Modifying a rear mech for road 1×10 and a 11-40t cassette (Part 5)

One final piece in this puzzle is the rear mech or derailleur: no Shimano road-shifter compatible mech was designed to reach a cog even as big as 34t, much less 40t. In fact, cassettes with 36t have only become common in the past few years when 1×10 mountain bike drivetrains started to appear. SRAM has this under wraps since their10 and 11 speed  road shifters are compatible with MTB 10 speed rear mechs as well as 11 road mechs (but not their MTB 11 and 12 speed mechs). With Shimano 10 speed road shifters, rear mech compatibility is restricted to 8, 9 & 10 speed road mechs and 8 or 9 speed MTB mechs, except for 9 speed Shadow: though the cable pull should be the same, I have not been able to get these derailleurs to shift correctly on a road setup (I tried several combinations).


What I ended up doing was modifying a Shimano XT RD-M761 mech, increasing both the main pivot tension and the B tension springs. Some blogs will suggest the use of a longer B tension bolt, but I find this rarely works as it falls off the tab it’s meant to push against on the mech hanger (see pic below). Dismantling a higher-end derailleur is pretty easy: to remove the B pivot, pop off a circlip. To remove the main pivot, there is a hidden allen bolt on the knuckle. When removing the main pivot, pay attention to what hole the spring was engaged in, and make note. You will want to put it back together using the other hole. It is my understanding that these two settings were designed for cyclocross/pave use, though it is also plausible they were a longevity feature for when the spring started to wear out. At any rate, clean off all the pivots, remove any dirt and grease, then regrease things before assembly.

RD-M761 spring tension modification


Modifying the B tension involves drilling a small hole about ¼ rotation from the original. I tried it further, but found the spring was under too much tension to assemble. If you remove the B tension bolt, the tab it screws into can be put in the jaws of a vice. Punch a centre for your drill bit, either using a punch or a wood screw.


To reassemble the B pivot, insert the spring into the main body of the mech, engaging its hole, then put the plastic washer on, then engage the modified plate in the correct hole. Using either plumber’s grips or vise-grips hold the tab you had in the vice and rotate it until it gets past its stop, and push the assembly together: hold with one hand whilst pressing the circlip back in place.


Reassembling the main pivot is similar, but much easier since the cage acts as a lever, and you only have to push a little pin in place to hold it together.

Tobias Feltus: