It's the width of the frame shell only. It'll be either 68mm or 73mm. Apart from the DH-orientated 83mm versions, most external-bearing cranksets will come supplied with fittings/spacers to allow it to be used with both 68mm and 73mm frames.

 

Thank you.
In that case its a 83mm shell.

 

It's the width of the frame shell only. It'll be either 68mm or 73mm. Apart from the DH-orientated 83mm versions, most external-bearing cranksets will come supplied with fittings/spacers to allow it to be used with both 68mm and 73mm frames.

........or Pivot's 92mm shell

 

I know I may not be of any help but neither is anyone on this site. A lot of talk and absolutely, positively no answers. Doesn't anyone know how to measure a bottom bracket shell with a readable structured formula that can come up to a concrete answer. Apparently Not. Not here or mostly any where else I looked. People always like to type putting one word next to another into a stream of endless strings of nothingness. I suppose I'll stick to looking for how to fix a flat on this site. Amateurs. Done!!!

 

Some pos rep for you to cheer you up. And for your insightful and useful reply to this question.

 

How bizarre.

FWIW, you simply measure your frame at the BB shell - you could have found out for yourself if you googled it... probably in about the same time it would have taken you to type out your inane rant. Hope this helps.

 

I know that if you have a bracket on the bike already you measure across the length of the bottom bracket shell from the left rim to the right rim and then from crank arm to crank arm. That's the cute easy answer if you have a bottom bracket on it already but if you have a new frame and an odd crank then there has to be a formula to get to a size. A chart just won't do. That comes after you get the result of the calculation. Tossing around manufacturer sizes isn't useful either because you need to narrow down the bottom brackets that will fit. So sorry no answer here and I haven't found anything of any clarity online either.

 

All you have to do is measure the BB shell. Nothing about crank arms. The figures that have been mentioned are the generic ones. However, if you face the BB, those dimensions can change.

As for the clarity you are seeking - you are either not asking the question you have properly or you are massively over complicating the issue. There is no chart or calculation that I am aware of. It is a straight measurement.

 

The only complication in bottom bracket sizing is with the near obsolete square taper BB's, where the spindle lengths might be anywhere from 108-122mm, but that is crankset dependent and you either need to get the information from the crankset manufacturer or measure an existing BB. Or try a spindle and measure the chainline to determine how much you need to shift your chainline. Square taper cranksets are prone to stretching the square hole so an old set of square taper cranks may require a longer spindle to maintain chainline.

ISIS BB's are available in 108mm and 113mm, but also crankset dependent. Typically 108mm is for road BB's and 113mm for MTB's, but Race Face made models of MTB cranks that used 108 as well as 113,

External BB's are adaptable to 68 or 73 shells with one spindle length.

 

So, let's say I bought a used frame that has no BB in it, just frame only and I want to purchase the BB and crank kit together. Am I just measuring the BB shell and ordering the corrosponding BB then pretty much any crank that fits that BB? Sorry for the confusion there, lol...

 

As long as you're not using a square tapered BB and crankset, then generally yes. However, there are still other differences such as those intended for fat bikes, dh, etc.

 

I just bought a used Trek 7000 with no BB, I measured the shell and it's the 73mm BB, but I have a brand new Shimano square taper crankset and need to know the spindle length of the BB... how do i do that without the old BB???? it will be set up running a 3 x 7 drivetrain.

 

The new crankset should indicate what spindle length it is meant for. If you post the model of the crankset, we can help you look it up.

Or sheldon browns bottom bracket data base may give you the answer, but its mainly only has older cranks:
https://www.sheldonbrown.com/bbsize.html#shimano