Concentric Bottom Bracket

Hi there. I've seen this mentioned in descriptions for bikes and was curious as to what it meant. Is
it a significant feature? What's a concentric bottom bracket?
--
Alexander Kahn

 

Alexander Kahn <[email protected]> wrote:

> Hi there. I've seen this mentioned in descriptions for bikes and was curious as to what it meant.
> Is it a significant feature? What's a concentric bottom bracket?

I suspect you mean ECCENTRIC bottom bracket. The spindle is mounted in an eccentric, which when
turned, will tighten and loosen the chain as appropriate. They are found wherever chain tension must
be adjusted in the absence of a sprung derailer, such as a single speed with vertical dropouts, or
the front chain on a tandem.

--
Ted Bennett Portland OR

 

"Alexander Kahn" <[email protected]> wrote in message news:[email protected]...
> Hi there. I've seen this mentioned in descriptions for bikes and was curious as to what it meant.
> Is it a significant feature? What's a concentric bottom bracket?

It's the default mode. As opposed to some loopy modern bikes which, having no other provision for
chain tensioning , incorporate a tandemlike eccentric BB. The eccentric holds the bearing assembly
and is rotated to move the spindle closer or farther from the rear axle.

--
Andrew Muzi http://www.yellowjersey.org Open every day since 1 April 1971

 

[email protected]
> I suspect you mean ECCENTRIC bottom bracket. The spindle is mounted in an eccentric, which when
> turned, will tighten and loosen the chain as appropriate. They are found wherever chain tension
> must be adjusted in the absence of a sprung derailer, such as a single speed with vertical
> dropouts, or the front chain on a tandem.

Aha, that is what i mean. Now I see what's going on on those Jericho frames and that Phil Wood
frame... Thanks. What tools are needed to adjust this type of bottom bracket (for the chain
tension)? Does the frame need to allow for a special bottom bracket?
--
Alexander Kahn

 

RE/
>What tools are needed to adjust this type of bottom bracket (for the chain tension)? Does the frame
>need to allow for a special bottom bracket?

Something called a "pin wrench" is the right way to do it. Hand pressure works too.

The frame needs a totally different housing down there.
-----------------------
PeteCresswell

 

> > I suspect you mean ECCENTRIC bottom bracket. The spindle is mounted in an eccentric, which when
> > turned, will tighten and loosen the chain as appropriate. They are found wherever chain tension
> > must be adjusted in the absence of a sprung derailer, such as a single speed with vertical
> > dropouts, or the front chain on a tandem.

"Alexander Kahn" <[email protected]> wrote in message news:[email protected]...
> [email protected] Aha, that is what i mean. Now I see what's going on on those Jericho frames
> and that Phil Wood frame... Thanks. What tools are needed to adjust this type of bottom bracket
> (for the chain tension)? Does the frame need to allow for a special bottom bracket?

Indeed the frame's "hanger" or "bottom bracket shell" is usually made much larger to accomodate an
eccentric. Ideally the eccentric is large enough to allow 1/2" of total fore/aft adjustment, the
length of a chain link. Eccentrics usually have normal ISO threading, so any crank bearing assembly
may fbe fitted. Most models are simply held in place with a pairof allen bolts.

http://www.yellowjersey.org/photosfromthepast/ECC_MUZI.JPG

Note the binder bolts on the left in the photo. They are regular seatpost binders. You can see how
the aluminum liner rotates to change the position of the crank spindle relative to the rest of the
bike. The binder bolts need not be tight at all - much less than a seatpost bolt's torque - as there
is no force acting to turn the eccentric. Santanas for example use a 5mm grub screw with a 3mm allen
key head and that's probably overkill.

--
Andrew Muzi http://www.yellowjersey.org Open every day since 1 April 1971

 

> RE/
> >What tools are needed to adjust this type of bottom bracket (for the chain tension)? Does the
> >frame need to allow for a special bottom bracket?

"(Pete Cresswell)" <[email protected]> wrote in message news:[email protected]...
> Something called a "pin wrench" is the right way to do it. Hand pressure
works
> too. The frame needs a totally different housing down there.

You're right about the "pin wrench", Pete and that's one of my pet peeves. There's no reason to
leave them with those diminutive inadequate dimples when you can sink a couple of nice ample 8mm
holes in the thing. http://www.yellowjersey.org/photosfromthepast/ECC_MUZI.JPG Who carries a pin
tool around anyway?

--
Andrew Muzi http://www.yellowjersey.org Open every day since 1 April 1971