OBSERVATIONS ON 155mm CRANKS

[Jay Kay]

The R&M cranks I received on my SD Mountain Rohloff were 175mm, and they were the same on my 2020 Delite and Susie’s 2020 Homage.

I did some research, and it seemed to me that shorter cranks had few drawbacks on an ebike, and obviously gave the pedals more ground clearance, so I purchased some 155mm Hope cranks to give it a go.

First up, I doubt anyone riding my bike would notice the difference if I didn’t say they were shorter. For me, knowing the numbers, I slightly noticed the shorter platform (distance between the two pedals when horizontal and standing on them with my rare off the saddle) and shorter turning circle, but even so it was more subjective than objective. My cadence number was up, which gave some objective data, and a higher cadence is not necessarily a bad thing. The cadence was never high enough that I felt like my legs were spinning too fast. In fact I only slightly noticed the higher cadence.

I achieved 2 cm great ground clearance for my pedals, and overall 2.3cm for the crank arm. Go have a look at your R&M cranks and you will see quite a bit of extra metal sticking down. The Hope arms are significantly shorter, with a flat bottom.

Verdict - definitely get shorter cranks on standard speed bikes!

Cadence on HS versions would need to be tested but will probably be fine with a derailleur or Rohloff! It might be too high with the Vario due to the narrower gearing ratio.

 

[bradl]

Thank you for this. I really appreciate folks who are willing to tinker and even more helpfully, report on their results.

 

[Cottwood]

Hi Jay Kay,
I am interested in how well this works for you. I test rode a delight or a charger at a dealer in Denver. I loved the bike, but when I jumped a curb the petal hit. This was a big turn off for me as I ride a lot of trails, bike paths etc. It sounds like this is a common problem with R&M bikes. I am anxious to hear if this eliminates the problem? Would love tho hear your input after a few hundred miles. Thanks!

 

[Jay Kay]

Hi Jay Kay,
I am interested in how well this works for you. I test rode a delight or a charger at a dealer in Denver. I loved the bike, but when I jumped a curb the petal hit. This was a big turn off for me as I ride a lot of trails, bike paths etc. It sounds like this is a common problem with R&M bikes. I am anxious to hear if this eliminates the problem? Would love tho hear your input after a few hundred miles. Thanks!

I ride 80% off road, including MTB singletrack. I will still get pedal strike if I am not careful, but so far (a week) I have gone up technical climbs without issue and with more confidence. The shorter crank raises the pedal by 2cm so you may have to make a call from there. The Mountain also has the most ground clearance of all the R&M models which also helps. I have another set of 155mm Hope cranks ordered for my wife’s Homage now I know they are worth it. Every cm helps.

 

[Jack K]

Hi Jay Kay!
Thanks for sharing your experience! Did you ask your Riese&Muller dealer, if the motor software could be adjusted to compensate for shorter cranks? I've understood that for some motor versions this should be possible, as written on Bosch's website: https://www.bosch-ebike.com/en/news/the-new-performance-line-cx-package/
I'm wondering if this could help keep the cadence more similar to having longer cranks, by adjusting the motor support curve.

I'm riding 165mm cranks on my mountain and road bike, but going to experiment with 160mm or 155mm on my upcoming R&M Multicharger HS. I would not want to use a higher cadence though to keep high speed commutes relaxed

 

[Jay Kay]

Hi Jay Kay!
Thanks for sharing your experience! Did you ask your Riese&Muller dealer, if the motor software could be adjusted to compensate for shorter cranks? I've understood that for some motor versions this should be possible, as written on Bosch's website: https://www.bosch-ebike.com/en/news/the-new-performance-line-cx-package/
I'm wondering if this could help keep the cadence more similar to having longer cranks, by adjusting the motor support curve.

I'm riding 165mm cranks on my mountain and road bike, but going to experiment with 160mm or 155mm on my upcoming R&M Multicharger HS. I would not want to use a higher cadence though to keep high speed commutes relaxed

Hi JK1,
I mentioned to my LBS that they would need to adjust the Bosch software, and they indicated they had done this for a 165mm crank before. I forgot to ask them if they actually did change anything, but my hunch is they did.

 

[amayers]

I don't understand how adjusting the crank length effects the cadence, and why you'd have a software adjustment for crank length. I get that shorter cranks would reduce your leverage, so you may choose to run in a slightly lower gear to have the same amount of strain on your legs. So in that case your cadence would increase. But I don't see why the motor would care. Isn't it just RPM and torque that it cares about matching?

 

[Nycturne]

I don't understand how adjusting the crank length effects the cadence, and why you'd have a software adjustment for crank length. I get that shorter cranks would reduce your leverage, so you may choose to run in a slightly lower gear to have the same amount of strain on your legs. So in that case your cadence would increase. But I don't see why the motor would care. Isn't it just RPM and torque that it cares about matching?

This came up in a different thread, and Bosch does have a setting that should be updated for short cranks. It looks like it does have an effect on the signals the motor gets from the torque sensors that is measurable and can be accounted for, with a larger impact on eMTB mode. But I think your comment about leverage is the key there, since what is leverage in the case of rotating cranks?

https://www.e-bikeshop.co.uk/blog/post/new-2019-bosch-ebike-updates-emtb/ (Under: New Update For Short eMTB Crank Arms). EDIT: JK1 also linked to Bosch's website discussing this as well.