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Double chainring ratio advice

Dick ScruttockDick Scruttock Posts: 2,533
edited November 2010 in MTB buying advice
If i were to run a double chainring setup up front what would people advise to go for ratio wise? I am running a 11-34 cassette and a regular triple 42-32-22 chainset.

Would 40-24 be to big a jump? Other options are 40-26, 38-26 and 38-24.

Only sticking some rings on to see how i feel about using a double instead of a triple before biting the bullet for a 2x10 groupset or a 3x10.


  • Buckled_RimsBuckled_Rims Posts: 1,648
    It's not really for other people to advise what gear ratios are best, it's for you to decide. It's comes down to your bike, your ability, fitness, leg strength, etc.... You are an individual and nobody is exactly like you.

    The way to do it is to keep a mark of what gears you use on the tracks. Then convert then to gear inches or ratios. Once you know the range that will be 98% acceptable you can then calculate which of the big rings you'll need. Basically, the fitter and less weighty you are, the more double become more advantageous.

    Quick calcs shows:

    42-32-22 gear inches on a 11 to 34 cassette (9sp) on a 2.1" tyre:

    99.0 75.5 51.9
    83.8 63.9 43.9
    72.6 55.3 38.0
    64.1 48.8 33.6
    54.5 41.5 28.5
    47.4 36.1 24.8
    41.9 31.9 21.9
    36.3 27.7 19.0
    32.0 24.4 16.8

    Double with 40-24

    94.3 56.6
    79.8 47.9
    69.2 41.5
    61.0 36.6
    51.9 31.1
    45.1 27.1
    39.9 23.9
    34.6 20.8
    30.5 18.3

    Shows you will only lose 2 gears, the highest and the lowest. But I doubt you'd feel the lowest gear ratio as it's only an inch and a half difference.
    Kona Jake the Snake
    Merlin Malt 4
  • The big issue is what ratios can i run up front without having shifting problems.

    I think the jump from 26-40 might be to big so therefore 24-40 is just not happening.
  • nicklousenicklouse Posts: 81,520 Lives Here
    are you looking at a new 2 ring set up or modding an existing 3 ring set up?
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • Modding a triple to run a double nick.
  • stu8975stu8975 Posts: 1,334
    40/26 will be for me...not sure if the bigger tooth jump of a 40/24 will cause front mech issues. You need to make sure you have the chainstay clearance when fitting a bigger middle ring as well.
  • How many of the gears on your 3x9 set up do you actually use ? How often do you use the big ring ? mainly on roads / fire roads ?
    As folk say it all depends on what kinda riding you do and level of fitness, but i run a 11-34 with a 24/36 upfront, mainly ride trail centers and in the lakes, don't think i've came across anything that i couldn't ride due to lack of gears, most folk i ride with run simialr as well.
    on the 24t ring i only tend to use the top 2 or 3 sprockets on the cassette, but use pretty much all of them on the 36t ring.
  • andyrmandyrm Posts: 550
    I run 26/36 on an 11-34 cassette, seems to work about perfectly for me as I am quite a spinny pedaller and never tended to make much use of the big ring. If you use your existing granny quite a bit at the moment it may be worth looking at a 24tooth perhaps? Horses for courses though, hence SRAM offering 2 choices as standard on their doubles.
  • Cheers Stu, after i posted this i remembered of a build that had converted to a double and it was your anthem so i checked the ratios you were running and promptly ordered the same.
  • njee20njee20 Posts: 9,613
    I'd say 24/40 would be horrible, you'll need to dump about 5 gears on the back at the same time as you shift. I always got on with 28/40 on a 9 speed set up, sure 26/40 will be fine, as Stu's found!
  • rhyko7rhyko7 Posts: 781
    njee20 wrote:
    I'd say 24/40 would be horrible, you'll need to dump about 5 gears on the back at the same time as you shift. I always got on with 28/40 on a 9 speed set up, sure 26/40 will be fine, as Stu's found!

    he has got a point, i ride 22/36 double with bash and you have to time your gear changes well to keep up when others shift hears. its quite a jump but you get used to it-you only have to dump about 2 gears on the back usually, or wait until your really grinding before you flick down.
    im going for 24/36 next time round.

    just a point-the diameter of the chainwheel is more important than the number of teeth when you work out the ratios. its about the ratios bewteen the diameters not the number of teeth that is what you need to think about

    do some rides and work out what gears you need.
    Dont look at it-ride it! they are tools not f*cking ornaments

    my riding:

    Some of my Rides Data/maps:
  • mooromooro Posts: 419
    quite keen on doing this aswell, i am mainly a cross country rider with a light bike and ride quite a bit of road so can climb a bit anyway.

    i have just bought a sram x-0 rear derailleur and some grip shifters (9 speed). my question is do i buy sram 2x10 front mech and then lock out the granny ring on my grip shift?

    Anybody see any problems to this working?

  • If you bought a specific bouble and bash to use would you be able to use all the gears unlike on a 3 ring setup and in the biggest front sprochet you can't use the biggest in the rear and vice versa?
  • njee20njee20 Posts: 9,613
    Mooro if you use a 2x10 SRAM chainset that'll work fine, although you use positions 1 and 2, rather than 2 and 3.

    Getonyourbike, yes if you run a double then the chainline is much better and you'll be able to use the whole cassette in both rings more effectively.
  • Just fitted the double conversion up front and should be testing it out later.

    I guess with a true 2x10 chainset the chainline is optimized further allowing use of granny and the bottom few rings on the rear cassette?
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