Shorter Cranks - loss of torque remedied

[AF1]

Hi
Bosch Gen 4 CX (non smart). While researching shorter cranks, I rang my LBS who said they can adjust upwards the torque to compensate! Good news for those who didn't know.

 

[Miket775]

I'm sure there's "short crankset" firmware version - I remember seeing something when I last updated my bike at the shop - maybe this is what they are referring to?

 

[AF1]

I'm sure there's "short crankset" firmware version - I remember seeing something when I last updated my bike at the shop - maybe this is what they are referring to?

Probably.

 

[Shark58]

Probably.

The short crank compensation is only available for the eMTB mode.

Bosch introduced an eMTB for short crank mode some years ago. As a dynamic mode influenced by rider power input it was quickly clear that it was difficult for lightweight riders to pedal with enough power in order to engage the higher support levels in normal eMTB. With the short crank mode this was fixed.

Bosch defines short cranks as being up to 165mm long. Anything longer is a standard crank.

 

[AF1]

The short crank compensation is only available for the eMTB mode.

Bosch introduced an eMTB for short crank mode some years ago. As a dynamic mode influenced by rider power input it was quickly clear that it was difficult for lightweight riders to pedal with enough power in order to engage the higher support levels in normal eMTB. With the short crank mode this was fixed.

Bosch defines short cranks as being up to 165mm long. Anything longer is a standard crank.

So this mode won't work if changing from 165 to 152mm cranks?

From 2019 Bosch Electric Bike System Updates – e-bikeshop.co.uk

Bosch introduces software updates via a Bosch eBike dealer, who can program each bike to suit the preferred crank length. This adapts the progression curve in eMTB mode and also offers increased support in Tour mode, offering maximum power up-hill.

 

[AF1]

Thanks to @Shark58
"When introducing the CX Gen4 it started with 75Nm max power and was later updated to 85Nm max power. The 85Nm update raised the support levels for all modes and gave us what we have today: 140% for Tour and 340% for Turbo with eMTB dynamically moving between 140% and 340%. There where still the two applications for eMTB with short or long cranks. Any dealer can choose which version to install, hopefully after asking the customer what he wants.

Fast forward to early 2021 and Bosch introduced the second dynamic mode called Tour+. That mode in general works like eMTB but dynamically moves between Eco (60%) and Turbo (340%) support. The rider also needs to input more force to reach the higher support levels. Now comes the caveat: If one wants Tour+ installed, you are forced to install eMTB for long (regular) cranks. Bosch hast dropped support for the eMTB short crank version with the installation of Tour+."

 

[Tepi]

Joten tämä tila ei toimi, jos vaihdat 165 mm: n kammet 152 mm: iin?

Boschin sähköpyöräjärjestelmän päivitykset vuodesta 2019 – e-bikeshop.co.uk

Bosch esittelee ohjelmistopäivityksiä Boschin eBike-jälleenmyyjän kautta, joka voi ohjelmoida jokaisen pyörän halutun kammen pituuden mukaan. Tämä mukauttaa etenemiskäyrää eMTB-tilassa ja tarjoaa myös paremman tuen Tour-tilassa, mikä tarjoaa maksimaalisen tehon ylämäkeen.

Omaan GEN 2 tehtiin päivitys 2020 vaihdettuani 160mm, kampiin.

 

[Bummers]

So this mode won't work if changing from 165 to 152mm cranks?

From 2019 Bosch Electric Bike System Updates – e-bikeshop.co.uk

Bosch introduces software updates via a Bosch eBike dealer, who can program each bike to suit the preferred crank length. This adapts the progression curve in eMTB mode and also offers increased support in Tour mode, offering maximum power up-hill.

No, by default you'll already be on the 'short crank' option.

 

[Shark58]

So this mode won't work if changing from 165 to 152mm cranks?

The eMTB mode for short cranks works for any lemgo crank. You can even have it installed with 175mm cranks and it will work.

