Warren said:
I am done with mid-drives through the pedal drivetrain. Absolutely loved the way it worked, but there is no redundancy, when a freehub fails, or a derailleur is destroyed. Had a chain link fail...no problem. Happened on our tandem too. That is why I carry extra links, a chain breaker, and quick links. Never had a problem in decades of just my effort. Double that, or more, and things break. Had the drivetrain fail twice in 33K miles on the mid-drive.
A left-side mid-drive is great, but again requires a chain, or belt. And I really like the idea of regen braking on both wheels. And having two motors and controllers makes for redundancy to get home.
Justin of Grin Tech did a presentation on this idea, minus being recumbent and pedal by wire. He thought it would make for the perfect bike. He said he would like to offer the progressive regen brake levers, but his volume is too small for suppliers to be interested.
Well... my personal experience with left-sided middrive suggests that once tightened, it is possible to remove the wheel, reinstall the wheel and force the belt back on no problem. It is even easier with a chain, because it is it not as finicky about tension.
Belt is almost silent, and if you don't go overboard with too small 'pinions' it should act a long time and need no maintenance. Chains are fine too if you wax them.
So, you have a one step reduction belt or chain, no tensioners, regen capable, no clutch to break. Even better performance with 20" wheel.
You might have seen some of my musings about using multiple *middrive* motors, including fixed gear, overrunning clutch and even centrifugal clutch.
The problem with coupling e-drive with human power is that their torques sum up and result in huge chain tensions, and human power is low RPM/high torque anyway, which is actually pretty hard on chains.
By going with high RPM left-sided middrive with large rear cog, you get relatively low chain tension but high 'thrust' due to torque multiplication, while with human power you actually have torque reduction (front chaining is larger than the rear).
Personally, I don't think that you need a really powerful regen brake, powerful enough to serve as 'panic brake', and on both wheels!
On my Lwb with overall weight (cause I'm frankly fat) approaching someone light on a decent-ish motorcycle, I can still get away with measly 20nm of braking torque (rather underpowered setup) that gets activated by brake handle to scrub speed before using front disk brake to come to a complete stop - I'm harvesting like 90% of energy possible (and with high efficiency!) and with similarly little brake pad wear.
And I can simply take an other belt with me in case something happends (a cheap HDT 5M belt!)
If my overall weight was less or I'd have a bit more braking power - I'd be able to harvest like 99% except 'panic braking' - and than unless you have a HUGE battery panic braking from high speed using regen alone can damage BMS/Battery, and hopefully that happends very rarely.
Now, if I was to travel with full fairings somewhere in steep hills but still maintain high efficiency - than yea, you'll need VERY considerable propulsion AND braking torques. I'll be experimenting with centrifugal clutch for that...
DD motors, however, must be very heavy to provide enough torque with decent efficiency in a bicycle-sized wheel.
Btw, I've checked the simulator and my current setup is pretty much functionally equivalent to Grin All Axle run with 20A controller, however it is 'underpowered' on purpose so I maintain peak efficiency, but I can absolutely maintain same regimes with less heat losses... yet it weights a 4x less more (700 gr motor and about a pound of hardware) and costs *literally* an order of magnitude less - again, including belts and laser cut/3d printed hardware that is dirt cheap nowadays.
By switching to only a bit more powerful motor and gaining about half a pound, I'm getting more torque AND more efficiency and that motor costs like 20$ more - and you don't have to deal with motor connectors and torque arms ever as well!
But if you want 'zero maintenance' and absolutely dead silent and don't mind some (ok, lots) extra weight - than yea, a couple of those Grin motors sounds about right.
You *will* get lower efficiency at cruise due to double cogging losses though, which is pretty significant (about 50w at 50 kmh as I understand, PER motor. Basically, at given generator losses, you'll need to pedal quite vigorously just to compensate for cogging
)
In my case that's 30w at 50kmh.
I think you really want a much lighter and much more torque-dense front hub geared motor, overall torque is going to be pretty damn huge for bicycle (already approaching moped territory), but zero cogging when off, with disk brakes on the front.