Dammit, you guys know how to take up a Saturday morning! I was compelled to go out to the shop to check this out. Amir's photos posted earlier s howed two different Campy 3-arm cranks, the alloy GS and the steel cotterle ss Sport.
I find I also have one of each. Both have alloy rings. On the older s teel Sport, the inner is riveted to the outer. On the GS, the inner is b olted to the outer, but with very small bolts, look like about 5mm thread d iameter. But whether bolted or riveted, the inner has a BCD which looks to be the Campy NR 144mm, or very close. Thus the smallest ring will be 42T.
As someone mentioned earlier, the GS has a notch for a ring only on the out side, since the stock inner ring bolted or riveted to the outer, not to the arm. So the easiest option is to find a GS ring pair. There might ev en be a few individual rings around where someone separated a pair, which w ould have been easier with the bolted ones. So you might be able to chan ge the combo, but in any case, the small ring can't be smaller than 42T
TA Pro cranks have the same BCD, so it would be possible to use TA rings, b ut a problem arises. I measured the thickness of the notched end of the GS cranks, which will determine the spacing between rings. It is 4.5 to 4.6mm. I measure a double notched Stronglight 99 at about 3.7 mm, while a TA Pro 3-arm measures only 2.8 to 3.0 mm. I think the difference in TA versus Stronglight is that the old TA 3-hole rings were thicker at the bol t holes than more recent 5 hole rings. Some of the alloy 3-arm cranks, i ncluding Nervar and possibly Stronglight had a single notch with the outer ring attaching from the backside of the arm, like the old steel cottered cr anks did. In this situation, the inner ring, whether bolted separately t o the outer or to the outer and crankarm, using a single set of boltrs, wil l be separated from the outer ring by a spacer, not by the crankarm, so one controls the spacing simply by using the proper thickness of spacer. But the Campy GS has the notch such that the outer ring attaches from the outside, like a modern 5 arm crank. Thus the thickness of the notched end on the arm sets the spacing between inner and outer rings. Thus from the above, using TA Pro rings on a Campy GS is going to space the rings 1. 5 to 1.8mm further apart than on a complete TA Pro crank.
Now as I found recently on a Schwinn Super Tourer with TA cranks, there doe s seem to have been some variation in chainring spacing on classic cranks. But the minimum spacing is what it takes to keep the chain from rubbing , while the maximum is what can be used without the chain dropping between rings on shifts. So unless the TA spacing is the absolute minimum, inc reasing it by 1.5 to 1.8 mm may well cause problems dropping the chain betw een rings. One solution could be to file down the backside of the Campy GS arms to reduce the spacing. Since the standard rings attach only from the front, this should not create any problem later reverting to Campy GS rings. It might also help to use Stronglight or Nervar rings, which may be a bit thinner than TA, which might eliminate or at least reduce the need to file the Campy cranks. Of course with the steel cotterless Campy Spo rt, filing would be more difficult.
Another solution may be one I mentioned earlier, using a 6 hole Stronglight outer with a 3 hole inner of the type with the inner webbing cut back to c lear the outer ring fixing bolts. I have such a setup on a pair of Stron glight cottered steel arms. So I just now duly removed the rings to see if they would work on the GS cranks. The webbing on the small rings is cut back to clear the outer bolts, not the ends of the crankarms themselves, as they need to do on the GS. As is they don't quite clear the GS crankarm s, but it appears with a little filing of the inner ring they would. Thi s would still leave plenty of inner ring material on the 38T inner I am usi ng and probably enough even on a 36T which is the smallest 116 BCD ring I h ave seen. These 6-hole outer rings show up on eBay fairly often, and oft en go cheap, as many people have no clue what they are for. The three ho le Stronglight inner rings I have I believe are from the Stronglight TS, a model which for a while in the mid to late 70's replaced the inexpensive s teel cottered cranks used on the Peugeot UO-8 and suchlike in the late 60's and ealry 70's. I got a few of these rins from Mike Barry. I believe though Mike has retired, he is still selling of some of his stock, so he c ould have a few of these rings still around.
Regards,
Jerry Moos
Big Spring, Texas, USA
From: David Bean <beandk@rcn.com> Subject: [CR]Thanks on the GS chainring Q To: "CR List (E-mail)" <classicrendezvous@bikelist.org> Date: Saturday, November 15, 2008, 9:12 AM
Thanks to all who responded. I now have an encyclopedia of 3-arm chainring knowledge! This relates to the Griffon bike I bought on ebay, which has a single ring GS crank. I wondered what my options would be if I wanted to add a front der and chainring.
David Bean
Arlington, MA USA
beandk at are-see-enn dot com