I'm looking into spec'ing out a couple of custom(ish) 'his and her' gravel rigs, to celebrate our partnership (both potentially cheaper and waaaay awesom-er than a wedding ).
We're both currently riding older cyclocross rigs. Mine has a 50-34 mated to a 9sp 11-32. She has a 50-39-30 (triple) mated to a 10sp 11-32. I've spent probably far too much energy worrying about gear ratios (this will become obvious), and was hoping for a little advice. Note, we live in Southern Ontario, Canada. We love attacking the short, steep hills; but there is not really any extended climbing.
We will definitely be using 2X systems (also for occasional road riding), most likely Ultegra 8020, with the mid-cage derailleur. There are a couple of standard chainset & cassette options available. We're not really interested in exploring the aftermarket chainsets (ex. Easton, Praxis) at this time.
We have a couple of targets:
1: achieve a lowest-gear ratio that no harder than her current 2nd gear which is 30x28 (12.5 k/h @90rpm and 700x32c tire)
2: the top gear is of little consequence. So long as we can comfortably hit about 45k/h on the road (@90 rpm) we'll be happy.
3: keep the shifting smooth and crisp, so not push the boundaries of what the rear mech can handle.
4: On the flats I spend the vast majority of my time in one of 2 adjacent gears: 50-24 and 50-21 (equating to 24.2 & 27.7 k/h respectively @90rpm with 700x32c tire). I'd desperately like to achieve similar ratios, but have a 3rd gear somewhere in the middle.
With these in mind, the 2 obvious options are:
1: 50-34 ‘compact’ mated to 11-32 (37-t range)
2: 46-36 ‘cyclocross’ mated to 11-34 (33-t range)
Target 1: the 36x34 & 34x32 low gears are almost identical (also the same as 30x28 with smaller wheel circumference), so no clear winner on target 1.
Target 2: Top speed @ 90 RPM are 53 & 49 k/h (using larger 700x38 tires) respectively, for the 50-11 and 46-11. Both are more than adequate.
Target 3: Option 2 seems to be the clear winner here as it has a significantly lower total tooth-count range, meaning shorter & tighter chain, less stress on the rear mech and crisper front shifts with the cyclocross crankset.
Target 4: The real advantages I see with option 2 are in the middle gear ratios. With a 46t chainring and 11-34 cassette, there are 3 cogs (19, 21 & 23) that span the 24 to 28 k/h speed range, while with the 50t chainring with 11-32 cassette there are only 2 cogs (22&25) that span the same range. Moreover, they are farther ‘up’ the cassette, thus not as good chainline with the compact vs. cyclocross chainset.
Shimano lists the 11-34 cassette with a different part number than the rest of the Ultegra cassettes. I believe this is because it is also the only (higher end) road cassette that is also compatible with 11-speed mtn bike freehubs (which are narrower than road freehubs). My understanding is that it comes with a spacer to be used for road bike applications. Its not clear on the Shimano ‘Dealer’s manual’, where this spacer goes (my guess is next to the spokes). One concern is that the cassette will have ~10% less overall ‘contact’ with the road freehub vs. a normal road cassette…so a greater potential for gouging a modern aluminum freehub (something I have wrestled with before).
Given all of this, I am currently heavily leaning towards the 46-36 chainset mated to 11-34 cassette option. Any insight into my logic or if you can share your own practical experience with different 2X combinations for a multi-surface bike (including specific experience with the 11-34 cassette) would be much appreciated. I also want to acknowledge the awesome gear calculator on SheldonBrown.com, which I have used extensively.