After a tedious search on the Internet for way to modify these little motorcycles, I didn't really find any ideas that I fell in love with, so I decided to make my own little adult conversion on a Razor MX650 that I picked up on Craigslist for $100.
Step 1: What Do You Want?
Step 2: Batteries
A vehicle of this size (with my setup) get around 0.45 to 1.00Ah/mile, with an average of 0.62Ah/mile on flat ground cruising at full throttle. So choose a battery with a capacity to suit you're range needs. But remember, if you're using lead acid batteries, the Ah rating on then is usually rated at 1amp, and when under a heavier load such as what is seen in an electric Razor, they become much less efficient. So if you are running 12Ah batteries, there may be as little as 8Ah of usable energy. The other issue with lead acid is that the voltage drops drastically under load which will hurt your top speed.
This is why I decided to go with lipoly batteries. Although these batteries are more volatile and care is needed to properly balance them, they are much more power, efficient, light, and energy dense.
If you are going to go with the stock motor but want more speed, the best thing to do is increase voltage. This will allow the motor to rev higher while not gaining anymore heat like gearing higher would since the load (amp draw) should be relatively equal if you exclude wind resistance.
By adding a speedometer that was properly calibrated, I was able to conclude that the original top speed of this bike was 16.8mph at the original 36V. Since I am going to use lipos (3.7V/cell) I figured that a 12cell pack which would output 44.4V nominal or 50.4v peak would reach my target top speed of 20mph without any problem. You can figure this out by taking the voltage you want to go with and dividing it by the original voltage. 44.4V/36V=1.23 times the original top speed. So if you take the original speed of 16.8mph and multiply it by 1.23, you get a new top speed of 20.72mph. And at fully charged, this will give you 23.54mph.
As far as range goes, I went with the largest pack I could fit inside the bike without doing any crazy welding to the frame. This came out to be 3 16Ah 4s packs from hobbyking.com that cost $80 each.
Step 3: Battery Assembly
Step 4: Wiring Batteries
Step 5: Controller Upgrade
After much research online, I found that there is pretty much only one brand that will give you a good small controller that will fit in the motorcycle and comes in a multitude of volt and amp outputs. The one I chose put out a max of 100A with a 50A continuos rating, and maximum nominal voltage of 48V (60V peak input). You can find these at Kelly Controller under "mini DC controller"
With this new controller my Razor is now a wheelie machine! Just sitting normally on the bike and you will be accidentally pulling wheelies every time you give it a little gas. No standing wheelies or popping up the font end needed.
WARNING Unlike increasing voltage, increasing the amperage will head up the motor, so be cautious of this as you can fry the motor windings or loose the magnetism of the magnets if the temperature gets to high! A good rule of thumb is to put your hand on the motor, and if it is even a little to hot to keep your hand there comfortably, the motor is to warm and needs to cool down before running it anymore.
Although my new controller is fully programable, I chose to keep the full amp output so the power is there when I need it. I manage the heat of my motor by not pushing it all the time and drawing high currents. (If you roll on the throttle slower, the motor will stay much cooler than if you're pulling wheelies and doing donuts in the dirt)
Step 6: Mounting the Batteries in the Frame
Step 7: On Off Switch
Step 8: Welding the Freewheel
Once you have the freewheel off, you can simply weld (tack) the inner threaded part to the out race that spins freely. (Just make sure not to weld the threads or it will never screw back onto the rim. I tacked my together in 4 places with a Harbor Freight welder at home and it hasn't broken yet!
After everything is welded up, screw or back on with your hands and put the wheel back onto the bike. The torque of the motor will tighten it up the rest of the way.
Once this is all done, the motor will be direct drive like like a real motorcross bike with no freewheel. And if you're worried about this messing up the chain tensioner, rest assured, everything will work perfect.
Step 9: Results
Top speed fully charged: 24.5mph (above calculated)
Top speed dead battery: 21.1mph (above calculated)
Range cruising: 35miles
Range at top speed: 26miles
Range on average hilly dirt trails: 20miles
Range on dirt trails going as fast as I can: 15miles (limited by motor temp)
Wheelies with new controller: yes, borderline scary
Wheelies with stock controller: yes, but you have to stand and pop the front up.
Waterproof: yes, take out the battery pack and hose it down like any other motorcycle.
Weight original: 98lbs
Weight after mods: 72lbs
If you have any other questions or comments, feel free to leave them. I'd be happy to help with your build. I can also make some videos if needed once I get a new go pro battery.