Bottom bracket change of axle length…

[CharlieB]

The time has come to change the bottom bracket on my loyal and reliable MTB.
It has always been sluggish in changing from mid to top chainring, and lots of experts and LBSs have fettled with it over the years without getting it perfect. The adjustment screw is about as far as it will go. Always kept clean and lubed, and cables are regularly checked.

If I bought a replacement bottom bracket with a slightly longer axle length (say 2-3mm), would this make a beneficial difference? Or are there other downsides I would need to be aware of?

 

[chillyuk]

Assuming you mean from the middle to the largest chainwheel, and your adjustment on the front mech is near it's limit, wouldn't a longer axle that moves the chainwheels further out make the problem worse?

 

[CharlieB]

Assuming you mean from the middle to the largest chainwheel, and your adjustment on the front mech is near it's limit, wouldn't a longer axle that moves the chainwheels further out make the problem worse?

Durrrr.

You're absolutely correct, now I think about it. Thankyouthankyouthankyou.

In that case, shorter by 2-3mm?

 

[Gerry Attrick]

Have you checked the chainline? It is usual for mid to large chainring to be a little slower changing. The chain has further to trave to complete the change.

 

[Cheule]

Have you tried altering the cable tension a touch?

 

[gwhite]

Have you checked the chainline? It is usual for mid to large chainring to be a little slower changing. The chain has further to trave to complete the change.

It's not the amount of travel but the fact that the FD works on the top section of the chain which is in tension, as opposed the the RD which shifts faster as it works from the bottom, untensioned section.
I'd check the chain-line before fitting a new BB myself. You can find instructions on the net or have a look at Sheldon Brown's site.