This instructable is perfect for someone with a razor MX650 or MX500 looking for the optimal way to modify their motorcycle. Since 1/1/16, California has now changed their laws for electric vehicles allowing for an "E-bike" with a speed of less than 20mph with no human power assist (no pedals required) to be ridden in the bike lane. This makes the Razor MX line perfect for getting around town and having a little fun while doing it. Unfortunately, these little bikes aren't meant for a full sized adult even though the weight limit is 220lbs, so a few changes will need to be made to truly make this motorcycle shine.
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?
Before going out and blowing a bunch of cash on a build, you should first decide what you want out of your motorcycle. For me, I decided to pay attention to weight, top speed, torque, handling, range, and of course price. Before I go on, I should say that this is not my first electric build. I have made an electric bicycle, supermoto bike, and a 20hp Honda Grom that is faster now that it was when it was gas powered.
Step 2: Batteries
Here is a little background information on batteries you may want to know if you're going to build one of these bikes.
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
Since I wanted to keep things simple and not have an onboard balancer with a specially designed charger, I designed the battery pack so that it was removable and each battery could be separately changed and balanced on a typical RC changer. To do this, the battery pack was double sided taped together (around the edge of where each pack connects in order to keep and dirt and debris from getting in between the gap of each pack and rubbing through the cell which could potentially cause a rupture). Then, the new 3 pack battery was rapped once with fiber tape in order to secure the battery pack from falling apart (see photo)
Step 4: Wiring Batteries
To wire the battery pack, I made a wiring harness that connected the 3 battery packs together in series along with a 4th connector to the controller that way the batteries can be connected together out of the motorcycle and then just plugged in once installed for ease of use. I also constructed a precharge resistor that can plug in between the batteries and controller in order to slowly charge the controller up which will stop any arcing from happening once plugged in and keep your controller working properly without a serge of energy. This was made by simply soldering a resistor between a male and female deans plug on the positive side and solder a plain wire between both negatives. To use this you simply plug it in between the battery and controller, wait 5 seconds, unplug it, and plug the battery straight to the controller. It's simple and may save you some money buying a new controller down the road. Although a precharge resistor is not mandatory, I highly recommend using one if you're considering upping the voltage past 36V.
Step 5: Controller Upgrade
Since I am not looking for all out top speed and more of something that can handle being a true offroad dirt bike that can handle sand, excruciating hills, and rough offroad trails, I opted to upgrade the controller to one with a bit more current. Although you do not need a different controller to make it faster, a new controller that will output more amps will drastically increase your torque output.
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
This is actually much simpler than you might think if you choose batteries that will fit in the original battery trays. Since my design requires that the batteries need to be taken out to charge, I simply ran some Velcro straps through the two screw holes located below the bottom battery tray. I left the foam pad below them for some cushion but that is about it. The top tray does not house any batteries, so that allows for top tray to house the the controller. A little double sided tape and the controller will be nice and solid.
Step 7: On Off Switch
Since the batteries need to come out after every ride, I did not put the original plastic box that looks like an engine around the batteries and controller. The only problem this caused was that I now needed to make a place to mount the on/off switch. To do this, I used a cutting wheel on a Dremal to cut a hole just behind the gas cap. This came out very clean and makes it a lot easier to turn the motorcycle on and off.
Step 8: Welding the Freewheel
This is VERY important! With all the added power, the cheap Chinese freewheel that mounts the sprocket to the wheel will break right away (mine lasted 0.4miles). To do this, you need to take the wheel off the motorcycle, unbolt the sprocket with a 10mm wrench and 5mm Allen wrench, and remove the freewheel from the hub. If you remove this before the power upgrade, you may be able to break it loose with a screwdriver and hammer, but if you run it first and break it, it will be much tighter and harder to remove. Id suggest taking the wheel to a bicycle shop and asking them if they could remove the freewheel for you. It took 2 people working at the bike shop and me to remove my freewheel after it broke since the torque of the motor tightened it so much on the hub.
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
After much testing and fun rides on some local mountain bike trails, here's are some specs to base your build off of.
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.