Gearchange problem - bottom bracket shield?

[Fastpedaller]

A longish flap on the front mudguard would keep it clean. Assuming you have mudguards.

Mud off the front wheel is the primary cause, so yes, I'll look at whether I can easily fit some damp proof course plastic between the mudguard and the existing rigid end of the Tortec mudguard. That could also minimise grit getting on the chain. The BB shield idea would only be a secondary measure, and may still be worthwhile.

 

[silva]

After discovering that my Octalink left crank restarted to loosen because of water ingress, despite front wheel having a rather big mudguard, even reaching alot sideways over the tyre, I made a bottom bracket specific mudguard.
I cut off a part of a blue water hose, in the length, then 2 different (front back) U - shaped sections out, so that it became a snug fit over the bottom bracket, then with a thin stainless wire held into place. Alternative is garden plant binder wire.
And bingo, problem solved - the shield "worked", no further loosening restarts.
Worth mentioning: also the REAR wheel throws water/mud towards the bottom bracket area and the chain, for the same reason that you see it spewing forwards from under the upper end of your front fender.

 

[Punkawallah]

A watering can to pour water over the cable runs after your ride?

 

[mickle]

I'm guessing that there's more going on than just crud in the bb cable guide.

The OP's maintenance, or lack of, probably extends to the cables and the front derailleur.