes are not really made for any real speed, but since this one is being pulled along in a safe manner, it is ok to increase the gear ratio to something more usable for distance riding.
After giving the completed frame a final check and cleanup, it was primed and painted. The final product looks as good as any bicycle store version and cost virtually nothing but a few hours of garage time.
Riding a bicycle with the Detachable Tandem connected is very easy, in fact, you can barely tell it's even there! The same rules should be followed as those for a stoker (passenger) on a regular tandem. Wait for the captain (driver) to get on the bike before the Detachable is mounted, and wait for the captain to give the signal to all passengers that it is safe to dismount. Although a kid's weight is not enough to topple you and your bike if they climbed aboard before you where ready, it doesn't hurt to have control of your ship!
The Detachable Tandem not only hooks up to a regular bike, but can also be connected to another Tandem as shown here, making a cycle train. Now we have a two-and-a-half horsepower vehicle!
This little add-on unit works great, and is a lot of fun for the kids since they can now feel like part of the team since they are doing some of the work as you ride. There is also no worry that they may wander off the trail into danger since they are only able to go where you go. Of course, the kids may be worried about where you plan to go!
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