In an article entitled “Sensible gearing for non-racing road bikes” 3 1/2 years ago I described one of the options:
Road Cranks With Large Cassettes
Shimano road cranks use a 130 mm bolt circle diameter (BCD) for the outer ring and 110 mm for the inner ring, which limits the smallest inner ring to 34T. Your only option of getting a lower lowest gear is a bigger cassette. Cassettes are limited by what number of teeth your rear derailleur (RD) can handle. Medium and long cage RDs handle bigger cogs as well as bigger differences between the small and large gears than do short cage RDs. A 9 or 10 speed road RD can also be replaced with a 9 speed MTB RD for more capacity, as they are pull compatible. This allows cassettes up to 36T. The drawback of bigger cassettes is extra weight and less closely spaced gears that require bigger cadence changes when changing gears.
Things have changed a bit with 11-speed groups and also with the 10-speed Tiagra 4700 group, as these newer generation parts use different rear derailleur geometries and cable pull than their 9 and 10 speed predecessors. Therefore 9-speed mountain bike (MTB) long cage derailleurs with enough chain wrap to handle 11-36 cassettes no longer work with 11-speed road groups.
Tiagra 4700
The medium cage Tiagra RD-4700 GS derailleur supports a 10 speed 11-34 cassette when used with a double crank (compact 50/34, mid-compact 52/36). It can handle an 11-32 cassette with a triple crank (50/39/30). We will be disregarding the short cage RD-4700-SS here, as it can only handle cassettes up to 11-28, not enough for low gearing.
The geometry of the Tiagra rear derailleur is compatible with its 11-speed siblings such as the 105 RD-5800-GS or Ultegra RD-6800-GS. The ratio of cable pull to sideways movement is the same, but each click of the Tiagra right shifter will pull more cable, causing a bigger sideways movement of the derailleur to match the more widely spaced 10 sprockets on the Tiagra cassette compared to 11 speed cassettes. What this means is that you can use an 11-speed derailleur with Tiagra 4700 shifters and vice versa, a Tiagra derailleur with 5800 or 6800 shifters. The number of gears will be determined by the shifter, not the derailleur.
105 5800 and Ultegra 6800, 105 R7000 and Ultegra R8000
Both the medium cage 105 RD-5800-GS and Ultegra RD-6800-GS derailleurs officially support 11-32 cassettes. On some bikes you will be able to use an 11-34 cassette by adjusting the B-screw on the rear derailleur. The next generation 105 R7000 and Ultegra R8000 medium cage derailleurs officially support up to 11-34, for 6% lower gearing than 11-32.
Shimano offers two 11-34 road cassettes (105 CS-HG700, Ultegra CS-HG800) that are functionally the same, but the Ultegra cassette is about 10% lighter. These 11-34 cassettes have the added benefit of also working with older 9/10-speed freehubs, unlike other Shimano 11-speed cassettes (12-25, 11-28, 11-30, 11-32). Thus they not only give you lower gearing but also access to a great variety of cheaper wheels! In particular, if you have an existing bike with 10-speed drivetrain including an 8/9/10-speed only freehub and you want to upgrade to 11-speed but don’t want to buy new wheels, these 11-34 cassettes may be just what you want!
All of the above 11-speed RDs can handle cassettes up to 11-40 if used with a Wolftooth Roadlink derailleur hanger extension. This little inexpensive gadget (about US$22) will lower the rear derailleur enough for it to clear bigger sprockets.
While Shimano doesn’t currently offer 11-36 road 11 speed cassettes, they are available from SRAM (PG-1170) and Sunrace (CSRX1).
The Roadlink will not change the length of the derailleur cage, so the chain wrap capacity of the derailleur remains unchanged. Therefore you could run into problems if you cross-chain too much (small/small or big/big) with a larger cassette than your derailleur was designed for.
If you shift down too far while on the big ring when your chain is not long enough, your rear derailleur may get sucked into the cassette and get damaged. This is a worst case you definitely want to avoid.
If you keep the chain long enough to be safe with big/big cross chaining, your chain may go slack and slip or fall off when you up-shift too far while on the small ring (small/small cross-chaining). The safe thing is to pick a chain length that covers the big/big case, just make sure you shift to the big ring at the front once you’ve up-shifted into the middle of the cassette at the rear.
Using a 10/11-speed Mountain Derailleur
If you want a rear derailleur designed for 11-36 or even bigger cassettes (and sufficient chain wrap for all cases), another option is to use a Wolftooth Tanpan (about US$40), which lets you use 10/11-speed long cage mountain derailleurs such as the RD-M786-SGS with 11 speed road shifters.
UPDATE (2018-09-21)
I have installed an 11 speed 11-34T cassette with a Shimano RD-R7000 GS rear derailleur on my Bike Friday. It was indeed compatible with my 10 speed Shimano Deore centerlock disk brake freehub. With a 52/36 mid-compact crank set it gives me virtually the same low gear as my previous 10 speed 50/39/30 triple crank with an 11-28T cassette (21.3 vs. 21.6 gear inches). This allowed me to switch from a 10 speed triple to an 11 speed double without giving up my low gears or having to buy new wheels. My tallest gear grew 4% taller (95.3 vs. 91.6 gear inches). The longer cable pull per click of the shifter ratchet of the 11 speed system makes for lighter shift action and less susceptibility to friction and cable stretching. Even though a 50/34 compact crank with an 11-32T cassette would have given me the same gears, the 11 speed 11-32T wouldn’t have worked with my freehub and a 10 speed 11-32T is not officially supported by the 10 speed 5700 GS or 6700 GS rear derailleur.
