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  1. #1
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    cx gearing - maximal range with low-end bias?

    I'm contemplating a new cx bike that I can use for gravel grinders and on the rare road rides. However, I am struggling to understand how to think about gearing, given my only background is with 1x MTB setups.

    On my MTBs (all equipped with power meters), I use the SRAM Eagle with a 30T chainring. On my 29er HT on a recent 55 mile gravel grinder that had 5.5k of climbing, I found even the 30T/50T gearing barely able to keep me at my target < 250 watts on steeper sections of climbs unless I let my cadence drop down into the low-60s. In that same race I would very quickly spinout the 30T/11T in the descents. This made me realize why in this race 2/3rds of the riders were on cx bikes....however, I did notice some of them dismounting on those steeper climbs, especially on the later hills.

    Because I would also intend to use the cx bike for the occasional road training rides, I'm thinking a 2x setup would be optimal. However, the 2x setups seem to be geared pretty aggressively...34/50 with 11-36 against my Eagle setup. I.e. at the low end for climbing a 34/36 is ~0.94 vs my 30/50 at ~0.60 ratio. Would the smaller wheel circumference or a cx bike lessen this vs the 29 x 2.2” I am used to?

    I open to any combination of Shimano or SRAM drivetrain, and don't mind a frankenbike build. I.e. I'm happy to pair any crank with any rear cassette with whichever derailleurs would allow me the greatest range. I.e. Easton Cranks, with XTR Di2 rear derailleur, a e*thirteen cassette, and whatever front derailleur…..
    What’s the art of the possible for maximal range in gearing, with modest bias to low-end for climbing? Why might I not want to do something like this?

  2. #2
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    Quote Originally Posted by L84Beer View Post
    I.e. at the low end for climbing a 34/36 is ~0.94 vs my 30/50 at ~0.60 ratio. Would the smaller wheel circumference or a cx bike lessen this vs the 29 x 2.2” I am used to?
    34/50 gives a pretty wide range. Yes, the smaller diameter wheels effectively lower the gear ratio. A cross bike is lighter than a MTB, wheels and tires are lighter. You can run a larger ratio on paper without experiencing a big difference out on the trail. (also using 175mm cranks will lower your ratio compared to 170 or 172.5)

    Cyclocross bikes (especially race oriented bikes) traditionally run 36/46t chainrings. Gravel bikes and some all-round cross bikes widen that range to 34/50. That should easily cover what your asking it to do. Low enough to climb comfortably, and high enough to not be spun out.
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    Quote Originally Posted by L84Beer View Post
    On my 29er HT on a recent 55 mile gravel grinder that had 5.5k of climbing, I found even the 30T/50T gearing barely able to keep me at my target < 250 watts on steeper sections of climbs unless I let my cadence drop down into the low-60s.
    Are you implying a 30/50 gear ratio was not easy enough?

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    Quote Originally Posted by wpccrunner View Post
    Are you implying a 30/50 gear ratio was not easy enough?
    He was trying to stay around 250w of work. That gearing was pushing him higher than 250 so he lowered his cadence.

    Could be an easy gear but he was working too hard.

    I wonder what kinda power you would see on these climbs on a gravel bike. I notice my power (work) is lower on my gravel rig.


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  5. #5
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    Quote Originally Posted by jkidd_39 View Post
    He was trying to stay around 250w of work. That gearing was pushing him higher than 250 so he lowered his cadence.

    Could be an easy gear but he was working too hard.

    I wonder what kinda power you would see on these climbs on a gravel bike. I notice my power (work) is lower on my gravel rig.


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    Correct. The 50/30 is a very easy gearing, but staying under 250 watts was critical. My FTP is 275, and I was targeting a NP for the 4.25 hour 55 mile ride of ~209w or ~75%. If I didn't weigh 199 pounds, I probably could push higher ratio....

    My weight is the biggest hurdle with the wattage I was targeting. As I think about it, I don't think there are material enough advantages of a cx bike on a climb vs a light weight MTB on steeper climbs:
    1) wind resistance on a climb would be a non-factor in most circumstances,
    2) rolling resistance of a 29 x 2.1 wheel is probably not large enough vs a 700x34c, and maybe even an advantage?
    3) wheel circumference is a slight advantage on the cx vs MTB for the above tires, but fairly small
    3) 21.1 lb HT MTB vs a ~17 lb CX bike is also probably not a large enough significance

    Maybe 2, 3 & 4 combined account for something, but I can't imagine it bridges a 0.6 to a 1.0 gearing ratio....maybe 0.6 to 0.7? I really don't know.

    I'm thinking the answer is a 2x11 set up with a 11-42, a medium length XTR di2 cage, and playing with front rings that work with that rear derailleur cassette combo. Will a XTR di2 work with the road side di2 shifters -- it seems like it should, but I would want to confirm before I start building and would want to know what experience has been if anybody else has dried this...?

