I recently got a new BH G4, and I had a really hard time setting up the front derailleur. I've never had trouble setting one up, but for reasons unknown I couldn't get things to work right. Some chain rub when in the cross-over gears is acceptable (the big-big and small-small sets that 'they' always tell you never to go into), but it was happening in the 4-5 smallest cogs/small ring combination. Part of the problem was that the frame has very short chainstays, which makes the chain run at slightly sharper angles. But even so, I knew there was something I was missing, so I contacted Mark Purdy of NYCVelo to give me a remedial lesson on front derailleur setup.
Step #1 for Purdy is to insure that the BB and crankset are both installed properly onto the frame. Next, mount the FD before installing the chain. Most carbon frames use a braze-on style mount, while most round tubed aluminum and steel frames use clamp-on style mounts. Some carbon frames have mounts attached to the frame with bolts and slotted holes instead of rivets (my BH has this), which is nice as you can tweak its angle so the FD tracks the chainring as closely as possible. If the mount can't pivot and the angle doesn't match well (this can be the case if you're running compact cranks), SRAM makes a wedged insert that lets you adjust the angle. Next set the height so that the closest point between the FD and Chainring is 1-3 mm. Remember that most chainrings have teeth that are profiled differently, so set up on a tall tooth.
Before you tighten the mounting bolt, set the middle part of the outer cage parallel to the chainrings. Later, if the chain is rubbing in crossover gears in the small ring you can angle the tail of the cage outboard. In drastic cases, Purdy will bend the outer cage to make appropriate clearance.
Next, roughly set the limit stops by eye. Now you can install the chain and set up the rear derailleur. RD’s are a whole class on their own, but that’s a story for another day. For now, let’s just assume that the RD is aligned and adjusted properly. Shift the chain onto the biggest cog on the rear, and double check the inner limit stop. Get the inner cage as close to the chain as you can without rubbing. Now it’s time to connect the cable to the FD. This may sound remedial, but make sure the cable is routed correctly through the BB cable guide. Incorrect routing can cause excess friction and sluggish shifting. Purdy suggests cleaning and lubing this spot frequently to keep it smooth.
Now, pull the cable as taut as you can with your fingers and tighten the pinch bolt. Purdy prefers to install without using a cable pulling tool, which he thinks can pull the cable too tight. The cable should have no slop, but shouldn't be as tight as a guitar string – the FD should move as soon as you move the shift lever. If the FD won't rest on the inner limit stop then the cable's too tight. Another indication that the cable's too tight is if the FD doesn't move slightly inward from the big ring when you trim it. That's a sign that the cable's under excessive tension as the FD is pushed against the outer limit stop. Unfortunately modern bikes have done away with barrel adjusters on the cable stops, so you may have to attach and reattach the cable a few times to get this just right.
Next set the outer limit screw. With the chain on the smallest cog on the rear and the big ring in front, set the outer limit to 1 mm of clearance between the chain and the inside edge of the derailleur cage. Some people think the FD should be able to travel a little past its outboard position to complete the shift, and then settle back a touch, but that isn't Purdy's preference. If your chain and rings are in good shape you shouldn't need to do this, as it can cause the chain to derail to the outside. 8000 miles later, if your chain won’t shift up to the big ring then you know it’s time to replace that chainring.
From this point there can be a lot of small adjustments to each of these settings to insure proper function. Once you get the shifting just right on the stand, go outside and ride it to make sure it actually works that way on the road, then tweak as necessary.
So what was my problem? I hadn't matched the angle of the derailleur to the chainring closely enough, so the tail of the derailleur had too much clearance, making it more prone to rubbing. I also hadn't tightened the FD mount enough, so the whole setup slipped a few times. It took a few tries for me to finally get things locked down. The last thing I wanted to do was strip the threads and render the whole frame useless.
“I really wish there were better rules for front derailleur setup”, Purdy says. “But the truth is, each bike has its own personality.” Some suggestions for maintaining your front derailleur function: Clean and lube the pivot points as well as the cable guide, keep your chain clean and lubricated, put a new chain on every 2000 miles and replace your large chainring at about 8-10,000 miles.
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With snark help from Schmalz