I’m thinking of replacing the 44 tooth outer chainring on my XT chainset with a 44 tooth Middleburn. The XT chainset (Octalink) is 4 arm with a 104mm bolt circle diameter (bcd) and I know that a 4 bolt ring with a 104mm bcd will fit. However, CRC simply sell their Middleburn rings as 4 or 5 bolt (no mention of bcd, unless I’ve missed it) whilst other sites sell them as 4 or 5 bolt and CD (I’m assuming Compact Drive) – all the CD ones seem to be 4 bolt. I think that the XT is compact drive and I guess it’s easy enough to email a retail and ask the bcd and compatability (as I’ve done with Stiff) but what is the difference between a 4 bolt, 44 tooth ring and a CD 4 bolt, 44 tooth ring? Are non-compact drive chainsets/rings road kit?

Compact Drive was Shimano’s answer to Campag’ and Suntour’s Microdrive system, both sharing the same BCD for middle/ outer rings in the 5 arm fittings used at the time. The idea was to save weight and increase stiffness by reducing the size of all the chainrings and regaining the high gearing by introducing an 11t sprocket in place of the previous 12/13t.The Standard Drive system it replaced became pretty much obsolete for everything bar spares many years ago.Your 4 arm system should be CD. As for road ‘rings, i’m fairly sure they have a different BCD again to the old Standard Drive ‘rings.