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80 volt Minibike Build

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  • 80 volt Minibike Build

    So, after posting a video of this thing in the off topic section, a couple of you requested a build thread. Not sure if it belongs in this section but I seem to remember other non-Miata stuff being posted here, so I'm going for it! If not, feel free to move thread.

    It started out life as a Monster Moto 80cc (actually 79cc, grrr deceptive marketing) mini bike, from Sears. I was just looking for a tool box, and then saw it on sale for $300 (originally $450) and had to have it. However, you can find these for sale on Craigslist for somewhere around $150-200 depending on condition.

    Sadly, I don't have a picture of it in stock form, but it looked just like this:

    Aaaaand the reason I don't have a picture of it is because it only lasted 1 day with the stock motor, before I decided it needed to be significantly faster and I pulled it off. I made a trip to Harbor Freight to get a Predator 212 (6.5hp) and stopped at Tractor Supply to get a clutch and some chain... and then turned it into this:

    NOTE: Sorry if any pictures are blurry. The OSS on my phone is broken, and sometimes sabotages my pictures, like above. It's the only picture I have :(

    After the swap, I kinda stopped riding it because the output seal of the HF motor was leaking oil, which the clutch would then pick up and sling all over your leg. I also had issues with the throttle linkage because the throttle on the mini bike is so crappy, it basically broke and never worked like it should, ever again.

    Fast forward a few months, I met somebody who had made their own "eBike", which had a range of ~15 miles and could go 45mph. I rode it, and immediately knew I had to do something similar, but not with a bicycle frame. I've never really gotten into bicycles.

    After struggling with several ideas, including buying a dirt bike frame and using that (kinda wish I'd done that, honestly) I decided I'd put the mini bike to use. Right around this time, I also obtained a really nice pressure washer that uses this same style motor for free, and it had a blown motor. I've been wanting a pressure washer for a couple years but could never bring myself to buy one, so this sealed the whole deal. I'd take the motor off of the bike and use it on the pressure washer, and then put an electric drive train on the bike! Win-win.

  • #2
    So, for the uninitiated, a brushless DC motor setup in its most basic form has the following components:
    • A motor controller
    • A motor
    • A throttle
    • A battery

    In addition to this, there are various switches that can (and in some cases, should) be added that will expand the feature set of the vehicle. For example, the controller that I bought can do regenerative braking, and can be programmed to activate regenerative braking based off of a switch being activated.

    These are the components I bought, with a little bit of my opinion mixed in:

    Controller: I bought a Kelly Controller KBS72051X with the high speed option. This controller can handle up to 90 volts and 25 amps continuous, or 60 amps for 10 seconds. This controller has a pretty big feature set, and I have no problems with it, at all. The one slight drawback is their program is sort of written in Chin-english, making some things difficult to understand.

    Motor: I went with an Alien Power Systems 80100 frame, 80kv sensored motor. A sensored motor is a REQUIREMENT for this application. If you don't have hall sensors, you will over heat the motor very quickly. I like this motor, the only issue I have with it is the lead time. The company only puts in orders once a month, so between my order date and the date it arrived at my door was around a month and a half. Luckily, I had a blown up motor at my disposal, so I could still work on my bracketry and such until it arrived.

    Throttle: For my throttle, I chose a simple thumb throttle from Kelly Controller. These fit standard 7/8" bars and, awesomely enough, come with some pretty crappy grips! However, they are still way better than the crappy grips that originally came with the mini bike. Thumb Throttle link, note, the grips I received were different from the picture. Mine are just smooth rubber. Very nice.

    Battery: My battery choice was limited. I could either get an off the shelf battery (my original plan) or build my own pack. I want to build my own pack in the future, but for now I went with a Greenworks 80v 2ah pack. These are used in electric lawn tools, like chainsaws/lawnmowers. I bought 2 of them, and when I bought them they were only $80 each. Link


    • #3 - just sayin... you got 7 months.


      • #4
        My next course of action was to start modeling stuff. I grabbed all of the important screw sizes from the motor, and made this model:

        Around when I finished that, I decided to pull the gas motor out of the bike and see where I wanted to put the electric motor. I had issues with the chain on the gas conversion, so I wanted to keep the chain as short as possible as well as make entirely sure the front and rear sprocket were exactly coplanar. With this in mind, I decided to put the motor in the little nook above the rear wheel, directly under the seat.

        Luckily, I had an idea for keeping the motor in line with the rear sprocket. I also was fortunate that Monster Moto decided to put 4 holes in the sprocket, appropriately sized to fit M10 bolts. These holes can be seen in this photo:

        For anyone who didn't see it coming, I decided to design a jig that would bolt to both the motor and rear sprocket, thus keeping them perfectly parallel to one another. That jig ended up looking like this:

        In order to get my dimensions, I simply held the motor in placed and used a level to measure the horizontal and vertical distances between the axle centerline and the center line of the motor. Shown here, my numbers were 4" horizontally and then 10.5" vertically:

        Another feature of this jig that you will note, is the 4 mounting holes for the motor are not perfectly vertical or horizontal. Instead, they are parallel with an imaginary line between the motor shaft and axle center. This is so that I can slide the motor up and down to adjust chain tension, as well as make slight adjustments to my bracket fitment.


