How to measure BCD?

[Darius_Jedburgh]

I'm sure this must have been covered but my searches show nothing.
I've never needed to do this before, but now I need to measure BCD on a set of cranks. Nothing stamped on there so how do I measure this across 5 arms?

 

[weareHKR]

How to...

 

[midlife]

https://sheldonbrown.com/measure-bcd.html

Sheldon Brown' page

 

[weareHKR]

I usually forget as well so have this print out pinned to the wall...

 

[Darius_Jedburgh]

Thanks, but Sheldon seems to contradict hinself🤔🤔🤔
I'll go into the garage for a proper play.

 

[Ming the Merciless]

Measure distance (centre to centre) between two adjacent arm bolt holes. Multiply the distance by 1.376 to get the approx BCD (allowing for measuring error etc). It should get you close enough to a standard BCD size.

Basically you’ve got a regular pentagon and the above formula gives you the relationship between the pentagon edge lengths and the diameter of the inscribed circle.

 

[DaveReading]

Measure distance (centre to centre) between two adjacent arm bolt holes. Multiply the distance by 1.376 to get the approx BCD (allowing for measuring error etc). It should get you close enough to a standard BCD size.

Basically you’ve got a regular pentagon and the above formula gives you the relationship between the pentagon edge lengths and the diameter of the inscribed circle.

I don't think that's right.

 

[Darius_Jedburgh]

Basically you’ve got a regular pentagon and the above formula gives you the relationship between the pentagon edge lengths and the diameter of the inscribed circle.

My 105 chainring does not have bolt holes in a circle. They are closer to being in a square formation!!!

 

[Ming the Merciless]

My 105 chainring does not have bolt holes in a circle. They are closer to being in a square formation!!!

Well that would be 4 bolt holes which would not form a pentagon. In that case it’s a circle inscribing a square , which is a different formula.

 

[Ming the Merciless]

I don't think that's right.

Well you have 5 bolt holes, equi distant which describes a regular pentagon. The formula gives the diameter of the inscribed circle for a pentagon. Bolt circle diameter (BCD) refers to the diameter of the inscribed circle.

 

[midlife]

If it has 4 holes is that 110 mm. bcd?

 

[DaveReading]

Well you have 5 bolt holes, equi distant which describes a regular pentagon. The formula gives the diameter of the inscribed circle for a pentagon. Bolt circle diameter (BCD) refers to the diameter of the inscribed circle.

Yes, I agree with your definition. Just not with your answer of 1.376 times. My maths makes it 1.7013 x.

 

[Ming the Merciless]

Ah just checked on a spare 5 bolt chain ring. If you measure between two non adjacent holes that’ll give you the BCD. So it isn’t the diameter of the inscribed circle!

 

[Ming the Merciless]

Yes, I agree with your definition. Just not with your answer of 1.376 times. My maths makes it 1.7013 x.

I refer you to, see Inradius

https://en.wikipedia.org/wiki/Pentagon#Inradius

 

[weareHKR]

What you can do is simply measure two adjacent bolt holes from centre to centre & times the figure by 1.699 (5 bolt holes)
So I've just measured one roughly at around 76.5 x 1.699 = 129.97 - so that would be a std BCD of 130 - I think that's a std Road double!