The only difference between eMTB short crank and eMTB regular crank is the reduction of power you need to input to reach the higher support levels. It was designed to compensate the 5.7% shorter lever between a 175mm and 165mm crank. When mounting a 152mm crank the even lower torque you generate will not be fully compensated.

 

[AF1]

The eMTB mode for short cranks works for any lemgo crank. You can even have it installed with 175mm cranks and it will work.

The only difference between eMTB short crank and eMTB regular crank is the reduction of power you need to input to reach the higher support levels. It was designed to compensate the 5.7% shorter lever between a 175mm and 165mm crank. When mounting a 152mm crank the even lower torque you generate will not be fully compensated.

Add that to my low cadence and I might regret doing it!

 

[Shark58]

Add that to my low cadence and I might regret doing it!

Short cranks should be used with higher cadence, otherwise you put to much strain onto your knees. Rule of thumb: Raise cadence by the same percentage your new crank is shorter than the original.

 

[AF1]

Short cranks should be used with higher cadence, otherwise you put to much strain onto your knees. Rule of thumb: Raise cadence by the same percentage your new crank is shorter than the original.

Old, not able to do high cadence!

 

[Shark58]

Old, not able to do high cadence!

High cadence with low torque normally should be easier than low cadence with high torque.

If absolutely impossible then don’t use very short cranks.

 

[AF1]

High cadence with low torque normally should be easier than low cadence with high torque.

If absolutely impossible then don’t use very short cranks.

Having read a lot about stature v crank length and being challenged in the leg dept, I checked the results of the various calculators to arrive at 152mm. It reduces the angular movement in hips and knees. The experts said that 165 is too long for shorter legged riders. I read that some ebike riders go to 127mm

 

[Shark58]

various calculators to arrive at 152mm

In that case you could try to raise your average cadence in small steps. After some time you will get used to it.

I use 160mm cranks on my eMTB to avoid pedal strikes on challenging trails. I have the eMTB for short cranks mode installed and that works great for me.

 

[AF1]

In that case you could try to raise your average cadence in small steps. After some time you will get used to it.

I use 160mm cranks on my eMTB to avoid pedal strikes on challenging trails. I have the eMTB for short cranks mode installed and that works great for me.

Apparently I already have Short Crank Mode active as I have 165mm OEM Cranks.

 

[Shark58]

Apparently I already have Short Crank Mode active as I have 165mm OEM Cranks.

Not necessarily. Many bikes with 165mm cranks where delivered with standard eMTB mode.
For a non smart motor you can see which version you have in a Bosch service report.

 

[KnollyBro]

Having read a lot about stature v crank length and being challenged in the leg dept, I checked the results of the various calculators to arrive at 152mm. It reduces the angular movement in hips and knees. The experts said that 165 is too long for shorter legged riders. I read that some ebike riders go to 127mm

Wow at 127mm... that would be like pedaling a kids bike and your stance/stability on the pedals would be affected noticeably! I am 172mm tall and ride 3 bikes with cranks at 175 (Park bike), 155 and 145mm (KSL and LSL) and really notice the difference when I first got on the short crank bikes for the first minute but really appreciate rarely hitting the cranks. My cadence is between 75-85 which seems fine to me. The only thing I have noticed are a little more battery drain with the shorter cranks and I feel more stable on the 175mm crank bike.

 

[Shark58]

For a non smart motor you can see which version you have in a Bosch service report.

@AF1 I‘m sorry, I have to correct this.

It is not possible in the Bosch service report to see which version of eMTB mode is installed. They both show up simply as eMTB.

You need to see your LBS and have him connect the bike to the diagnostic system. There is a screen where he can select which container to install onto the DU. There are a total of 7 different containers to choose from and two of those install eMTB for cranks 165mm or less. One with 85 Nm and the second choice with 75 Nm (yes, one can still select to have the original 75 Nm version installed). There is only one container to install TOUR+ which will force the install of eMTB for standard cranks and 85 Nm. Please note that all containers are mutually exclusive. Installing eMTB replaces SPORT. Installing TOUR+ replaces TOUR.

