Going to a larger front chain ring on a gen 4 motor

Deacon Blues

Well-Known Member
@stephen-Bosch Team, or anyone else that has a definitive answer to my question:

I have a Cannondale topstone Neo Lefty 3 that comes stock with a 42T front ring. I've purchased a 46T front ring and I've been told that the motor will need to be reprogrammed (or something like that) before the bike/motor will function properly.
Is this true?
 

William - Bosch Team

Well-Known Member
Hello!
Officially, anytime you change gearing on the Bosch System, the OEM needs to approve it and remotely adjust the gear ratio values programmed in the drive unit. Generally, when you go down in size, this doesn't really seem to be an issue. When you go to a larger gear though, the DU may trigger an error code depending on the amount of change you made. Willingness to approve/assist varies by OEM and justification. Also worth mentioning for future readers of this thread- your shop should always verify proper/safe clearance for the larger sized ring, as well as installing a longer chain if necessary.
 

Deacon Blues

Well-Known Member
Thanks for the feedback. The larger ring shouldn't be a problem (I hope), as the more expensive Cannondale Topstone Neo Lefty 1 comes stock with the 46T ring.
 

William - Bosch Team

Well-Known Member
Thanks for the feedback. The larger ring shouldn't be a problem (I hope), as the more expensive Cannondale Topstone Neo Lefty 1 comes stock with the 46T ring.
You're welcome!

A few things to keep in mind...

- Likely will not be a chainring/chainstay clearance issue, assuming it's the same exact frame.
- Chain will likely need to be longer because chains are sized based on exact drivetrain components, etc.
- The model that comes stock with a 46T will have different gear ratio values programmed in the DU by Cannondale.
 

Xenium

New Member
Region
Europe
What about the Bosch gen 3 motor. Can the front chainring be changed to a 36T for example. The granny ring they come with are a real pain in the butt.
 

TrevorB

Active Member
What drivetrain do you have eg 9, 10 or 11spd and brand. What type riding do you do, commute, tour or MTB?

If you have 9spd maybe worth upgrading to 11spd drivetrain with 11-46t cassette which handle 99% of hills on and off road.
 

Xenium

New Member
Region
Europe
Hello Trevor
Just to be clear, I am talking about an eBike with Bosch Gen3 motor
I have a 12 speed SRAM 15-50T Eagle
Want to change the front chainring to the same as the Gen 4, 38T but not sure it will work. Using a 15T chainring in the front is more spinning than momentum therefore wasting energy
 

William - Bosch Team

Well-Known Member
Hello!

If your drive unit has a 15T currently, it is a Generation 2 Bosch drive unit. All Gen2 DUs have internal gearing that increases the ratio by 2.5 times. If your DU has a 15T, it performs identically to a 37.5T ring (obviously you can't have half-teeth, but your ratio is right between 37T and 38T for the sake of gear-inch calculations.)
 

Marci jo

Well-Known Member
Hello!

If your drive unit has a 15T currently, it is a Generation 2 Bosch drive unit. All Gen2 DUs have internal gearing that increases the ratio by 2.5 times. If your DU has a 15T, it performs identically to a 37.5T ring (obviously you can't have half-teeth, but your ratio is right between 37T and 38T for the sake of gear-inch calculations.)

William,
Thanks for the great info. Have a question regarding my Trek class one with gen 3 Active Line Plus motor.
Currently it has a 38 tooth chain ring which I wanted to bump up to 42 or 44 tooth. Based on my cadence and "feel" I was only using the smallest 3-4 sprockets in the rear and would like to use more middle sprockets in the rear. So rather than changing the rear sprockets I was hoping to only change to front chain ring.
However, when I brought to bike to my lbs the mechanic said a bigger chain ring might trigger an error code on the Bosch system and subsequently lock up the motor, thinking the motor has a speed override device.
So my question; On an ebike with a Bosch gen 3, is it possible to change the front chain ring without triggering an error code?
 
@Marci jo

Hi Marci Jo, anytime a gearing change is made on a Bosch drive unit, the OEM (in your case Trek) would have to approve of the gearing change. A chainring swap to your desired 42 or 44T may prompt an error code from the DU. Housekeeping wise, you would also want to have your LBS to inspect that the chainstay could accommodate a chainring that large. It also would most likely necessitate a longer chain.

I hope this info helps, let me know if you have any more questions!

🤠cheers
 

FlyingScot

New Member
Region
United Kingdom
City
Motherwell
You're welcome!

A few things to keep in mind...

- Likely will not be a chainring/chainstay clearance issue, assuming it's the same exact frame.
- Chain will likely need to be longer because chains are sized based on exact drivetrain components, etc.
- The model that comes stock with a 46T will have different gear ratio values programmed in the DU by Cannondale.
I am VERY Confused, so confused I joined this forum to ask this question, and that is, why does the DU need reprogrammed if you change the size of the front chainring?

