Newbie trying to build a cheap mid-drive ebike conversion

[sylph]

Hi folks.
I've been lurking here for years, and riding one of those noisy converted road bikes with a gas engine.

I've been meaning to start working on an ebike for ages, and finally I'm getting around to it.
I'd ideally like to keep things extremely cheap and simple, even if that means losing some performance, and was hoping for a little advice. At the moment my ideal plan (probably pretty flawed) is to buy one of those 'my1020' motors from aliexpress, bolt a 36+ tooth sprocket on to the cranks, and have the motor thus pulling the cranks for me. I know that ideally I should fit a freewheel, but I think doing so will require welding beyond the scope of a simple, quick, dirty ebike conversion. I'm of the understanding that this might not have enough torque to do much, given that the ideal RPM of these motors is about 2800rpm. That would mean rotating the cranks at nearly 1000rpm for optimal power right? Is there a cheap(ish), commonly available reduction that can easily be fit on most mountain bike frames? Or is this not going to save any time and money in the long run, and I'd be better just getting something pre-made for mid drive (bafang? Or is there a product that's cheaper?)

Second, I wondered whether a controller is necessary?

Third, I wondered if, with appropriate reduction, it is even possible to have a mid-drive pulling the cranks without a freewheel and getting a decent output?

Thanks tons in advance. I know these probably sound really misguided, but I'm in quite a bit of confusion about how electric motors perform. I used to ride motorbikes, and am rather used to the power of petrol.

 

[amberwolf]

First, a simple cheap $200-$300 hubmotor kit is going to be WAY easier to do, and probably cheaper, than a DIY middive via experimentation.


Second, if you don't go that route, I highly recommend you read around the non-hub motor forum section at the various DIY builds, to see what people have successfully done, and what it took to do it, to get the kind of performance you are after, before you go any further. Middrives are generally not going to be all that simple to come up with from scratch; Among other things, you need to calculate ratios for sprockets based on the voltage you're using, and the motor kV (RPM per volt), the gear ratio to the rear wheel, the rear wheel diameter, and your desired speed and torque.

You can, of course, just play with stuff till it does what you want, but it is likely to take a lot of experimenting to get the desired results. (it may also break a lot of parts...I burned up a fair bit of stuff doing it this way).

At the moment my ideal plan (probably pretty flawed) is to buy one of those 'my1020' motors from aliexpress, bolt a 36+ tooth sprocket on to the cranks, and have the motor thus pulling the cranks for me.

The number of teeth on the crank sprocket needs to make a ratio with the sprocket on the motor that lets the motor spin at the right speed (without being bogged down) for the speed you want at the cranks to get the speed at the wheel you are after.

Without a reduction, unless you run the motor at very low voltage, you need a huge sprocket on the cranks (possibly one so large it will hit the ground or worse; see below), so the motor doens't burn up.

know that ideally I should fit a freewheel, but I think doing so will require welding beyond the scope of a simple, quick, dirty ebike conversion.

A freewheel on the motor sprocket simply means that the motor won't be spun by you pedalling.

A freewheel on the pedals simply means that the pedals won't be spun by the motor.

Third, I wondered if, with appropriate reduction, it is even possible to have a mid-drive pulling the cranks without a freewheel and getting a decent output?

Freewheel has nothing to do with the output levels (speed or power). Only the two things noted above.

I'm of the understanding that this might not have enough torque to do much, given that the ideal RPM of these motors is about 2800rpm. That would mean rotating the cranks at nearly 1000rpm for optimal power right?

If you spin the cranks that fast, you will probably severely injure your legs when you hit them with the pedals (which will happen at some point even if you ride bowlegged with your feet way out away from the frame, in the air).

It will also require that you use some huge chainring on the pedals to drive the chain to the rear wheel, perhaps one so large it will actually touch or be below ground level when the front and rear wheels are level on the ground. And a sprocket on the rear wheel that is so small it may not have enough chainwrap to physically keep the chain on it, and be noisy, etc., otherwise your wheel will want to spin far too fast. Since it won't be able to do so, as those motors won't have the power for it, the motor will stay in it's inefficient zone and overheat and burn up, exacerbated by having no controller (and thus no current limiting).

Is there a cheap(ish), commonly available reduction that can easily be fit on most mountain bike frames?

You can look up Cyclone, GNG, BBS, Recumpence (DaVinci) drive, etc., for some of the commercially available stuff. Except for the BBS you can probably get the reductions without getting their motor (but then you have to adapt your motor to it).

There are also quite a few DIY reductions, or repurposings of non-bike reductions, in various threads around ES (mostly in the non-hub section). But all of those will take some work to adapt, and some will require two stages of reduction.

Second, I wondered whether a controller is necessary?

Yes.

Otherwise you have no speed or torque control except whatever gears you have on the rear wheel.

Additionally, you then must use a power switch that is very large and probably expensive, because it must handle the full current the motor will draw under load, and must be able to break that current under load without arc-welding it's contacts together (preventing you from turning it off at all).

but I'm in quite a bit of confusion about how electric motors perform.

I recommend going to http://ebikes.ca/simulator and reading the whole page, then playing with the simulator itself with the hubmotors that are there, to get an easier understanding of relationships between RPM, torque, voltage, current, load, etc. It's not super complicated, but it's a lot easier to see on the simulaotr than it is to explain in words.