DIY KIDS ELECTRIC TRIKE
Here is a fun little electric trike for your young DIY enthusiasts that can be built in a single evening from a pair of children's bicycles, a few bits of scrap tubing and an old stand up electric scooter. Actually, this project is so simple that you can make it out of practically any battery powered DC motor and any scrap bicycle parts you may have lying around the shop. Since speed is certainly not the goal here, even an old cordless drill will make a fine power source for this vehicle, allowing your kids to run around the yard for as long as the batteries hold out. You can purchase one of those small plastic electric kids cars at just about any toy store, but the all plastic construction and low wattage motor may not keep up with the kids demanding driving habits. If your yard or park is not perfectly smooth and hill free, the department store vehicle may wear out in a hurry, or simply fail to traverse the terrain. By using a pair of kids' bikes for the wheels and forks, this electric trike becomes a high quality vehicle, capable of driving on gravel, up hills, through the mud, and even on the grass.
Since kids' bikes come in many sizes, with wheels measuring 10 inches, 12 inches, 14 inches, and 16 inches, you should not have any problem scrounging up the parts for this project by visiting the local dump or hitting the yard sales around the neighborhood. The parts are really not critical, but you will want the two front wheels and two front forks to match, at least for size and shape, since they will be placed side-by-side on the trike.
This photo shows the basic parts that will be needed in order to put together the kid's trike. You will need three bicycle wheels (two of which should match), three front forks (two of which should match), one head tube and bearing set to match one of the front forks, and some type of DC motor that can power one of the wheels. I chose an old stand up electric scooter motor as the power source because it could be placed against one of the trike wheels to make the vehicle move, thus requiring no transmission, gears or chains. These little stand up scooters are also plentiful at yard sales and scrap piles, since it is often the frame that bends before the motor fails.
As shown here, you will need these components for the front of your trike - a front fork and matching head tube with all included bearing hardware. This head tube actually came from the electric scooter, and it just happened to fit the kids bicycle fork stem perfectly. With all of the bearing hardware installed, the fork will spin freely and without friction, which is a sure sign that all the hardware is properly matched. If you take your front forks and head tube from the same bicycle, then you will be certain that all hardware matches.
To make the kids trike, you need two wheels and forks for the rear and one front fork, wheel and head tube. For this reason, the two rear wheels and forks should match, and the front wheel can be whatever size you want. On my trike, I decided to use three matching wheels since they were easy to locate, and I also used a front fork that was slightly larger than the two at the rear. You will also notice that all the wheels are actually front wheels, since there is no drive chain needed you will not need a wheel with a sprocket attached. You could use a rear wheel if you cannot locate three front wheels, but a little force will be needed in order to widen the fork legs to take the larger diameter rear hub and axle.
Most cheap stand up electric scooters have a 100-300 watt motor connected to a small rubber wheel, like the wheel you would find on a road skate. These motors will typically move the scooter at a speed of 15 to 20 miles per hour when using a good 24 volt power source. For use on a kid's trike, I decided to use only one large 12 volt battery, reducing the top speed to about 10 miles per hour, but extending the run time by many hours. Since the motor already turns the small rubber drive wheel, all you need to do is let it rub against one of the trike wheels, and you will have a simple friction drive transmission system. Whatever speed the scooter would be capable of will be the top sped of your trike, since there is no gear reduction. As long as the scooter drive wheel is turning the bicycle wheel by contact at the edge of each wheel, there is a 1:1 gear ratio, so wheel size makes absolutely no difference to the final speed of the vehicle.
Cut the stem from each of the rear forks just above the crown as shown here, being careful not to nick the top of the fork leg tubing. The cut area will be the point at which you will attach your new frame tubing, as this is the strongest area of the fork. If for some reason your forks are not exactly the same, try to cut each stem so that the distance from the axle to the cut area will match on both forks.
Since I am building this kid's trike from parts I already had around the garage, I simply placed the two rear forks on the ground and found a pair of 1 inch diameter square tubes that would create the triangular frame as shown here. I think the square tubing came from an old table I hacked up, but since the trike is only going to hold the weight of a child, you don't need to use heavy tubing. As for the size of the frame, simply have your young pilot sit on a chair, and make some basic measurements of how much room they will need in order to sit comfortably between three wheels. I went for a wheelbase about the same as the original bicycle, and a width of about the same as the wheelbase, creating an equilateral triangle footprint.
The basic triangular frame needs three tubes to hold it together, one at the rear to join the two rear forks, and two more tubes at the front to form a triangle between the fork crown area and the head tube. Shown in this photo is the tube that will join together the two rear forks, creating the frame to whatever width you like. A fishmouth cut is ground out at the ends that mate with the round fork tubing. The tube should be placed about half an inch from the end of the fork leg tube so there is room to make a weld.
