Triple crankset bolt size and compatability?
 

Hey all,

I just discovered that the crankset (Sugino VP 165mm) on my commuter bike is a triple, even though it only has two chainrings. It's a 1980s Centurion Sport DLX, with a 6 speed freewheel on the back.

I've got a 30t chainring from an old Tiagra triple crankset (came off a 2008 Cannondale CAAD9 R6).

I was wondering if I could run just attach that to my crank, readjust the FD, and possibly fix the chain length in order to have a triple. The bike comes with DT shifters, so shifters won't be an issue.

The end result would be a 50-42-30 with a 14-28 in the rear. Would this work? Also, I don't have any bolts to attach the small chainring to the crankset. Any idea what kind of threads it uses?

If I could get this working, it would save me from buying a new bike as a commuter. I sold my previous bike, because I needed a triple. I'm going to be living up a hill that's a third of a mile long and averages 10% grade.

http://img529.imageshack.us/img529/9462/90309447.jpg

You need inner chainring bolts with spacers, a new front derailleur that can handle a triple crank, and a slightly longer bottom bracket spindle.
The spacer size will depend on the crank - the inner ring should be the same distance from the mid ring as the mid ring is from the big ring. Using a straightedge and a caliper to calculate this would be a good idea.
Going from double to triple usually means a BB a size up is needed. (eg. If your old one is 115mm, the new one will need to be 118mm.)

You might need a longer cage on your rear dreailleur and a triple front derailleur. If the chain is setup properly it won't need any adjustment. Also make sure that you have enough clearance by the chainstay or you might need a longer BB spindle.

Thanks, I checked the clearance on the chainstay with the original spindle. Seems like it'll work. I'm hoping the original RD will work.

How important is spacing between the chainrings? Also if the RD or spacing didn't work out, could I just get shorter chainring bolts and use the only the 42-30 chainrings.

I got it installed today. I actually didn't need to buy anything except spacers (which I used washers). FD is adjusted and works perfectly, the RD works fine except for if I'm in the 52-28, which is fine, because I don't usually cross chain. The 30t ring fits nicely, with almost a mm of clearance.

The only problem I have is that the chain doesn't seem to want to stay on when I'm in the 30-28. It's a little better in the 30-24, but still makes some noise. In the 30-21 gear, it works perfectly fine. Any idea to why this is? When I slowly turn the crank I can see the chain wrap around the teeth, but then slightly outwards towards away from the BB. I feel like I may have installed the 30t chainring backwards. FD is properly adjusted along with the spacing between chainrings.