DIY SHORT WHEELBASE RECUMBENT BIKE TUTORIAL
Here is a very simple 20 inch wheel short wheelbase recumbent bike that you can build in a few hours using nothing more than a kid's bike and a few lengths of round tubing. This project makes a good starter recumbent and is a good experimenter's platform for those who want to try out various seating positions and angles. The completed bike doesn't weigh much more than the bike used as parts, and can be stored in the same space as a regular bicycle. Of course, this simple project lacks many of the more advanced features offered in our DIY Plans, but you could always add your own modifications to expand on this project.
You can build a 20 inch wheel short wheelbase recumbent from practically any kid's bike or BMX bike. You will need to start with a working cycle or at least one that has functioning wheels, cranks, and a transmission system. The rear wheel should have a multi-speed freewheel so that you can have multiple gears, but you could also build the SWB around a coaster hub and just make your bike single speed. If you want to climb a hill or reach any decent top speed, then you will need a rear wheel that has at least a 5 speed freehub.
Besides a working 20 inch bike, you will also need a few feet of round tubing. You can cut up another bike frame for the tubing, or just use some thin walled electrical conduit (EMT) for this project. Basically any square or round tubing with an approximate 1.25 inch diameter and a wall thickness similar to bicycle frame tubing will work. The other part shown in the photo is the bottom bracket and crankset that will be placed ahead of the front wheel. You can cut the bottom bracket from another frame since the original bottom bracket will be unused and left on the frame. For more information on bottom bracket sizes and assembly, see the tutorials on our main page.
This simple recumbent bike is made to fit the rider, so it has no adjustable seat or bottom bracket. For this reason, we will make the seat first so that you can later use your body as a measuring system in order to determine the optimal placement of the cranks for your leg length. Chop off the top of the seat tube as shown in the photo and then find another 12 inch long tube that can be welded over the top of the cut-off seat tube. This tube will become your seat back tube, and it will support the back of the seat as well as set the recumbent angle of your seat.
The angle of the seat is completely up to you. Anything from 25 degrees to 35 degrees (taken from the horizontal ground) will work. If you lean your seat back too much, then you will have to hold up your head, and if you set your seat to far forward, your knees will have to bend a lot while pedaling. Try a 30 degree angle and make only tack welds on the seat support tubing until you can sit on the frame and see if you like the angle. On a project like this, it is easy to make adjustments, and since your frame parts will differ from the ones shown here, you will need to experiment.
To support the seat, the 12 inch long seat tube is welded to the top of the cut-off seat tube and then braces with any small tubing such as a seat of seat stays cut from another old frame. When you are testing your seat angle, you can get away without the bracing as long as you don't put any hard pressure against the back of the seat. For riding though, you will definitely need the seat tube bracing.
The seat will be held to the frame by a set of tabs that have holes drilled for woodscrews. Any 1 inch wide 1/8 inch or similar flatbar can be used to make the seat mounting tabs, and they are made 6 inches long so that there is about 3 inches of material on each side of the frame tubing to mount the seat. Weld the seat mounting tabs to the frame so that the woodscrews will fasten each part of the seat in the approximate center of each board.
The seat mounting tabs shown here are welded to the frame so that each seat board will be held to the frame by a set of woodscrews. If you are not yet sure of your final seat angle, then leave out the seat back support tubing until you have had a chance to sit on the frame and see if your seat angle is comfortable. You could also make an adjustable seat back by hinging the seat back support tube and installing some kind of clamp that will allow the seat back support tubing to be moved along the frame and then locked into position.
The seat is made of 3/4 inch thick plywood and foam as shown here. Feel free to make your seat any width and height you like, but for a reference, I like to make my seat base 10 inches wide at the back, 8 inches wide at the front, and 10 inches in length. I usually make the seat back 10 inches wide at the back, 8 inches wide at the top, and 12 inches in length. To further enhance the seat, another top section can be added to support your upper back, but for now, this basic seat will get you started.
For seat padding, I used a firm 1.5 inch thick foam that has been cut to fit onto the plywood seat boards. You will need to install your seat padding before moving on with the frame design.
Fasten the two seat boards together where the meet using a pair of bent L-shaped shelf brackets so that your seat becomes ones ingle assembly. You can then glue the foam to the plywood using some spray adhesive. The shelf brackets will be strong enough to hold your seat boards together yet still allow some give if you want to force the seat into another angle later as you make modifications to your bike. Once your seat is made, fasten it to the frame by using the four woodscrews through the seat mounting tabs installed on the frame.
The main difference between this bike and a regular upright bicycle is the fact that the cranks are placed ahead of the front wheel rather than between the wheels. This recumbent position lets you push your full energy into the cranks and offers both a comfort and aerodynamic advantage over a regular bicycle seating position. To get your cranks in front of the frame, you will need a set of tubes to create a triangulated boom and a bottom bracket to weld to the end of these boom tubes. Unfortunately, you can't easily remove the unused bottom bracket from your frame and transplant it here since it is holding all of your frame tubes together, so you will have to salvage one from another frame.
If you have to choose between a bottom bracket for a single piece cranks set and a 3-piece crankset, then choose the threaded bottom bracket for a 3-piece crankset as these will offer the most selection when choosing a crank set (see out crank and bottom bracket tutorials on the main page for more information). Clean up your bottom bracket shell with a grinder and then make a fishmouth cut on one of your boom tubes to conform to the bottom bracket as shown here. Before cutting the boom tube length, read ahead to see how the length of the boom tubing will be determined.
The goal is to position the cranks over the front wheel in such a way so there is no pedal or crank arm interference with the front wheel as it turns. If the crank arm were to hit the front wheel, the steering would be locked up, resulting in a loss of control. So basically, you want to position the bottom bracket as far ahead and as low as possible while at the same time ensuring that the cranks do not interfere with the front wheel and so that they are placed the optimal distance from the seat for your leg length. To find the best bottom bracket position, sit on your seat (with shoes on), and extend your leg while a helper holds the crank arm with pedal installed. Your helper can then take a measurement from the head tube to the bottom bracket or crank center so you know the correct distance from the head tube to install the bottom bracket.
The lower front boom tube can then be cut and tack welded to the head tube as shown in the photo so that you can test the crank position for both your leg length and clearance over the front tire. You may need to make fine adjustments to meet both requirements, so take your time and get it right.
Once you have the lower front boom tube solidly tack welded in the correct position, you can measure the distance from the bottom bracket joint to the top of the head tube and cut the upper boom tube, that will form a triangle between the bottom bracket and head tube. This tube is also fishmouthed to fit the bottom bracket joint and the head tube to create a good joint for welding. The resulting triangulated front boom will be extremely strong, able to withstand any amount of pedaling forces.
With the front boom installed, you can now install the cranks and rear derailleur to make up the long chain needed. You will need a chain about twice as long as a normal bicycle chain, but this is no problem, since you can simply join together two chains to make a new one. For information on chain joining and sizing, see the tutorial on our main page. When sizing the new chain, set it on the largest front chain ring and on the middle rear chain ring and then make it long enough to pull the derailleur into the position shown in the photo, with the top idler wheel just over top of the lower idler wheel. You will also notice that the chain is in the way of the front wheel, but this will be fixed with the installation of the idler pulley.
An idler pulley designed for a 1/2 inch v-belt can be used to reroute the lower side of the chain over the front wheel as shown in the photo. There is never any tension on the lower (return side) of the chain, so it can be routed behind the top (drive side) of the chain, out of the way of the front wheel. Place the idler pulley on the lower front boom tube so that the lower chain will clear the front wheel and not rub on the top side of the chain. Any steel or nylon 2.5 inch to 5 inch diameter idler pulley that includes a ball bearing will work. You can purchase a suitable idler pulley at many yard and garden supply stores.
Once you have found the optimal place for your return chain guide pulley, weld the bolt to the boom tube as shown in the photo. Also shown is the completed welding on all front boom tubing joints.
Complete any leftover welding, and then clean up the welded areas. Your short wheelbase frame is now completed, and only needs a steering system to be installed for test riding.
You will need to create a long gooseneck that will allow your handlebars to be placed in a comfortable position that will allow room for your knees to pass underneath. Handle bar position and style are a matter of personal preference, but you will definitely need to extend a regular bicycle gooseneck to gain the height necessary to clear your knees. Find a set of handlebars you like and then sit on the bike, turning the cranks around while a helper takes a measurement from the handlebar center to the top of the head tube for you. Using this measurement, you can create the extended gooseneck, using some bicycle frame tubing or electrical conduit as shown in the photo. Cut the clamp section from the original steel gooseneck and then insert the new tube between the base and the clamp to create a new extended gooseneck. Remember to also consider the angle of the extension tube in order to position your handlebars in the most comfortable position. You may need to tack weld and test, making slight adjustments in order to get the angle just the way you want it.
Once you have tack welded your extended gooseneck together and tested it on the bike, complete all the welding, making sure to weld the entire joint on both the top and bottom. Clean up and inspect your welding to ensure that there are no gaps - you don't want your steering system to come apart during a ride!
Now that all of your frame and steering parts have been completely welded, you can prime and paint your new bike. I always use department store spray paint to complete a project, and if you take your time and follow the directions, you can achieve a very good quality paint job for only a few dollars. Primer is first added and let to cure for a day or two so that there is a good base coat for the paint to adhere to. Only a light sanding is necessary on bare or painted metal, just to remove any welding spatter or edges of old paint that remain. It is not necessary to spend days scraping every last bit of paint from the parts to be painted. Once the primer has had a day or two to cure, apply the spray paint, and again allow a day or two of curing before assembling the bike.
A front and rear caliper brake were added, using the original parts from the donor bike. For more information on installation and adjustment of brakes and shifters, see the tutorial on our main page. Install all cables along the frame and avoid any tight bends so that the levers and shifters work with minimal friction. Brakes and derailleurs need to be aligned for optimal use, especially when taking them from one bike and putting them on another.
Besides the laid back seating position, riding a short wheelbase recumbent bike is virtually the same as any other bicycle. Get onto the seat, place your best foot on the top of the cranks and then launch. If this is your first recumbent bike with pedals placed ahead of the front wheel then it may take some time to get used to the fact that your cranks stay pointing forward during a turn. You may have the "wobbles" for the first few launches and corners, but after a few hours the bike will feel completely natural.
Recumbent bicycles offer an aerodynamic advantage over upright cycles due to the reduced frontal area hitting the wind. You also have a massive strength advantage as you can now deliver the full power of your legs directly into the cranks as your back is pushed into the seat. On an upright cycle, you can only deliver as much force as you weigh, and any more will just lift you off the seat. Having the cranks so high from the ground also means that you can blast into corners without slowing down or worrying about where your pedals are. The only limitation on the corners will be your tire traction and courage to take them at full speed.
Now that you have been bitten by the recumbent building bug, why not try a more advanced project, or try your own modifications to this one? Add an adjustable seat, a sliding bottom bracket, electric assist motor, or even a front fairing to cheat the wind. The possibilities are only limited to how much time you have to spend at your workbench!
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