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Hello fellow cycling enthusiasts.

Intro

I have bought, just 2 months ago, a new Cube Hyde Pro 2020 bicycle with belt drive (Gates, rear cassette out of metal, CDC with 22 teeth) Shimano Nexus 8 internal hub (model SG-C6001-8D). I mostly ride in the city and sometimes for fitness purposes. Bicycle link with specs: cube.eu

Problem

Overall I am happy with the bike. It rides quiet and comfortable but I have noticed that under high torque situations like short steep inclines or fast take offs a single sharp and unpleasant clunking noise can be heard from the gear hub. This happens in all the gears. I have tried to catch this noise on camera and took a video of it on my daily commute on a short steep incline and gearbox in 1st gear: Youtube video

What I tried

  1. First I made sure that the tension on the gear shifting cable was correct and that 2 yellow marks on the hub line up when the 4th gear is selected (see the picture)

enter image description here

  1. Second step was that I checked the belt tension with the Gates frequency measurement app on the phone and it shows ~35HZ tension which is within the range recommended by the manufacturer

  2. 3rd step was research and it seems that few other users have noticed such behaviour even on this website. I actually found 2 threads with similar problem but with no clear solution:

I would greatly appreciate any help or input from you. Even if you don't have the solution but have observed the same behaviour please reach out and maybe we can find the solution together.

Thanks!

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    Two months, don't attempt any repair by yourself. The product is still under warranty. Take that bike back to the shop or arrange a return if you bought online. – Carel Oct 30 '20 at 18:17
  • Whats the mileage? Alfine are recommended an oil change at 1000 km, and then every 5000 from then on. I suspect Nexus will be similar. – Criggie♦ Oct 30 '20 at 22:20
  • Thanks for the reply! Yes I will not try any major repairs, just the adjustments. If its serious I'll take it to the dealer. Mileage is low, 200 km tops – user2523745 Oct 30 '20 at 22:25
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There are some Nexus and Alfine bikes that have made it out into the world with a spec mismatch between the over and under style cassette joints and the shifter.

The cable pull sequence works in reverse order between the two shifter types. Because the cable pull for this system varies with each click, everything gets screwed up if you then use the setup marks. Something like what you're experiencing can result.

To check this, get your exact cassette joint and shifter models and make sure they're listed as compatible on si.shimano.com.

If that checks out, test it with less belt tension.

If that's not it, get oil dunk gear and the Nexus/Alfine notchy tool, pull the core, wipe, inspect for anything out of place or prematurely damaged or worn, dunk, grease, reassemble, and see if that fixes it.

One perfectly likely explanation is that the pawls that drive the shell are having engagement problems. If one or more is under-lubricated, deformed, or out of place, it could cause a partial engagement problem, which also fits the symptoms. This can lead to a situation where the tips of the pawl(s) get damaged and the problem gets worse. This is easy to inspect for once the core is removed. Note that despite the ostensible complexity of the hub, the pawl system in question is the most exposed part of the whole deal, being the interface between the drive ring inside the hub shell and everything else. If it does the same behavior with the cable disconnected completely, I think this pretty likely is what's happening.

It is true that everything about this should be covered under warranty. Between parts shortages and the usual awkwardness of getting warrant support to happen for mail order bikes, it may get hairy at the moment.

Also check dropout alignment slash sync up slider fore/aft if applicable, all of which can in theory cause hub badness.

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    thanks for your input! I checked the compatibility and the shifter is compatible. I tried adjusting the cable tension away from the yellow mark. And by away I mean 0.5-1 turn of the adjuster. Also I decreased the belt tension to ~25-30hz and so far so good! I can take the same slope without the loud bang, and shifting feels a bit smoother. I will continue observing it's behaviour and still try to avoid applying to much torque to the system. Its crazy how sensitive these hubs can be and also how mark can be a bit off and misleading. Thanks for suggestions! – user2523745 Nov 2 '20 at 0:05
  • Glad it worked. It's somewhat of a known issue that the published tension specs for Gates can be punishing to Shimano IGH hubs. – Nathan Knutson Nov 2 '20 at 0:08
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    @user2523745 would you be so kind to write your solution in an answer yourself? I don't think your method is included in Nathan's answer yet. – gschenk Nov 2 '20 at 0:50
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After several suggestions and trials I think I have resolved the issue.

I tried adjusting the cable tension away from the yellow linup mark. And by away I mean 0.5-1 turn of the adjuster. Also I decreased the belt tension to ~25-30hz which is a bit below the recommended 35-50hz range and so far so good! I can take the same slope without the loud bang, and shifting feels a bit smoother.

I will continue observing it's behaviour and still try to avoid applying to much torque to the system. Its crazy how sensitive these hubs can be and also how the yellow mark can be a bit off and misleading.

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  • Great work! I found mine had to be backed off a couple turns from the "right" position, but that was on a bike with suspension, so sitting on it re-tensioned things. Very curious that yours wanted to go the other way. – Criggie♦ Nov 2 '20 at 23:26

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