Bracket sizes + crank requirements = anger and confusion

ajoten

Sequence of events:

1. Bought new cranks, needed longer bottom bracket than currently installed (110) 2. Bought new BB of correct length (125), then found current BB seized. 3. Bought BB tool and penetrating oil, still couldn't shift it.

So I've already wasted several £. Now I could either waste further £ on pro BB removal (which presumably might fail) or just get new cranks that fit.

However, when I check eBay sellers who are selling cranks, they generally shrug and say they don't know what size BB is required.

So the question is, how does one measure the correct BB needs of crank x?

Or of course if anyone has any cheap old cranks for square taper 110mm BB for me single speed project, let them declare it.

Bronzie

Sheldon has a helpful BB sizing page:
http://www.sheldonbrown.com/bbsize.html

ajoten

Yes - but sadly that doesn't help with unbranded (or obscure brand) cranks.

topdude

As you already have a new BB of the correct length for your new cranks you might as well take it to your LBS and have it fitted. The old BB will not last forever so will need to be removed sometime anyway.

ajoten

Which is in fact what is going to happen, on the assumption they can indeed get the damn thing out. I decided I was better throwing money away on LBS services than further useless parts.

But my question remains though: is there a way of measuring some bit of the crank to find your BB needs?

John.T

Not easily. If you have an old BB you could slip the crank on to it and measure the distance from the middle ring (triple) or ring gap centre (double) to the BB centre. This should give the chain line on that BB. You would then have to try various BBs to give the correct chain line for the chainset or a standard one if you do not know this. This would be quite difficult and only a bit better than guessing.
BBs vary a bit as well. I had 2 Shimano ones, a 107 and a 110. Both were the same length on the drive side, the extra was all on the non drive side. I did not even have to adjust the front mech when I changed it. It did save me buying a new one though.
Just noticed that you are going single speed. In this case you may find that you need a shorter BB anyway. Put the new crank on the bike and if the crank nips up without binding on the BB check the chainline. It may not be far out.
As regards actually just measuring the crank, no chance.

topdude

But my question remains though: is there a way of measuring some bit of the crank to find your BB needs?

It will be determined by "Q" factor and "Chainline"
This is either specified in the tech info for the intended crankset or you have to do some working out for yourself as below.
Measure your current setup to determine distance from inside of each crank to centerline of bike, this gives you the "Q" factor.
Measure the distance from centre of cassette to centerline of bike, this gives you the ideal "Chainline"
Measure how far cranks push onto the BB axle.
From this you can estimate the axle length required.

ajoten

John.T wrote:

Just noticed that you are going single speed. In this case you may find that you need a shorter BB anyway.

Ah, but bought an SS crank but it was rather bowl-shaped and the ring touched the chain stay even before it was fully on the axle. I emailed the seller for a recommended BB length and got what I was told.