With the parts sitting on a flat surface, tack weld the rear frame tube to the two rear forks as shown here so that the fork legs are at 90 degrees to the rear frame tube. Because both forks are sitting on the same flat surface, vertical wheel alignment will be almost perfect, and as long as you get both forks running parallel, horizontal wheel alignment will also be perfect. If the two rear wheels are badly out of alignment, there will be scrubbing on the road, which will decrease battery run time, so try to get the wheels running as true as possible.
Once you have completely welded the rear frame tube, join the two rear forks together so that you can continue the frame layout, adding the two front frame tubes that will form the triangle as shown here. The two front frame tubes will determine the wheelbase (length) of your trike, which should be fairly close to the original bicycle wheelbase so that your pilot can reach the handlebars comfortably. Also shown in this photo is the front head tube which should be set at 90 degrees to the frame tubing, which will result in a final angle of about 65 degrees once all three wheels are installed. The joint between the head tube and the two front frame tubes needs to be ground out (fish-mouthed) to conform to the head tube so that you can make a decent weld along the joint.
Once the head tube has been welded to the frame, insert the forks and three wheels to check your rolling frame. The head tube angle is now close to that of a standard bicycle (between 65 and 68 degrees), so your trike will handle just like a regular bicycle. Now you can finish all the frame welding to secure the main frame triangle to the head tube and rear forks.
The biggest complaint when it comes to kids and their electric vehicles is how quickly the battery goes dead. On the original stand up scooter, you could barely go around the block once before the battery started to give it up. By using a much larger battery and half the voltage, the new trike will run almost all day on a single charge, and allow the vehicle to get up hills that would normally exceed the smaller battery packs immediate discharge capacity. Sure, the large battery is quite heavy, but that simply adds stability to the trike, so it won't tip over as three kids try to stand on it as they roll over the curb. Another very important thing to note when choosing a battery is to ensure you use Gel cell, or non-spillable battery, which is a must for a kid's vehicle. Unlike a typical lead acid battery, this battery will not leak any dangerous chemicals, even if turned upside down, so don't use any other type of battery.
The battery will live between the two rear forks as shown here, supported by a few bits of flat bar which have been welded to a battery support base made of angle iron. The battery box base should place the battery at least 4 inches from the ground, so it doesn't bang on small objects which may roll under the trike as your kids drive it through hostile territory.
A pair of tall BMX style handlebars will probably work best for this project, as you will require a little height rather than having to bend over to steer. This photo shows a typical 8 inch tall set of BMX handlebars and a matching gooseneck to hold them.
Also note that goosenecks come in two sizes, and although the larger sized gooseneck will not fit into the smaller sized fork stem, the smaller gooseneck will indeed fit into the larger fork stem, but will eventually work its way free or fall out. This is more evil doing by the secret non-standards committee, so be careful. There should be no play between the gooseneck tubing and the fork stem.
To stand on your scooter, you will require some kind of foot peg or platform. There are many options, including BMX pegs or even a skateboard plank, so use your imagination. This photo shows two options I considered - a standard round BMX peg and a platform mate of some 1/4 inch thick flat bar. I opted for the flat bar platform as I wanted to use the foot pegs for some other project later.Remember to round off all sharp edges on your projects, especially ones that are guaranteed to be smashing into you at high speed like this one will. The pavement hurts enough, so you don't need to be cut to shreds by your vehicle as well!
The seat will be placed over the battery, helping protect the wiring, and to keep meddling hands out of the electrical bits. Using some 3/4 tubing, or whatever scraps you have in the junk pile, make a basic seat frame like the one shown here that can be welded to the frame, allowing the seat to cover the battery, yet also allow the removal of the battery. An old kitchen chair with metal legs is a good source of metal for the seat frame as well as the seat cushion.
There is nothing critical about the seat frame other than allowing a comfortable sitting position and easy removal of the battery for charging or swapping. The simple seat frame shown in this photo allows the battery to drop into the battery box from the front of the trike once the seat cover has been removed.
Once you finish all of the welds and paint your trike, the young pilot will be able to hit the trails on a long running environmentally friendly vehicle. Of course, you have to do a bit of electrical wiring first in order to transfer electrical power from the battery to the motor, but since it is a simple matter of adding a switch in series between the two, there is no need for a circuit diagram. Just run one wire from the battery to the motor and then one from the other battery terminal to you switch, and then from your switch to the other wire on the motor.
If your scooter came with a throttle switch like mine did, then simply install the electrical system exactly the way it was on the scooter. If you have no throttle switch, then find a contact switch that can handle your motor's power (5-10 amps typically), and you are ready to roll. I also added a front pull brake to the completed trike shown here although it was probably not really necessary due to the limited top speed and the fact that it stops moving within a few feet once the throttle is off.
The completed kid's electric trike runs for many fun hours on a single battery charge. Top speed is limited to a "kid safe" level due to the lower voltage, and even if the trike stalls, the motor will not burn out because there is a limited amount of slip in the friction drive. This trike is so well behaved that it could be used indoors, and if you install white rubber tires, there will be no track marks on hardwood or linoleum floors. Oh, and yes, the trike can also move a fully grown kid around, as I have found out! Be safe and have fun.
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