A Bottom Bracket/Chainline question

[TomYoung]

Hi all:

I have a complete 9-speed Ultegra triple setup that I mount on my
roadbike once a year for a very hilly Century. Now, I'm thinking of
getting a touring bike and mounting the Ultegra setup on that, but the
bike I have in mind has 135mm rear dropout spacing instead of the 130mm
on my road bike.

The Ultegra bottom bracket has a listed length of 118.5mm. I've never
measured the chainline when the triple setup is mounted on my current
road bike, but shifting has generally been fine, so I'm assuming, at
least for sake of this discussion, that the chainline is "just right."

Offhand, it seems like the chainline could be out of whack if I mount
the Ultegra stuff on a bike with 135mm rear spacing since the cogs are
now going to be sitting 2.5mm to the right of where they'd be on my
current road bike, meaning I should be going to a longer bottom bracket
axle, which doesn't seem to be available from Shimano.

Am I thinking about this correctly? Or is there any general approach
to these chainline issues without actually fitting components to the
bike?

TIA.

Tom Young

 

[* * Chas]

"TomYoung" <[email protected]> wrote in message
news:[email protected]...
> Hi all:
>
> I have a complete 9-speed Ultegra triple setup that I mount on my
> roadbike once a year for a very hilly Century. Now, I'm thinking of
> getting a touring bike and mounting the Ultegra setup on that, but the
> bike I have in mind has 135mm rear dropout spacing instead of the
>130mm on my road bike.

The touring bike frame will probabaly have longer chainstays.

> The Ultegra bottom bracket has a listed length of 118.5mm. I've never
> measured the chainline when the triple setup is mounted on my current
> road bike, but shifting has generally been fine, so I'm assuming, at
> least for sake of this discussion, that the chainline is "just right."

I recently installed Shimano triples on 2 different bikes. They both
theoretically required 122mm BBs. I had to go to 118mm on both of them.

You can use a spacer behind the "fixed" or right hand cup on most bottom
brackets to adjust the chainline..

> Offhand, it seems like the chainline could be out of whack if I mount
> the Ultegra stuff on a bike with 135mm rear spacing since the cogs are
> now going to be sitting 2.5mm to the right of where they'd be on my
> current road bike, meaning I should be going to a longer bottom
bracket
> axle, which doesn't seem to be available from Shimano.

2.5mm shouldn't make that much difference on the chainline with a lomger
chainstay. Generally you should avoid extreme crossovers for good chain
life, big - big, or little-little sprockets.

> Am I thinking about this correctly? Or is there any general approach
> to these chainline issues without actually fitting components to the
> bike?

I had to install the cranks on both frames. in my situations.

> TIA.
>
> Tom Young
>

Chas.