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Chainring Bolt Pattern(5 posts)

Chainring Bolt Patternt-bill
Aug 7, 2002 12:41 PM
What does a chainring bolt pattern actually measure? For instance, if the pattern is 112, is that 112mm between bolt holes?

Thanks!
It's the measurement that you can't do easily.Spoke Wrench
Aug 7, 2002 12:54 PM
It's the "Bolt Circle Diameter." Since most cranksets have five bolts, you can't just measure across to the opposite side of the circle to find the diameter. Fortunately, there is a fudge factor you can use.

For five bolt chainrings, if two adjacent holes are:
79.5mm apart, the BCD is 135mm
76.4mm apart, the BCD is 130mm
43.5mm apart, the BCD is 74mm
It's the measurement that you can't do easily.Atombomber
Aug 7, 2002 9:03 PM
Couldn't you just measure from the center of the crankarm fixing bolt to the center of one of the chainring bolts, and then multiplying by 2?
Not if it's not on the bike. (nm)Spoke Wrench
Aug 8, 2002 5:14 AM
Granny gear BCD couldn't be measured that way either. Some of the granny gear BCD's require a pretty precise measurement. A few years ago some cranks were produced which used 56mm and 58mm BCD's. Almost, but not quite interchangable. Uh - that's the ring that wears out the soonest on mountain bikes too.
Not if it's not on the bike.Spoke Wrench
Aug 8, 2002 5:15 AM
Granny gear BCD couldn't be measured that way either. Some of the granny gear BCD's require a pretty precise measurement. A few years ago some cranks were produced which used 56mm and 58mm BCD's. Almost, but not quite interchangable. Uh - that's the ring that wears out the soonest on mountain bikes too.