Tuesday, 9 February 2010

Thorn's Eccentric Bottom Bracket

I've adjusted the Raven Nomad's Eccentric Bottom Bracket (EBB) once so far, to re-tension my slightly elongated chain. It was a painless affair and took 2 minutes. I will have to do this about once every 1000 - 1500 km to maintain reasonable chain tension.

Although the first adjustments of the chain tension will be easy to perform, I'm not convinced (at the moment) that they will always be so. This is because of Thorn's choice of using a Set Screw EBB.

Thorn's Set Screw EBB

To lock the EBB after adjustment, you screw in two large semi-pointed set screws (bolts) underneath the bottom bracket. Once tightened, the set screws make indentations directly into the EBB casing enabling the screws to grip the EBB and keep it firmly in place. Every time you adjust a virgin EBB, you get two new (permanent) indentations. Over time, these indentations build up. I can think of some possible niggles with this method of locking the EBB.

Firstly, you will only be able to adjust the chain tension in quite coarse steps. If you try to make a very small adjustment, the set screws will engage the edge of the existing indentations and 'deflect' the EBB back to the previous position. (This has been mentioned on the Thorn forums). Not a big issue, but some people like to have a 'perfect' chain tension all the time and this is not achievable with a Set Screw EBB of this type.

Closeup of set screws underneath the bottom bracket

Once you've gone through one chain, you'll have a nice set of indentations in the EBB. When you fit a new chain, you'll need to reset the EBB. This should be pretty easy to do, as a new chain is of a standard pitch, so by naturally tensioning the chain you should 'find' the first EBB indentations again. After that, though, could it become more hit and miss? Unless you re-tension the chain at roughly the same slackness as the first chain (easy to judge?), the EBB will accumulate further indentations. Eventually, you could unintentionally deflect into existing indentations and move the EBB from your intended position. This might niggle if the deflection over-tensions the chain slightly, perhaps?

This could also happen if you use different combinations of chainrings and sprockets, on the same EBB, to develop different gear ratios (E.g. 40x16 and 38x16). Each ratio will require a different EBB starting position and probably, separate adjustment indentations.

Perhaps it's worth marking the outward face of the EBB, every time you adjust the chain tension, so that later at least you know where the existing indentations are?

There are other designs for an EBB. One design uses a split bottom bracket shell with 'pinch bolts' that clamp the shell around the EBB, rather like a brazed-on seat collar clamping a seat pin. Another (patented) design is the Bushnell Eccentric. Instead of using two set screws to lock the EBB, it has an allen key fitting, on the outward face of the EBB, which drives internal cams apart that spread the EBB inside the bottom bracket shell. Another similar design is the Carver EBB. The Gary Fisher EBB operates in a related way and looks like an oversized 'quill' stem! (Note, the Bushnell is 53.8mm in diameter and won't fit the current 51.3mm Thorn BB. The other 3rd-party examples probably won't fit either as Thorn's EBB seems smaller than 'standard').

The alternative designs certainly appear to offer a finer granularity of EBB adjustment and solve the potential niggles I've discussed, but perhaps introduce different compromises and problems? (I've read of some creaking and slipping issues).

What do people think? It would be interesting to know the underlying reason(s) why Thorn choose the older style Set Screw EBB ahead of other potential designs...


Stuart said...

Mmm I think I better start reading my Nomad owners manual. I've only just graduated from the hammer and WD40 school of bike maintainence, this looks a tad trickier!

Interesting article.


Shaun said...

It's not tricky, per se, but it may niggle at some point if you're fussy about chain tension...

p.s. I changed some text since you read the post.

Gearóid said...

You should be able to see the previous indentations if you look into the holes.

Anonymous said...

This issue may be the one thing that keeps me from buying the Nomad. I seems to me that Thorn ought to offer an "Upgrade" to this system, assuming it's possible.