KIDS TADPOLE TRIKE DIY TUTORIAL
This easy to build mini tadpole trike is a nice weekend project that does not require any machined parts or wheel building, just a pair of kid's bikes and a few extra bits and pieces from your scrap pile. The Kid's Tadpole Trike has two front wheels, but uses the standard front forks and head tubes so you do not have to worry about axle strength or custom brake parts. You can build this little trike using any size wheels you like, although I am not sure it would be well suited for an adult, so wheels of 20 inches in diameter or less would be most appropriate.
Let's start with what you will need in order to build the Kid's Tadpole Trike: two identically sized front wheels, forks, head tubes, and a similar sized frame for the rear that includes a crank set and rear wheel. You do not have to match the front wheels and rear wheel, but it does make for a better looking final product. the photo above shows the three identical 14 inch kids bike frames I found at the city dump scrap pile. These department store starter bikes are so common at the dump and garage sales that I probably have at least 10 more like them in my huge scrap pile. Again, only the fronts of the bikes need to match, as the trike is basically a bicycle with "two heads" as you will soon see.
Remove the front fork hardware from the two matched frames so you can ensure that all parts are in good working order, and that the forks are very close to the same size and shape. If they are identical, then great, but you could probably improvise if one of the fork stems is a bit longer than the other. The goal is to have the front of the trike balanced and symmetrical, so hunt around for the best matching set of forks and front wheels that you can scrounge. This photo shows my two front forks, which just happen to be identical minus the color difference.
You will also need a crank set, bottom bracket and all the included hardware as shown here. Clean all old grease and rust from the bearings to ensure that there is no damage, and replace any excessively worn parts.
Since the Kid's Tadpole Trike has two front ends, you will need to cut the head tube and down tube from both matching frames as shown here. The top tube is cut right at the head tube joint, and the down tube is cut right at the bottom bracket joint so you have the most material to work with. When making the first cut, be careful if using a grinder, as the frame may tend to collapse on the disc and pull the grinder from your hands. If you want to play it safe, cut the last little bit using a hacksaw to release any tension in the frame. Make the same cuts to both frames.
This photo shows the result of butchering both frames - a pair of matched head tubes with an included length of down tubing attached. Again, both parts should be almost identical so the angles on each side of the trike are the same when completed. You could cut one head tube to adjust it, but that is probably more work than simply sourcing the two identical donor frames. Grind away any excess material at the joint of the original top tube as well, filling the small holes in the head tube if you feel the need.
The two head tubes will be joined together so that they are perfectly parallel to each other as shown here. A length of angle iron is being used as a guide to ensure that the head tubes are perfectly aligned and that the joint at the ends of the down tubes fit together with minimal gap. Don't worry about the width (track) of this trike; just take as little material away from each end of the down tubes as necessary, and the trike will be plenty wide enough for a safe ride.
Using a piece of angle iron or a flat surface as a guide, position both head tubes so they are parallel and tack weld your down tube joints as shown in the top half of this photo. With the parts tack welded, do a visual inspection of the head tube alignment from all angles and then complete the entire weld as shown in the lower half of this photo. You can also tack weld the head tubes to your guide metal to ensure that they remain aligned as you complete the welding.
Here, you can see how the rear of the frame must be cut, removing the head tube at the end of the down tube and the entire top tube. You can also clean up any excess top tube material from the seat tube joint as well at this point.
Since the end of the down tube on your rear frame must mate with the joint between the front parts of the trike, you will need to cut the appropriate fishmouth at the end of this tube as shown here. This process is a simple trial and error procedure, so take your time and rough it out with a grinder disc, finishing up with a round file. Take only as much material away as needed, or you will shorten the wheelbase of the trike. The next photo shows how the front and rear of the frame come together.
As you can see here, the Kid's Tadpole Trike is nothing more than a standard kid's bike with two heads instead of one. All angles and clearances should remain the same, so you don't have to worry about head tube angles and bottom bracket clearances. Inflate all tires, and prop up the parts as shown here, so you can check the fishmouth cut made between the rear of the frame and the front. It is also a good idea to make the tack welds as the trike is sitting like this, to ensure that the angles are correct.
A few tack welds made as the parts are propped up should allow you to safely manipulate the parts if slight adjustments are needed. Check the trike from all angles to make sure the parts are in the correct place, making sure that the angle between the front and rear tubing is at 90 degrees. What about head tube angles? Well, if you look directly at the side of the trike, it should look like a regular bicycle, with the head tube angle the way it was on the original bicycle. Of course, don't get to fussy about this angle, just make your best guess, or look at any other bicycle as a guide.
Once you are happy with the alignment between the front and rear of the frame, weld the entire joint right around the fishmouth cut. At this stage, the frame is not ready for any weight, so do not let the eager young pilot jump on the frame as it may bend the down tube where it joins with the bottom bracket. The frame needs a few more small tubes in order to form a triangle, the strongest of all structural shapes. By hacking up the leftover frame tubing, you will be able to make the two tubes needed to form a triangle between the front and rear of the frame. This photo shows the seat stay tubing cut from the rear of the two frames that were used to make the front of the trike. Sure, you could use any scrap tubing with a diameter of 3/4 inch, but recycling is the way to go.
There should be just enough tubing in those seat stays if you join them end to end to make two longer tubes as shown here. The top half of the photo shows how the tubes were welded together, and the bottom half shows the completed tubing after cleaning up the joints using a flap (sanding) disc.
The two tubes made from the recycled seat stays will create a triangle in your trike frame as shown in this photo, giving it tremendous strength. I will refer to these tubes as "side rails", since the seat will be placed between them. Cut the side rails to whatever length you need to make them run from the top of each head tube to the joint at the seat tube at the rear of the trike. Once welded, you frame can now take its full weight safely.
Although an adjustable bottom bracket is optional, it does make fitting your rider to the trike a heck of a lot easier, and since kids grow like weeds, you won't have to cut and re-weld the frame every time they grow an extra inch. A simple adjustable bottom bracket can be made using a section of seat tube, a seat post or similar diameter tube, and the bottom bracket from one of the frame leftovers. This photo shows the parts you will need in order to create a sliding boom adjustable bottom bracket.
The idea is very simple. The bottom bracket will be welded to the tube that fits into the seat tube and the original seat post clamp will lock it in place. You will need about 12 inches of seat tube, an 8 inch length of seat post or same diameter tubing, and the bottom bracket you plan to use for your trike. A fishmouth cut needs to be made in the end of the tube that will slide into the seat tube and then it is welded to the bottom bracket.
This photo shows the completed adjustable bottom bracket after welding the seat post tubing to the bottom bracket. Now, the bottom bracket can be adjusted either way to about 6 inches, to accommodate different rides, or small growth spurts. This assembly will now be called the "bottom bracket boom tube".
The bottom bracket boom tube will be welded to the front of the trike frame as shown in here, directly ahead of the rear frame joint and at an angle roughly parallel to the ground. Since chain routing is also a consideration, it might be best to read ahead and understand how the boom angle will affect chain routing.
The chain needs to pass under the front frame tubing on its way to the rear wheel, so some type of guide pulley will be needed to do this job. Any 1/2 V-belt idler pulley with a center bearing will work for this purpose, as well as a grooved out skate blade wheel, or any other idler you might find laying around your shop. A center bearing is a good idea, as a simple brass bushing may introduce extra friction into the transmission, robbing your young pilot of valuable top speed. Here you can see my 2 inch diameter V-belt idler pulley and a bolt that will go through the hole in the center ball bearing.
