bottom bracket sizing guide?

se99paj

So I decided to try and build my own mountain bike, I managed to get a frame and a few other bits off eBay, I'm going to use some parts from an old bike to try and keep costs down.

But I'm going to need a new bottom bracket and I'm a bit confused, how do you know what length it needs to be? I know the shell needs to be 68mm but what about the spindle? Some websites make it sound like trial and error.

Is it going to make more sense to just take it down to my local bike shop?

nicklouse

it all depends on the cranks.

se99paj

They are triple cranksets, but I'm guessing its a bit more complicated that that. Is it possible to measure the crankset to be absolutely sure?

nicklouse

no it depends on what they are. make model year and part number.

cooldad

Techdocs for the cranks should tell you the axle size needed.

se99paj

Just had a quick Google, my crank is an ofmega sierra, but couldn't find much info.

The gears and derailleurs are all shimano, I'm thinking buying a shimano crank might make more sense as I can get decent info on the bb.

YeehaaMcgee

If you get a hollowtech II crankset from Shimano, then it makes everything far easier.

pete_s

It doesn't look good news as far as Sheldon's concerned:http://sheldonbrown.com/bbsize.html

Get a Shimano SLX second hand. Going rate for one is about £80 and the BBs are cheap and easy to install.

The Rookie

pete_s wrote:

the BBs are cheap and easy to install.

Cough Shell facing cough.....

If you don't know the axle width required it is trial and error, see if you can borrow one locally (start with 118mm as most common) assemble, measure chain line, buy the right length (unless 118mm works!).