Installing larger chain ring on a gen 4 motor

Deacon Blues

Well-Known Member
I'd like to install a larger chain ring on my ebike, with a Bosch gen 4 motor, but the bike shop says that it's more complicated than just installing the larger chain ring and adding a new longer chain, or adding links to the old chain. They said the motor had to be adjusted somehow.
The shop, where I bought the ebike, usually sells high-end road bikes and have limited experience with ebikes.

Has anyone here put a larger chain ring on their Bosch motor? @Ravi can you give me your opinion on this?
 

rich c

Well-Known Member
Why the larger chainring? Are you pedaling out before hitting the high limit? That's not been an issue for me.
 

Deacon Blues

Well-Known Member
I'm finding that I often ride with the cassette in the smallest or next smallest cog. On the flats I'm okay, but on a slight downhill grade I often run out of gears (and I'm way past the motor cut-off).
I ride a Cannondale Topstone Neo Lefty 3, which has a 42T chain ring and an 11to 42 cassette. The Lefty 1 (more expensive model) has a 46T chain ring, with a 10 to 50 cassette.
I want to replace my 42T chain ring with the 46T chain ring found on the Lefty 1. I figure it should be an easy swap.
 

fooferdoggie

Well-Known Member
Humm I wonder maybe ask one of the bosch guys here to see. I know I put a smaller one on mine I had just the opposite problem I could peddle to about 35 mph. so I put a smaller one on but mine is the second j=gen with the small chainrings.
 

Ravi Kempaiah

Well-Known Member
I ride a Cannondale Topstone Neo Lefty 3, which has a 42T chain ring and an 11to 42 cassette.

Bosch system is programmed for very smooth power delivery and also shift sensing. So, it requires OEMs to input the drivetrain ratio into the system.
The shift sensing works differently for derailleur vs IGH systems. On an automatic IGH like the E-14 or Enviolo automatiq, it is a bit more subtle and reduces the power when the cranks are in 6'clock position to allow for more smoother shift.

If you go from 40 to 48T, you might encounter an error code because the ratios are getting messed up but within a certain range, you could do it. Since the left 1 has 46T and 10-50T cassette, ask them to look at the gearing ratio used in the system and then reprogram yours to that ratio. Then you will be able to use the same drivetrain that is on the Lefty 1.