The scooter is geared for a top speed of about 25 mph, has around 3 horsepower, and can go 8-12 miles per charge.
Step 1: Parts and Tools
- Donor kick scooter frame - mine was a Royce Union Transit
- Wheels - I used 12.5" low-speed pneumatic wheels from Northern Tool
- Motor(s) - 3x CIM motors, which are generally used in FIRST robots
- Gearbox - CIM motors are fairly high speed/low torque motors, so a gearbox was required in addition to the reduction from the chain drive. My franken-gearbox is a mashup of two Andy Mark Toughboxes from 2005.
- Motor Controller + throttle- I went a bit overkill, and chose a Kelly KDS72200E, 72V, 120 A continuous, 200A peak controller.
- Batteries - 8 x Turnigy 5000 mAh 4s LiPo packs
- Aluminum - angle channel and flat bar in a variety of sizesd
- Shock absorbers - 2x mountain bike rear shocks, 2x mountain bike fork shocks
- Nuts and bolts - too many and of too many different varieties to list here
- 1/2" threaded rod with matching nuts
- 1/2" and 8mm stainless steel rods + shaft collars- for the pivot points in the suspension
- Large power switch
- Fuse/Fuse holder
- High current terminal connectors
- Drill - with lots of large bits, up to at least 1/2"
- Hack Saw - if you have a bandsaw with metal cutting blades, please save yourself and use that
- Drill Press - Not strictly necessary, but it makes drilling precisely aligned holes much easier than with a hand drill
- File - for smoothing sharp corners and enlarging holes
- Propane torch and aluminum-zinc brazing rods
- An assortment of clamps
- Bench vice - mine was literally a workbench that was a vice, but the kind that bolts onto a bench would be even better
- Soldering iron + solder - one with a very heavy tip for soldering large power connectors and battery leads
- Lithium Polymer battery charger with power supply
- At least two adjustable wrenches
Step 2: Donor Scooter Teardown and Layout
I started out by stripping the scooter of its original wheels and suspension, so that I could figure out an approximate layout of the final scooter.
For use on softer and less regular terrains than asphalt, the original wheels needed to go. I got new ones from Norther Tool. They sell a version with a 60t sprocket included, but I managed to scavenge a 60t sprocket with the same bolt hole pattern from my school. The best part of these wheels is that I can later upgrade the tires to these snowblower tires if I want to use the scooter in snow.
To keep room for suspension travel and keep a good steering geometry, the wheels had to be placed completely under the deck of the scooter. This means that the rider's platform is well over a foot above the ground, and the clearance under the scooter is around 7 inches.
Step 3: Rear Suspension Assembly
Since the original scooter had suspension, designed the new suspension system to use the same pivot points as the old one. The original used 8mm bolts, and I had some lengths of 8mm stainless rod pulled form old printers that became the new pivots. I found that one of the 8mm rods was flexing significantly under load, so I drilled out those holes to 1/2", and replaced the 8mm rod with a 1/2" steel rod taken from a large flat bed scanner.
I made the left arm of the suspension assembly about two inches longer than the right arm, because I later would need to bend the aluminum around the rear sprocket. To bend the massive aluminum bar without a heavy duty bending brake, I heated the area I needed to bend with a pair of propane torches, and then used a vice, clamps, and brute force to bend the metal.
Step 4: Fork
For the new fork, I started out by tearing apart the fork from an old mountain bike. Fortunately, the scooter used a 1" threaded headset, which I was easily able to replace with the mountain bike's 1" threadless headset.
I pulled the springs and dampers out of the mountain bike fork, and used them to create a new pair of shock absorbers with pivots at each end. The pivots allowed me to build a leading-link style suspension arm, which is much easier to build than a telescoping fork.
I reused the fork crown and the top of the fork legs form the mountain bike, and bolted the 1/4" x 2" aluminum bars that made up the new fork to the tops of the old fork legs.
In my original fork design, the front wheel was was centered in front of the axis of the steering column. While this design did work, I did a few human-powered test rides of this configuration and found the handling to be poor. The wheel's forward position made it impossible to lean while turning, because if you lean into a turn, the wheel naturally wants to swivel away from the turning direction. Additionally, at higher speeds I realized that there would be the risk of the front wheel "castering" - trying to swivel around 180 degrees.
I rebuilt the fork, using only the parts from the original iteration, so that the front wheel was properly positioned for good handling. This did raise the front end of the scooter a couple inches, but it was a worth while tradeoff.
Step 5: Wheels
Step 6: Gearbox
The gearbox was mounted to the scooter using the original bolt holes built into the gearbox and some aluminum angle bolted to the scooter's frame. Finally, a 21 tooth sprocket for #35 chain was fixed to the output shaft.
Step 7: Chain Tensioner and Idler Configuration
In addition to a chain tensioner, the scooter required an idler sprocket to rout the chain under the aluminum frame of the rear suspension arm.
In version 1 of the tensioner/idler configuration, both the idler and tensioner were fixed relative to the rear suspension arm. The chain tensioner was very simple, made of one idler in a pivoting arm attached to a spring, that pulled the chain tight.
This design worked okay on roads, but I found that riding over big bumps, especially on trails, caused the chain to slip off the drive sprocket. I was able to rationalize two reasons why this happened. When riding over a bump, the movement of the suspension puts additional tension on the top of the chain. Since the mounting of the idler sprocket was only made from 1/8" aluminum, the extra tension could have caused the idler to deflect to one side, causing the chain to slip off the drive sprocket. Additionally, the first chain tensioner design actually could not store enough extra chain to allow for full suspension travel, so over large bumps the suspension may have bottomed out and overstretched the chain, causing even more tension and idler-bending. At least that kind of makes sense. I couldn't actually see what was happening, so I don't really know if it's true.
