Measuring up for the correct bottom bracket length?

[Joe_Rides]

An amatuer question i know, but are there any tips for measuring up for the correct BB length?

I've recently bought a frame (GT) which did not come with a BB, so i have no reference.

The frame measures 73mm so i'm cool with that, its the length associated with the crank clearance which im unsure about?

With ranges between 110mm and 122.5mm i'm a little overwhelmed!!

Any help is much appreciated.

Joe

 

[FluffyChicken]

Re:

They start at 107 for triple MTB.

But the start is to pop your pics of cranks and preferably a make and model number.

The other way is to get a few lengths. Preferably 122 and fit the crank and measure the chain line then work back from there.
Sheldon Brown bottom. Use in a search engine should get you a good start of some possible lengths and how to measure chain line.

There are a few more intricacies but we can come to them.


Though we're in the 98 section so age of bike also helps.

 

[Joe_Rides]

Thanks Fluffychicken,

Just checked the Sheldon Brown website - my cranks are Truativ and it says that they are all 113 (square taper), link below:

http://www.sheldonbrown.com/bbsize.html

I forgot to mention that the frame i took the cranks off had a 113mm BB (specialized Hardrock). I just wasnt sure whether using a 113mm BB on my new GT automatically means i get the right crank clearance?

Edit: I'm unsure of the date of the GT - think it's 98/99ish. The specialized hardrock i removed the cranks from is a bit later, circa 2004, the cranks were original so 2004 also.

Joe

 

[FluffyChicken]

Re:

The length is linked directly to the crank, not the frame.
Only if the shortest length for the crank doesn't work (arm hit or rings dig in) would you move to the next chainline size.

 

[Joe_Rides]

Ahhh i see.

Well everything points to a 73 x 113mm so i've ordered one.

Thanks for your help!