Originally Posted by
mawn
yes, it's 42.. behind the Crank, there is "Shimano SG A-52-42-30".
Well 52-42-30 implies that it is a 3x capable crank and for some reason doesn't have the inner ring. So if your STI for the front is 3x, then you only have to find an appropriate inner ring and add back to it. Then you'll have plenty of low gear ratios to choose from as well as higher ratio gears.
And if the STI is 3x, then I'll just assume a previous owner took off the inner ring and that if the rear DR is also original to the bike that it will handle a front difference on the front from 52 to 30. But you should find that part number on it and be certain what it is if you are wanting to change gearing.
Of course after adding the small ring back you'll need to adjust the front DR and maybe the rear too to get perfection.
I don't understand very much about PCD or BCD. so I think I need to check with a Bike Mechanic.
Thank you
PCD is pitch circle diameter and BCD is bolt circle diameter. They tell you the same thing with the same number, but they are determined differently. Essentially it is the diameter of the circle on the chain ring that the chain ring holes are drilled on. So you have to know what those are to be certain you get the correct rings for it. The inner rings on 3x cranks usually were a different PCD or BCD and may have required inner ring with holes for it to attach to or some other method.