Thursday, March 6, 2008

What's in the stand today...

Have you ever put your boxers on backwards, drink a lot of water and then rush into the ...? Oh, yeah, um, me neither. But hey, look what's in the stand today. A very nice man-sized Steve Potts Break-Away cross bike. It gets used as both a road bike and cross bike with two sets of wheels. Road wheels get road gearing, but the owner wanted to run the cross wheels with a 34t large cassette cog and with the Dura Ace 27t max capacity derailleur. Just slapping on the wheels in the frame with the stock Dura Ace derailleur and the chain length such that big/big is just possible yielded an upper pulley wheel that rode on the 34t like ... well, let's just say, there was a lot of noise.

There are ways to get a derailleur to work with a freewheel that has a larger big cog than what the derailleur is rated for, but a 34t is pretty big for a road derailleur. On some frames, simply screwing the B-tension screw all the way in is sufficient. On others, running the stock B-tension screw in from the back side works. Still others it may be possible to simply run a longer M4 B-tension screw. On this frame, however, once a longer B-tension screw got to the point where the derailleur was being pulled back sufficiently, the screw was hitting the screw stop at such an obtuse angle that the screw was slipping past the stop. I probably could have bent the screw slightly so it hit the stopper perfectly, but if you needed to remove the screw it would be a pain.

Plan B (pun intended) involved modifying the derailleur main pivot spring tension itself. Here's an untouched Dura Ace upper pivot area. Drilling a hole where the arrow is will yield a higher spring tension once it's reassembled. And this is just what I did - drilled a new spring fixing hole, reassembled the unit, ran the B-tension screw all the way in and it worked perfect. That is how to convert a 27t max cog Dura Ace derailleur to run quietly on a 34t cog. (and no, the derailleur taken apart in the photo below is not the same one that is on the bike as you can tell by the smaller pulley on the one in the photo.)

Look at that upper pulley wheel nicely pulled away from the large cog - quiet.

This is what it looks like when you run the B-tension screw in from the back side.

(What's playing: Leonard Cohen Tower of Song. Number of times B-tension is mentioned: 5.)

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