4kW dropout mounted RC drive

Squilt

100 mW
Joined
Sep 16, 2022
Messages
44
Location
Hagerstown MD
I interrupted my recumbent trike build-in-progress to make an e-bike for my roommate to get to work. The two goals were maximum power to cost, and ease of operation and maintenance. I don't remember how I settled on each component. There was a lot of guessing around the power figure but I'm very happy with how it turned out. I managed to complete it with ~$700 including the bike. The result is a 62lb 4kw e-bike with a 750wh battery. It has a range of 20-25 miles w/o pedaling and a top speed of 35mph.

I started by searching for the cheapest bike I could get with full suspension. I got an Ironhorse MTB on Craigslist for $75. It was very lightly used and then kept in a garage for years. Starting weight ~40 lbs. It had a crappy front shock and dangerously weak brakes. I upgraded to a 180mm front rotor ($18) and replaced the caliper and plastic lever with ones I had lying around. I was going to replace the shock but it seems to be getting better with use.
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Next to buy were the cells I found on batteryhookup.com for a good price. I ordered 100 Samsung ICR18650-22p cells for $93. That's $120/Kwh or 8Wh/$. With that and a $62 Daly BMS, $42 of nickel, and $12 of tape, I made a 14s7p pack.
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Next on the purchase list was the ESC and Motor. I went with the Spintend 100v 100a Ubox based on Vesc($147), and the Freerchobby 63100 Sensored outrunner($140). I could have gone with a hub motor for a much simpler setup, but as I have access to shop tools and free aluminum, an RC drive is much cheaper for the same power output. I also anticipated this being more reliable and easier to service than a hub motor that's on the cheap side. Not to mention the efficiency and weight savings.

Sprockets, chains, and bearings: $93

I mounted a sprocket where the rear brake rotor used to be and mounted the drive unit where the rear disc brake caliper used to be. It is held on by the M6 caliper bolts, 3/8 bolts through holes in the dropouts, and the quick-release axle. It is extremely rigid and hardly flexes at full throttle. The drive unit is a 16:1 two-stage chain drive. With a #25 chain in the first stage and a #35 chain in the second stage. I could have gotten away with smaller sprockets but for the sake of reliability, I followed the standard recommendation of no less than 17-18 teeth on the smaller sprocket.
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Adding the cost of an XT-90 splitter, barrel jack charge port, and the headlight I'm putting on brings us to a total of $713. The rest of the foam, coroplast, connectors, heat shrink, hardware, and the charger, were things I had on hand or acquired for free.

The result here is a competent commuter that's a blast to ride. I told my roommate the build wouldn't take much more than a month. That was four months ago. What took the longest was the battery. I'll admit that's because I was afraid to work on it. More afraid to find out this pack wouldn't work for this bike than afraid that it'd blow up. But it just barely fits and it's holding up well after a handful of cycles.

The deal was that I get compensated for the parts but not my work, but since I like this stuff it doesn't feel like work. I'm happy with this arrangement since I got to take on a design challenge and make something new for free, and when he no longer needs it, I get a kick-ass e-bike for free.

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Woah, are you cutting your own nickel strips for spot welding with a water jet? That's a first for me! Nice build. What material are you using on the sides of the battery?
 
That was my question also, how did you cut the nickel strips?

Very clean buildup, a lot of thought went into this. I'm presuming it's not to be ridden in the wet conditions?
 
Woah, are you cutting your own nickel strips for spot welding with a water jet? That's a first for me!
I used a CNC mill to cut them out. It was a pain, I wouldn't do it again if I can avoid it.
What material are you using on the sides of the battery?
Corrugated plastic sheet. The stuff lawn signs use.
 
I'm presuming it's not to be ridden in the wet conditions?
I wouldn't trust it in the rain, but it can handle wet roads since I added a small mudguard on the rear. The battery should be pretty water resistant. If I enclosed the ESC and wrapped some wires, I could maybe get it weatherproof. But my friend wouldn't want to ride it in the rain anyway.
 
Whats the noise like from the rc and all those gears?
Interesting to see the 50kv option for that motor at a reasonable $110 price point from my random search.

With that motor sticking so far out to one side have you had any issues?
 
Whats the noise like from the rc and all those gears?
It's a nice sound IMO. Not as loud in person as the video might make it seem.

Interesting to see the 50kv option for that motor at a reasonable $110 price point from my random search.
Yep, I considered it for a less steep reduction but I wouldn't be getting the most out of the motor if I spun it slower. So I went with the 146kv. Though it doesn't seem to really be 146kv. At 52v I'm only getting about 6500 rpm. I emailed them about it but it's been nearly a month with no response.

With that motor sticking so far out to one side have you had any issues?
Not besides looking silly. I don't expect it will. Doesn't stick out far past the pedals. I thought about adding a scraper that protrudes in front of it in case it gets laid down.
 
Very cool DIY build, lot's of neat things going on here.
 
Installed a headlight before handing it off.
 

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