Carlton Pro Am 12 Bottom Bracket

[ZG862]

Whilst we're nit-picking terminology:
* The bottom bracket shell is the piece of the frame into which the down tube, seat tube and chain stays are affixed (brazed/soldered/glued/welded)
* The bottom bracket cups are the threaded pieces that screw into the bottom bracket shell. Usually the right hand cup is fixed and has a left hand thread to stop it being unwound by the action of pedalling. Usually the left hand cup is adjustable (winding it into the frame tightens the bearing) has a conventional right hand thread and has a lock ring around it. Pretty much every manufacturer likes to invent their own tooling for holding the cup and moving the lock ring.
* The bottom bracket axle is the steel/titanium bit to which the cranks attach, usually as a press fit with 4 tapered flats on each end. 2 nuts (in your case I think) or bolts stop the cranks from wandering off. (A popular misconception is that that nuts/bolts hold the cranks to the axle but let's not go down that rabbit hole just yet.)
* The bottom bracket bearing comprises the cups, axle and bearings which can be loose ball bearings, caged bearings or any variety of sealed needle/ball bearing cartridges.
* The average Joe's inability to grease/adjust a cup/cone/ball bearing bottom bracket plus the significantly faster fitting time has led to the almost total replacement of the bearings described above with the "modern" design.

I assume the 125mm relates to the length of the axle.
There is a great article on axle lengths/offsets here: https://www.renehersecycles.com/bottom-brackets-demystified/

Z
Edit: Link updated.

 

[drgj]

Thanks for that. The link doesn't work but I think I have seen something similar.

I think I may just convert to 6 speed as I said earlier.

 

[ZG862]

Thanks for that. The link doesn't work but I think I have seen something similar.

I think I may just convert to 6 speed as I said earlier.

Great idea if terrain permits. Less weight, fewer things to go wrong. If you want to get the chain line right you'll need a new axle of course and I'd imagine will want to revisit the gearing with perhaps a slightly smaller chainring. A 48T with a 13T sprocket will still give you nearly 100" gear at the top end and you should be able to get into about the 40"s with a fairly normal block. Best of luck.

 

[drgj]

Re:

Thanks for the reply. If I go to 6 speed I thought that as the bottom bracket axle seems too short for two chainwheels then I could use the same one for just one?

Dave

 

[drgj]

Re:

My bike has become 6 speed. I shortened the chainwheel bolts to work with a single one. I used the 42 tooth and moved it to where the larger one was. However, when I put the bike into the lowest gear the chain kept coming off. I sighted down the chain wheel to the cassette and it was too far over towards the high gear side. I had to turn the bottom bracket axle/ spindle round so that the short end was on the drive side and then it worked ok.

Funny that the chain didn't come off the large chain wheel when in that position but maybe there were just enough teeth to keep a hold of it.

Dave