Bottom bracket size? How to tell without removing.

[Cyclopathic]

Is there any way easily tell what size a bottom bracket is? I know the length is usually printed clearly on the unit itself (113mm or what have you) but is there any other way to tell?

Also how important is it to get exactly the right size. Could it be the case that a mm either way would not matter in that it would not be detectable in the ride quality. I assume this is to do with chain-line as well as clearance for the cranks on the chain-stays.

All advice happily received and time wasters most welcome.

 

[smokeysmoo]

What bike is it?

 

[Spoked Wheels]

Is there any way easily tell what size a bottom bracket is? I know the length is usually printed clearly on the unit itself (113mm or what have you) but is there any other way to tell?

Also how important is it to get exactly the right size. Could it be the case that a mm either way would not matter in that it would not be detectable in the ride quality. I assume this is to do with chain-line as well as clearance for the cranks on the chain-stays.

All advice happily received and time wasters most welcome.

How about measuring the shell of the bb and then measuring the spindle length. That would put you in the right track.

 

[Cyclopathic]

What bike is it?

It's a ladies Giant CRS 2.5 commuting/hybrid sort of thing. I've looked it up as far as I could on t'web but got no more detail than "enclosed bottom bracket".
I just wondered really if there was some obvious way that I'd missed that everybody else knew but it doesn't look that way.

 

[Cyclopathic]

How about measuring the shell of the bb and then measuring the spindle length. That would put you in the right track.

This is probably the best idea. Thanks.

 

[Andrew_P]

This is probably the best idea. Thanks.

http://www2.giant-bicycles.com/en-gb/bikes/women/1432/31746/?collections_id=3
BOTTOM BRACKET Shimano BB-UN26


Maybe email Giant and ask the width?

 

[I like Skol]

Just pull the cranks off and measure the axle length, as per RRSODL's suggestion above.

 

[smokeysmoo]

^ this, plus it will be a 68mm BB shell

 

[RecordAceFromNew]

Way to figure out without removing anything is:
1) identify chainset make and model (probably on inner surface of crank arm) precisely, and
2) narrow down type of bb, e.g. whether it is square tapered or otherwise by examining where spindle enters chainset, and
3) measure front chainline

Chainset's specific model, if it is from a major manufacturer, will allow you to narrow down the couple of possibilities what possible bb type is (e.g. various Shimano and sram chainset models can be either square tapered/Octalink/ht2 specific or square tapered/ISIS/gxp specific respectively). Many major manufacturers also provide tech docs showing you the suitable spindle length of bb type for delivering particular chainline. Hence knowing chainline, bb type and chainset make/model could id bb geometry precisely (with the help of requisite tech docs).

 

[Cycleops]

Rather than trying to emulate Hercule Poirot simply buy a cheap vernier caliper ruler, you can pick them up for a few quid, then you can measure it accurately.

 

[mickle]

On square taper bbs the first number is shell width and the second number is axle length - like 68-113 for eg. Take it out, read it (or measure it), order a new one (next day delivery) and fit it. No?

 

[I like Skol]

On square taper bbs the first number is shell width and the second number is axle length - like 68-113 for eg. Take it out, read it (or measure it), order a new one (next day delivery) and fit it. No?

No, Lets use our powers of deduction to try and work out the size then, when it turns up and is the wrong size, measure the original and order another one that's the right size

My guess is 73 - 118

 

[Cyclist33]

Am I being really thick, or isn't it just a case of identifying the specific model number of the chainset? I mean they all seem to be designed to fit one or other exact specific BB, so once you know the chainset, you can just go onto google and find the shimano/sram/campag tech documents or brochure or whatever and go from there? What year is the bike from?

I bet a million big cahunas the BB is square taper because at the price of that bike, they all are.

 

[RecordAceFromNew]

Am I being really thick, or isn't it just a case of identifying the specific model number of the chainset? I mean they all seem to be designed to fit one or other exact specific BB, so once you know the chainset, you can just go onto google and find the shimano/sram/campag tech documents or brochure or whatever and go from there?

That was wot I thought and replied accordingly above, without noticing the OP specifically inviting time wasters, of which we can see there is clearly no shortage around here...