Electric Minibike

December 17, 2023

I picked up a Burromax tt250r second hand because it is small minibike with full suspension, which from my research will hopefully result in a smoother ride despite the small wheel size. 

Next placed the dead 24v lead-acid batteries for 33.6v of LiPo packs wired in series, which was able to go about 15 mph. I knew I was going to replace the entire powertrain but I wanted to test the current motor so if weight becomes an issue later, I could return back to the original motor.

December 18, 2023

I disassembled both the Burromax as well as a second minibike I bought at the same time for a later project and more parts, and experimented with different wheel sizes on the Burromax frame.

I also test fit the new motor which supported my existing plan to extend the rear swingarm both for more space and to make the bike more stable and less prone to wheelies from the added torque. 

December 18, 2023 continued

Next I created multiple initial sketches of various ways I could go about modifying the rear swingarm to fit the newer motor.

From the test fit and these sketches I realized I would have to relocate the rear suspension as well, so I drew up some initial sketches for how I might go about that as well.

December 30, 2023

I milled down the front wheel hub on a rotary table to be able to mount a brake rotor. First I took 140 thou off the top of the hub, while keeping the center a friction fit with the inside of the rotor for ease of centering and mounting later. Next I drilled and tapped the mounting holes for M6x1 threads. 

I also created a CAD model of rear swingarm in Fusion 360 to start figuring out dimensions and experimenting with various motor placements.

December 31, 2023

Once the mounting location for the motor and suspension was determined I cut the old swingarm apart with a portable band saw, and found some scrap pieces of metal for the longer side rails and crosspiece.

December 31, 2023 continued

I also cut the previous lower suspension mount off and used a belt sander to get it to the correct angle. I also cut in half and ground down a piece of the same stock used for the longer side rails for the upper shock mount.

December 31, 2023

Next I TIG welded the rear swingarm back together, and my friend welded the upper shock mount onto the frame so I could continue working on other parts as it was getting late.

January 5, 2024

Once the frame was together I moved onto modifying the motor. I removed the stock 14 AWG phase wires and soldered 50% larger 10 AWG wires on instead. I had to Dremel the access hole larger to fit the larger phase wires.

January 5, 2024 continued

I drilled the holes on the end plates out for more airflow, and cut down and glued screen into the holes to stop dirt from entering.

Then I designed and 3D printed a mount for an 80mm computer fan to mount to the rear of the motor to increase airflow, and mounted a screen to the outside of it too.

I also mounted a heat sink to the motor which I have found to be very helpful in the past.

January 11, 2024

Next I cut down a piece of 3/16" thick Aluminum plate to the correct width and drilled holes to mount the motor into both it and the swingarm. As this was the first time the motor was placed into the assembled frame, there was a lot of test fitting to make sure it was in the best location to not hit the suspension and still have the chain properly tensioned.  

I also cut out a notch for the chain as it was hitting the motor mount plate when I mounted it.

January 12, 2024

First test ride! I replaced the cable actuated front brakes with 4 piston hydraulic mountain bike calipers which will be necessary for safety at the speeds this will be going. For the first ride I left the old rear brakes as I didn't have any other hydraulic brakes that were bled.

January 15, 2024

Modifying the throttle.