Choosing proper gear ratio (motor and crank)

[markz]

I am right in the middle of building my DIY crank freewheel for a Leaf 1000W or 1500W mid-motor setup that I will be using to climb stairs and steep hills 20%+ for 50m+, cruise at slow speeds 35km/h and under.

I have the option of going 48/32/22 or 44/32/22 on the crank, with an additional 4th chain ring used on motorized bicycles.
I can find 58T, 56T, 54T, 52T and 50T.

Leaf 1500W

Here are some figures for speed per volt in a 26" wheel:

I measured my no load at 645rpm at 49 volts ( 13.16 kv ), so keep that in mind. By that math:
4T: 631rpm / 48v = 13.16 kv ( measurement by me, adjusted down 1 volt )
5T: 485rpm / 48v = 10.1 kv ( fluxshifter's measurement )
6T: 336rpm / 48v = 7kv ( assumption based on 3kV steps per winding )

Rough idea of 48v speeds in a 26" wheel:
4T: 39mph ( observed )
5T: 30mph ( calculated but not observed )
6T: 21mph ( assumed )

What tooth count should I put on the motor?
What tooth count should I put on the crank?

If I go ultra small count crank sprocket for my 4th drive chain ring, I go even faster. Larger count sprocket, I get tons of torque.

Do I just buy a bunch of different sprockets, like lets say 50T, 54T and a 58T, or is there a way to get a general idea of the speed and torque I will get from different motor sprockets and different 4th chain-ring drive sprocket.

Here is a diagram I found

 

[amberwolf]

If you look at Sheldon Brown's site, he talks about gearing and ratios and such, different methods of looking at it and working it out. Takes a little bit to sink in, but well worth the time to read (and maybe re-read).

The only way to know what torque and speed you'll get is to know what youd' ahve with the motor as direct drive (1:1) to the wheel, and then multiply those by the ratio you get when you use different gearing.

Figuring out what the "best" gearing is depends on the range you want out of it, between speed vs torque.

If you use your rear gear cluster as the limiting point, for how many teeth are on it's smallest and largest sprockets, then you can figure out what chainrings you need on the front triple (cranks) to get the speed you need in highest gear if you used 1:1 gearing from the motor to the 4th chainring on the cranks.

The same is true for torque, though you'd invert the ratio for that, AFAICR.

Once you know what those are for 1:1 gearing from motor to cranks, then you can decide if you need different chainring gearing, or just different motor gearing.


The chainring gearing directly affects both your pedalling (with or without motor), and the motor.

The 4th ring gearing to the motor only affects the motor's abilities.