Technical FAQ: GRX wide range, Planck’s constant, and Campy a Ergopower overhaul

Dear Lennard,
I am about to purchase a new gravel bike if I can find one. I am trying to get the easiest gear I can for the more or less endless climbs around Seattle. I have seen articles, and/or videos of Shimano’s GRX group with the long cage derailleur working on rear cogs as large as 40 teeth. Shimano only rates it for 34. Is it safe to assume that 40 teeth is possible, or should I just plan on an XT/XTR rear derailleur? I am definitely planning on a 2x setup. If I need to go with the mountain bike rear derailleur, is it compatible with a GRX front derailleur and crankset? Is there some unpleasant little landmine I am missing? Something completely indecipherable such that an eleven-speed XT derailleur will work with the GRX front set up, but the twelve-speed XTR will not? Shimano has worked awfully hard to make this hard to figure out. For extra credit, is it worth considering one of the aftermarket long-cage setups?
— Dan

Dear Dan,
The reason you have seen those articles/videos is that a 40T large cog is indeed possible with GRX; the Shimano RD-RX812 GRX rear derailleur can handle an 11-40 or 11-42 cassette. Thing is, that is actually the shorter-cage RD option; its total gear capacity is 31 teeth, quite a bit less than the 40-tooth capacity of the RD-RX810 GRX rear derailleur built for an 11-34 cassette. In other words, the RD-RX812 is not designed to work with a front derailleur, so if you’re dead set on a 2X, then this one is not an option for you. If you’re talking Di2 rather than cable-actuated shifting, the Di2 RD-RX815 has the same limits as the RD-RX810, and the Di2 RD-RX817 has the same limits as the RD-RX812.

As for using a cable-actuated XT or XTR rear derailleur: You can forget that. It’s the GRX shifter you have to worry about compatibility with, not the front derailleur or crankset unless you want electronic shifting. Shimano cable-actuated mountain-bike derailleurs are not compatible with Shimano road/gravel dual-control levers, and vice versa. And if you’re talking Di2, then the front derailleur and crankset do come into play: GRX Di2 shifters (and any road Di2 shifters) will only work with a Di2 MTB rear derailleur if the front derailleur is also an MTB model, and MTB front derailleurs are not compatible with GRX chainrings (it’s the larger chainring diameters that are the issue; the radius of curvature of the XT/XTR front derailleur cage is too small to work with them).

What you could do is use XT/XTR Di2 front and rear derailleurs along with an XT/XTR crankset with GRX Di2 shifters; you would get the super low gear you seek, but you would be lacking on the high end. To get around this, you could use a SRAM 10-42 11-speed cassette (the wheel would have to have an XD freehub). Even though it is not rated for it, I know from personal experience on my own mountain bike that an XTR Di2 rear derailleur with a 2X MTB crankset works fine with a SRAM 10-42 cassette. That’s the setup I rode for years on my bike that’s on the cover of Zinn & the Art of Mountain Bike Maintenance.

As for aftermarket long cages for GRX like this one for the RD-RX812, I have no experience with them and can’t comment on their functionality.
― Lennard

Dear Lennard,
Regarding your column from February 2nd regarding chain tension:

It is already 2 years as the unit of mass is redefined to not use the slug anymore. Now it is defined via the Planck Constant, namely, the kilogram is such mass that a value of the said constant is exactly 6.62607004 × 10^-34 ⋅ m² ⋅ kg / s.

From the practical point of view, nothing changes.
— Oleksandr

Dear Oleksandr,
Hey, that is cool! And to think that if it weren’t for the mention of the slug in this column, I still wouldn’t know about this! Even in pre-pandemic 2019, paying attention to the news was like drinking from a fire hose, and this little gem escaped my notice. That’s big! No more physical references for any of the SI units! It’s a bit sad that the gorgeous little platinum-iridium IPK reference kilogram has become an irrelevant relic in a double bell jar! I wonder what will happen (or has happened) to the IPK, its six copies and 10 working copies in Paris and all of national prototypes around the world?
― Lennard

Dear Lennard,
Since I wrote to you about using a Redshift Shock Stop to buffer thumb pain, my 10-speed Campy Ergo right shifter broke a G-spring. I’ve rebuilt it 3 times in 47,000 miles, so I wasn’t surprised, although it was pretty silky before it broke. While looking for rebuild parts, I noticed that Campy now markets a complete replacement shifter core for Ergo 10-speed. It comes without the brake handle and clamp/clamp-bolt but drops right, in about half an hour. Since the cost of the individual rebuild parts was pretty steep (I had never replaced most of the other springs), I got the module. Not good! I should have paid more attention to Andy Pruitt’s comments. Finger shifting was fine, but the thumb button was incredibly stiff. My right thumb/wrist got pretty beat up. I figured out an approach that avoids the pain, but it’s iffy if I don’t do it exactly that specific way. Fortunately, it seems to be getting easier to shift as the internal parts “lap in,” but I was really concerned for a while.

Other than the thumb issue, the new module works… well, like new.
— Larry

Dear Larry,
Thanks; I used to love replacing little gubbins inside old Ergopower levers until they worked like Swiss watches again; that’s why I was still racing cyclocross on Campy 10-speed Ergopower up through 2013, and that’s why I have sections devoted to that in Zinn & the Art of Road Bike Maintenance. It’s nice Campy makes an entire shifter core available for those.

When I jumped a railroad track in Colorado Springs in 1979 with my right heel back so that it pushed my Super Record rear derailleur into my spokes while I was airborne, my appreciation of Campagnolo for making it possible to keep our old parts on the road began. My derailleur was ripped into multiple shards, with pieces flying every-which-way when I landed, and my rear wheel became D-shaped from its collision with multiple spokes. Campagnolo used to offer bike shops an elegant chest of metal drawers chock full of Campy small parts, and I was able to get every little piece I needed to rebuild that derailleur, as well as the wheel, at Criterium Bike Shop. I was riding that derailleur again the next day, and it changed my life. I like finding serviceability in our current chuck-it-out-and-get-a-new-one world.
― Lennard

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Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.” He holds a bachelor’s in physics from Colorado College.

Follow @lennardzinn on Twitter.