Spare parts E-BMX Build "Deathbike Junior"

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Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » Apr 02 2018 6:19pm

I have a lot of spare parts laying around, and my GF's 12 year old has been drooling over motor scooters, so I'm going to build up a quick and low power BMX e-bike. I cut up a small pink BMX bike, 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, which is good because this kid is VERY good at breaking things.
ebmx.jpg
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I found some nice leftover chunks of 1.5" chrome moly to braze to the cheeseball pink bike rear end and front end. It will be a bit longer and higher than stock, which is good because it was tiny. I have the BMX wheels that went with the motor. The rear wheel has drives on both sides of the hub, so it can be pedaled and driven as an ebike, or both. I wish I could find more of those hubs! I'm planning on brazing on a disk brake bracket for a bit of additional safety.

I'll update this thread as I make progress.

Warren

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by liveforphysics » Apr 02 2018 9:21pm

Nice build and good luck!
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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » Apr 12 2018 11:31am

Thanks, Luke.

Looks like I will need to keep the wheelbase as short as possible rather than stretching it way out so the kid can reach the handlebars.

I need to spread the rear triangle to fit the dual drive hub. I'm going to try the scissor jack from my car.

I found a braze-on disk brake mount which I'll use on the front fork.

Also I need to buy more acetylene and O2 so I can braze it up.

Warren

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by amberwolf » Apr 13 2018 4:50am

www.recumbents.com wrote:
Apr 12 2018 11:31am


I need to spread the rear triangle to fit the dual drive hub. I'm going to try the scissor jack from my car.
how about an old axle with nuts on teh inside of the dropouts, to spread them by turning the nuts. Use a couple of pieces of something like baling wire from the axle near each nuts starting point to the BB shaft ends to keep the axle from wandering backward out of the dropouts.

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » Apr 13 2018 2:48pm

Oooh. Yes I have done that before and have the axle all ready to go. Thank you for jogging my memory Amberwolf!

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » Apr 18 2018 9:28pm

I used the axle from on of my jigs to crank the dropouts open further.
spreading_dropout.jpg
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Here's the dual drive wheel in dropouts
wheel_in_dropouts.jpg
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Got the wheels clamped into my jig so I can be reasonably sure it will be straight and started brazing up the frame. It's really hard to make it look nice in a dark and cold garage!
frame2.jpg
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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » Apr 23 2018 5:56pm

Got the bike all tacked together and stuck the seat and handlebars on it for a picture. Wheelbase is 40 inches, which is pretty long for a BMX bike, but I needed that space for the battery. It's still a tiny bike. Should be fun.

The motor says 450 watts but topped out at above 600 watts when I used it on a previous bike. The old ping delivered about 1000 watts tops when I used it last, so it should be pretty well matched.

Image

I bought this ebay 24V 30 amp generic lifepo4 BMS, so 30A * 24V = 720 watts. If it's not junk it should be perfect.

Image

Also picked up some brass filler rod to finish brazing up the frame.

-Warren
Last edited by www.recumbents.com on May 04 2018 10:21am, edited 1 time in total.

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » Apr 23 2018 8:39pm

Finished brazing up the frame tonight. Next I need to build the motor bracket and get the front disk brake brazon mounted.

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by liveforphysics » Apr 23 2018 8:46pm

Nice brazed joints and nice work on a cool project!
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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » Apr 25 2018 12:37pm

Thanks Luke!

I got all the pink paint off the bike last night with the wire brush on my angle grinder. My neighbors must love me...

After that I attempted to braze the disk brake bracket onto the front fork. Lined it up and held it in place with an old disk drive magnet. Twice. The first time I neglected using flux and the braze refused to stick. Second time I did use flux and tacked it in place but then during post tacking QA I noticed it had shifted. Got disgusted and went inside to veg out to an Amazon Prime movie...

Warren

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » May 04 2018 10:19am

I was away from home for almost a week visiting relatives, so didn't get to do any building.

Last night I got the disk brake bracket brazed on. This time I lined up the bracket, marked the location with a magic marker, and then clamped it into place with a C clamp, which worked much better than the magnet. Whatever steel they used did not like to be brazed so the brazing is not pretty.

Image

I received the (tiny!) LiFePo4 BMS and the charger, so I should have all the parts. Now I just need building time!

Next I will mount the motor!

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » May 11 2018 8:20am

Here's the bike assembled enough that I can sit on it and push myself around. It's still tiny!

Image

While fitting the motor I found that the rear stays hit the chain, so I had to cut the left one away from the seat down tube and bend it out a bit.

Image

Motor is now mounted and I have reattached the chain stay to the seat down tube, but I'm out of acetylene. I'll have to run out for more today, plus some 5mm bolts.

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » May 11 2018 9:14pm

Motor is bolted in and I ended up having to tweak the lower stay too to have clearance for the chain. Motor chain is on and now I need to get more cheap BMX chain for the human power side. I was hoping they’d both be tensioned properly but I will just have to slot the motor mount holes instead.

