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44Dwarf UltraDork
12/6/16 4:58 p.m.

Tack weld some "key stock" to the drop out. if your wheel diameter has changed you can use this to lift or lower the the axle centerline. key stock all sizes

problemaddict HalfDork
2/17/17 11:56 p.m.

update time!

Okay, so we left off with needing some welding done. After the holidays, and when it was actually warm-ish enough to bother heating up the garage, I did work.

As 44Dwarf suggested, I looked into key stock, but found that McMaster.com had normal steel in all the same measurements for much cheaper. I don't need the high strength of key stock because the motor's axles are made out of chinese c̶h̶e̶e̶s̶e̶ steel and there are instances where hardened steel has just chewed through ebike axles.

So, I got some 1/4 by 1/2 inch stock and some 3/8 by 3/4 inch stock. I used the 1/4 for my new dropouts and used the 3/8" as a spacer. My axles have 10mm flats and 3/8" is 9.5mm or so. I figure I'll slowly file the dropouts until the axles install with a bit force and have a nice and tight fit. Used some washers to space things and setup for welding:

I knew my little Craftsman MIG wouldn't have enough juice to do the finish welds, but I figured it would be enough to tack everything in place. The multiple welds shown were me just playing with different settings. Later I realized I only had 0.025" wire in the MIG. I should have tried it with some .035". I had plans to bring it to my buddy who is a certified welder.

Now, on to the front fork. I got my spacer game on point and the disc hub installed.

The brake calipers have adjustable pads which allowed me to clamp the caliper onto the rotor. Now I could stare at the rotor, caliper, and fork for a long time. I could use this time to contemplate how I should connect all these pieces together... Maybe some beer would help... mmmmm...beer...

Anyway, after all this contemplating, I got to work. Starting with some magnets, then, some cardboard:

I'll stop here, for now....

AngryCorvair UltimaDork
2/18/17 6:51 a.m.

Mike! Good to see you back, maing!

problemaddict HalfDork
2/18/17 12:26 p.m.

Hey Pat! I've always been lurking. Mainly focused on 2 wheeled adventures these days...

84FSP Dork
2/18/17 12:31 p.m.

Have not seen this done to a bmx before. Another reason why the GRM hive knows all.

problemaddict HalfDork
3/19/17 8:22 p.m.

Seeing as I've been scooting around the neighbourhood on this thing, I should upgrade the thread.

Got my buddies help with the welding. Used a Lincoln tombstone arc welder (thanks Les B.!) that had a hard time starting an arc. This is by far the best looking weld.

My first time assembling a spoked wheel went far easier than expected. Horror stories abound, but thanks too YouTube I found it rather straightforward.

Dished with soda cans and some change.

True'd with zip ties:


We got a roller! Starting on the prototype stage of mounting the important bits and wiring.

Got this doohickey from Thingaverse. It's a plug for the Echo battery. Had it 3d printed by a sorta local library. Built a temporary power harness.

Wire it all up, a bunch of zip ties and Velcro, and BOOM! Functional ebike prototype!

With more speed out of the box than I expected!

The controller is pulling 35.5 Amps from the 52v battery. 1846 watts = 2.5 horsepower! This motor is easily capable of 4500w continuous.... Oh, and the bike weighs 52lbs as you see it here.

Stay tuned for videos!

problemaddict HalfDork
4/1/17 6:54 p.m.

Built a harness to parallel two battery packs, and some hardware store brackets and more zip ties and I'm good for 12 miles or so.

That also brings the amp draw down to 15.2A per pack which should help them last much longer.

I got regen braking working, but it's too strong and chirps the rear tire every time I pull the lever. Also, I found this controller has a high speed mode! With that mode selected and the wheel of the ground it spins a good but bit faster, but with me riding I am still at 32mph top speed. I have to look into that further...

Waiting on some 3d printed parts to show up. Then I can mount everything up permanently, clean up the wiring and do some touch up paint.

problemaddict HalfDork
4/5/17 8:49 p.m.

Video of the first day of messing around.


Embarrassing wheelie attempts!!

Click Subscribe!!

Nick (Bo) Comstock
Nick (Bo) Comstock MegaDork
4/5/17 11:02 p.m.


That looks like a blast!

problemaddict HalfDork
4/30/17 9:18 p.m.

Woo! The "high speed mode" I spoke of earlier randomly kicked in on a ride today and I got a significant increase in top speed.

My goal was 40mph. I thought I'd have to assemble a higher voltage battery to achieve it, but i guess not. So I'm stoked!

problemaddict HalfDork
5/30/17 3:31 p.m.

How I spent my 3-day weekend:

eBMX build Time Lapse YouTube

jere HalfDork
6/15/17 9:15 p.m.

Oh man I did not need to see this right now... Been daydreaming about how to make gravity bikes viable on places other than hills...

problemaddict HalfDork
6/27/17 8:50 p.m.

In reply to jere:

That'd be pretty rad! Do it!

problemaddict HalfDork
8/20/17 5:23 p.m.

How to build an electric BMX bike and hurt yourself with it!

Episode 1 of my YT channel is up! I hope you enjoy it...

NBraun Reader
12/13/17 11:37 a.m.

Do you ever find  yourself wishing for more power/a  bigger motor etc? I've been doing quite a lot of research and had plans to build a bike with a 2000 watt qs205, 20s lipo etc. I wanted to build a pretty quick bike that could cruise at 25-35 but ended up decided against it just because of the cost. Now i'm back looking at e bikes again and wondering if i should try to build a cheaper/slower bike first, but i keep getting caught up with, "if im going to spend that money on this part i may as well pay a little more and get the faster version".


I've never even ridden one, so maybe I'm getting ahead of myself.

problemaddict HalfDork
1/8/18 8:40 a.m.

Sure!  Who doesn't want more power!?!  laugh. My bike is putting down @ 2000w, and for bmx-y type stuff (easy trails, grass, parking lots, bombing around the neighbourhood) the power is plenty...  for commuting, you might want more only because a steady 35mph feels pretty uneventful.  But on a bmx, anything above that feels sketchy as Hell.

Now, as far as your qsmotor on 20s lipo, that would be a seriously capable setup.  First thing to keep in mind is never believe the wattage rating of ebike components.  For whatever reason they are usually seriously understated.  For example, my motor is a Leafmotor "1500w", but it is capable of 6000w peaks and over 3500w continuous.  There is a vid on YouTube of the same motor I have, but in a mountain bike, pulling a 60mph top speed.  My motor controller, a "Sunwin 500w" isn't even getting warm at 2000w.

The qs 205 should be a good deal more powerful than my motor.  The widest version, 205 50h, can do over 8000w peaks.  At 72v, you'll be well over the 25-35mph you mentioned.

What bike will you use and what kind of performance are you looking for, ultimately?  I can throw you some setup suggestions.


Finally, I'm sure you're already checking it out, but if not, all the info you need is at www.endless-sphere.com

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