Gear Ratios

Road Red
Road Red Posts: 232
edited December 2009 in Road buying advice
Is there a calculation or table for working out gear ratios?

I'm trying to see if there would be much difference between a triple with a 30 12-27 or a compact with a 34 12-28 set up.

Yes, I know the real answer is to go and test them, which I will!

Comments

  • morrisje
    morrisje Posts: 507
    This seems to be the popular calculator that is usually quoted

    http://www.sheldonbrown.com/gears/
  • JamesB
    JamesB Posts: 1,184
    Chainring size / sprocket size x 27 is the formula I believe is used for 700c wheels (even though it does not give the fully correct result). You can set up an Excel table with the formula in it and play to your hearts content on adjusting chainrings / sprockets :D
  • dennisn
    dennisn Posts: 10,601
    The 30-27 is the LOWER gear. If you used a 34-28, with a standard 700 tire, each full revolution of the crank arm would move the bike about 8 inches FURTHER down the road
    than would a single revolution of a crank with 30-27 gearing, thereby making hill climbing
    in the 30-27 a bit easier(which really isn't the right word) by letting you spin a little more
    while traveling at comparable speeds. I say easier isn't the right word because no matter what gear you're in it takes the same amount of work to get to the top of any one hill,
    whether you're spinning at a high RPM or grinding away at a lower one.
  • gsk82
    gsk82 Posts: 3,526
    i'm looking at increasing my top end ratio. atm i've got a 34/50 chainset and 12-27 cassette. using that link above i think i'd prefer changing the cassette to an 11-25 over changing the chainset to 36/52 based soely on ratios. it's also the cheaper option.

    is there anything else i need to bare in mind? effect on other components, ease of fitting etc?
    "Unfortunately these days a lot of people don’t understand the real quality of a bike" Ernesto Colnago
  • dennisn
    dennisn Posts: 10,601
    gsk82 wrote:
    i'm looking at increasing my top end ratio. atm i've got a 34/50 chainset and 12-27 cassette. using that link above i think i'd prefer changing the cassette to an 11-25 over changing the chainset to 36/52 based soely on ratios. it's also the cheaper option.

    is there anything else i need to bare in mind? effect on other components, ease of fitting etc?

    An 11 will definatley give you more top end. Question is "can you actually push a 50-11?"
    This is a huge gear that the only time you may even be able to us it is downhill. So the question becomes "why even buy an 11 rear cog?". All that will happen with an 11 tooth is that it will sit out there at the end of the cassette and never, or very rarely, get used.
  • FitzM
    FitzM Posts: 232
    dennisn wrote:
    gsk82 wrote:
    "why even buy an 11 rear cog?". All that will happen with an 11 tooth is that it will sit out there at the end of the cassette and never, or very rarely, get used.

    Surely if you want an 11 cog, you should buy one. I'm about to buy a compact and two cassettes 11-23 & 12-25.

    I rarely buy Champagne but when I do..... it sure tastes good :D
    Klein Quantum, Cervelo Soloist Team, Boardman SLR 9.0S, Boardman SLS 9.8, Kinesis Racelight 4S, DengFu FM028
  • FitzM
    FitzM Posts: 232
    dennisn wrote:
    gsk82 wrote:
    "why even buy an 11 rear cog?". All that will happen with an 11 tooth is that it will sit out there at the end of the cassette and never, or very rarely, get used.

    Surely if you want an 11 cog, you should buy one. I'm about to buy a compact and two cassettes 11-23 & 12-25.

    I rarely buy Champagne but when I do..... it sure tastes good :D
    Klein Quantum, Cervelo Soloist Team, Boardman SLR 9.0S, Boardman SLS 9.8, Kinesis Racelight 4S, DengFu FM028
  • gsk82
    gsk82 Posts: 3,526
    dennisn wrote:
    gsk82 wrote:
    i'm looking at increasing my top end ratio. atm i've got a 34/50 chainset and 12-27 cassette. using that link above i think i'd prefer changing the cassette to an 11-25 over changing the chainset to 36/52 based soely on ratios. it's also the cheaper option.

    is there anything else i need to bare in mind? effect on other components, ease of fitting etc?

    An 11 will definatley give you more top end. Question is "can you actually push a 50-11?"
    This is a huge gear that the only time you may even be able to us it is downhill. So the question becomes "why even buy an 11 rear cog?". All that will happen with an 11 tooth is that it will sit out there at the end of the cassette and never, or very rarely, get used.

    when riding in a group at speed or on slight decents on TTs i run out of gears/ leg speed, so definately gonna need a bit more
    "Unfortunately these days a lot of people don’t understand the real quality of a bike" Ernesto Colnago
  • dennisn
    dennisn Posts: 10,601
    FitzM wrote:
    dennisn wrote:
    gsk82 wrote:
    "why even buy an 11 rear cog?". All that will happen with an 11 tooth is that it will sit out there at the end of the cassette and never, or very rarely, get used.

    Surely if you want an 11 cog, you should buy one. I'm about to buy a compact and two cassettes 11-23 & 12-25.

    I rarely buy Champagne but when I do..... it sure tastes good :D

    Buy anything you wish, but buying an 11 and actually using it are two different things.
    There have been more than a few people on this website who claim to be able to PUSH
    a 50-11 or NEED a 50-11 or can SPIN a 50-11. I say, bullsh*t. Sorry, but that's how I see it.
    Then again you're allowed to buy anything you want, but buying 11 and then using it, for anything other than the occasional downhill, is not something most riders are capable of.
  • Going from previous experience on a 50 -12 downhill, that was way too fast for me :shock:

    There's also the factor of cadence to remember. If (for example) you're able to pedal at 80rpm on a 50-11, you might actually be going quicker on 100 rpm on a 50-12. Anyone know a calculator for that?
    CAAD9
    Kona Jake the Snake
    Merlin Malt 4
  • you'll be faster on 50-12 @100rpm

    80-100 increase of +25%
    12-11 is about 16% (rough mental maths)
  • you'll be faster on 50-12 @100rpm

    80-100 increase of +25%
    12-11 is about 16% (rough mental maths)

    There you go, proof you don't need an 11, just spin them legs faster :wink:
    CAAD9
    Kona Jake the Snake
    Merlin Malt 4
  • JamesB
    JamesB Posts: 1,184
    Yes re calculator :):) ----your speed in mph can be calculated from (Chainring / sprocket x 27 (this gives gear ratio in inches)) x 3.14159256 (ie pi, this gives distance travelled per turn of pedals in given gear)) x cadence (rpm) x 60 (to convert minutes to hours) / 63 360 (to convert speed in inches per hour to mph)---this can also be approximately represented as your gear ratio (chainring / sprocket x 27) x rpm x 0.003

    So pedalling a 50 x 12 at 100 rpm and you`re doing ~ 34 mph :D