Shimano UN54 bottom bracket - BB Mount

[yello]

I need a new bottom bracket for a bike. The Shimano UN54 is the one I want but I see it advertised (on wiggle) as BS Double, BS BB Mount and BS Triple.

Now I assume 'BS' is 'British Standard', 'double' and 'triple' speak for themselves (but why the difference??) but 'BB Mount' just defeats me. What is it?

 

[Steve Austin]

Some front mechs bolt onto the BB hence the need for a BB mount BB
A double might be shorter than a triple. why its not listed by size is beyond me though?!

 

[yello]

Some front mechs bolt onto the BB hence the need for a BB mount BB
A double might be shorter than a triple. why its not listed by size is beyond me though?!

Thanks Steve. The size is listed as well! I can't think how it matters whether it's a double or a triple!

 

[k-dog]

A triple BB sticks out further (although the cups are the same distance apart as a double as it is the BB shell that dictates that) so that you can fit 3 rings in - the outer two rings are attached to the cranks and the inner ring is attached to the middle ring - so it sits a little further in than the inner ring of a double setup.

If you used a double BB with a triple chainset you could find that the granny ring rubbed the chainstay which wouldn't be clever.