Novice Q: specifics on swap of 42T for 39T
 

Novice question: I have mid-1980s Campy Victory or Triomphe 52-42T chainrings on my crank, and I'd like to swap the 42T for a 39T. I've done some cursory research on the subject, but am a bit confused about whether what I have is a "135mm" chainring or not. Or what. Put bluntly, I don't know anything about it at all.

I'm hoping that visual cues will help. Here are two views of my cranks:

1. The 42T in a single speed configuration, which is how it was when I bought it:
http://i81.photobucket.com/albums/j2...ainring42T.jpg

2. With the additional 52T ring (works fine with a modern Shimano 105 9-speed chain linked to a 9-speed Ultregra cassette, I might add).
http://i81.photobucket.com/albums/j2...ring52-42T.jpg

Would the Salsa 135mm pattern chainring (see Lickbike page here) fit on my crank? Could I cruise eBay for any 135mm chainring, then?

Victory and Triomphe don't use 135 BCD or 144 BCD, they use 116 BCD (not commonly available).

Have a read here http://www.sheldonbrown.com/gloss_bo-z.html#bcd

Thanks for setting me straight on this!

It appears that the 116mm BCD rings were only made in the 42T and 52T varieties, so I don't think I'll be able to find a 39T ring.

So my next question is: would a new Campy crank fit onto my 1980s bottom bracket? Would it simply slip on because these things are a standard size?

Yes it would probably slip on but there is no guarantee the chainline will be right. Some cranks will mount much farther in or out from the frame, hence all the different lengths of BB spindles. Sheldon's site has some data on the chainline of various cranks, if you find yours and one that matches it you should be good to go. Otherwise, you'll need a new BB (or at least a new spindle) to go with your new crank.

Chainline. Hmmm...this is something that I'd never considered before. The page on Sheldon Brown's site with all of that information is a bit intimidating, to be honest. In any case, the help from people in this forum has been terrific.

Thanks!
R

Have you thought of leaving the crank alone and extending the cogs in back? How many speeds is it? 6?
Perhaps you can find a freewheel that has some larger cogs to give you the lower gearing you seek.

The frame, the crankset, the headset/stem and the seatpost are the only original pieces of the puzzle left on this one. I've actually got a 9-speed 12-21 cassette on the back there, which I enjoy immensely, as there aren't many huge hills where I am, but I'm still thinking that moving from a 42 to a 39 would be salutary.

http://i81.photobucket.com/albums/j2...23_13small.jpg

keep going back to Sheldon's site and rereading the info there. I found much of it intimidating at first but it slowly sunk in and I am able to understand some of it now :)