I've now done a bit more research and experimentation with alloy rings on steel cottered crankarms, and I'm becoming increasingly convinced that the three-hole alloy rings and some 6-hole alloy rings made by Stronglight were originally made to fit the old 116 BCD three-arm steel cottered cranks.
The old steel cranksarms were usually threaded, with fixing bolts threading in from the backside, i.e. the BB side. A typical setup seen in the 50's is that the arms attach not directly to the rings, but to three arc-shaped carriers, each providing two holes, to attach one or two 6-hole chainrings farther out than the actual 116 BCD crankarms. The actual ring BCD looks to be about 158mm, with the samllest ring 46T. I have a setup like this with Stronglight steel 116 BCD arms, Simplex carriers, and Simplex alloy rings. I think such alloy rings were probably already avilable in the 50's, although in the early 50's steel rings might have been more common.
However, the later 116 BCD three hole (9mm bolt hole) alloy rings from TA, Nervar and Stronglight (and maybe Campy) will attach directly to the 116 BCD steel cranks, eliminating the carriers and allowing a much smaller small ring. Never seen a three hole 116 BCD Simplex ring, so I don't know if these exist. I have a set of Stronglight steel arms with a single TA alloy three hole ring attached directly to the arms, using the same bolts that would have been used to secure the carriers.
Because the bolts that thread into the back of the arms were typically used to secure the carriers, they are only long enough to secure a single ring directly to the arms. But I recently bought from Mike Barry in Toronto some longer bolts with which I mounted two alloy 116 BCD rings, using a spacer between the rings.
But there seems to have been another way to mount two 116 BCD alloy rings to steel crankarms. Stronglight made 116 BCD 6 hole alloy rings with the same 9mm bolt holes as the 3 hole alloy rings. I have a few of these and long wondered what the 3 extra holes were for. Now I've concluded that one set of 3 holes was used to attach the 6 hole ring to the steel 3 arm cranks, while the other 3 hole are used to attach an inner three hole ring to the 6 hole outer. I have a couple of pairs of 116 Stronglight rings with identical logos and evidently from the same era. The larger ring is 6 hole, while the smaller one is 3 hole. Attaching the inner to the outer would allow using the same single-length bolts originally used to secure carriers to the arms to instaed secure the outer ring to the arms. And what makes this conclusive for me is that the 3 hole Stronglight smaller rings have three arcs cut away from the inside of the webbing, exactly at the positions of the additional 3 holes in the larger 6 hole rings. It seem to me this can only be designed to allow the inner 3 hole rings to clear the bolts used to hold the outer 6 hole rings to the steel crankarms. I'm going to try to actually set up some steel Stronglight arms with 6 hole/3 hole Stronglight rings and post some photos.
Anyone else familiar with this stuff? Anyone have a different interpretation?
Jerry Moos Big Spring, TX
Jerome & Elizabeth Moos <firstname.lastname@example.org> wrote: In my investigation of 3-hole chainrings spurred by changing the ratios on my newly rebuilt 1974 Raleigh Competition with TA Professional, I've come across Stronglight 6-hole 116 BCD alloy rings. These can be used on 3-arm cranks simply by using half the holes. But one then wonders what the extra 3 holes were for. I think TA did make an adapter with a 116 BCD 6-hole bolt circle, but as I've never actually seen one, I have trouble believing all the 6-hole 116 BCD rings out there, which do show up fairly frequently, were for a fairly obscure TA adapter.
I therefore have another theory. As many know, the old steel cottered cranks had both rings attached inboard of the arms. Typically a fairly short bolt would thread directly into threads in the backside of the arms to attach a chainring "carrier". The carrier would have holes at a much larger diameter, where the rings would attach, either both rings to the carrier, or the outer ring to the carrier and the inner to the outer. Could the 6-hole 116 BCD alloy rings have originally been designed to upgrade the old cottered steel cranks? It's been observed here before that the best quality steel crankarms of the 40' and 50's were often not much heavier than newer alloy arms, which are much thicker. But there would have been a significant weight savings in upgrading the steel rings to alloy ones. So were the 116 BCD 6-hole rings designed to bolt directly to the 116 BCD steel cranks, eliminating the carrier?
But the original bolts designed to secure the carrier would only be long enough for one chainring. Now the ring manufacturers could alway have supplied new bolts, and perhaps they did. But an option would have been to drill another set of holes to attach a 116 BCD inner ring to the outer one. Was anyone on the list working at a shop in the late 50's/early 60's, when this type of setup might have been introduced? Anyone have or have seen steel cottered cranks with alloy 116 BCD rings?
Big Spring, TX