On Friday, May 19, 2017 at 1:30:12 PM UTC-4, Tim Wescott wrote:
On Fri, 19 May 2017 10:25:58 -0700, rangerssuck wrote:

On Thursday, May 18, 2017 at 2:10:55 PM UTC-4, Tim Wescott wrote:
I use "thingie" in the title to not give anyone a false impression of
what I do or don't know.

I'm working with a client who's using one of these in his specialized
bike:

https://www.amazon.com/dp/B01KYKEP2Y...xx_P1400_1000?
tag=dradisplay-20&ascsubtag=c73ee0e485a804ac76001596b2b029f4_S

He's currently using the 250W version, but we're having problems of
insufficient torque -- this is partially due to re-purposing a board,
and we're on top of that part, but I'm thinking ahead (well, borrowing
trouble, in proper engineering fashion). So we thought -- hey! We'll
use the 350W version (that's the one in the link).

The problem is that the 350W version actually has a lower current-to-
torque constant -- it gets its higher power through pulling more
current for the torque, and delivering more speed.

So -- any thoughts on an easy way to gear the thing down? I just
recommended to the customer that they put two chain rings on the crank,
so that they can drive the pedals at one speed and the bike at another..

--

Tim Wescott Wescott Design Services http://www.wescottdesign.com

I think I'm missing the problem - looking at your question, you refer to
driving the pedals. Looking at the picture on Amazon, they are driving a
separate sprocket on the rear wheel. What's the problem with changing
that driven sprocket (on the wheel) to a larger size?

But do like the idea of the Sturmey-Archer 3-speed. Just get that
mounted in the middle of the drive train, and you're good to go.

Customer can't easily get bigger sprockets for his chosen rear hub.

That is on the table, but I'm looking at alternatives.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Oh, I dunno, take a look (for example) he http://www.rollerchain4less.com/35-P..._1013-1-1.html They have a pretty big selection. You can, with some machining & welding, get these to fit pretty much any hub. You could even cnc mill an opening in the larger sprocket to be a snug fit over the existing sprocket and either weld or mechanically fasten them together.