Rohloff / Thorn eccentric bottom bracket shell

BobT said:

I'm thinking about building a commuter bike using the Rohloff 14 speed hub.
I've seen the Thorn frames with an eccentric bottom bracket shell for chain
tensioning. Is this a better approach than using a standard bottom bracket
shell and the Rohloff chaing tensioner device?

Yes, but the best alternative is probably using a bike with Rohloff OEM
adjustable dropouts.

The tensioner is best avoided because it adds friction to the
drivetrain and increases the likelihood of dropping the chain. It also
makes wheel changes somewhat fussier and more complicated.

An eccentric BB is rather heavy and adds a point of likely mechanical
failure. Its main advantage over sliding dropouts is a cleaner
appearance.

Chalo Colina

Quoted message said:

BobT said:

I'm thinking about building a commuter bike using the Rohloff 14 speed hub.
I've seen the Thorn frames with an eccentric bottom bracket shell for chain
tensioning. Is this a better approach than using a standard bottom bracket
shell and the Rohloff chaing tensioner device?

Chalo replied.

Quoted message said:

Yes, but the best alternative is probably using a bike with Rohloff OEM
adjustable dropouts.

An eccentric BB is rather heavy and adds a point of likely mechanical
failure.

"Likely"? Haven't had a lick of trouble with mine. I've worked on lots
of tandems with eccentric bottom brackets, and have never seen any
serious problems with any that use the simple grub screw system as Thorn
does.

It's true that the wedge type sometimes gets stuck if normal maintenance
is neglected.

Quoted message said:

Its main advantage over sliding dropouts is a cleaner
appearance.

The sliding dropouts would appear to me to be at least as "likely" a
failure point as an eccentric bb.

With the sliding dropouts you need to worry about getting them set
evenly or the wheel will wind up crooked...not a big issue, but not an
issue at all with an eccentric.

With an eccentric you also gain an option of vertical adjustability,
which may be of value to some riders.

An eccentric bottom bracket also permits fine-tuning for perfect
chainline. Given that the Rohloff 54 mm chainline is not all that
common, this feature can be quite desirable. Straight chainline is,
after all, one of the chief benefits of internal gear hubs.

Sheldon "Happy Thorn Raven Owner/Dealer" Brown
+--------------------------------------------------+
| Pohl's law: Nothing is so good that somebody, |
| somewhere, will not hate it. |
+--------------------------------------------------+
Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041
http://harriscyclery.com
Hard-to-find parts shipped Worldwide
http://captainbike.com http://sheldonbrown.com

Sheldon Brown said:

Chalo said:

An eccentric BB is rather heavy and adds a point of likely mechanical
failure.

"Likely"? Haven't had a lick of trouble with mine. I've worked on lots
of tandems with eccentric bottom brackets, and have never seen any
serious problems with any that use the simple grub screw system as Thorn
does.

Ok, I'll accept that. My shop experience is only with the binder-bolt
kind, which can damage their pinch bolts, and the internal-expanding
type, which become an onerous problem if you strip a thread.

It was partly due to the shortcomings in those two types (and to the
nasty mess I've seen set screws make of seatposts) that I machined my
own BB for my Rohloff-hubbed bike that uses an eccentric shell. I made
the body with a 24tpi thread on the end and a massive lockring to clamp
the BB onto its shell. I had to machine a pair of 2.5" aluminum crow's
foot wrenches so I'd have reasonably compact tools to use for adjusting
the BB.

Quoted message said:

Quoted message said:

Its main advantage over sliding dropouts is a cleaner
appearance.

The sliding dropouts would appear to me to be at least as "likely" a
failure point as an eccentric bb.

If you compare set screw BBs to to sliding dropouts, I'm sure you are
correct. But expanding eccentrics have a well-earned reputation for
getting stuck and defying reasonable attempts to extract them. Failure
of an adjustable dropout is a heck of an easier problem to fix than a
stuck expander.

Quoted message said:

An eccentric bottom bracket also permits fine-tuning for perfect
chainline.

Perfect chainline with offset cranks, maybe. I chose to use cranks
with adjustable chainline and tread spacing. My BB does not have any
lateral adjustability anyway. I like the eccentric system I have just
fine, but if I had it to do over again I'd probably choose sliding
dropouts and an American shell.

Chalo Colina

Per Chalo:

Quoted message said:

Yes, but the best alternative is probably using a bike with Rohloff OEM
adjustable dropouts.

I've gone both ways and also prefer the Rohloff adjustable dropouts.

Lighter - although that's probably moot on a 32-pound bike...

My main problem with the eccentric BB approach (elegant-looking as it is...) is
that it eats into my fore-aft saddle adjustment range. There's only about an
inch of that to begin with and who needs the BB adjustment reducing it further?
--
PeteCresswell