I was having problems matching up chain rings and cranks for a cross-over drive set up on the 1985 (Is that C&V?) tandem I'm rebuilding. I tried measuring bolt hole centres but my calculations always seemed to end up between standard sizes - eyesight ain't what it used to be perhaps!

After I'd finally finished and got it all working fine, I was lurking on the Classic Rendezvous list and someone had very kindly posted this link to a really great bolt circle template at:

https://www.m-gineering.nl/techdexg.htm

I've pasted bits of his post in here so you guys can also get your own copy. While this is primarily for five arm cranks, its possible you could get a good estimation of a
three arm circle.

OK so the words are in Dutch (Flemish?) but the numerals are clear and it all makes sense. Lay your chain ring or crank spider on top of the template centred around the cross in the middle, turn it until all the bolt holes have numbers under them - the numbers are the BCD of the crank or chain ring in mm.

I've printed off and laminated a couple of these and they really do work - oh, the frustration I could have saved if I'd had this a few days earlier!

Anyway, I thought it would be good to share this really useful find with you guys & girls - Enjoy!

By the way, the BCD Template is on the left hand side menu under "Technical", listed as 'Chainwheel bolt - something or other"

here's a direct link

https://www.m-gineering.nl/steekcirkels.PDF

it'll be accurate as long as your printer doesn't scale it to fit the page.

Thanks Zaphod, that's a lot less trouble to use.

I downloaded that when I saw it on CR--and then realized it doesn't include 86bcd, one of my favorites (e.g., Stronglight 99)!

Neal

Awesome! Thanks for the link.

I just measure bolt-to-bolt and use Sheldon's table. This is pretty cool though.

WHA!?

No 151 ???? bummer. I guess I can extrapolate.

Good stuff! I too found this on CR a while back, it has been pretty handy.

Cheers,
Chris

Marten Gerritsen is a great guy. Not only a frame builder, he also collects old bikes. He has a really good eye for nicely made bikes and often finds marques which are often not heard of beyond their native borders - primarily The Netherlands, Belgium and sometimes Germany.

I'd love to meet up with him if I ever get back to Holland.

Yes! Definitely useful info on Sheldon's website.

Sheldon mentions different diameters commonly used by different manufacturers and the smallest available chainring size for each BCD in his tables on this page of his Glossary: https://www.sheldonbrown.com/gloss_bo-z.html

This info is extracted from the entry:



Regarding the last (6-bolt) measurement above. Yes, it would seem obvious that you could simply measure straight across a 6-bolt chainring... But, that's not always easy if a chainring is already fitted on a crankset mounted on bike. (And, I'm also an idiot when it comes to calculating such simple 6-point geometry which may seem truly elementary to many of you...)

For example, shown below is an odd (long obsolete) chainring size. The BCD of the steel 3-arm crank is the once very common [virtually universal] 116mm... but... the outer Simplex alloy chainrings attached by the 3-to-6-bolt outer extensions have a 157mm BCD.

I would have guessed it to be the same as older TA "Criterium" type chainrings which was the same diameter originally used with Stronglight alloy "Adapteur" rings during the 1940s and 1950s. And even when placed fairly close, the two chainring sizes do look VERY similar in diameter. But, I would have been completely wrong... those were 152mm. Sheldon's simple info above had quickly saved me a lot of wasted searching (and expense) for a wrong size chainring - and without first removing the chainrings.