Home Forum Bike Forum Can someone please explain BCD?

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• Can someone please explain BCD?
• bones76
Member

As above really, am wanting to run a set of Rival cranks that at the moment have a 53/39 set up on 130BCD

But i am wanting to if possible use the cranks on my next cross bike build but run a 46/36 set up 110BCD

Can i do this? and what is BCD? is it the gap between the bolts?!

Rorschach
Member

No.Bolt Circle Diameter.Yes

tonyg2003
Subscriber

BCD bolt circle diameter. Distance between bolts.

It’s the pitch circle diameter of the mounting bolts. Hence, you can only use BCD that is the same as yours.

matthew_h
Member

If you draw a circle through all the bolt holes, then BCD (Bolt circle diameter – sometimes also called Pitch circle diamter) is the diameter of the circle.

You can’t mix BCDs of chainrings and cranks because the holes won’t line up.

It’s not distance between bolts, it’s the diameter of the holes.

It’s the diameter of the circle you would get if you drew one that intersected the centre of the bolts on your chainset. Just measure the distance from one to the one on the opposite side and that’s the BCD.

Beaten to it by just about everyone there.

It’s not distance between bolts, it’s the diameter of the holes.

I’m not sure if you’re wrong or you’ve just worded that very badly.

It’s the diameter of the bolt circle. For a 4-arm crank, that’s also the (centre-to-centre) distance from one bolt to the opposite one.

billyboy
Subscriber

It may not be the distance between the bolts but you can measure it by measuring the distance between the bolts, so it is very like the distance between the bolts even if you say it isn’t, which it may not be, strictly speaking………………………..

Dibbs
Member

It’s the center of the axle to the center of the bolt x two. ðŸ˜€ (Bolt Radius x 2 Diameter, BRx2D)

tinribz
Member

It may not be the distance between the bolts but you can measure it by measuring the distance between the bolts,

Good luck with that with 5 bolts.

edhornby
Subscriber

Smallest inner ring you can get for 130bcd is 38t

eskay
Member

Distance from centre of spindle to centre of a chainring bolt x2.

dirk_pumpa
Member

I’ve never heard ‘bolt circle diameter’ outside of the cycling industry. It sounds stupid.

As far as i’m concerned it’s a bolt hole array on a pitch circle diameter.

+1 Mr Pumpa, I’m thinking BCD is the US phrasing of the British PCD, could be wrong though. I’m an ex Toolmaker, and never heard of BCD outside of cycling.

emanuel
Member

specialites TA boxes are handy because they have nearly all bcd printed on the back as a template.

BCD is common term used in engineering for pipework flange sizing. This is offshore mind, so not sure if it’s used in the UK or not, but it’s pretty widespread.

JoeG
Subscriber
mtbmatt
Member

You can get 38t chainrings for 130BCD if you look hard. Not common though.

scaredypants
Subscriber

Now I’m not an engineer (which is my point, really) but I’ve never heard the term “a bolt hole array on a pitch circle diameter” before this morning

BCD works for me as a punter

Rorschach
Member

/\ Winner!

ndthornton
Member

Can’t believe there is a tool for this ðŸ™„

matthew_h
Member

Never mind that, isn’t it nice how the OP has come back to his thread to thank us all for answering his query.

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