Technical FAQ: The endless bonfire of drivetrain compatibility questions

The drivetrain compatibility questions keep pouring in, and Lennard Zinn has answers for them all.

Dear Lennard,
Are Dura-Ace SL 7900 shifters compatible with a 10-speed Campagnolo Chorus RD and Campagnolo 10-speed cassette or do I need something like a Jtek Shiftmate 3 for compatibility? I assume there wouldn’t be an issue with a Campagnolo FD since shifter is in friction mode.
— Mike

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Dear Mike,
No, they are not compatible. Yes, some way to bring the shifter’s cable pull in line with the derailleur’s activation ratio is necessary, and then you have the added complexity of a cassette with Campy 10-speed cog pitch. Here’s a bit more on those subjects, introducing the following formula:

Cable pull x Derailleur shift-activation ratio = Cog pitch

In the 5th (current) edition of “Zinn and the Art of Road Bike Maintenance,” I have a table on page 116 of cable pull, shift activation ratio, and cog pitch for a number of generations and speeds of Campy, Shimano and SRAM shifters, rear derailleurs, and cassettes. From that chart, the Dura-Ace SL 7900 down-tube shifter has the same cable pull as Shimano 10-speed STI road brifters, namely 2.3mm. The 10-speed Campagnolo rear derailleur has an activation ratio of 1.5, so,

2.3mm X 1.5 = 3.45mm cog pitch.

However, the 10-speed Campagnolo cassette has a cog pitch of 4.15mm. No go.

A Jtek Shiftmate 3 likely will do the trick. You can also try hooking up the cable on the other side of the cable-fixing bolt and see if you’re satisfied with the shifting (you can also experiment with this with the Jtek Shiftmate installed, if the shifting with it is still not as crisp as you’d like).

And yes, with a frictional down-tube shifter, you can use any front derailleur you want, including that one.
― Lennard

Dear Lennard,
I have a Fuji Gran Fondo 1.1 with DA except for shift levers (Shimano R-685) and calipers (Shimano 805 hydraulic). Would upgrading the levers to the new DA9120 and/or the new 9170 calipers result in improved performance of either over the OE equipment? And would just the 9120 shifters be compatible with the OE calipers on the Fuji if the bigger difference would be in shifter performance?
— Bruce

Dear Bruce,
I doubt that the shifting of your ST-RS685 hydraulic dual-control levers would be any different than that of the ST-R9120 levers. The R9120 levers would be lighter. I don’t know if there would be any difference in longevity.

I think the same goes for the BR-R9170 flat-mount disc brake calipers compared to your existing BR-RS805 flat-mount disc brake calipers. And yes, the levers and calipers of those systems are cross-compatible (no pun intended).
― Lennard

Dear Lennard,
I have a 2015 Campy Record 11-speed compact group with a 12-29 cassette. Can I use the new Campy Potenza 11-32 cassette with my existing Campy? Or do I need to go to the complete Potenza group?
— Joseph

Dear Joseph,
Maybe. It depends on your frame. If you have a long derailleur hanger on your dropout, it might.

Campagnolo introduced a 72.5mm-long “medium” cage for the Potenza rear derailleurs in order to ensure that any frame/chainring combination would work with its new 11-32 sprocket. By contrast, your Record derailleur, and the standard Potenza rear derailleur (which replaces Athena), would have a 55mm-long cage.

And as you probably are well aware, the b-screw on Campagnolo rear derailleurs is not at the mounting bolt as it is on Shimano and SRAM derailleurs; this b-screw increases the chain gap by rotating the derailleur clockwise when tightening the screw against the tab on the derailleur hanger. With those derailleurs, you can turn the b-screw around (or get a longer one) if you need just a bit more chain gap to accept a bigger cog.

Campagnolo rear derailleurs instead adjust the chain gap by means of a screw under the lower pivot that adjusts the tension in the lower-pivot spring. There is no way to jury-rig this to increase the chain gap.

