Crank drive query

Electrically assisted bikes, trikes, etc.
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Crank drive query

Postby hercule » 13 Oct 2019, 6:08pm

Looking at options for motor assist on a tadpole recumbent trike, I had dismissed crank drive as I would lose the gear range I currently enjoy. However I was looking at the Bafang motors which seem to have a square taper interface just like any other square taper bottom bracket. What’s to stop me fitting a standard triple chainset?

I have my doubts that a SA CS-RF3 gear would stand up to the power of a motor drive, and also worry about the chain idlers midway, but those are separate issues.

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Re: Crank drive query

Postby stodd » 13 Oct 2019, 7:04pm

I'm not sure about the front chainset. It may be that the bottom bracket on the Bafang isn't wide enough to fit a triple.

You don't necessarily need such a wide range of gears with electric because of the help the motor gives. That is especially true with crank drive, as the motor is benefiting from the gears you do have. On our tandem with front hub drive we still have (and need) all the original 24 gears; that is partly because we have a not that powerful motor (XF07) to assist two people (180kg without bike itself). The motor is a huge bonus, but we still need to provide quite a bit of effort on the hills. One problem with hub gears is that as you slow down on hills so does the motor, and it can't deliver its top power at slower speeds.

The SA should manage manage a motor drive unless you really stress it out. Our (shared) solo bike is a Raleigh Motus with Bosch crank drive and Shimano Nexus 7 gears. The Nexus 7 is probably quite a lot less strong than the SA3.

Also worth asking at ... n.42/where there are lots of very clued up people (but sadly also some who just profess to be clued up).

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Mick F
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Re: Crank drive query

Postby Mick F » 13 Oct 2019, 7:22pm

My CS-RF3 on my Moulton gets some welly from my legs.
No, not electric drive at all, but I'm strong and tough and will get up any Cornish hill you want.

My CS-RF3 is connected to a triple 10sp system giving 90 gears from 16" up to 135"
All work perfectly and I have no issues with any of the ratios pushing as hard as I want ...... and can.

Electric Assist would be easily coped with.
Mick F. Cornwall

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Re: Crank drive query

Postby hemo » 13 Oct 2019, 11:26pm

With Bafang mid drive or any other clones like Mxus , Akm or even the tsdz you are limited to about 32t as the smallest ring before it interferes with the internal drive gear cover and you can use/fit a double with an adapter.