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runrideski

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Reply with quote  #1 

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 [biggrin]).

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)

My analysis:

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.
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clarksonxc

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Reply with quote  #2 

You really did your research, and I can't see any obvious errors in your analysis.  So I'll offer up another option, which I have used extensively with Ultegra 6800:  You can buy the 46/36 crankset, and pair it with 34T inner ring.  The 34T isn't much extra money ($15 USD?) and it shifts just like the 46/36.  I've also used the 50/36 combo as a result of the swap, and that shifts perfectly as well.  I live in WNY, and we have a bunch of people from ON come down and race/ride in my area.  I really think the 46/34 with 11-32 would be the ticket for the type of riding you're describing.  That said, since you are building up two bikes, you can try it both ways and experiment between the two!  Get the 50/34 x 11-32 for you, and the 46/36 x 11-34 for her... Mix it up after a few months and see if the change makes much difference (and check the freehub body as well, which I think will be fine since the cassette spacer is very narrow in the grand scheme of things).

 

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runrideski

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Reply with quote  #3 
Thanks for your insights.

I had already considered both scenarios that you mentioned:  buying a new inner 34Tchainring and switching 46-34, or getting one bike with compact and the other with cross chainsets and swapping inner rings.  

I think the former is a viable option, particular if she finds the 36-34 low gear a little tall.  Going 34-34 (the venerable 1:1) is about a 6% reduction.  As you said, the inner ring is not much $$.  Very good to know that it doesn't really harm the shifting though.  

My main reason for steering clear of the 50-36 option, with a 11-32 in the back, was the improvement in the 'sweet-spot' gear ratios that I was seeking.

I think it would also be cool to have the exact same gear ratios.  Since we both have similar pedalling styles, it would seem to make the most sense to match gearing as well.  Also, it will be super interesting (for the geek in me) to see how often we do/don't use the same gear at the same time!

thanks!
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Slim

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Reply with quote  #4 
You said you are using 9 speed? Then the 11-34 cassette is a XT(mtb) cassette. They are affordable, light and shift great. They use an alloy carrier for the larger coggs, so gouging is not an issue. All high end mtb cassette have this alloy carrier, Only road cassette(with their small coggs) and cheap mtb cassettes have riveted stacks of individual coggs, which, as you noticed, are prone to gouging the casette body.

yes, spacers go between the cassette and the spokes.

Your rear derailleur might not handle the 34t big cog. If it doesn’t Shimano 9 speed is interchangeable between road and mtn, so you can just swap in a mid cage 9 speed mtn rear derailleur.

If you go for the CX chainrings, you might need to get a CX front derailleur from Shimano, the Aare shaped to match the smaller radius of the 46t chainring better and clear the chainstay better too.

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runrideski

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Reply with quote  #5 
I am currently using 9-sp, new bikes will have Ultegra 11-sp.

I wasn't aware that Shimano had specific CX front mech.  Maybe this was for older generations, because I can only find one front mech for current Ultegra:  FD-R8000 which covers the outer chainring sizes from 53 to 46 with a max 16t difference between outer and inner.

I hadn't thought about chainstay clearance, so I'm glad you brought that up. I actually have this issue on a full-suspension Ellsworth MTB where the front mech, if set up within manufacturer's specs, contacts the chainstay under full compression...not good!

I guess, depending on the frame design, a compromise might be needed in setting the front mech height, such that it is above the point where it could contact the chainstay.  In extreme cases, this might be higher than the 1-3mm clearance suggested between the outer plate of the mech and the large chainring.  
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Schadenfreude

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Reply with quote  #6 
My Cannondale Slate 105 came factory fitted with a Shimano CX-70 front derailleur - officially for 10 speed but the Slate is 11 speed.
Also, the 105 comes with 52/36 front rings out of the box and the Cx-70 worked fine with those...
I've since changed to 46/30 up front and still working.

As a middle step between getting a new bike for her why not try doublifying her triple...
Was a thing few years ago before Compact Plus chainset became more available:
Replace the 39 with a TA Specialities Alize 130mm 46 tooth middle and the 50 outer with a BBG bash ring.
She'll have 30-32 bail out climbing gear - just need to wind down the top screw on the front derailleur so it doesn't shift up to the removed large chain ring.
The chain line is a little off but if anything it will favour the lower gears
At least she could feel how the set up is before splurging on a new bike.
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drwelby

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Reply with quote  #7 
I know you want to stick to Shimano cranks, but if you want 45 kph top speed at 90 rpm and a 1:1 low gear then a setup that might be worth considering is an 11-28 cassette with Sugino OX-601D cranks in 28-42. Here's what it looks like compared to a 34-46 with that 11-34.
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runrideski

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Reply with quote  #8 
Thanks for all of your suggestions.  They helped reassure me that my ideas were sound.
We pulled the trigger today, ordering new a Kind Human Kensuke (a local Toronto brand, I posted separately about it under the 'bikes' heading) for each of us.  
We settled on the 46-36 chainsets with 11-34 cassettes.  We'll replace the 36 inner chainring with a 34 if the need for a (slightly) easier gear arises.

drwelby, thanks so much for that link to the gear calculator.  I love how well it helps you visualize the different gear ratios!


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widerisfaster

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Reply with quote  #9 
Thanks so much for sharing this gearing tool. It really helped me visualise where I spend time riding on road and off road, and I'll be buying different cassettes for my new build as a result!
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