Nubra 

Hello,
I recently received a group of TA chainrings from a friend whose father used them in the 70's. I thought that the crank that his dad used was a TA triple touriste. But don't know much about the TA line, so just guessed. I now have 12 chainrings ranging from 52 down to 26, all with the same 5 hole BCD.
Using the method of measuring from the center to center and multiplying by factor of 1.701 for 5 hole bolt pattern, For 38mm between two adjacent bolt holes, I got a BCD of 66.33 If I used 39mm, 64.6. Does this sound right? I cannot find this BCD anywhere! What am I doing wrong?
Please see attached screen shot for how I determined my calculations.


I do plan to offer these rings to BF members in the future and want to have correct BCD.

Many thanks in advance.

38-40mm rough estimate from center to center.

bikemig 

TA outer ring is 50.4 bcd but it made different inners. The cylotourist inners are 80 bcd. I thought that the smallest inner was a 26. In any case if the bolt hole diameters all line up with your small chainring, then they are all 80 bcd chainrings. The other sizes of TA rings were 152 and 116 bcd.

https://www.bikeforums.net/classic-v...otouriste.html

Sutherlands 6th has the smallest chainring at 26 teeth. It's a 6 bolt pattern. The BCD is 80; from hole center to hole center, the measurement is 40.

Nubra 

How did my calucations go so wrong? I added a screen shot of the directions I followed to determine the BCD. Are these NOT TA rings?

bikemig 

I added some more info from Sutherland's 6th. 40 mm hole center to hole center is a 80 bcd and that's consistent with your measurements. Also it's a 6 bolt pattern.

thumpism 

T/A outer is the only one that is a 5-bolt, and all the ones that connect to it, the middle and inner rings, do so using a 6-bolt pattern that is also in the large ring's spider. The 46T ring in your first photo is one of those middle/inner 6-bolt rings.

bikemig 

Right, 6 bolt pattern. Also the TA randonneur which has a 116 BCD goes down to 36; the TA Criterium which has a 152 BCD goes down to 43. You have 6 bolt TA chainrings smaller than 36.

Also with a 6 bolt pattern, you should be able to measure right across the chain ring using holes that are opposite to each other and get 80 BCD.

Nubra 

Whoa, that is really really embarrassing? How could I miscount? Because I have never seen a six bolt pattern before this! Still no excuse.Thanks a million.