Hub motor vs. belt / chain drive

[atlason]

Hi there,

I'm working on a light motorbike (no pedals) with a couple of buddies.
The idea is to create a bike that is made predominately for urban use, so there is a lot of stop and start vs. cruising at the same speed for a long time.

We are considering,

Battery: Lithium Manganese Dioxide 72V, 50 Amp.h, C rate=2
Motor: Zelena Vozila's Cromotor, pancake style, 6000 W, top speed=72km/h

I'm curious to know if there is some economy or advantages to be found in using a motor with a chain/belt drive rather than the Cromotor?
We know that some economy is lost in chain friction, there is simplicity in less parts etc. but not sure about if there is economy to be found in selecting specific gearing.
We'd appreciate some insights / comparisons / recommendations.

 

[spinningmagnets]

If you don't have any hills (or are stuck with a silly 250W power limit like Europe), a hub motor will be cheaper and simpler. Whatever issues that a hub may have compared to a non-hub installation, using a hub frees up the entire frame to hold the massive 50-Ah battery you are interested in.

 

[atlason]

What we are concerned with is, how motor choice will affect range. If a hub motor is somehow less efficient? Or if a smaller motor with the correct gearing can produce the same / similar output with less energy. Having a hard time wrapping my head around this.

 

[parajared]

In my experience my hub motors have been more efficient than my mid-drives (GNG gen1 and mid-drive hub), but much less off-road capable.

 

[ARod1993]

It honestly depends; having a mid-drive setup can let you run a smaller, lighter motor at at high RPMs to pull down a lot more power than you'd get otherwise. Basically, you can run a $80 Hobbyking Turnigy SK3-6374 at 5000-6000RPM and gear it down by a factor of 10 or 20 to 1; done properly you can pull 3-5kW peak from that motor and something like half of that continuous. The gear reduction can be a bit tricky to assemble without access to a machine shop, but you're looking at assembling a 2-3kW power system for something like $250 for the motor, controller, Hall sensor boards, and throttle; the whole thing will also weigh maybe 5-10 pounds, and the weight is low down in the middle of the frame, which will improve handling. A hub motor means (especially for big 3-8kW systems where the motor alone can weigh north of 15-20lbs) having all that weight unsprung in the back wheel, which puts extra stress on the spokes and can make handling more difficult.

 

[spinningmagnets]

There is an efficiency benefit to having a higher Tangential Magnet Speed. Its a fancy engineering term that just means, motors are inefficient if they are loaded down when spinning at low speed. The other issue that having a non-hub motor helps is unsprung weight in the wheel.

Compare two systems, one is a Cromotor in the hub, and the second is a Cromotor mounted to the top of the swingarm with a chain to the rear wheel.

The non-hub can change its gearing with a simple sprocket swap, the spokes are longer (to absorb bumps) and the wheel itself is much lighter. If you want a 20-inch wheel with a moped tire? easy if that's what you want.

If the Cro is in the wheel, it doesn't just free up all of the frame for the huge battery, it also allows the wheel base to be as short as possible. Putting the Cro on top of the swingarm requires the wheelbase be stretched about a foot.

 

[amberwolf]

Although having a longer wheelbase could help prevent wheelies and aid in front-end traction at high torque on the rear wheel.