Thorn Cycles Forum

Community => Rohloff Internal Hub Gears => Topic started by: mickeg on January 06, 2022, 12:17:53 AM

Title: Adjusting the Chain Tension on the Nomad Mk II Bottom Bracket Eccentric
Post by: mickeg on January 06, 2022, 12:17:53 AM
There currently is a short debate on a different thread about the Nomad Mk II bottom bracket eccentric.  I thought I would start a new thread on how I adjust the chain tension on my Nomad Mk II in case others are curious, or they may buy a used Mk II and are unsure how to do it, or perhaps they have a different bike with that type of eccentric.  This way there is a new thread that can easily be found in the future.  I own a Nomad Mk II, but no other Thorn Rohloff bikes, thus I can't cite a list of other bike models that this applies to.

The Nomad Mk II has two set screws that tighten to hold the eccentric in place, these set screws are conical where they press into the eccentric and leave a permanent small conical depression (I call that a divot) in the eccentric.  Thorn has been clear in their instructions that you should avoid putting the divots too close to each other, as a groove can result.  And a groove could allow the eccentric to rotate, which would allow the chain to loosen when you do not want it to.

Years ago on this forum, someone described how he did it, he removed one eccentric bolt so he could see where the divot in the aluminum eccentric was before he adjusted it.  I have been using that method ever since and I think that is the key to adjusting the eccentric on the Nomad Mk II eccentric.  I built up my Nomad Mk II in 2013, or about eight years ago.  This method has worked well for me for most of those years.

When it is time to adjust the chain, I remove one of the pinch bolts and loosen the other.  I can see inside where the divot into the eccentric is, that divot is centered in the hole where you removed the pinch bolt.

This is the pinch bolt:
https://www.sjscycles.co.uk/bottom-brackets/thorn-m8-extra-fine-conepointed-screw-for-eccentric-bottom-bracket-shell/

The diameter of the pinch bolt is 8mm.  Thus the radius of the pinch bolt hole is slightly less than 4mm.  I always look inside the hole and rotate the eccentric so that the divot from the last adjustment is on the edge of that hole and then tighten one pinch bolt there.  Thus I put the new divot slightly less than 4mm from the previous divot, not closer. That prevents a groove from forming because all divots are well spaced out on the eccentric.

And after you wear a few chains out, you have put several well spaced divots into your eccentric at distances that should match all of the adjustments you need for your chain.  If however you change a chainring or sprocket size you may find you will be adding more divots to the eccentric.

My complaint about the eccentric design is that I would prefer infinite adjustability, but I have learned to live with the lack of that on my Nomad Mk II.  There have been a few times I started to adjust my chain and found that I could not move the eccentric as far as I wanted to for optimum adjustment without putting a new divot closer to an existing one than I wanted to do.  In that case, I restored the chain to the previous adjustment instead of tightening the chain.  One result of that is that I no longer attempt to adjust the chain as often as I used to.

Several years ago one of my pinch bolts was a bit loose and I noticed that before it could fall out.  Since then I have wrapped a rubber band around the two eccentric bolts so that the bolts can't rotate unless the band is removed.  The band became a good cable guide too.

I do not use a torque wrench on the pinch bolt, perhaps if someone knows the correct torque setting to use, they could cite that in a comment below.
Title: Re: Adjusting the Chain Tension on the Nomad Mk II Bottom Bracket Eccentric
Post by: Danneaux on January 06, 2022, 12:28:11 AM
Quote
...perhaps if someone knows the correct torque setting to use, they could cite that in a comment below...
Per Thorn's Bike Owner's Manual PDF available for viewing or download at http://www.sjscycles.com/thornpdf/ThornBikeOwnerManual2Web.pdf ...
Quote
Retighten the eccentric screws to 10 17 Newton meter (torque) 7.38 to 12.54 foot-pound-force.

Best,

Dan.
Title: Re: Adjusting the Chain Tension on the Nomad Mk II Bottom Bracket Eccentric
Post by: UKTony on January 16, 2022, 10:50:03 AM
I do use a torque wrench and so far have found that  tightening to max 12Nm is sufficient. I normally wait for chain wear  slack to reach at least 60mm (ie T1-T2  on page 12 of the Thorn guide referred to above). On one occasion I left tensioning until there was about 65mm slack, the one occasion the chain jumped off the chain ring when I was freewheeling slowly over a patch of poor road surface.