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Rohloff / Thorn eccentric bottom bracket shell
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? BobT |
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Rohloff / Thorn eccentric bottom bracket shell
BobT wrote:
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 |
#3
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Rohloff / Thorn eccentric bottom bracket shell
BobT wrote:
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. 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. 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 |
#4
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Rohloff / Thorn eccentric bottom bracket shell
Sheldon Brown wrote: Chalo wrote: 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. 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. 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 |
#5
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Rohloff / Thorn eccentric bottom bracket shell
Per Chalo:
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 |
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