Electric Assist Recumbent bicycles
 Deathbike Junior e-bike
By Warren Beauchamp
A youngster I know had been drooling over motorbikes for a few years. Since he was too young for any of those, and $$, that didn't happen. It did however start me thinking about unused e-bike parts from past projects that were languishing in my garage. An idea was born. Take a tiny girls BMX bike that had been rusting away next to the garage and convert it into a fun little e-bike. It would be too small for but but for the youngster it would be just the right size.

In the fall of 2017 I hacked the pink BMX donor bike apart, and it sat waiting while I completed my trike project. Now it's spring and time to get this project rolling.

4/2/2018
I found my 24VDC mongoose motor, controller, and throttle. I'll use half of my ancient split pack 20Ah 48V Ping LiFePo4 pack along with a BMS and power supply I have to buy. Even 1/2 the ping pack is a large battery, so it should last pretty well and be very safe and bulletproof

The bike needed to be stretched significantly to accommodate the motor and the battery. That rectangle of cardboard in the picture represents the battery. I found some cut-off 1.5" thin wall 4130 tubes in my junk box that were the right lengths for the new frame.

4/18/2018
One of the first issues that I ran into was that the rear dropouts were way too narrow to fit the dual sided hub rear wheel that I wanted to use, which was left over from a previous project.

I was able to spread the dropouts using a threaded rod and nuts. I just kept cranking the dropouts apart until they were the right distance apart.

Note that this only works with steel frames, not aluminum frames.

Here's the wheel into the widened dropouts. You can see the two drive cogs. One is used for human power and the other is for electric power. They are both free-wheeling.
I clamped the bike into my frame jig and started cutting and shaping tubing and brazing it into place.

I had to move the seat post tube forward and reduce the angle to provide clearance for the motor.

4/23/2018
I ordered an inexpensive 30 amp battery management system (BMS) from China to allow the battery to be charged properly and to prevent it from being over-discharged.
Here's the frame tacked together. You can see how upright the seat tube is. Normally the seat is well behind the bottom bracket. I suspect it wont be pedaled much, so that's ok.
5/4/2018
This bike will be about 25lb heavier when I'm done, so it will need better brakes. I had a braze-on disk brake bracket from another project, and a cheap disk brake caliper so it was a no-brainer.

It was difficult to braze the brake onto the fork so the brazing is not pretty. They must have used some very cheap metal. I tried a couple methods to keep the disk brake bracket lined up and in place and ended up marking the location with a magic marker, and then clamping it into place with a C clamp. This worked much better than the magnet.

5/11/2018
I finished wire wheeling off all of the pink paint and put the bike together so I could sit on it and wheel around. Whee!
While building the motor brackets I discovered that the chain hit the frame. I had to cut the frame, re-bend the left side stays, and then re-build the stay extenders. I also had to remove the rear brake bridge and make a new one.
5/14/2018
Both chains are on and I rode the bike up and down the driveway. I still need to find a half link or make it so I can adjust the motor position a bit so both chains are tensioned properly. It seems to handle like a tiny BMX bike! Wow it's already feeling heavy.

I disassembled the old 48V ping pack from my cruiser bike, cut the Lexan battery box in half because this is a 24 volt motor, and fit it to the BMX frame. I still have to make some brackets to attach the battery box to the frame robustly.
5/17/2018
Connected the power wiring, charge port, fuse, and power switch.

 

Jammed the battery in the rebuilt box (tight fit!), mounted the controller, and temporarily wired it up to the motor to make sure everything still works after sitting for many years. It works! And the throttle even has lights on it so I don't have to figure out how to make an LED work with 24V.
 
5/19/2018
Finished the wiring, connected the BMS, crammed all the wires into the battery box and took it for a test ride up and down the block a few times. I added a cover for the motor sprocket to make it safer.

The bike weighs 50 lb with the battery box. Performance with me on it is not spectacular, and top speed is under 20 MPH. Perfect!

5/20/2018
Here's me on the unpainted bike.

The kid was itching to ride it even though it wasn't quite ready, which meant that he had to ride it without using the pedals because that chain was still very loose. He rode it all day, until the BMS cut the battery off. I'm not sure how far he rode but I'm pretty happy with the range. He asked me what the other switch was for on the battery box and I told him it was for the headlight. "Headlight?" he asks. "So basically this is a motorcycle?" Yes. Yes it is. LOL!

5/25/2018
Took the bike all apart for painting. The quill was stuck in the steerer tube and I had to use the torch to free it. I think that happened during the extended testing last weekend.

I finished capping the tubes and the rest of the brazing, cleaned up the joints, and shot a layer of black primer onto the frame.

6/1/2018
Painted it with metal flake midnight blue auto paint, re-assembled it and adjusting the chains. I had to use a half-link on the human powered side to get the chain tension right on both sides.

Painted the chainring black after this picture was taken after I was informed that the flowers weren't appropriate for a boy. LOL.

Delivered the bike on 6/2.

6/27/2018
He's been riding it every day he's been home and it wasn't raining. I added a bike computer to see how many miles he put on it. I also added an "always on" bright tail light that's powered by the e-bike battery so he knows to turn off the battery. The LEDs on the throttle are rather dim so it got left on occasionally. After he complained that it didn't get very far before cutting off, I had to explain that it really needed to be charged overnight to allow the battery to balance. It's been fine since. The headset was loose and the chains were very loose so I adjusted those. Looks like it goes about 16MPH, which is slower than I thought, but keeps it pretty bulletproof. He hasn't asked for more speed yet and I could get a cog with an extra tooth or two easily but he doesn't wear a helmet so I'm a bit apprehensive about that. Kickstand is not working great, spring may have gotten hot and lost its temper. I will need to fix that.
   

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