Originally Posted by
ehcoplex
Zombie revival! There's a pretty nice looking Raleigh Competition GS for sale not too far from me. My size, looks like it's in good shape, etc etc etc. Considering it has got me diving into the ins and outs of the Campy GS group. I wondered if anyone ever actually did try drilling the arms of a GS 3-arm crank to 'triplize' it with a small RH 3-bolt ring, as suggested above? Seemed like a kind of cool/interesting idea. I really like the way these 3-arm cranks look, but 42/52 doesn't really work for me where I live. I do have a Cyclotourist compact double laying around, but why do the easy thing?
Here's one I did in about 1979, using a 27t TA ring as the granny.
I managed to break one of the spider arms, but it didn't have anything to do with the conversion, it broke nowhere near where I drilled it for the granny ring bolt. And I was abusing it horribly at the time. I converted my 1933 Excelsior to a Marin County-style klunker with derailers, drum brakes etc, and used it for Observed Trials.
Worst Trials bike ever, but I had fun, riding over giant fallen trees, picnic tables etc. until I made my custom trials bike in '81. I smashed it into a rock or something, I forget the details, bent the outer chainring something terrible and cracked the one spider arm that's pointing down in the pic. The break is near the base of the spider where it joins the crank It was a "displaced fracture", but I bent it back to where it looks like a little crack, but I wouldn't trust it for riding.
The Raleigh you're looking at has the alloy version of the crank, correct? Aluminum is weaker, but IIRC there's more of it, so my guess is it'll be strong enough for road riding. Just don't hop over picnic tables on it.
As you can see, I counterbored the holes so the heads of the "nuts" sit flush. This isn't necessary and I removed more steel from the spider arm that way, so it would have been stronger if I hadn't. Since your alloy crank doesn't have "windows" there, you can probably forego the nuts altogether, and just tap threads in the spider, for screws (M6 or larger) that come in from the back.
Campy's granny adaptor bolts use an M6 thread but there are 5 of 'em, so you might want to up that to M7 or M8 for a three-arm. I have no way to calculate what strength is needed, I'd just have to try it and see. Err on the side of larger bolts if you're risk-averse.