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)

http://popup3.tok2.com/home/mizuguchi/mtb/ruckus.htm

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

http://www.gtbicycles.com/mountain/tech/idrive.php

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

sittingduck said:

http://www.gtbicycles.com/mountain/tech/idrive.php

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
http://www.habcycles.com
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
http://harriscyclery.com
Hard-to-find parts shipped Worldwide
http://captainbike.com http://sheldonbrown.com

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:
http://www.bikeboard.wyd.pl/images/porady/gt-i-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