eMTB with Custom Build Drive Kit

[gkasev]

The design of this Drive Kit has been heavily influenced by the work that Ron did with his 750W Chain Puller. After studying his work, and spending some time figuring out how it worked and what parts he used, I decided to design my own kit. I briefly considered buying a ready to install mid drive kit, but non of the ones I found seemed to meet the sweet spot between quality and price, and I like building stuff myself.

My basic idea for the build was to make exclusive use of sheet metal, 3d-printed and off-the-shelf parts to keep the cost of the mechanical part of the build as low as possible, with no CNC-machining and no welding. In the same spirit, all electrical component had to be cheap repurposed off-the-shelf parts. My aim was to build this kit for about 1000€ with batteries included (no pun intended).

After a couple of iterations, the design converged to what you see below. The kit will fit a bike with a 83mm BSA bottom bracket. All electric components, including batteries and controls, are integrated in the kit. The batteries can be easily removed for charging, by swiveling the mud guard plate.











The central piece of the kit is a Neugart PLE60 planetary gearbox with a gear ratio of 32. This gearbox can fit different motor shaft diameters, ranging from 6mm to 11mm (also inch size shaft diameters), depending on what kind of adapter you use — see image below. The motor I use is a very affordable NTM 50 60 from HobbyKing, 270KV, 2400W. I considered using an Astro Flight like Ron, but those are very expensive and would have busted my budget right from the get-go. That being said, the design can fit the Astro Flight with small modifications. I use two Turnigy Graphene 10000mAh 4S 15C LiPo batteries connected in series for a 29.6V nominal system voltage.



Being on a tight budget, I was able to procure/purchase all parts month by month and finish the build within a year. This includes all parts for the kit, as well as building the bike itself, which I did not have prior to starting the project. You can see the final build below.











The kit ended up costing me 1212€ with the batteries, throttle handle and both chains, which was slightly more than my initial budget. It has a couple of rough edges, mainly cable management and the chain tensioning device, but overall it works pretty well. It has so much power, that it feels a little scary. I have to test the range of the bike yet, but so far it seems to be pretty solid.

UPDATE Mar 6 2022: I was able to go for a spin today and test the range of the bike. I rode it outside the city on flat, paved road with almost no stop-and-gos and moderate deceleration/acceleration. With minimal headwind throughout the whole ride I was able to go for close to 5km purely electrical (no pedaling). I expect the range of the bike to be around 20km, if I use the electric drive to pedal-assist me, as intended. This kind of range was what I planed for all along and I couldn't be happier.

Additional posts about the controls and the drive train side of the kit will follow as sub-posts to this one. Feel free to get in touch, if you would like to build something similar and I can provide the 3d-model of the kit.

P.S. The most frustrating part of the whole build was soldering the XT90 connectors. This was just a nightmare. Make sure to read about it before trying and also be prepared to invest money in a proper soldering iron.

 

[neptronix]

Hey, congratulations. Traffic is flooding in from ycombinator to this thread and we're getting slashdotted
I just now had to upgrade the server to a huge one temporarily.
No biggie, we've just never seen this much traffic that *wasn't* a DDOS :lol:

Nice build by the way!!

 

[gkasev]

Thank you for the friendly message and I'm sorry for the inconvenience.

 

[pwd]

Nice build; nice to see a DIY mid drive setup. Why did you go with a single speed setup?

 

[SafeDiscDancing]

Nice build; nice to see a DIY mid drive setup. Why did you go with a single speed setup?

I use a similar sized motor at about 2,500 watts and drive directly to the rear wheel with a separate #219 chain (more like the China gas bikes at 50cc) and you just don't need gears except for when you hit a hill steeper than 10% grade.

Assuming you gear it for 28 mph as your top speed anyway.

My guess is a lot of people are wildly off on gearing and are high by 25% - 50% so they have troubles but if you hit it right you can get rid of the extra gears.

This type of motor tends to very efficient too.

My setup is 11t to 100t (#219) and a 6384 motor with a newly rewound 94 kV going direct drive to a 26" rear wheel.

Another huge benefit for his build is you can use a BMX CHAIN which is much stronger than the narrow chains these days which would likely make the chains last five times longer.

---------------

Wild Idea

Has anyone tried a complete swap of bicycle chains to #219 go kart chains?

I mean you can buy the sprockets easily for the front chainring side, but you would have to build your own rear sprocket somehow.

My "guess" is if you started with an oversized rear freewheel that had say 22 teeth you could grind it down and transform it into something that would fit the #219 pitch.

Then you have infinite chain life... or nearly infinite.

 

[gkasev]

Nice build; nice to see a DIY mid drive setup. Why did you go with a single speed setup?

I plan to use the bike on flat, off-road terrain. The thinking behind using a single speed was that I would have a lot of power to my disposal to accelerate to 20/25kmh and can then pedal with constant high cadence. I also do like the cleaner look and simple setup. I need to revisit my calculations, but I had a cadence between 70 and 80 rpm at 25kmh with this gearing.

 

[BalorNG]

Nice build; nice to see a DIY mid drive setup. Why did you go with a single speed setup?

I use a similar sized motor at about 2,500 watts and drive directly to the rear wheel with a separate #219 chain (more like the China gas bikes at 50cc) and you just don't need gears except for when you hit a hill steeper than 10% grade.

Assuming you gear it for 28 mph as your top speed anyway.

My guess is a lot of people are wildly off on gearing and are high by 25% - 50% so they have troubles but if you hit it right you can get rid of the extra gears.

This type of motor tends to very efficient too.

My setup is 11t to 100t (#219) and a 6384 motor with a newly rewound 94 kV going direct drive to a 26" rear wheel.

Another huge benefit for his build is you can use a BMX CHAIN which is much stronger than the narrow chains these days which would likely make the chains last five times longer.

---------------

Wild Idea

Has anyone tried a complete swap of bicycle chains to #219 go kart chains?

I mean you can buy the sprockets easily for the front chainring side, but you would have to build your own rear sprocket somehow.

My "guess" is if you started with an oversized rear freewheel that had say 22 teeth you could grind it down and transform it into something that would fit the #219 pitch.

Then you have infinite chain life... or nearly infinite.

A well maintained and properly *waxed* bike chain lasts pretty much indefinitely anyway under human power AND allows for gears which is really good for efficiency.

I'm using a HDT belt on a similar setup, but I also have a chain like this, too, used briefly with, ehehe, 3d printed cogs which held fine and not surprizing because some of the better sprockets for cars are also composite thermoplastic.

I'm not entirely happy with a belt due to finicky alingment issues and tendency to skip sometimes when regen braking, maybe I should also try that setup again (I have 92T sprocket and 120KV 6384 motor of the cheaper variety, I have low-voltage setup and intend to fix low end torque deficiency by going dual drive with similar 'crank drive' arrangement).

This way you can use a bicycle chain for gears with a crank drive, and still have good lifespan because you'll only be using it very intermittently.

 

[BalorNG]

Nice build; nice to see a DIY mid drive setup. Why did you go with a single speed setup?

I plan to use the bike on flat, off-road terrain. The thinking behind using a single speed was that I would have a lot of power to my disposal to accelerate to 20/25kmh and can then pedal with constant high cadence. I also do like the cleaner look and simple setup. I need to revisit my calculations, but I had a cadence between 70 and 80 rpm at 25kmh with this gearing.

It would indeed be better to go fixed gear, left-side drive with a large sprocket, replacing the brake disk and serving as rear brake as well. This way your ditch a few reduction steps, setup is still light and contribute little to unstrung mass, but indeed there would be a slight torque deficiency and adding double reduction in a typical bicycle rear trinagle form-factor is a major hassle...

 

[neptronix]

Thank you for the friendly message and I'm sorry for the inconvenience.

Hey, no problem. Thanks for making the world aware of the site, and i hope you stick around.

 

[gkasev]

One detail that took me a long time to figure out was how the controls of a system like that work. After a little bit of searching, I was able to figure out that some of the builds described on this forum use the excellent Domino electric throttle. The question then was how to connect it to the ESC in the most effective way, given that it's signal is an analog potentiometer value and the ESC expects a PWM signal.

For this purpose I use an Arduino Nano running a simple routine that reads the potentiometer value and translates it to a PWM signal. I use the BEC output of the ESC to power the Arduino board and read the throttle signal, this way I don't need any additional power source only for the Arduino board. The BEC output voltage of the ESC is set to 7.4V. The controls setup is given below.



The Arduino routine is very simple and just maps the current potentiometer value from the throttle to a PWM value. The minimum pulse that the ESC expects is 1000 milliseconds (no throttle), and the max is 2000 milliseconds (full throttle). The potentiometer read value is between 0 and 1023.

