Short Cranks on Bosch DU

[Doomanic]

I swapped over to shorter (152mm) cranks last week and the bike felt odd on my first ride yesterday. It was OK when going for it, but seemed to lack oomph at lower cadences.
I'm sure I read something about updated Firmware to allow for short cranks, can anyone enlighten me?

 

[Fat Rat]

That’s one of the updates for 2019 cx motors and can be updated on 2016 on cx motors for cranks less than 165mm

 

[Doomanic]

I thought I’d seen something along those lines. Is the update likely to cure the issues do you think?

 

[Fat Rat]

I would have thought so as your losing leg torque because of the shorter arms
So the software compensates for the loss

 

[Charliefox]

I swapped over to shorter (152mm) cranks last week and the bike felt odd on my first ride yesterday. It was OK when going for it, but seemed to lack oomph at lower cadences.
I'm sure I read something about updated Firmware to allow for short cranks, can anyone enlighten me?

Did you change to a shorter crank because of knee problems? I did, having no choice after a replacement knee left me with a max knee bend of 90 degrees. So now I ride with a high saddle position and changed or shortened cranks. ( I did think of only shortening the offside but did both in the end.) I soon got used to having a little less leverage by going down a gear or using the throttle a little more. strangely enough my grown up daughter never seemed bothered by the shortened cranks of my ordinary MTB! Apparently many riders, especially older ones have cranks that are too long as these days there is not much choice....175 and 170mm being almost universal. My shortened cranks cost me around £50 and the shorter ones are bought were cheapo ones, probably for kids bikes.

 

[Doomanic]

Bringing this thread back to life.

I've had the update done and fitted 152mm cranks for no other reason than I fancied a change.
However, since doing this I seem to be 10% or so down on range. Has anyone done the mod and found similar?

 

[Fat Rat]

I would say that was too be expected
You have reduced your input by using a shorter crank so the update increases motor effort to compensate
It will also be a lot more effort now over the cut off point
Hope this helps with your question

 

[Doomanic]

Yeah, it's what I was thinking.

Does anyone know if I need to go back to the previous firmware to run the long cranks again?

 

[soundwave]

Yeah, it's what I was thinking.

Does anyone know if I need to go back to the previous firmware to run the long cranks again?

no the bosch software aint that good lol just put the old ones back on.

try change the front chain ring if you want higher or lower gears

 

[Fat Rat]

Yeah, it's what I was thinking.

Does anyone know if I need to go back to the previous firmware to run the long cranks again?

Agree with SW
You will get a bit more assistance for your effort going back to standard crank length more than standard anyway obviously because of the software tweaks
No harm thou

 

[Doomanic]

It must be slightly clever if it needed the update in the first place.

 

[Doomanic]

It was never about assistance, it was about pedal strikes on technical singletrack. I didn't get many anyway and the number proably hasn't changed so I'd rather have the range I've lost back.If that means changing the firmware back I'm fine with it.

 

[Fat Rat]

It must be slightly clever if it needed the update in the first place.

No
It was just a power tweak
If it was clever it would of updated itself lol

 

[Fat Rat]

It was never about assistance, it was about pedal strikes on technical singletrack. I didn't get many anyway and the number proably hasn't changed so I'd rather have the range I've lost back.If that means changing the firmware back I'm fine with it.

Yeah I get that
Try it first
I would think the loss of range was just down to less human efficiency because of the cranks

 

[soundwave]

try a smaller front sprocket as that will give you more power with the shorter cranks.

you only get max power from the motor at 120rpm at the cranks

 

[Doomanic]

Yeah I get that
Try it first
I would think the loss of range was just down to less human efficiency because of the cranks

My concern is that the old firmware didn't work with short cranks and totally ruined a ride so the new firmware might not work with long ones. Richard at Raceco mentioned something about it but I wasn't paying full attention and he's on holiday this week.

 

[soundwave]

firmware does not care what length the crank arms are or what size chain rings you use.

the only option when you could change it was the wheel size tho a dealer can change this but thats about it.

software updates dont change the power settings as there locked in from factory so if a dealer is saying different they not got a clue.

and 152mm is tiny so the motor cant sense the max force you could put in with normal length cranks so you are getting less support than the 300% in turbo mode.

 

[soundwave]

that made the bike unusable as its so big i could not put the force on it needed or get the rpm high enough so was slower than ever.

 

[Gringo]

The quote below is taken from the CX page of Bosch's website.

"Whether you have long or short cranks, Bosch provides the right support, according to the torque. Short cranks give you extra ground clearance for eMountain Biking. Your specialist dealer will be able to make the corresponding adjustment."

From this I can only assume that the dealer can adjust the setting within the software. So if set correctly for the shorter crank you should get max assist without any more effort than with the longer cranks.
All things being equal and the software working correctly, if a bike is set for short cranks is then fitted with longer ones, maximum assist would be achieved with less rider effort.

 

[Gringo]

The quote below is taken from the CX page of Bosch's website.

"Whether you have long or short cranks, Bosch provides the right support, according to the torque. Short cranks give you extra ground clearance for eMountain Biking. Your specialist dealer will be able to make the corresponding adjustment."

From this I can only assume that the dealer can adjust the setting within the software. So if set correctly for the shorter crank you should get max assist without any more effort than with the longer cranks.
All things being equal and the software working correctly, if a bike is set for short cranks is then fitted with longer ones, maximum assist would be achieved with less rider effort.

I shoul add, I ride with short cranks and can only dream of the above torque adjustablility, alas I have the active line plus's motor But that cool as its still a cracking bike to ride