Tensioner V1.5 added a chain guard around the drive sprocket, to prevent the chain from slipping off it. The guard was made by cutting small angled sections out of one side of some angle aluminum at regular intervals, and then bending the aluminum at these points to form a roughly circular guard. The seams were then brazed over and sanded down. This worked well at first, until the chain pulled so hard against the guard that it cracked one of the brazed joints and bent the chain guard out of the way.
Tensioner V2 failed so miserably it isn't worth posting pictures of or mentioning andy further.
V3 had the idler sprocket fixed relative to the gearbox, rather than relative to the rear wheel, so it completely prevented the chain from coming off the drive sprocket. The chain tensioner was fixed relative to the rear sprocket, so when it pivoted to lengthen the chain, it could not come out of alignment with the rear sprocket. Also, the tensioner used two sprockets to direct the chain in an "S" shape, similar to the rear derailleur on a bicycle, in order to store more extra chain. It was constructed from entirely 1/4" thick aluminum, making it extremely solid, and used actual shielded ball bearings for a pivot, rather than a bolt.
Step 8: Brake
The right brake pad is fixed to a rod that passes through the caliper, springs, and aluminum frame of the suspension, and the pad and rod can slide through all those bits. Since the spring is split in the middle, the brake is applied, and the brake cable pulls the two halves of the caliper towards eachother, both halves move equally in opposite directions, so that they both move towards the sprocket.
Normally on kick scooters, you brake by pushing down a lever above the back wheel with your foot. The lever rubs against the wheel, slowing the scooter down. I wanted the disk brake to be actuated the same way, so I built a brake pedal out of some 1" U and angle aluminum. Two pivoting segments are attached to the brake pedal, which, when fixed to the scooter frame, form a kite shape with hinged joints. When pressure is applied to the pedal, the kite is deformed, which lengthens its long diagonal. Since the brake cable is attached across the long diagonal, the cable's housing is pushed up the cable when the pedal is actuated
Step 9: Handelbars
The hall-effect throttle grips I ordered with the motor controller fit exactly on the handlebars, and the cable was routed down the steering column.
Step 10: Deck
In the spirit of building as much of the scooter as possible from parts I had on hand, I made a new deck out of some carbon fiber left over from my bamboo bicycle. Since the carbon fiber was in the form of 12K tow, I had to form it into cloth myself. I built a wooden frame with nails spaced every 1/2" around its edges, and used it to wrap two orthogonal layers of carbon fiber at a time. Once I applied epoxy to the fiber, I clamped the frame to another piece of wood to squeeze out excess epoxy and create a smooth, flat sheet. After 5 or 6 repetitions, I cut the deck to shape. I glued a layer of 1/8" polycarbonate on top of the carbon fiber, because carbon fiber is not very impact or abrasion resistant. The new deck was screwed on top of the old one with countersunk stainless steel screws.
Step 11: Electronics Mounting and Wiring
All electrical connections were wired with heavy duty bolt-through terminal connectors soldered to two parallel lengths of 14 ga wire. Because the motor controller was mounted with its contacts close to the underneath of the deck, its connectors had to be attached at right angles to the top of the controller. Because of this, the middle connection had to be raised, which I did with a small copper block soldered to the bottom of the connector.
The three motors were wired in series, using 5mm bullet connectors between each motor, so that individual motors could be easily removed if they got damaged.
Step 12: Battery Pack
I paired off the battery packs, and wired the pairs in parallel, including the balance connectors, so I had four battery modules. To charge the pack easily, I made an attachment for my charger that connects all the packs in parallel so they can be charged simultaneously. When charging is done, the packs can easily be reconnected to each other in series.
Step 13: Battery Holder
Step 14: Final Pictures and Videos
Step 15: Thoughts and Improvements
The biggest issue with the scooter is the motors. CIM motors are designed to be 12V motors. With about a 60V battery pack and 3 motors in a series, each of the motors is getting around 20V. While the extra voltage in and of itself is not especially a problem, the extra current that the extra voltage entails is. ~1.7 times the voltage means ~1.7 times the current through the motors, which means ~2.9 times the power lost through heat in the motor's windings. Since CIM motors are not very efficient motors in the first place, the risk of overheating and damaging the motors is very serious. Going up a long, fairly steep hill, I managed to burn out two of the motors simultaneously. Fortunately, the motors are really cheap and I had an extra (a second spare came from HammockBoy), so it wasn't too much of a problem.
One way to solve the motor-burning-out problem would be to use the motor controller's programmable current limiting to reduce the current available to the motors. However, less current = less torque = less acceleration = less fun. The ideal solution would be to just buy a much nicer motor, like a short magmotor, which would be more powerful, more efficient, and lighter than the three CIMs.
For using the scooter in anything but completely dry conditions, it would need fenders around the wheels to prevent water and grime form being sprayed into the electronics and mechanical bits and onto the backs of my legs. Also, the connections for the electronics would probably need additional (mechanical, not electrical) shielding.
Finally, the bearings in the Norther Tool wheels are meant for low speed use, and are generally pretty terrible. Pulling them out and replacing them with proper sealed bearings would be a good upgrade.
If you like this project please vote for it in the Hurricane Laser Contest. If I won a laser cutter I would set it up at MITERS, MIT's student-run shop/hackerspace, so that any student, regardless of department, could have free, easy access to a laser cutter.
This is also entered in the Back to School contest. I was able to ship the scooter from Atlanta to Cambridge(due to it's folding ability), and I plan on using it as my method of transportation around campus and grocery-getting vehicle.
Finally, it is also entered in the Fix and Improve It contest, as I certainly regard this as an improvement on a boring old kick scooter