Road Cycling: Diagnosing and Fixing a Chain Line Issue in a Modern Road Bike (Easily, Quickly, and Permanently).
So, I had an interesting conundrum pop up with the Venge. At the end of last season I bought and installed two new chainrings for the 10 speed drivetrain. I also picked up a new rear derailleur, a cassette (11-28) and a new Dura Ace chain for the refurbishing of the bike as the crankset required some hefty work and new bottom bracket bearings. I figured if I was going to get the bottom bracket fixed, I may as well go all out and really give it the business. I had knockoff SRAM chainrings on both my Specialized Venge and my Trek 5200 that faired quite well but wore out quickly. The rings on the Venge were still quite good but the rings on the Trek started giving me skipping problems when I tried to climb a hill.
I opted for Shimano 105 chainrings so I could have a full Shimano 105 system on the Trek and a 105/Ultegra mix on the Venge.
The change on the Trek was flawless. Not so much on the Venge. For some reason, on the Venge, the new 105 chainrings rode outboard of the knockoff SRAM chainrings they replaced. This meant a chain skip when the bike was shifted to the big chainring and big cog in the back. Now, for those puritans among us, I am quite aware we’re not supposed to cross-chain and ride in that particular gear but I’m also a realist. There will be five or six times a year where I need that one last gear to crest a hill without shifting to the baby ring. I will cross-chain in that situation. Every time. If I’ve got a skip, though, I’m worried about the chain dropping from the big to the little chainring whilst, and at the same time, putting some decent power to the pedals. That just won’t do.
The fix for this is simple, but a little complex. You have to change a lot, simply.
Now, if the chainrings need to move outward, a shim at the crank will work well. If, however, the chain line needs to move in, toward the bike, we’re limited by the crank. The easiest fix is to move the cassette out and to do this I simply added a shim to the cassette body. With the cassette moved out, I had to change the set screws on, at the very least, the rear derailleur, but possibly both front and rear (I did both in this case). If you don’t check the set screws, two very bad things will happen. First, you’ll be able to shift the chain beyond the last, biggest cog into the spokes of your wheel. This can be a costly mistake. So, for the big cassette cog, you adjust the outer set screw for the rear derailleur clockwise to move the pulley wheel outbound so the pulley wheel lines up directly below the biggest cog. To check that it’s out far enough, turn the pedals and shift all the way to the smallest cog. Then, without touching the shifter, turn the pedals slowly and operate the derailleur so the chain slides up the gears to the big cog. If the pulley wheel is set correctly you won’t be able to push the chain beyond the big cog (if you can, be very careful here – if you push the chain into the spokes, the chain will damage them big time which is why you were pedaling slowly). Then you have to adjust the pulley wheel for the smallest cog as well, especially if you’re experiencing a little click that won’t go away when you shift into the smallest cog and the barrel adjuster won’t make it go away – or the adjuster will make the click stop but the bike won’t shift properly in the rest of the gears. For this adjustment, you turn the inner set screw counterclockwise until the jockey wheel is just outboard of that smallest cog.
Then adjust your front derailleur if necessary.
At that point, Bob’s your uncle. Give the bike a spin to make sure the fix is right (there’s a chance you may need to double check the set screw adjustments).