Can some one explain BCD?
- Thread starter tintin40
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Bullpup
Senior Retro Guru
BCD or Bolt Circle Diameter is a descriptive term to help determine compatability. It literally is the diameter of a circle that runs through the centre of the securing bolts.
As you know the middle and large rings are siamesed on the outer holes while the small ring uses the inner holes hence the difference in BCDs. BCDs also differ between 4 and 5 bolt chainrings for the same reason.
As you know the middle and large rings are siamesed on the outer holes while the small ring uses the inner holes hence the difference in BCDs. BCDs also differ between 4 and 5 bolt chainrings for the same reason.
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Different BCD's on MTB chainsets allow you to run different ring sizes, for instance you would never be able to run a 34t middle on a 130 bcd, as there is not enough material to make a viable ring.
Where as with road chainsets it was a competition and sales thing, I see very little point in 130, 135, 144, and the myriad other large BCD sizes as they all allow the same ring sizes :facepalm:
Different BCD's on MTB chainsets allow you to run different ring sizes, for instance you would never be able to run a 34t middle on a 130 bcd, as there is not enough material to make a viable ring.
Where as with road chainsets it was a competition and sales thing, I see very little point in 130, 135, 144, and the myriad other large BCD sizes as they all allow the same ring sizes :facepalm:
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At some point there is strength/ weight/flexibility optimisation for the size of the ring.
At some point there is strength/ weight/flexibility optimisation for the size of the ring.
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Cranks are designed around a set number of teeth range.
In retro days it was 46/48 tooth and (24) 26/28 granny
With 36/38 middles.
So the middle ring decided the maximum size the BCD they could use, 5 arms would have been the best strength to weight to manufacturing ability at the time.
Granny would be the largest they could get for 26 or 24 as it turned out.
The sizes where dictated by the cogs available at the back.
Basically that 12/13 tooth small cog as that dictates the maximum speed and hence the largest cog at the front.
Cranks can be designed stiffer than thin ring.
Now with one or two front rings the largest isn't very large and the rear can be 11 or 10 tooth. So new BCD come out optimised for holding the one or two rings. Or back to single ring style splined fitting.
There are so many now as ideas and competing theories come.from different manufacturers.
Top end stuff does not need to be standard, it's all about the money, I mean performance, no compromise.
144 in road will have been around when ring where large, rear cogs not very small, perhaps 14 tooth lowest at the rear.
Cranks are designed around a set number of teeth range.
In retro days it was 46/48 tooth and (24) 26/28 granny
With 36/38 middles.
So the middle ring decided the maximum size the BCD they could use, 5 arms would have been the best strength to weight to manufacturing ability at the time.
Granny would be the largest they could get for 26 or 24 as it turned out.
The sizes where dictated by the cogs available at the back.
Basically that 12/13 tooth small cog as that dictates the maximum speed and hence the largest cog at the front.
Cranks can be designed stiffer than thin ring.
Now with one or two front rings the largest isn't very large and the rear can be 11 or 10 tooth. So new BCD come out optimised for holding the one or two rings. Or back to single ring style splined fitting.
There are so many now as ideas and competing theories come.from different manufacturers.
Top end stuff does not need to be standard, it's all about the money, I mean performance, no compromise.
144 in road will have been around when ring where large, rear cogs not very small, perhaps 14 tooth lowest at the rear.
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On the track, why are they not using a fully integrated chainset ?cce":2q7mhfqo said:
Where the ring is the crank itself?
I don't follow track or road much (or mtb these days much unless someone asks about it or I need a new part.)
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