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » May 14 2018 8:54am

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, and fit it to the BMX frame. There is very little or no clearance to the chain ring. I didn't think about that. I can just offset it a little bit in the back to provide clearance. Now I have to make some brackets to attach the battery box to the frame robustly. I'm starting to get excited as it will be ready to test ride under power soon.


Image

I have two controllers in my parts box that I'm trying to decide between. One is the 20A one that came with the motor that also has a brake cutout switch, and the other is a 30A one with no brake cutout. I'm leaning toward the 20A one because this kid is going to run it wide open all the time until it breaks. He breaks EVERYTHING! This weekend it was the USB cable to his PS4 and the toilet handle. Apparently I need to teach him to fix everything...

More pics of progress later...
Last edited by www.recumbents.com on May 17 2018 7:41pm, edited 1 time in total.

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by amberwolf » May 14 2018 2:17pm

www.recumbents.com wrote:
May 14 2018 8:54am
I'm leaning toward the 20A one because this kid is going to run it wide open all the time until it breaks. He breaks EVERYTHING! This weekend it was the USB cable to his PS4 and the toilet handle. Apparently I need to teach him to fix everything...
Sounds like me. :oops:

Thats one big reason I learned to fix stuff in general (other than not being able to afford new stuff)--Im really hard on everything, even my clothes. :oops: All I have to do is touch it and it could fall apart! (this happened yesterday, for example, when I put the slime Raine had ordered into his trike tires: the valve core in the left rear tire disintegrated as I removed it, but left a piece inside, so I couldnt replace it with a new one, and had to use one of the RV adapters off my trike to give it a valve to keep the air in, until I can replace the tube. The right rear valve stem itself blew out rigth where it joins the tube, but not while I was working on it--it happened sometime between then and this morning.... (both were perfectly ok until I touched them)

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » May 15 2018 1:26pm

Yeah Amber, he already does try to fix things so the aptitude is there.

Last night I worked on the battery mounts and capped the last tubes and ground off some of the ugly "brazing in a dark garage" issues.

It's getting very close to a powered ride and I should probably get the front brake working now so I won't be tempted to test ride it with no brakes!

Wow, it's been almost 10 years and 4 e-bikes since I joined ES.

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » May 17 2018 10:52am

Made the brackets to mount the battery box last night. It's very solid now. I had to make a small cutout in the Lexan box where the chainring was too close. That should be fine. A single half link costs about $5, and an entire half link chain costs $20. Ordered the single half link...

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » May 17 2018 7:47pm

Jammed the battery in the box (tight fit!), mounted the controller, and temporarily wired it up 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.

Image

Now I have to hook up the BMS and finish cleaning up the wiring. And brakes. I need to add the brakes!

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » May 19 2018 11:45am

Today I remade the rear brake bridge that I removed when I terrorized the rear triangle to provide motor chain clearance. Brazing when it's light out is so much nicer! I also reinforced the motor mounts and added the front brake cable, so I have brakes! The goal for today is to take a powered ride around the block. The half link isn't arriving until next week so I'm leaving the pedal powered side disconnected for now. I still have to finish wiring up the battery
and BMS.

It weighs 35 lbs with the motor but no battery. That's less than I imagined. It should weigh about 45 pounds with the battery. That's heavy but managable.

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » May 21 2018 9:33am

I worked most of the day Saturday on the bike. 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. It weighs 50 lb with the battery box. Performance with me on it is not spectacular, and top speed is under 20 MPH.

Image

Still to do:
- Wiring still needs to be cleaned up
- Install half link to keep human power chain from flopping around.
- Bright power light.
- Speedo?
- Paint!

Image

Sunday the kid rode the e-bike, without using the pedals. All day. He rode it 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! I was going to take off the knobby tires and add some street tires to make it a little faster, but it seems fast enough for him. Maybe later.

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » May 22 2018 3:51pm

Installed the half link last night. I think the chain tension on both sides is now good. Yay!

I'm charging the Ping battery for the first time as a 24V battery, and the cheap LiFePo4 charger seems to be working, but it buzzes loudly. Is that normal?

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » May 23 2018 9:42am

The battery did finish charging with the new BMS so that's all working great.

That was the first time I wired up a BMS myself and it was relatively painless, so I think I will now look for one for my home built 18650 pack .

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » May 25 2018 1:39pm

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. :roll:

Anyway, I shot some black primer on it and now need to go look for some midnight blue metal flake paint.

Image

I could have spent more time to make the brazing pretty but the rider cares more about when it will be done!

The battery box is all back together and the wiring up to the handlebars was cut down to the right length and re-soldered.
Also I had to shorten the kickstand a bit.

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by www.recumbents.com » May 29 2018 8:04pm

Painted the bike rattle-can midnight blue metallic. I should have it back together and "done" by the weekend!

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Re: Spare parts E-BMX Build "Deathbike Junior"

Post by wturber » May 29 2018 9:56pm

I hope this kid is appropriately thrilled with this thing. Heck. I'd be thrilled with it. ;^)

When he breaks it (that seems inevitable), it would be cool to involve him in the repair.
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