        • #5
          The cool part of jigging to the sprocket and having the motor mount attached by bolts in slots, is that I can slide my motor mount up and down on the jig as well as rotate the rear wheel slightly to make the pieces line up perfectly for welding.

          Next, I took my motor dimensions and designed the bracket the motor would mount to.

          Added some gussets for strength:

          As shown here, the motor fits nicely:

          and has room to slide up and down...

          May as well throw in the bolts.... (I need to do this to make sure I space the sprocket out far enough to where the chain will clear the bolt heads)


          • #6
            Here are what all of the parts look like so far, all together:

            If I throw a chain link into the assembly, I can see about how far I will have to space the sprocket from the motor mount face in order to clear the bolt heads:

            This needs to be a multiple of 0.125" since the stock I'm using is all 1/8". So, I ended up going with exactly 0.5", shown here:

            These spacers ended up just being 3 circles with holes in them, since 0.125*3 is 0.375 and the thickness of the jig is also 0.125, which is 0.5" overall. Like this:

            Last edited by twolf; 03-24-2017, 04:05 PM.


            • #7
              With everything pretty much designed, I started cutting stuff out and test fitting to make sure my measurements were correct...

              This picture really demonstrates how the jigging shines. You can see how in order to get that top edge perfectly parallel with the chassis tube, I can simply rotate the wheel slight forward or back and then slide the bracket up:

              Here is a picture attempting to show the gap (rather, lack of a gap) between the motor mount and chassis plate. Not a great picture, but it also shows the 3 spacers used.

              The next thing I needed to design was a plate to hold my key switch. I wanted it to go right in front of the seat:

              So I simply stretched a piece of paper across the tubes, marked where the inside of the tube was on the top and bottom of the paper, then designed this:

              As shown by the dimension on the right, my piece of paper measured 5.95" from top to bottom (It was actually a 6" notepad) and the measurement between the tubes on top was 0.98" and 2.8" on the bottom. The hole is appropriately sized for my key switch.

              It ended up being a great fit. I could put it in place and it would hold itself via friction. The joys of working with triangles, any mismeasurement and I can simply move it up/down a little bit and it will fit, since all I really did by measuring was get the angles correct.


              • #8
                My method of holding the battery is rather simple. I bought an extra charger and opened it up, and soldered some 10 gauge wire directly to the lugs. This way, I can slide a battery in and ride, and if i want, I can plug the bike itself directly up to the wall!

                Once that was done, the welding began. I didn't take many pics, sadly, because my camera really started to die...

                First test fit of the motor:

                This was really the bulk of the work. The rest was just wiring the controller up and all that jazz. I didn't take any pictures at all because I get really frustrated by wiring.

                One thing I forgot to mention is that this motor is all metric. The shaft is 12mm, and the bike uses ANSI chain. This is of course an issue because it's impossible to find an imperial sized sprocket with a metric bore. So, I bought an imperial bore, put it on the lathe, drilled and reamed it to 12mm and then broached the 3mm keyway into it. When I went to install the sprocket, I just put a 1/2" gauge block on the motor mount face (Remember from earlier when I designed it to be spaced 1/2" out?), and slid the sprocket on until it butted against the gauge block. The chain runs perfectly true, and I'm really happy I did everything the way I did.

                Here's a little video of when I first cut the chain:

                video, sharing, camera phone, video phone, free, upload

                As you can see, camera is reaaaaaally messed up.
                Last edited by twolf; 03-24-2017, 04:28 PM.


                • #9
                  And of course, here is the video:

                  video, sharing, camera phone, video phone, free, upload

                  Interestingly enough, I am only running around 2000 watts, which is less than 3 horsepower, yet it is such a little torque monster that it is faster than the 6.5hp gas motor without question. This is also largely in part to the fact that there is no longer a centrifugal clutch.


                  • #10
                    Neat! The racers would kill for a pit bike that out accelerates their race cars. :-)
                    David Luney - Suwanee GA
                    [email protected]
                    04 RX-8, 10 Black MS3, 08 Red MS3, 06 550i, 05 325i 92 Spec Miata and a parts car or two.


                    • #11
                      Loved all of this. Thanks for sharing. The laughter in the video was contagious. You need to GPS this, very curious of top end.


                      • #12
                        Enjoyed the thread. Thanks for sharing.

                        | 10AE-T | BEGi S4 | GT2560r | Enthuza | 6UL | MS3Pro PNP|
                        | 2001 Track Rat | Enthuza | Jenvey | MS3Pro PNP | Exocet in Progress|
                        | 1992 323 | Wrong-wheel-drive Miata Endurance Racecar|


                        • #13
                          This is awesome!! I appreciate quality fabrication and this is no exception! I love seeing some good CAD work too. Are you using a CNC mill to cut the pieces out?


                          • #14
                            Thanks for the kind words everyone!

                            Originally posted by Trick2789 View Post
                            This is awesome!! I appreciate quality fabrication and this is no exception! I love seeing some good CAD work too. Are you using a CNC mill to cut the pieces out?
                            Everything was done on a waterjet cutter, but there are a lot of services out there where you can send out CAD files and they will send you parts back for pretty fair prices, like


                            • #15
                              nice work! and good documentation for the next fool ;-)
                              '01 BRG #2807 FS: 4 nice black Chaparral's, $400

                              Build log: