Actually both Apollo
have made very valid points regarding my post; It is indeed confusing as often information differs.
To upate my post the TA distributor has a new website (you will see the link in the above post above no longer works) which is http://www.chickencycles.co.uk
, the older website had details of how they measured their TA chainrings, (their new one hasn't got the same information yet) I had simply duplicated the same information on our website so that we were the same as theirs (duplicated as in litterally, I copied and pasted the informatiom from their website to the description of BCD in my diagram), in an attempt to be consistant with the supplier of our chainrings. It is also another reason why I use a diagram so that even if you don't agree with what our supplier and as such us are refering to as BCD (and as you can see from the picture from Sheldon Brown below you have good reason to) you can at least see what I am refering to.
When customers ring us for advice I always ask for the number of chainring bolts and measurement between the nearest two bolt centres. Whatever you call the measurements just make sure they match up with who you are buying them from, which is why like Chicken and Sons I listed my stock the same way they do, personally
I prefer what I had listed on our web site before I amended it to match our supplier which was:
Sheldon Brown lists BCD as Bolt Circle Diameter
so as far as he is concerned he has called BCD what I have called PCD
To quote SB, "Measuring BCD: This 110 mm BCD ring measures 64.7 mm center-to-center.
Multiplying this dimension by 1.7 will give you the actual BCD for a 5 bolt chainring"
He quites PCD as: Chiefly British variant of B.C.D, "P.C.D." stands for "Pitch Circle Diameter" but "B.C.D." ("Bolt Circle Diameter") is preferable, because the term "pitch circle" has a different meaning when speaking of sprockets.
(The primary, more correct meaning of "pitch circle " is the circle running through the centers of the chain rollers of a chain engaged with the sprocket.)
(edit I now work at http://www.candncycles.co.uk
Sorry, your'e wrong. Paul Smith is right. I didn't mention PCD initially simply so as not to confuse things.But BCD and PCD (pitch circle diameter) are distinctly different things.
BCD = distance from centre of bolt circle to next nearest bolt circle
PCD = distance going across centre of crankset from any bolt centre to another bolt centre.
Though normally when ordering replacement rings, it is sufficient to quote BCD size, hence why I didn't cloud the issue with this info' before!
(Though saying whether it's for a four or five arm crank spider also helps!).
As regards the thickness of the crank spider, if the changer worked before, it will do so again. The replacement rings won't be that much different if at all.
Hope this helps.