Giant Lafree ST larger motor sprocket

[freewheeler]

Hello,

I got an original Lafree comfort ST, one of the final models, the one with the SRAM S7 hub gear. Somehow it has survived unused. I don't have the correct charger but managed to get it charged using an Imax B6 RC charger.

The bike rides very nicely, I intend to fit a smaller rear sprocket to increase top speed but I would also like to be able to spin the cranks faster before the power starts fading out so i'm wondering if this would be possible.

The current setup is:
47-559 tyres
SRAM S7 hub gear
19t rear hub sprocket
37t chainring
14t motor sprocket

Would a larger motor sprocket give the desired effect? I have access to a lathe so could potentially make one up.

Thanks in anticipation!

Mark

 

[flecc]

The answer is yes Mark. The power reduction is Hall sensor controlled by a ring of magnets in the motor output shaft gear enabling sensing RPM, so increasing its chain sprocket size will increase the pedal cadence at each of the four power step down points between 9.4 and 15 mph assist with the standard sprocket set.

The room to increase that motor sprocket size is a little restricted by the proximity of the chainwheel and idler follower wheel sizes of course.
.

 

[freewheeler]

Thanks Flecc

So is it the case that the percentage increase in tooth count (e.g. 7.14% from 14 to 15) will give a corresponding increase in the ramp down candence speed?

 

[flecc]

Thanks Flecc

So is it the case that the percentage increase in tooth count (e.g. 7.14% from 14 to 15) will give a corresponding increase in the ramp down candence speed?

Yes. There's 24 magnets read by the Hall sensor per rotation of the output shaft so those will give a quite consistent change.
.

 

[freewheeler]

How exciting! I just offered up an 18t sprocket and it looks like it would clear the chainwheel. It should be quite possible to turn down the original sprocket and silver solder it inside the 18t. I'll report how well this works out - 4 extra teeth is 28.5% increase so the characteristics should be well different.