May contain nuts.

Nice little 2× drivetrain gear ratio quirk

Warning, this may be a bit wordy and nerdy.

I have been researching drivetrain options again—don‘t judge me—because I found myself wanting a bit more evenly and tightly spaced gears on both my road bike and my gravel bike.
The area I moved to a while back (southern Black Forest, Germany) is an absolute dream for cyclists: I can climb to my heart‘s content either on- or off- road and when I just want to go fast on the flat, a short trip into the wiiide Rhine valley and it‘s go-time – yes, I went there.

Current setup

On Panthor, my road bike, I presently have a 1× (single chainring) setup using a 36-tooth chainring and a 12-speed 9–46 e*thirteen TRS+ 2nd gen cassette. For the 11 gear shifts, I get an average gear ratio change of 13.8%. The gear ratio in the lowest gear is 0.78 and in the highest gear it‘s 4.0.1

On Battle Cat, my gravel bike, it’s a 1× setup with a 32-tooth chainring on the front and a 11-speed 9–39 e*thirteen XCX cassette on the back. That gives me a solid 0.82 gear ratio on the bottom end of the spectrum and 3.55 on the top end, as well as 10 gear changes with an average gear ratio change of 13.6%.

Changing things up

Digging into some good combinations of cassette and chain rings, I had two sets of requirements 1. A closer grouping of the gears on the road bike is desirable but with at least one climbing gear with a gear ratio below 1. 2. A low climbing gear on the gravel bike is a must given the steep and quite technical terrain I‘m riding more and more often but I also want fairly evenly spaced gears throughout the range, to tackle longer rides across varied terrain more comfortably.

There‘s an incredible amount of choice in the market for realising what I wanted on both bikes but in all cases I found out that a 2× (double chainring) setup was probably the only way to get there.

Most cyclists know or have heard that a 2×11 setup typically offers 14 unique gears. This can vary depending on the choice of chainrings and cassette in the back. I‘ve compared a lot of cassettes and chainring combinations using a nifty little online gear calculation tool by Jürgen Berkemeier; everything from the e*thirteen and Shimano cassettes I have at home to options from SRAM, Sunrace, Garbaruk, KCNC, etc.

Similarly, I looked at various double chainring options, ranging from insane 53/38 to the 46/36 I still have on my Rotor 3D30 crankset to 46/30. Most of those combinations when paired with almost any cassette end up giving me 14 unique gears, a few give me 15 and even fewer only 13. Interestingly, depending on the difference in teeth between the big and small chainrings, I‘ve seen cassette + double-chainrings combinations where the shift from big to small or small to big chainring (and compensating in the rear) results in a 2%–4% gear ratio increase. I‘d consider this barely usable, which means that these combinations really only offer 14 instead of 15 or 13 instead of 14 unique and usable gears.

Now here‘s the funny bit.

46/30 chainrings are the only ones I found that, paired with the right cassette in the back, end up with 15 proper, unique gears and a change from the small to the big front ring (again while compensating 3 gears on the cassette), results in an actually usable jump of 7%–9%.
I checked other variations of 16-tooth difference chainrings but none seemed to work out with such a high gear ratio increase.2

It gets even funnier looking at the cassettes. Only two cassettes I‘ve found, have cog size spacing that results in 15 proper gears: * Shimano CS-R8000 11–32 * Shimano CS-M8000 11–40

What now?

I‘m going back to 2× on both my bikes. – GASP!

Panthor is going to receive a 46/30 chainring coupled with the 11–32 cassette. This gets me a gear ratio of 0.9375 in the lowest and 4.18 in the highest gear and and average gear ratio change of 10.11%, a reduction of ~ 3.6%. This average value may not sound like too much but looking at the gear ratio changes across the current 11 and soon 14 shifts, it‘s substantially closer gearing that‘ll give me a much more consistent cadence.

Battle Cat will also get a 46/30 chainring connected to a 12–40 cassette.3 The low gear will offer a ratio of 0.75—*WOOOOO! Insane climbs here I come!*—and the high gear is going to have a value of 3.83, which is plenty. The average gear ratio value will be ~ 11% (down from 13.6%) across 14 gear changes (up from 10).

I’m not sure how long this is going to stick, if I like the setups for riding, and to a lesser degree from a visual perspective – vanity in cycling is a thing, y‘know.
Thankfully I still have a set of Ultegra R8000 brake shifters and an Ultegra R800 front derailleur at home. The remaining parts like the new chainrings for both bikes were fairly easily found. The Rotor 46/30 direct-mount chainrings are already here and the Easton Gravel Rings, the left GRX brake shifter and the second front derailleur should be here by the end of the week.

Looking forward to the tinkering and trying these two new setups.

  1. Quick foot note: the reason the road bike has such a very unusual setup for now, is because it‘s also my shopping and commuter bike, that I sometimes pull my dog in a trailer in. Now not only my needs and wants are shifting, I‘ve also become a stronger cyclist over the past few months. It‘s time to experiment a bit. [return]
  2. I honestly haven‘t had the inclination to dive into the math why the product of certain combinations of chainrings, all with 16-tooth jumps, didn’t result in the same usable amount of gears on the cassettes I looked at. [return]
  3. I‘m using the XT CS-M8000 cassette but replacing the 11-tooth cog with a 12-tooth cog. I simply don‘t need the speed the 11-t cog offers on the gravel bike. [return]