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-   -   bottom bracket & crank (https://www.bikeforums.net/showthread.php?t=1003169)

oj. 04-13-15 02:26 PM

bottom bracket & crank sizing
 
I have found this frame that I am interested in (Fixie/Fixed gear/Single Speed Bike Frame with Fork Matt Black 52cm | eBay), but I have been confused by bottom bracket sizes. The frame says that it needs a 68mm-107mm bottom bracket (which i have found - Shimano BB-UN55 MTB / Road Bike Bottom Bracket 107mm | eBay). The question that i am asking is can i just put any square taper cranks on or do i have to get a set of cranks specific to the bottom bracket dimensions? P.S I know this is probably a really easy to answer question but this is my first foray into anything other than bmx.

DMF 04-13-15 03:23 PM

Depends on your REAR components. You want the front chain ring in the same plane as the rear cog. In general you can mount any square taper crank on any square taper axle (there are nuances), but the crank will have its own geometry.

So what's on the back?

DMF 04-13-15 03:24 PM

BTW, is 52cm the right size for you?

Bill Kapaun 04-13-15 04:24 PM

You buy the BB to fit the crank used. Not the other way around.

hueyhoolihan 04-13-15 05:18 PM

yes, the vast majority of modern square tapered double or single-speed road crank will fit.

i'm equivocating, but still... don't quote me. :)

D1andonlyDman 04-13-15 05:33 PM


Originally Posted by hueyhoolihan (Post 17716375)
yes, the vast majority of modern square tapered double or single-speed road crank will fit.

i'm equivocating, but still... don't quote me. :)

Not necessarily. Certainly the 68mm dimension is nearly universal on current road bikes, but the 107mm, which is for the spindle (axle) length, is not. In fact, a 107mm spindle seems rather short for most cranksets, other than single chainring cranks. Typically, most double square taper cranks would use a spindle ranging from 108 to 115mm, and most road triples would use a spindle ranging from 115 to 127mm.

Here's Sheldon Brown's database on bottom bracket compatibility with cranksets. It's not complete, but it's as good a source as I've found on this subject:

Sheldon Brown's Bottom Bracket Size Database

Camilo 04-13-15 06:21 PM


Originally Posted by Bill Kapaun (Post 17716187)
You buy the BB to fit the crank used. Not the other way around.

Just want to repeat this - the crank determines the type of crank (including spindle length), not the frame.


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