This photo shows the chain idler doing its job as well as the general horizontal position of the bottom bracket boom tube. As long as your chain does not rub on any part of the frame, your idler pulley is installed properly, although just behind the front frame tubing seems to be a perfect place to weld the bolt. You will also need to adjust the length of the chin to suit the best position of the adjustable bottom bracket, using the rear dropouts to fine tune any slack. You will be able to adjust the rear wheel to account for about 1 inch of chain slack before needing to add or remove a chain link.
The two front forks must turn together, taking into account Ackerman steering angles, so a pair of steering control arms are made from some 1 inch pieces of flatbar. The steering control arms are 2 inches long with a rounded end that will conform to the shape of the front forks, where they are to be welded. You will also need the appropriate sized bolts to pass though the ball joints and the control arms.
Since this trike has such a short wheelbase, and will never see any high speeds, Ackerman steering angles are not such a concern like they are on our adult sized DIY Tdapole Trikes such as the StreetFox and Warrior. Simply weld the steering control arms on a 45 degree angle to the front forks as shown here, and your steering will work just fine. The angle of the steering control arms should make them point towards the rear axle center, but 45 degrees will be close enough for this trike.
This photo shows the steering control arms installed as well as the steering rod and ball joints, which make both front wheels turn at the same time. The ball joints are small hardware store items which have been welded directly to the 1/2 inch rod that makes up the steering rod. Ensure that both front wheels are tracking perfectly straight, then measure the hole to hole distance between the steering control arms to find the optimal alignment position. Once the steering rod is at the correct length, nothing will ever go out of alignment.
The pilot's seat is a nice wide seat from an exercise bicycle, held in place by a small bit of seat post tubing that has been welded to the frame. Your seat should not interfere with the steering rod, and should be placed so the rider's heels do not strike the frame tubing. Some adjustment of the bottom bracket may be needed to find the best seating position, which is why it was a good idea to make it adjustable. This photo shows my seat installed in what seems to be the optimal position for balance and comfort.
Depending on the angle of your seat, you may want to create the optional back rest as shown here. Since the rear frame still has the seat tube and clamp installed, you can simply add a bit of wood and padding to a seat post and install it right into the frame. This photo also shows how the seat post was cut and re-welded in order to put the backrest at a more reclined angle. Now, the young pilot will not fall off the rear of the trike if he or she accidentally lets go of the handlebars during some vigorous riding.
The handlebars for this trike are made to install directly into the front forks, adding a handle to each side of the trike. Since the two front forks are forced to turn together by the steering rod, the two handlebars will also do the same. This photo shows the bits that I will be using to make up the two handlebar halves; two equal length goosenecks and a mountain bike handlebar. The two goosenecks should have equal length stems, and fit properly in your front forks. There are two different diameter gooseneck stems, so make sure you choose the right ones.
The position and angle of the handlebar halves should allow easy reaching by your pilot, so tack weld them in place and determine if they seem comfortable and functional before completing the welds. Here you can see the position and angle of the handlebars which allowed an easy reach and full range of steering with no interference. The goosenecks will be adjustable left-to-right, so you have the ability to set them up for best comfort. A slight upwards angle of each handlebar seemed to make the most sense ergonomically.
Here you can see the completed Kid's Tadpole Trike, complete with luggage rack (buddy carrier) and chain guard. The cool little trike required no machined or custom parts, yet looks as good as any adult sized trike. The frame is strong enough to carry as many kids that can pile onto it and it may even be able to handle the abuse of an adult rider. OK fine, I rode it too!
This photo shows the rear of the completed Kid's Tadpole Trike, complete with buddy carrier and adjustable back rest. Since brakes are included in the rear coaster hub, there are absolutely no cables to worry about, and the trike has plenty of stopping power, so your crazy young test pilots can fishtail around the corners.
Tanner puts the Kid's Tadpole Trike through the paces, handling the trike with the accuracy of a highly trained jet pilot! The completed trike is so much "cooler" than the run of the mill kid's bike, that every kid on the block will want one. Do you have enough time and spare parts to build a dozen more?
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