# "120/90mm BCD" ...wait, wut?



## Bob Ross (Apr 18, 2006)

So I bought the missus a 'cross bike (Cannondale CAADX 105 Disc) since she wanted something that could handle a little bit of dirt/gravel but mostly so she could ride in less-than-ideal weather, and also so she could have a dedicated commuter bike.

Since she has no intention of ever actually doing a cyclocross race, the stock gearing won't work for her; she wants to change the 46/36 chainrings to 50/34. [Edit: Or, at the very least change the inner chainring to 34, or 32, or 30.]

Okay, methinks, no big deal, I'll just buy the correct [sic] chainrings, raise the FD clamp a feww mm, swap out chainrings and badda-boom badda bing, Bob's yer uncle.

Then I looked closely at the crankset that came on the Cannondale, an FSA (Gossamer, I think?) with BB30: On the chainrings it says "120/90mm BCD"

Okay, either I'm a complete newb, or I'm completely crazy...what's the deal with two different numbers for the Bolt Circle Diameter? Is this some newfangled spec? As far as I can see there's only one set of bolts holding both chainrings to the crank. Is it 120mm or 90mm? (And fwiw there are only four bolts, in case that makes a difference.)

What do I need to specify to find alternate chainrings that are compatible with this crank? Or am I screwed?

Thanks.


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## Dan Gerous (Mar 28, 2005)

120 is the BCD for the big ring, 90 is the BCD for the small ring (bolts holding the big ring are almost hidden behind the small ring).

FSA seems to create 13 new BCD's each year for odd reasons... I'm not sure if anyone but FSA makes chainrings for this particular one but I have seen them in other sizes including Compact 50/34t but in theory, the 90 BCD would allow smaller rings than 34 to fit so maybe 32 or 30 exist.


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