I am having cadence issues at high speeds and for sure would love a larger front chain ring on my FOCUS JARIFA² 6.7 NINE

Since the DU has no feedback from the back derailleur and does NOT know what gear you are in, why does the DU need reprogrammed?

And IF it actually does :-( can your LBS that is Bosch approved do this for you

Does the DU actually look at and measure and compare RPM's of the motor vis a vis the RPM of the back wheel
 

William - Bosch Team

Well-Known Member
I am VERY Confused, so confused I joined this forum to ask this question, and that is, why does the DU need reprogrammed if you change the size of the front chainring?

I am having cadence issues at high speeds and for sure would love a larger front chain ring on my FOCUS JARIFA² 6.7 NINE

Since the DU has no feedback from the back derailleur and does NOT know what gear you are in, why does the DU need reprogrammed?

And IF it actually does :-( can your LBS that is Bosch approved do this for you

Does the DU actually look at and measure and compare RPM's of the motor vis a vis the RPM of the back wheel
The DU looks at wheel speed and motor output, among many other things. It is able to determine what the max speed should be based on the max gear ratio, etc.

If a larger chain ring is installed, it may trigger an error code as the max wheel speed won't "make sense" to the DU if the new gear ratio is much larger than stock gearing.

The OEM must approve the change, as well as remotely adjust the value in your DU. Your dealer will assist with this process, but is not able to complete it on their own. The OEM must be involved.
 

FlyingScot

New Member
Region
United Kingdom
City
Motherwell
The DU looks at wheel speed and motor output, among many other things. It is able to determine what the max speed should be based on the max gear ratio, etc.

If a larger chain ring is installed, it may trigger an error code as the max wheel speed won't "make sense" to the DU if the new gear ratio is much larger than stock gearing.

The OEM must approve the change, as well as remotely adjust the value in your DU. Your dealer will assist with this process, but is not able to complete it on their own. The OEM must be involved.


Sure;y Shirley it must look at the wheel speed to determine max speed since it cannot possibly know what gear you are in, and then it will need to know wheel and tyre size

And what happens, then you change the cassette? I am looking to move from a 11 - 36 9 speed cassette to a 10 - 40 11 speed cassette

Are you really saying that I cannot change anything on my drive line set up on 'MY BIKE' without Bosch or my bike manufacture's permission and approval
 

TrevorB

Active Member
@stephen-Bosch Team, or anyone else that has a definitive answer to my question:

I have a Cannondale topstone Neo Lefty 3 that comes stock with a 42T front ring. I've purchased a 46T front ring and I've been told that the motor will need to be reprogrammed (or something like that) before the bike/motor will function properly.
Is this true?
 

TrevorB

Active Member
Changing cassette and derailleur shouldn't be issue. I've upgrade Performanceline Gen3 with 9 spd to 10spd with 11-42 cassette and Gen2 CX 11-42 to 11-46 cassette. CX came with with 15t but I've run 14 and 16t with dongle without any issues.

10t on rear is only likely to used passed engine speed cutoff so shouldn't cause problem.
 

William - Bosch Team

Well-Known Member
Sure;y Shirley it must look at the wheel speed to determine max speed since it cannot possibly know what gear you are in, and then it will need to know wheel and tyre size

And what happens, then you change the cassette? I am looking to move from a 11 - 36 9 speed cassette to a 10 - 40 11 speed cassette

Are you really saying that I cannot change anything on my drive line set up on 'MY BIKE' without Bosch or my bike manufacture's permission and approval
Minor changes in gearing generally don't cause any programming issues or error codes, but we share the official response so that there are not any surprises should you encounter an issue. The OEM designs their product to behave in a certain way, Bosch is just a component supplier, so it is up to the OEM to approve any changes to their design. The OEM's decision is often influenced by the engineering considerations the OEM made when designing a safe, reliable, and legally compliant product.

I would imagine that anybody is welcome to make just about whatever modifications they like to a product they purchased, but some modifications may affect safety, reliability, legal compliance, warranty claims, etc. I am not a lawyer, this is just my informal understanding.

From what I've heard from dealers in the past, dropping from a 11t to a 10t on your cassette likely won't be an issue.

If you have not already, I suggest speaking with your local Bosch Certified dealer and asking them about making the gearing change. They should be able to check with the OEM on policy, inspect your bike for compatibility, etc.

I hope this helps!
 

FlyingScot

New Member
Region
United Kingdom
City
Motherwell
I just had TWO LONG chats with bike people, one sensible one with someone in a London bike shop and one totally crazy one with someone in what I thought was a top Glasgow bike shop

Let's look at the Glasgow one first

Last week I went in there to enquire about fitting a Shimano Alfine 11 to my Focus Jafira 6.7 NINE

NOT a problem, bring the bike in and let us have a look at it.

TODAY BIG BIG PROBLEMS I CANNOT make ANY changes to MY BIKE with the Bosch CX GEN4 Drive system at all without FOCUS and BOSCH input AND PERMISSION

They had called Bosch and been told no no no no.

