By Jim Langley

RBR reader Andrew K. wrote us recently about a topic that rings true for many of us as we get older but still want to be able to do the "big rides" we've done for years.

Andrew wrote: "As I age, I feel the need to go from a mid-compact to compact gearing to participate in some gran fondos. Would you consider discussing changing chain rings and implications for needing to adjust the derailleur, chain, etc? I specifically use a Shimano Ultegra Di2 11-speed groupset."

Going from a mid-compact to compact gearing may not be as complicated as you think it might be, Andrew.

The fact that you have the Ultegra Di2 group is great for what you want to do, because your Ultegra crankset should be Shimano's newer 4-arm design. One of its chief advantages is being able to accept many different chain ring combinations.

So, if you’re running the mid-compact 52/36 rings now, you can just remove them and put on the 50/34 to lower your gearing a bit more for those hilly gran fondos.

And, since there is only a 2-tooth difference between a 36- and 34-tooth chainring, there’s a good chance that you won’t have to change your chain length.

So, I would buy the new rings, install them and then check the chain when you are on the new 34 and the smallest cassette cog in back. As long as the chain is not now rubbing on the rear derailleur pulley – which would mean you’d likely see some slack in the lower run of chain beneath the chainstay – the chain length is fine.

If the chain does sag and is so loose it hits the pulley, you might be able to tighten the B tension screw – the one on the back and top of the derailleur that points to the rear. Tightening that adjustment screw pulls the derailleur back and increases chain tension and can take a little slack out – sometimes just enough for a small change in chain ring size like this.

If the B tension screw doesn’t solve the issue, then you will need to slightly shorten the chain. Usually, removing an inch of chain is enough – (which is one full link). To do that on a Shimano chain, you’ll need a chain tool and a special Shimano chain pin to reconnect the chain after you remove the links. Or you can use a master link instead of the chain pin, in which case you would remove two links, replacing one with the master link.

All these things are easy to do at home as long as you have the tools. I hope this gets you on the road with some nice low gears.


Jim Langley is RBR's Technical Editor. He has been a pro mechanic and cycling writer for more than 40 years. He's the author of Your Home Bicycle Workshop in the RBR eBookstore. Check out his "cycling aficionado" website at http://www.jimlangley.net, his Q&A blog and updates at Twitter. Jim's streak of consecutive cycling days has reached more than 8,000. Click to read Jim's full bio.

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