Ask a Mechanic | Short Chainstays and Cross Chaining With Di2

Question: I have a Specialized Tarmac with Ultegra 6870 Di2 and I can’t get the drivetrain to work in the 34-tooth chainring and in the two smallest cogs in the back without rubbing on the front derailleur. I’ve tried adjusting the front derailleur trim and I still can’t make it work. What am I doing wrong? From: Sun

Answer: You’ve brought up a feature that is new to Shimano’s 6870 and 9070 front derailleurs that wasn’t available in the earlier versions of Di2. That feature is a front derailleur trim adjustment option. So let’s start by going over the whole Di2 front derailleur adjustment process, including the trim adjustment.

Start by checking the position of the front derailleur relative to the large chainring when it is shifted into the small chainring. Make sure that the outer plate of the derailleur is parallel with the big chainring and is 1-3mm above the tallest tooth on the big ring. Once you have the derailleur in position, tighten down the derailleur fixing bolt to between 5 and 7 newton meters. Then adjust the support bolt so that it just touches the frame without moving the derailleur out of position.

Now we’ll set the low limit. Start by shifting the rear derailleur into the largest cog. Turn the 2mm low limit screw clockwise to move the derailleur outboard, and counterclockwise to move it inboard. You want to set the inner cage of the derailleur as close to the chain as possible without rubbing. Shimano calls for just 0.5mm of clearance between the chain and derailleur cage.

With your inner limit set, shift the front derailleur to the big ring and the rear derailleur to the small cog. Turn the 2mm high limit screw clockwise to move the derailleur outboard, and counterclockwise to move it inboard. You want to set the inner cage of the derailleur as close to the chain as possible without rubbing. Shimano calls for between 0.5mm and 1mm of clearance between the chain and derailleur cage.

For the trim adjustment, with the front derailleur still shifted into the big ring, shift the rear derailleur into the big cog. This is the gear combination that Shimano says you should be in to adjust the trim function. I realize this is counterintuitive, but I can assure you that this is the way it should be. Put the system in adjustment mode by pushing the adjustment button on the “A” junction box and then use the front shifter buttons to move the derailleur cage until the inner cage plate is between 0 and 0.5mm away from the chain. That’s it. There is no requirement to adjust trim at the other extreme with the chain shifted into the small ring and the small cog.

Something to keep in mind as you make these adjustments is that your frame’s geometry can affect how well your drivetrain functions when cross-chained in the small-to-small gear combo. Shimano says that 415mm long chainstays are required in order to prevent the front derailleur from rubbing on the chain, or to prevent the chain from rubbing on the large chainring, in the 34/11 gear combo. Shimano also says that chainstays must be at least 405mm long in order to achieve optimal shifting. Your Tarmac is right at 405mm. So, it is likely impossible for you to get that gear combo to work on your Tarmac without any rubbing at all.

But look on bright side, you have a duplicate gear available midway up the cassette when shifted into the big ring, you’ll just need to make a few shifts to get there.

Welcome to our Ask a Mechanic column where our expert mechanics Daniel Slusser and Greg O’Keeffe answer your bike maintenance questions. If you have a question for us, please post it on the Art’s Cyclery Facebook Wall or e-mail Daniel directly at daniel.slusser@artscyclery.com. To see more great how to videos click on the highlighted link to subscribe to our YouTube channel and stay up to date on each episode of the Art’s Cyclery/VeloNews Ask a Mechanic Series.

2015-12-16T15:25:28-08:00