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    Quote Originally Posted by L84Beer View Post
    Correct. The 50/30 is a very easy gearing, but staying under 250 watts was critical. My FTP is 275, and I was targeting a NP for the 4.25 hour 55 mile ride of ~209w or ~75%. If I didn't weigh 199 pounds, I probably could push higher ratio....

    My weight is the biggest hurdle with the wattage I was targeting. As I think about it, I don't think there are material enough advantages of a cx bike on a climb vs a light weight MTB on steeper climbs:
    1) wind resistance on a climb would be a non-factor in most circumstances,
    2) rolling resistance of a 29 x 2.1 wheel is probably not large enough vs a 700x34c, and maybe even an advantage?
    3) wheel circumference is a slight advantage on the cx vs MTB for the above tires, but fairly small
    3) 21.1 lb HT MTB vs a ~17 lb CX bike is also probably not a large enough significance

    Maybe 2, 3 & 4 combined account for something, but I can't imagine it bridges a 0.6 to a 1.0 gearing ratio....maybe 0.6 to 0.7? I really don't know.

    I'm thinking the answer is a 2x11 set up with a 11-42, a medium length XTR di2 cage, and playing with front rings that work with that rear derailleur cassette combo. Will a XTR di2 work with the road side di2 shifters -- it seems like it should, but I would want to confirm before I start building and would want to know what experience has been if anybody else has dried this...?
    I wonder if your gonna have to come to terms with the fact that you won't be able to work that low on climbs?

    I am a terrible example as I'm a power masher but I wouldn't think I'd carry that low of wattage on a nasty climb like you have described.

    I'll have to go out and do some climbs and see where my wattage sits on my gravel bike. I am running 40-36 gearing.


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    Quote Originally Posted by L84Beer View Post
    As I think about it, I don't think there are material enough advantages of a cx bike on a climb vs a light weight MTB on steeper climbs:
    1) wind resistance on a climb would be a non-factor in most circumstances,
    2) rolling resistance of a 29 x 2.1 wheel is probably not large enough vs a 700x34c, and maybe even an advantage?
    3) wheel circumference is a slight advantage on the cx vs MTB for the above tires, but fairly small
    3) 21.1 lb HT MTB vs a ~17 lb CX bike is also probably not a large enough significance

    Maybe 2, 3 & 4 combined account for something, but I can't imagine it bridges a 0.6 to a 1.0 gearing ratio....maybe 0.6 to 0.7? I really don't know.
    Check out this calculator- Bike Calculator

    It's not perfect but it's fun to play around with and shows that weight isn't much of a factor until the climb gets really steep and/or long. It can't accurately account for rolling resistance but I would guess that a wider mtb tire usually more efficient off road.
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    You are giving it lots of thoughts about this!!! Lots of Analysis about gearing and the power output.

    But you forgot about rotation weight of the CX wheels and it will probably lower than your mountain bike's wheels. You may not need a really low gear to get you up a steep hill. The chainstay of the cross bike will probably be 1cm shorter than your hardtain. It may not sound like much, but it will make the differe on how much power will transfer from your pedals to the rear wheel. Every gram and mm will be crtical on how a bike will preform over all.


    When I used to race CX with gears, my gearing was 46/36 and 12-28 at the back while my mountain bike's gearing was 30 with 12-36. I think you are overthinking about the gearing. I am not going to stop you in using 50/30 with 11-42 but I think it may be overkill for what you are planning to do. There are times, you will quicker if you dismount and walk the the bike than spinning in a lower gear.

    If you haven't, go test ride or rent or borrow a show room stock cross/endurance/road/gravel grinder and try out the gearing and the different bike geometry.
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  9. #9
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    For me what i have done to determine gear inch values is find a crank/cassette/tire size combo that produces a 70 gear inch (GI) which is my middle of the road/flat surface gear. Shooting for low gears i try to get into the mid 20s GI so i have a bailout/climbing gear.

    Using this methodology my first experiment with CX/off road gearing was using a converted ultegra triple crank using bash/46/30 and a 11-32 cassette. On my recently built pinnacle Arkose im using a FSA 48/32 crank, 10 spd 5700 shifters, 9 spd xt rd and a 11/36 xt cassette. In both examples im using 700c 35c to 1.8" tires and 40c the most.

  10. #10
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    I'm running a CX bike with SRAM 1x.

    40t ring with a long-cage rear mech. This allows you to simply swap cassettes as the ride dictates.

    Gravel/Climbing mode = 10-40 cassette (XTR to keep the weight reasonable)
    Cross mode = 10-32 cassette

    Given the LC Rear Mech, you could even go to a 10-42 cassette without going to Eagle.
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  11. #11
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    10-32 cassette? Which one?

  12. #12
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    Quote Originally Posted by global View Post
    10-32 cassette? Which one?
    Pretty sure it's a SRAM PG1130 or 1170... it was a take-off from my wifes full SRAM Force 1x Cross bike.

    The bigger cassettes get very heavy fast, so you have to spend a few bucks. I found the XTR to be the best weight/cost. Seems to work fine with my SRAM drivetrain.
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