Gearless hub motor that runs 300-400 RPM at max efficiency?

[Fastolfe]

Hello all,

After electrifying my Quest velomobile with good success, I'm looking to apply the same recipe to my QuattroVelo. That is, a motor driving the cranks that "sees" all the gears. Only this time, instead of using a geared Headline motor, I want to install a low-power (read legal 250W or less) gearless hub motor, Stokemonkey-style, for the following reasons:

1/ The motor powers the cranks, so I can run it at its most efficient speed to match my prefered pedaling cadence at all time, regardless of the gear I'm in. I don't need a lot of power to climb anything or reach a decent speed on the flat, since it goes through all the gears.

2/ I use the e-assist very seldom, When I don't, I want to keep my on-board battery topped up to run the indicators, lights, satnav, cameras etc. On my Quest, I have a separate generator and a separate battery to do that. On my QuattroVelo, I want to try and use the hub motor as a generator when it's not used as a motor, and use the e-assist battery as my sole battery. Hence the need for a gearless motor. I'm aware that such motors have drag and cogging when they're not powered, so maybe (probably) it won't be practical. If it isn't, I can always make some kind of disconnector to disconnect the motor from the drivetrain entirely on the fly, and install a separate generator later. Or perhaps run just a little power through it to overcome the drag when I'm in "bike mode". But I'd like to try.

So I need a gearless hub motor with a 6-bolt disc rotor mount to install a sprocket on the left-hand side that'll drive the chainring on my left-hand side crank.

The problem I have is, the chainring in question is 75 teeth, and can't be any smaller because of the gearbox I use. So to keep the sprocket reasonably small, and to improve the motor's efficiency, I want to go for a motor that spins faster. Depending on the sprocket I choose, I'll need a motor that spins between 300 and 430 rpm at maximum efficiency. I figure a hub motor designed to be laced into a 20" wheel would do what I need. The trouble is, I've looked at many potentially suitable hub motors on various Chinese manufacturers' websites, but very few of these manufacturers publish the RPM/power/efficiency curves for their motors.

So I'd like some advice from you specialists: can anybody recommend a suitable, good quality smaller wheel hub motor that would meet my requirements?

Thanks!

 

[amberwolf]

It's not exactly what you're asking for, but you can "make" efficiency curves yourself for any motor on the Grin Tech http://ebikes.ca/simulator. It's actually better than what you'd find at a manufacturer site, because you can setup the simulator for your specific system's parameters.

Since you're using it as a non-hubmotor, you just need to calculate the effective wheel diameter created by your gearing and drive wheel size, to use the simulator in an equivalent way.

Unfortunatley it's only for the motors already in the simulator....but if you can use one of those, it'll work.

 

[Fastolfe]

It's not exactly what you're asking for, but you can "make" efficiency curves yourself for any motor on the Grin Tech http://ebikes.ca/simulator. It's actually better than what you'd find at a manufacturer site, because you can setup the simulator for your specific system's parameters.

Indeed, what a great tool. Thank!

Since you're using it as a non-hubmotor, you just need to calculate the effective wheel diameter created by your gearing and drive wheel size, to use the simulator in an equivalent way.

That's the thing: because the motor sees all the gears, there is no concept of effective wheel diameter: I more or less always pedal at the same speed (around 85 rpm when using e-assist) so I only need to match the sprocket and motor so it works "with me" at its most efficient speed. Then with all the gears in my bike, with that fixed motor speed, I can climb 25% slopes at 1 mph or bomb on the flat at 40 mph.

But no matter: the Grin tool lets you switch to RPM. So it gives me the exact answer I need. Brilliant.

Unfortunatley it's only for the motors already in the simulator....but if you can use one of those, it'll work.

Well, Grin have a wide selection of motors already, they know their stuff, and I'll need to order a Cycle Analyst and Cycle Analogger from them anyway. And they're great to deal with. So I think I might just order everything I need from them and make things simple. China is cheaper but it's more headaches, and the Grin simulator you pointed me toward is the clincher I think.