The fact that you’re using compact chainrings makes it less likely to work as well, because the jump in teeth between the chainrings is larger, thus requiring the rear derailleur to take up more chain slack. But since Campagnolo designed the Potenza medium-cage derailleur to work on all frames with any foreseeable chainring combination by means of an increase in cage length of only 17.5mm, if your frame has a long derailleur hanger, your Record RD might work, with the lower-pivot-tension-adjustment screw tightened all of the way in.
― Lennard

Regarding increasing Ultegra Di2 gearing range

Dear Lennard,
Since Jeff’s wife has UDi2 (with I assume a long cage derailleur), I think Jeff should consider putting on a SRAM 11/36, or at a minimum, a Shimano or SRAM 11/32. She won’t even notice the difference if he upgrades her from 28 to 29. I find it takes at least a 10-15% change to be truly noticeable. She would still have the wasted 50/11 combination, but that can be cured as I found with a 46T ring, which will take care of any slack chain issues with the 34T ring (if he opts for the 11/36).

If Jeff does use a SRAM cassette, I would recommend he use a SRAM chain. My buddy Bruce had issues with his first XTR chain getting caught between cogs in his 11/36 SRAM cassette. The shop put on a SRAM chain, and it’s been flawless ever since. That ever so slight different in the plate width seems to make a surprising difference.
— Steve

Regarding SRAM Exact Actuation and stuck shifters

Dear Lennard,
I wanted to add one more log to the endless bonfire of tech questions regarding compatibility between shifters/derailleurs and cogs.

I have a Soma Double Cross currently repurposed for life as a mullet gravel grinder (TRP HY-RD disc-brake front, Avid cantilever-brake rear) but needed wide gearing because of decades of chronological enhancement. I’m still rocking first-generation SRAM Rival 10 speed Double Tap, because that is how I roll. My drivetrain included an FSA Gossamer 50-36 crank, Force 1st generation front derailleur, but I needed an 11-36 in the back. I’m still on 10 speed cassettes.

Since both the SRAM Road 10 speed and mountain have Exact Actuation, I put a SRAM GX MTB 2×10 long cage rear derailleur on to see if it would work. Turns out … it does. Fantastically well. I had to cobble up an inline adjuster for the rear derailleur (MTB RDs have direct cable routing and all adjustments are at the shifter), but once that was dialed in, I’m happily spinning up hill and dale.

But it gets way better than that.

In a fit of curiosity I also ordered a 1×11 Apex RD, long cage. As you have often mentioned, SRAM 11 speed is ALSO Exact Actuation. I mounted it on my vintage 1×10 MTB (Titus Racer-X) with an 11-42 Sunrace derailleur and a rear GX 10-speed shifter. I had previously been running a 2×10 RD (because the GX 1×11 RDs are X-actuation and thus incompatible) but even with a long B-tension screw the shifting was mediocre onto the 42T cog. I happened to have an old Avid Rollamajig (still awesome after all these years) to shorten the rear derailleur housing loop into the Apex RD (with advertised capacity for a 42T large cog). After some tweaking (I think return springs are different spring rates) it shifts AMAZINGLY well. I’ve ridden GX 1X11 and the quality is similar.

So now I have a gravel grinder with an MTB 2X10 RD, and an MTB with a Road 1×11 RD. Both are 10 speed rear cassettes using the requisite SRAM 10 speed shifters, and both shift great.

Sending this in for those in your audience still wishing to eke some life out of serviceable, stout 10 speed SRAM drivetrains.
— Noel

Dear Lennard,
Although I don’t have Campagnolo levers, your response to “stuck Campagnolo shifters” helped me address misfires with my Dura-Ace 9000 rear derailleur shifter. I couldn’t understand why the small shift lever would move freely with no-click or shift in gear. Based on your feedback, I realized that this happens every time that I put pressure on the big shift lever. I found it interesting that the same mistake (i.e., pressure on one shifter while actuating the other) can produce completely different results in different groupsets.
— Guillaume