Gearing Thoughts

Y'all, I can't stop thinking about gears, cogs, chainrings, and shifters.

I'm building up a Salsa Cutthroat for a Tour Divide attempt in 2021. I've got the bike and it's got the stock SRAM Apex components, which I'm not a fan of. Swapping the FD to a Shimano XT made a huge immediate improvement, so I'm excited to see how the bike feels with all Shimano kit.

But what Shimano kit?

There are so many options!

Shimano Road 2x

GRX just came out, and the 2x rear derailleur (reportedly) handles 11-40 stock, and would work decently well with the 2x 28-42 chainrings I've got now. This gives me a gear range of 0.70-3.82, or 545% range. This is basically exactly what I have now, which I know that I like. It would be running the GRX out of spec, but only a little, and I've done this with a Tiagra 4700 rear derailleur that handles it fine.

Of course, the Ultegra RX8000 rear derailleur is basically the same thing, but very slightly different - the GRX is slightly beefier, slightly heavier (+6g), has slightly more range (+1t).

The GRX brifters are particularly appealing, but they only come in hydraulic, which is both more expensive and potentially not as reliable as I'd want for a touring/bikepacking bike. Maybe that's silly, and I should just go for it. 

The Ultegra 8000 shifters are common to find as take-offs or used, and they show up in both hydraulic and mechanical for much less than the GRX shifters.

Shimano Mountain 2x

I can run a Shimano XT 11s rear derailleur and use a Wolf Tooth Tanpan to get compatibility with 11sp road shifters. This covers the 11-40 case well within spec. Indeed, it'd allow me to go up to 11-42, or even 10-42, as the XT RD reportedly handles that just fine, and a Goat Link improves that even further.

A 10-42 cassette and 42/28 chainrings get me a range of 0.67-4.20 or 627%. That's actually a taller gear than a 46x11! That is pretty intensely cool.

Weight-wise, this is a pretty good way to go. The Tanpan is 17g, and the XT rear derailleur is 272g compared to the 255g of the GRX, so we're looking at 29g of added weight. Shifting to an XD driver saves about that much weight, fortunately. The 11-40 Sunrace CSMX8 cassette is 412g, while the 10-42t cassette is 458g. Switching to a SRAM 1150 cassette gets the weight to 394g, without any price increase.

Could I go SRAM XD and 10-42 with the GRX setup? Possibly, but there have been mixed reports of success and skipping on the 10t. I think I'd need a longer cage on the GRX derailleur to keep 2x. The 1x would work fine, but then I'd be looking at bigger cassettes.

Get the new Cutthroat

The new Cutthroat comes with 46/30 gearing with GRX components already. The old Cutthroat is limited to 42/28 gearing, and that standard is becoming somewhat hard to find parts for. So I could upgrade to the new model, get a wider range on my chainrings, and get all the other goodies.

Eh. I don't think so. I like my Cutthroat a lot, and I'm not sure that the Boost spacing and increased Q factor would really improve the bike as a Tour Divide rig (or as the 'do basically everything' bike that it is now).

What about a 1x?

A 1x saves a pretty significant amount of weight. I can drop 200g by switching to a Force 1 crankset, drop 155g off my front derailleur (plus cables etc), about about ~100g by using a brake-only left lever. 455g - that's a little over a pound!

Well, sort of. If we're replacing cranksets, I can get an XTR MC-985 on eBay for around $150. That saves 212g from my current setup, and is cheaper. So the weight savings (like-for-like investment) is only 243g.

The big killer is range. I've currently got a range of 545%, and the biggest in spec 2x range I can achieve is the 627%. The Eagle 10-50 cassette (500% range) are only available in 12 speed, or obscenely expensive 11 speed offerings from Garbaruk/OneUp/e*thirteen/etc. e*thirteen offers a 9-46 (511%) cassette, which is good. SunRace offers a 11-50 cassette, but at 560g (plus the extra ~30g or so for Shimano Hg driver), it's nearly 196g heavier than the 10-42 SRAM cassette - that's almost all of my weight savings gone right there! Considering that I'd likely need a Goatlink or extender to get the XT derailleur working, we're looking at only no weight savings and a 27% loss in range.

The tallest gear the Cutthroat can run as a 1x is 38t. The tallest gear I can get - 38x9 - has a gear ratio of 4.22, which is even taller than a 42x10 (which is taller than 46x11). A 38x46 gear is only 0.82, though, which is equivalent to running a 34t cog on a 28t chainring. I'd run an oval, which brings me down to a 36t chainring (0.78 - 4.00), roughly equivalent to a 40x10 (or 44x11) to 28x36 range. So that's kind of disappointing.

"But Matt, you can just run a different chainring for unloaded stuff and loaded rides." Yeah, I guess so - swapping chainrings on a per-ride basis wouldn't be the worst thing. And 36x9 would be faster than the 42x11 that I have now and am happy with. But the e*thirteen cassette is reportedly not very durable, and I'd want a durable setup for bikepacking... and at $250/ea for the cheaper one, that's a little too expensive for me to be replacing frequently.

Do you feel better now?

Yeah. Thanks for letting me vent. I think the mountain 2x is the way to go now. Just need to get the shifters now!

UPDATE 1-1-2020:

I got some take off Ultegra shifters for a good deal and did the upgrade. It's so good. V happy.