I attach a screenshot from the diagnostic system with all containers listed. It is in German but easy to understand: „Kurbellänge“ is crank length.

 

[AF1]

Wow at 127mm... that would be like pedaling a kids bike and your stance/stability on the pedals would be affected noticeably! I am 172mm tall and ride 3 bikes with cranks at 175 (Park bike), 155 and 145mm (KSL and LSL) and really notice the difference when I first got on the short crank bikes for the first minute but really appreciate rarely hitting the cranks. My cadence is between 75-85 which seems fine to me. The only thing I have noticed are a little more battery drain with the shorter cranks and I feel more stable on the 175mm crank bike.

Some would say 145mm is extreme too!

 

[KnollyBro]

Some would say 145mm is extreme too!

Agreed, but it is both expensive and difficult to find short cranks for a Levo SL gen 1 motor so I modified a set of 175mm cranks to make some 155mm but could only get some 170mm cranks, to make a second set, which ended up being closer to 145mm. 150mm was the recommended size for my inseam. I happy with the increased ground clearance and have not really noticed a stability issue so far.

 

[AF1]

Agreed, but it is both expensive and difficult to find short cranks for a Levo SL gen 1 motor so I modified a set of 175mm cranks to make some 155mm but could only get some 170mm cranks, to make a second set, which ended up being closer to 145mm. 150mm was the recommended size for my inseam. I happy with the increased ground clearance and have not really noticed a stability issue so far.

Have you noticed a reduction in assistance due to shorter cranks?

 

[KnollyBro]

Have you noticed a reduction in assistance due to shorter cranks?

Since I only use the assistance to climb steep, long, loose trails and FSRs (and not to go fast), the cassette's gearing handles all the assistance I ever need. YMMV.

 

[fasterjason]

@AF1 I‘m sorry, I have to correct this.

It is not possible in the Bosch service report to see which version of eMTB mode is installed. They both show up simply as eMTB.

Screenshot above is second page of diagnostic for a Bosch System 2 CX motor. The first character in the application ID code lets you know which container file is installed. The "Q35389" file is emtb, <165mm, 85Nm, regular tour mode, if it began with "N" it would be tour+. The screenshot you shared has the most up to date container files for System 2 motors, e.g. Q34071. I am currently running a 160mm crank and the H34071 file because the Q file was not working well with the Kiox controller I installed.

I run 155mm cranks on my Rail 9.8 for pedal strikes because I stroked the shock, and with the flow app you can change almost everything about the support modes.

 

[Shark58]

Screenshot above is second page of diagnostic for a Bosch System 2 CX motor.

Interesting, is this from the new diagnostic 3 system?

My latest service report from 07/2022 looks completely different. It tells me I have a Bosch Performance Line CX motor with software version 1.1.5.0 installed, but no application ID. There is absolutely no reference to the container file used.

The report was generated with diagnostic system 8.1.6.0.

with the flow app you can change almost everything about the support modes.

Yes, but for the flow app to work you need a smart system motor.

 

[fasterjason]

Interesting, is this from the new diagnostic 3 system?

The screenshot I shared was an old report for my Rail7 generated with Bosch diagnostic tool 8.1.6.0. The diagnostic 3 tool was updated about a month ago by Bosch and part of the upgrade is the ability to use it with the System 2 motor, battery and accessories. I have only connected a system 2 motor to the diagnostic 3 tool once to have a remote reprogramming performed. The report and interface are totally different between the two tools. Functions too, for example you can test smart system components with diagnostic 3 tool (see below) but not system 2 components.

Yes, but for the flow app to work you need a smart system motor.

My 9.8 with the 155mm cranks is (for better or worse) a smart system. Here is a snip of a report with diagnostic 3 tool v. 9.7.13, and the most recent version v. 12.6.9 looks the same. You have to activate the support modes to customize them with the Flow app.