Bicycle Mechanics - BB and Crankset

I am considering a new crankset and bottom bracket. What determines the length of the bottom bracket? Is it the frame or is it is the crankset or is it a combination of both of them. For example, my current bb is 68 X 110; does that mean that I will always need that length for that particular frame? Thanks.

PS I know this a probably a newbie question, so I apologize.

The first number is the width of the bottom bracket part of the frame. The second number is the length of the axle between the cranks. The 68 part will always be the same, and the axle length can vary based on the cranks you choose. You need to maintain the correct chainline which is another ball of wax.
Start digging around http://www.sheldonbrown.com for information on chainline.

68 is the shell width, it can not be changed. 110 is the spindle length, it determines how far the crank is from the frame. If you can, put the new crank on the old BB to get an idea of what spindle length you will need. This is determined by how well the FD lines up with the crank and also how straight your chainline is. Some cranks also have spindle length/chainline info. readily available.

Go here (http://sheldonbrown.com/glossary.html) if you need help with any of these terms.

EDIT: Wow, Thurstonboise beat me by 5 minutes. Guess my computer is slower than I realized.

68 is the shell width, it can not be changed. 110 is the spindle length, it determines how far the crank is from the frame. If you can, put the new crank on the old BB to get an idea of what spindle length you will need. This is determined by how well the FD lines up with the crank and also how straight your chainline is. Some cranks also have spindle length/chainline info. readily available.

Go here (http://sheldonbrown.com/glossary.html) if you need help with any of these terms.

EDIT: Wow, Thurstonboise beat me by 5 minutes. Guess my computer is slower than I realized.


Exactamundo!