Eccentric BB on derailer mtb: Why?

Rick Onanian said:

Rode tonight with a group including a guy whose mountain
bike (derailer equipped, single bike not tandem) had an
eccentric BB. The BB shell was HUGE, and the BB was _very_
eccentric. The BB was probably four or five inches
diameter, and the cranks very close to the rear of the BB
shell. What's the purpose of that?

Maybe not quite five inches, could be a fish story in my
mind, but definitely huge.

Are you sure it was an eccentric, as in adjustable? I think
some full-suspension MTBs have suspension pivots that
surround the BB that way...

Sheldon "Maybe..." Brown +--------------------------------------------------------
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On Thu, 13 May 2004 22:49:55 -0400, Rick Onanian <[email hidden]>

Quoted message said:

Rode tonight with a group including a guy whose mountain
bike (derailer equipped, single bike not tandem) had an
eccentric BB. The BB shell was HUGE, and the BB was _very_
eccentric. The BB was probably four or five inches
diameter, and the cranks very close to the rear of the BB
shell. What's the purpose of that?

Maybe not quite five inches, could be a fish story in my
mind, but definitely huge.

Was it a GT Full Suspension? They have an eccentric bottom
bracket on their I-Drive models to isolate pedaling forces
from the suspension. Personally I'm not sure how good an
idea it is (lots of moving parts, seals for dirt to destroy,
etc.) but those who ride them generally seem to like them.

"Rick Onanian" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Rode tonight with a group including a guy whose mountain
bike (derailer equipped, single bike not tandem) had an
eccentric BB. The BB shell was HUGE, and the BB was _very_
eccentric. The BB was probably four or five inches
diameter, and the cranks very close to the rear of the BB
shell. What's the purpose of that?

Maybe not quite five inches, could be a fish story in my
mind, but definitely huge.
--
Rick Onanian

Was it one of these? (scroll all the way down)

popup3.tok2.comruckus.htmOpen ↗

Like the guy above me said, it was probably a GT I-Drive
setup, their (popular) attempt at a bob-resistant
suspension design.

Chris

Chris said:

GT I-Drive

gtbicycles.comidrive.phpOpen ↗

There is even a video on it there....

sittingduck said:

gtbicycles.comidrive.phpOpen ↗

There is even a video on it there....

whoops, thats the newer version, looks like they took out
the big bottom bracket.

Rick Onanian said:

Rode tonight with a group including a guy whose mountain
bike (derailer equipped, single bike not tandem) had an
eccentric BB. The BB shell was HUGE, and the BB was _very_
eccentric. The BB was probably four or five inches
diameter, and the cranks very close to the rear of the BB
shell. What's the purpose of that?

Maybe not quite five inches, could be a fish story in my
mind, but definitely huge.

If it's not the GT, it's probably a frame that's set up to
do singlespeed duty if necessary. The eccentric BB is a
cleaner solution to setting chain tension than horizontal
dropouts, at least for a MTB (where it's more likely the
rear wheel will slip with a horizontal dropout). Plus I
suppose some folks would like to play around with the
"geometry" of their bike.

Mark Hickey Habanero Cycles habcycles.comhabcycles.comOpen ↗ Home of
the $695 ti frame

Mark Hickey said:

The eccentric BB is a cleaner solution to setting chain
tension than horizontal dropouts

I was getting ready to switch to single speed on my vertical
dropout bike, and bought 2 chain tensioners... once I
finally made the switch, GUESS WHAT? the chain is PERFECT
with no tensioner, I hit it just right by blind luck. 🙂

Was it a SS with verticle drop-outs? All the folks around
here (Cental PA) that are SS'ers are moving to the ecentric
BB setup so they can run disk brakes.

"Sheldon Brown" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Rick Onanian said:

Rode tonight with a group including a guy whose mountain
bike (derailer equipped, single bike not tandem) had an
eccentric BB. The BB shell was HUGE, and the BB was
_very_ eccentric. The BB was probably four or five
inches diameter, and the cranks very close to the rear
of the BB shell. What's the purpose of that?

Maybe not quite five inches, could be a fish story in my
mind, but definitely huge.

Are you sure it was an eccentric, as in adjustable? I
think some full-suspension MTBs have suspension pivots
that surround the BB that

way...

Quoted message said:

Sheldon "Maybe..." Brown +--------------------------------------------------------
---------+
| This message has been sent to you using recycled
| electrons | exclusively. Please do not discard them
| after use, | send them along and help conserve these
| irreplaceable | sub-atomic resources for future
| generations. |
+---------------------------------------------------------
--------+ Harris Cyclery, West Newton, Massachusetts Phone
617-244-9772 FAX 617-244-1041 harriscyclery.comharriscyclery.comOpen ↗ Hard-to-
find parts shipped Worldwide captainbike.comcaptainbike.comOpen ↗
sheldonbrown.comsheldonbrown.comOpen ↗

Quoted message said:

On Thu, 13 May 2004 22:49:55 -0400, Rick Onanian
<[email hidden]>

Quoted message said:

Rode tonight with a group including a guy whose mountain
bike (derailer equipped, single bike not tandem) had an
eccentric BB. The

Was it a GT Full Suspension? They have an eccentric bottom
bracket on their I-Drive models to isolate pedaling forces
from the suspension.

Bing! You are correct. I googled images for GT I-Drive and
got this: bikeboard.wyd.plgt iOpen ↗
drive/zblizenie-2.jpg

That's definitely it. I remember the FD-clamp-protrusion.

Quoted message said:

Personally I'm not sure how good an idea it is (lots of
moving parts, seals for dirt to destroy, etc.) but those
who ride them generally seem to like them.

Well, he was a hell of a lot faster than me...although
nearly everybody was.
--
Rick Onanian