How to choose the right bottom bracket?

[ibie1983]

Hello everyone,

I am custom building my own bike, basically I am building a bike from scratch. I've got the frame and I have measured the shell width which is 68mm. I have bought a single speed 44 Tooth chainset/crankset. Now how do I know which size bottom bracket would fit the crankset? Would I have to buy 3 or 4 different size bottom brackets and try each one out to get the right bottom bracket? Somebody please help.

Thank you
Ibs

[Painkiller]

(08-25-2013, 01:38 PM)ibie1983 Wrote:  Hello everyone,

I am custom building my own bike, basically I am building a bike from scratch. I've got the frame and I have measured the shell width which is 68mm. I have bought a single speed 44 Tooth chainset/crankset. Now how do I know which size bottom bracket would fit the crankset? Would I have to buy 3 or 4 different size bottom brackets and try each one out to get the right bottom bracket? Somebody please help.

Thank you
Ibs

generally the BB width is spec'd by the crankset, this would only be bad if you are doing something freaky with the rear hub. some BB's are adjustable to 2 or 3mm. If everything is normal check the specs on your crankset and go with that first

[ibie1983]

Thank you for your reply. I bought the crankset on ebay and it doesn't specify anything on it. All it reads on the crank arm is RC-170-5.

[Painkiller]

(08-25-2013, 03:53 PM)ibie1983 Wrote:  Thank you for your reply. I bought the crankset on ebay and it doesn't specify anything on it. All it reads on the crank arm is RC-170-5.

try to pull the ad up again and send it to me, or find the seller and he may have more than one. it may have said in his post and u did not catch it. Try to find the brand name and model. It may have claimed a chainline dimension too to work from. Keep digging

[nfmisso]

Somehow you need to get a baseline measurement with the crank mounted from the chainring center line to the bike center line. And the same information for your rear wheel. Then knowing the length of the BB you used for the measurement, through arithmetic, figure out the length of the BB you need. Your measurement can be made with any BB on any bike, as long as you know be measurement the BB that you are using for measurement - and the new crankset will fit.

Ref: http://sheldonbrown.com/chainline.html