I was then given a load of rubbish about why this would not work probably damaging the Motor, the 9 speed cassette and the Alfine 11 hub NOT that they would now fit one to my bike and no I could not change to an 11 speed SRAM 10 - 40 cassette either!!!

ALL POLITICAL and rubbish about needing trained and they as Bosch dealer could not undertake any of the modification to MY BIKE (with a 2021 CX GEN4 DU) which is NOT under warrantee as it was bought from a friend after they damaged their leg and the warrantee is NOT transferable NOT that I care about that.

But they then went onto the liabilities that they MIGHT incur if they did the work for me without permission from Bosch and Focus................................. they also said I was going to make the bike ILLEGAL by changing the front chain ring or the back cassette !!!. TOTALLY BONKERS I was also chastised for buying an E-MTB as my use is mostly somewhat hilly tracks and flattish canal banks and dedicated cycle tracks as the gearing on my Focus Jafira 6.7 Nine is for REAL mountain biking. I was also told that I CANNOT change my back tyre to one of a different section with less nobly bits on it. NOT that I had asked that question!!!

As a BIG F1 enthusiast I know that they 'play with tyre pressures' I guess I better make sure my back tyre (hard tail E-MTB} {with suspension seat post retrofitted by me with no permission from anyone} is fully inflated as a soft underinflated back tyre 'might' upset my CX GEN4 DU :-(

TOTALLY NUTS!!!!

The London guy was exceptionally helpful and talked so much sense unlike the LBS guy

I won't name the bike shops in Glasgow or London, but he said NO ISSUES at all changing from a 36T chain ring to a 38T chain ring but a 40T may be an issue {but he thought not} and a 42 would/should definitely need the diagnostics run, and the gear ratios reprogrammed AND this he said was EASILY DOABLE!!! He also said that after changing to a bigger chain ring the DU would not produce an error as none of the external bits was liable or able to generate an error condition, He also said that since I was looking at getting higher speeds PEDDLING well above the CX GEN4 cut off speeds (just think I could fit a SPEEDBOX 3 tuning device and not worry about spinning) and not just spinning like a top inputting no torque/driving force to the back cassette and back wheel that he could totally see no issues and no problems whatsoever as by then the GEN4 DU would have been in idle mode as I would be way past the cut off speed.

Being a heavy E-MTB any slight downhill slope, and it is off like a bat out of hell BUT with me unable to pedal to keep up OR go any faster.

PRE buying this lovely Bike with really an excellent CX GEN4 drive system I was going to convert my Cannondale using a BAFANG BBS02B or BBSHD drive unit, seeing all this CRAP and HASSLE I am experiencing re a simple modification to customise MY BIKE to the way I ride it I am now wondering if that would have been a more sensible option, FOR SURE tons LESS hassle and miles cheaper as well if not so aesthetically pleasing and not having torque sensing unless I used the Tongsheng TSTZ2 DU

I was, and probably I am going to upgrade the display from a Purion to a Kiox or Nylon I do wonder what sort of hassles that is going to generate

Please Mr Bosch and Focus can I change my display please

And Mr Focus I am going to fit Blumells 75 mudguards and rack and a RFR KSA 40 Pro Bike Stand is that OK?

GOING OUT ON MY BIKE NOW take care and have fun
 

FlyingScot

New Member
Region
United Kingdom
City
Motherwell
Minor changes in gearing generally don't cause any programming issues or error codes, but we share the official response so that there are not any surprises should you encounter an issue. The OEM designs their product to behave in a certain way, Bosch is just a component supplier, so it is up to the OEM to approve any changes to their design. The OEM's decision is often influenced by the engineering considerations the OEM made when designing a safe, reliable, and legally compliant product.

I would imagine that anybody is welcome to make just about whatever modifications they like to a product they purchased, but some modifications may affect safety, reliability, legal compliance, warranty claims, etc. I am not a lawyer, this is just my informal understanding.

From what I've heard from dealers in the past, dropping from a 11t to a 10t on your cassette likely won't be an issue.

If you have not already, I suggest speaking with your local Bosch Certified dealer and asking them about making the gearing change. They should be able to check with the OEM on policy, inspect your bike for compatibility, etc.

I hope this helps!
THANKS, that was all sensible but read my post re the LBS which are Bosch and Shimano dealers vis a vis the London based BS which talked real logically based sense

As a RR trained engineer with many disciplines under my belt, ALL I am TRYING TO DO is customise a nice bike to make it 'better' suited to my riding style and use I make of it.

I HATE reinventing the wheel here, and while I understand manufacture's positions, even the bike shops position when Bosch say NO!!! {not being a party to that conversation I assume {always dangerous} that they did say no as that was what was told to me} I am not making a massive change to this bike just a slight change to make it a bike that meets my needs in as many ways as is possible and to do that simply and without any major issues or hassles. All I seek is an intelligent logical response and IF there are REAL reasons why this is not a good idea, explain them in an open easily understood manner, so I can then make my decision based on facts not maybe yes maybe no and this might or not happen.