LWB recumbent mid-drive (RC brushless?)

[bambuko]

I am making LWB recumbent and want to install power assist to help me up the hills.
So far I have made the frame:

As you can see from the photo front crankset is LH single chainwheel.
Intermediate bottom bracket has two freewheels on LH side and "normal" front/rear deraileur on RH side.

So far simples , but when I started thinking about motor choice it got a bit more complicated...
I don't want "electric only" moped, I don't want overpowered rocket either.
I would like it to be legal (or thereabout) in UK, where the rule is "maximum continuous rated power which does not exceed 250 watts"
not only because it's legal, but because it makes sense (to me at least).

I can get 3200 watts(max), 60 KV brushless RC motor, but this would be way too much for what I want.
Smaller RC brushless motors with more appropriate power rating have too small shafts and far too high KV to be practical.
It would seem 63mm outrunner is about the smallest sensible choice?

I am not the first to ask similar question:
Best RC motor for lightweight utility bike build
The lightest low KV RC-motor for 500w middrive
but I haven't found real answer...

And if I choose 63mm RC motor as above, can I run it at half or one third of it's power to get what I want?
Will it not defeat the objective of using RC motor by destroying it's efficiency? (If I get it right, they are best at around 75% of their max power)

So, any suggestions would be appreciated - thank you.

 

[bowlofsalad]

I've looked into exactly the same idea you have, I haven't really found much without going to an extreme in the reduction ratio. I don't know what else can be done beyond re-winding a motor yourself.

Have you checked out a Q75? Next step up in capacity is probably the Q85, Q100 or Q100h. All of these geared hub motors freewheel, meaning you won't feel drag while you pedal only. This concept likely solves a lot of other problems all at once. The down side is that you need to make stronger drop outs and build another wheel.

 

[katou]

I would choose the motor for the desired kV and limit the power through the controller.

Having more power than you want isn't that much of a problem. You could even put a mechanical stop on the throttle so that you can't open it up.

As you've figured out, there isn't much out there to do what you want. Since you have the extra BB back there, have you considered a Stokemonkey type of setup? Mechanically, it will be a bit tricky to implement, but you could use a very small hub motor, which might get you where you want to be power-wise. Driving through the gears will of course get you massive torque which it sounds is more important to you than high speed.

My 2c,

Katou

 

[amberwolf]

If you didn't mind the weight, you can look at my original CrazyBike2 powerchair motor drivetrain. You can get "250w" brushed powerchair motors with rightangle gearboxes (I used a 350 and then a 600?ish), cheap controllers (I used a 4QD), and have torque enough to destroy your drivetrain and wheels if you like. (not that this EVER happened to me :wink: )

Pretty easy to make brackets to hold it all on the bike, add sprockets to the gearbox output, etc. Also very cheap to do. Might even find wha tyou need for free if you look in the right places.

 

[bambuko]

Thank you guys,
I was hoping for efficient, lightweight, brushless solution, but I suppose I can always use one of these things:

Quick, easy, heavy, inefficient, and cheap

 

[bambuko]

...All of these geared hub motors freewheel, meaning you won't feel drag while you pedal only. This concept likely solves a lot of other problems all at once. The down side is that you need to make stronger drop outs and build another wheel.

This thread is about mid-drive not about hub motors (I am not interested in hub motors).

 

[bambuko]

I would choose the motor for the desired kV and limit the power through the controller.
Having more power than you want isn't that much of a problem. You could even put a mechanical stop on the throttle so that you can't open it up...

I thought that motor efficiency drops off the cliff if you use it below it's optimum settings?

...As you've figured out, there isn't much out there to do what you want....

Indeed, hence the question here

...Since you have the extra BB back there, have you considered a Stokemonkey type of setup? Mechanically, it will be a bit tricky to implement, but you could use a very small hub motor, which might get you where you want to be power-wise. Driving through the gears will of course get you massive torque which it sounds is more important to you than high speed.....

Not sure what is "Stokemonkey type of setup" although vaguely recall seeing something like this.
For me it is simple:
on LH side of the intermediate bottom bracket I have two freewheeels. One to be powered by motor and second one to be connected to front crankset where I will pedal.

The only thing I need is an efficient, quiet, low power motor that I can gear down to my usual cadence of between 50 and 80

 

[bambuko]

If you didn't mind the weight, you can look at my original CrazyBike2 powerchair motor drivetrain. You can get "250w" brushed powerchair motors with rightangle gearboxes (I used a 350 and then a 600?ish), cheap controllers (I used a 4QD)...

Yes, you are right and I know that - it is my fallback plan if I don't manage to conjure something a bit more sophisticated

...and have torque enough to destroy your drivetrain and wheels if you like. (not that this EVER happened to me :wink: ) ...

Yes, and that is exactly one of my concerns
AFAIC with the setup I have I probably need no more than about 150 watts! (for what I want to do).

 

[pguk]

How is this coming along?
I wonder if you found the Mac mid drive conversion thread on here? The Mac is a robust motor, electrically difficult to drive - lebowski has managed to run it with his controller. Could be a solution for you?

 

[bambuko]

I have put it all together, but it wasn't a success, not because e-bike side of things but because of LWB recumbent...
LWB recumbents are rear heavy (typically 70/30 axle loading split).
Adding all this weight to the rear, made things far worse to the point where it was affecting the handling of the bike - front wheel was so light that it was dangerous.
I also made it very difficult by making rear triangle very small, so there was little space for motor etc.
Plus I didn't like having "front" deraileur behind me (where I couldn't see what was going on)

So I chucked it all out and I am currently using the bike without power assist (and using conventional layout) - I am happy with it, the handling is great and all the minor problems were sorted out.

Current plan is to add motor and the batteries at the front of the bike using double front freewheel arrangement.
It will be lighter than the original arrangement with intermediate shaft, plus all the weight will be where it helps the bike handling - at the front.