You are correct. I have accumulated a number of these recently. 116 BCD was the standard, and most used 9 mm bolts as compared to 10 mm for 5-arm cranks.
One nice aspect of this is that you can actually equipe the old Stronglight 3-arm steel cranks with more recent alloy rings. This also allows much lower gearing as the old steel rings and even many older alloy rings attached to a carrier that attached the actual rings at a BCD greater than 150 mm. The Stronglight steel arms were very high quality and elegant, and, when equiped with alloy rings, really aren't that much heavier than modern cranksets.
Most of the variation is in how a second ring is attached. The steel arms usually had threaded holes to attach the steel ring carriers to the arms from behind. The rings themselves, alloy or steel, are 157 BCD. TA Professional 3-arm alloy cranks attach both rings directly to the arms with one set of bolts, one outside the arm, one inside, just like 5 arm cranks. Nervar alloy 3-arm are a little different in that both ring attach inside the arm.
Campy Sport (cotterless) steel and Grand Sport 3-arm alloy attach the outer ring to the outside of the arm, but then the inner ring attaches to the outer at the 144 BCD circle used on the 5 arm Campy NR/SR. This means the smallest inner ring is 42T (well theoretically 41T, but I doubt a 144 BCD 3 hole 41T ring was ever made). On the old Campy Sport cotterless steel I have, the inner ring is riveted to the outer. On the alloy GS 3-arm crank I have the inner is bolted to the outer. The inner ring seems to typically have been 42T.
One thing that initally puzzled me is that Stronglight made a 6 hole 116 BCD ring with 9 mm bolt holes. Were these for some 6-arm Stronglight crank I never heard of? I've now concluded these were made for 3-arm 116 BCD arms, probably initially the Stronglight steel cranks. Three holes attach the ring to the arms, the other 3 attach the inner ring to the outer. One interesting detail supports this. Stronglight 3 hole 116 BCD inner rings, unlike TA, have the inner web cut away at a point halfway between each pair of bolt holes, i.e. exactly where the extra 3 holes would be on the 6 hole rings. Why? Well, when these inners are used with 6 hole outers, the cutaways allow clearance around the bolt heads where the 6 hole rings are bolted to the inside of the Stronglight steel arms.
Ain't this old French stuff interesting?
Regards,
Jerry Moos Big Spring, TX
Amir Avitzur <avitzur@013.net> wrote:
I just checked a bunch of 3-Arm cranksets for interchangeability.
All are 116 BCD and, surprisingly, all have 9.2 ~ 9.4 mm holes for 9mm diameter chainring bolts.
* TA Pro 3 * Nervar * Campagnolo steel GS * Stronglight steel (threaded with 9mm lip for chainring)
* Stronglight Aluminum rings * Cyclo Steel Rings
I may be mistaken, but all the classic 5-arm cranks I know have 10mm Dia chainring bolts.
Funny, the distinction.
Amir Avitzur
Ramat-Gan, Israel