Change the gear ratio of a e-bike.

[Ferrobiker]

Hi, my bike mounts a BOSCH PERFORMANCE CX LINE (2017) motor; it have a 15 teeth sprocket in the motor and 11-42 sprocket at the rear wheel.
Is it possible to change the gear ratio of a e-bike to increase speed? (for example, putting a 18 sprocket instead of 15). People said that the Bosch system detect this and launch an error, is it true?. Please, have a ligth onto my question.

thanks a lot.

 

[steve_sordy]

You will not increase the power cut off speed by anything you do to the gearing. The speed is derived from a sensor that is triggered by wheel rotation. You are of course free to ride the bike as fast as you can above the cut off speed provided you have the legs for it.

I have no idea what systems Bosch have to detect any change to the front ring (if any?)

 

[Ferrobiker]

Sorry, but I'm afraid that I haven't explained properly.
I have read in other forums that changing the gear ratio of a e-bike is not possible, because the motor software has been factory programmed on each e-bike model to a certain gear ratio.
For example, If you change the rear sprocket 11-42 / 11 speed for a 10-50 / 10 speed, the system detect that the gear ratio is not correct and launch a display error.
But all this I say is theory, no one knows if is true. That's the reason I aks to this forum.

 

[aarfeldt]

I have 2 bikes (Focus Jam2 with Shimano E8000 motor and a Specialized Turbo Levo with Brose 2.1 motor).
On both bikes I can change front ring and casette gearing without any problems.

 

[Gigha]

When I had to buy a replacement Bosch CX motor [outwith warranty], I had to supply the make of bike it was being fitted to, the wheel size, the front sprocket size along with the min and max derailuer sprocket size.

I was told this information was required so that the motor firmware was properly configured/tuned for my bike. It is hard to know if all motors of a certain type are the same, or if the do actually have different power characteristics depending on the expected gear ratio.

My original front sprocket was 16T, I now have a 14T sprocket fitted and have not experienced any errors. I know of people who have gone the other way and fitted larger 18T front sprockets without issue. [If I remember correctly there used to be a company that sold a dual front sprocket for the Gen 2 Bosch CX motor]

The wheel size is certainly important as there is a limit on how much you can tune the size within the onboard options. [if you have a Nyon or Kiox controller.] The gear ratio issue may be similar in that there may be a range where it it is OK, but if exceeded then it may throw an error.

But how would the motor actually detect the incorect gear ratio [Cadence/rear wheel speed ratio ? but what would happen if freewheeling down a steep hill while slowly turning the pedals]

 

[Ferrobiker]

When I had to buy a replacement Bosch CX motor [outwith warranty], I had to supply the make of bike it was being fitted to, the wheel size, the front sprocket size along with the min and max derailuer sprocket size.

I was told this information was required so that the motor firmware was properly configured/tuned for my bike. It is hard to know if all motors of a certain type are the same, or if the do actually have different power characteristics depending on the expected gear ratio.

My original front sprocket was 16T, I now have a 14T sprocket fitted and have not experienced any errors. I know of people who have gone the other way and fitted larger 18T front sprockets without issue. [If I remember correctly there used to be a company that sold a dual front sprocket for the Gen 2 Bosch CX motor]

The wheel size is certainly important as there is a limit on how much you can tune the size within the onboard options. [if you have a Nyon or Kiox controller.] The gear ratio issue may be similar in that there may be a range where it it is OK, but if exceeded then it may throw an error.

But how would the motor actually detect the incorect gear ratio [Cadence/rear wheel speed ratio ? but what would happen if freewheeling down a steep hill while slowly turning the pedals]

Thanks for your information!. So, the wheel circunference is related with the gear ratio?. in this case, you can play with it between two values (min-max), and replace the 15th with a 18th without error. it would be great if there was a gear ratio calculator to change the values and they were within range.