/*
  Created on 21.08.2021 by Georgi Kasev
  https://howtomechatronics.com/tutorials/arduino/arduino-brushless-motor-control-tutorial-esc-bldc/
*/

#include <Servo.h>

// create servo object ESC
Servo ESC;
// create variable to store potentiometer value (throttle position) 0 (0V) to 1023 (5V)
int potVal;

void setup() {
  // put your setup code here, to run once:
  // connect ESC on ditital pin 9
  ESC.attach(9,1000,2000);
  // for debugging
  // Serial.begin(9600);

}

void loop() {
  // put your main code here, to run repeatedly:
  // read potentiometer value
  potVal = analogRead(A6);
  // sclate potentiometer value to use with servo library
  potVal = map(potVal, 0, 1023, 1000, 2000);
  // send scaled potentiometer value to ESC
  ESC.write(potVal);
  // for debugging
  // Serial.println(potVal);

}

 

[peter100]

Hello @gkasev,
I sent a DM yesterday. Did it go through?

 

[gkasev]

Hello @gkasev,
I sent a DM yesterday. Did it go through?

Yes, thank you for your message. I just replied to it. Feel free to contact me, if you need anything else.

 

[gkasev]

Here is an update about the range and the overall feel of the bike.

My daily commute to work is approximately 18,5km of flat terrain with approximately 50m elevation change, so basically as flat as it gets. Roughly one third of it is in the city (paved roads, heavy traffic with lots of traffic lights), one third in the outskirts of the city (paved roads, almost no traffic lights) and another third on cross country roads (unpaved road with loose gravel) as shown below. I usually ride a hardtail mountain bike (carbon fiber frame, 100mm front travel) to commute to work.



Yesterday I switched the hardtail for the eMTB. Starting with a fully charged battery, I was surprised to arrive at work and find out that the battery was still at 80%. I used the electric drive mostly to accelerate to a speed at which I can pedal with high cadence of 70-80 rpm. Whenever I felt unpleasant resistance, as for example along the cross country part of the route, or when headwind occurred, I used the electric drive continuously alongside pedaling. With this style of riding, I can definitely see the bike going for 60km and more on a single charge with plenty of battery left. This is just fantastic, given the compact size of the drive kit and the batteries, and it clearly exceeded my expectations.

Overall the concept that I had in mind - accelerate to a speed at which you can pedal with high efficiency using the electric drive and then just pedal - works really well. In this state the bike feels almost effortless to pedal, although you are expending a lot of energy (more about this below). I think this effect is due to the high impulse of the bike at speed (the bike is heavy, so it doesn't slow down easily) and the efficiency of the human body to pedal at high cadence. My hardtail clearly doesn't feel like that.

As a side note, keep in mind that the electric motor is connected to the rest of the drive train through a freewheel, so there is almost no added friction to your pedaling when the motor is not running. This is clearly where a lot of commercial e-bikes fall short, where once you reach a certain speed, the pedal assist drops off and you feel the high resistance of driving the motor when pedaling.

Another plus for me is the added comfort of a full suspension bike, compared to a hardtail. The bigger tires also play a significant role here, as well as the upright position. Riding the new bike made me aware of how much the carbon fiber frame of my hardtail wears me down. The bike is really fun to ride on the unpaved sections of the route.

I also wanted to see how both bikes compare terms of physical effort to commute to work. Is it easier and quicker, if I ride the eMTB? As you can see from the data below, I was quicker when I rode the eMTB, but a lot of this depends on the traffic light situation. My commute can easily take 5 minutes longer, if I hit all red lights along the way. So, while I was quicker and got home in less than 50 minutes, something that I could never achieve on my hardtail, I think the difference in speed is only marginal. 5% to 10% at most. On the other side, I had to expend more energy when riding the eMTB and even though it felt effortless, I was sweating more when pedaling with high cadence. The big disappointment was that when I came home, I felt as tired as with my hardtail.



So the question then is: Is it worth it to build a bike like that? Fun and comfort aside, right now I would say - not that much. At least not in this form. I would clearly like to keep the effortless property of this bike with my next build, but I don't know how to achieve this without the added weight, which I don't like to have. A full suspension frame is a must, bigger wheels (29") and tires as well. Maybe a tiny torque generator of 50W-100W is the way to go. The batteries would be so much smaller and lighter, and the system could be tiny compared to this.

Let me know what do you think in the comments below. What is your experience in riding an ebike to commute, compared to a normal bike?