Hey guys first time poster. I have a Raleigh Marathon road bike and I am pretty sure it’s an ‘85. The crank arm has been falling off so I have decided to just replace the whole crankset. I have been looking into getting a 12-speed Shimano 105.
I was wondering what bottom bracket I should get to be compatible with the crankset as well as the Raleigh Marathon bike itself. Any help is appreciated!
To make the crank go in the bike, you need any ISO aka BSA threaded Shimano road Hollowtech 2 bottom bracket. Put differently, any of them that isn't Italian.
You have to be concerned with the chainline situation, because the chainline on the new crank will likely be further out than the bike really wants, since it's presumably a 120mm or 126mm spaced bike. Chainline basically can't be modified on external Shimano road cranks. The two possible areas of consequence are how well the front end chainline plays with the rear end, and whether the FD you have can get out far enough. Measure your current rear chainline and see what the comparison looks like to the prospective new crank, whose chainline you can look up. If it's pushing too far, the main options are either modernize the rear end as well as the front (spread the frame and run a 130 wheel), or use a crank you can tune the chainline on.
If you're running 8-speed chains, you won't be able to drop in a 12-speed crank without rub and involuntary pickup against the big ring while riding in the small ring and the smaller end of the cassette. This is easy to remedy on 10 and 11 speed cranks using chainring spacers under the small ring, typically a 0.6mm on each tab. I haven't done it on a Shimano road 12 crank yet, but it will probably work.
