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Compact Drive (MTB triple)
A system of using smaller-than-normal sprockets front and rear. This has the advantage of saving a small amount of weight, improving chainwheel clearance for mountain bikes, and perhaps making a slight improvement in shifting. This is done by going to a smaller bolt circle, typically 94 mm/58 mm or 104 mm/64 mm B.C.D.
The downside of compact drive is significantly reduced chain life, and decreased interchangeability of parts. I consider compact drive to be a very bad idea for mountain bike use, but it seems to have become ubiquitous.
For mountain bike usage, compact drive typically uses 22/32/42 tooth chainwheels as opposed to the 48-38-28 or 46-36-24 used on earlier mountain bikes.
The original mountain bike cranks were mostly 5-bolt 110 mm/74 mm BCD. "Compact Drive" cranksets usually use the 94 mm/58 mm BCD 5 bolt, or 104 mm/64 mm 4 bolt cranks, as opposed to full-size systems that have 24-28 tooth lows and 46 or 48 tooth highs on 110 mm/74 mm BCD cranks.
