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Gear ratios

Stick8267Stick8267 Posts: 154
As I understand it you calculate a gear ratio by dividing the teeth on the front cog by the teeth on the rear cog. This means that with, for example, a compact you can ride the same ratio using the 50 or 34 tooth by selecting the appropriate rear wheel gear.

My question is for any given ratio is there an advantage in using the 34 tooth ring as the point of force application to the chain is further from the pedal? This means there should be a lever advantage compared to using the 50 ring.

So is this right? Is the effect significant? Am I spending too much time thinking about this sort of thing? Should I have concentrated more in physics?

Posts

  • boggybrnboggybrn Posts: 29
    Hmm... where to start!

    It makes almost no difference provided you keep the same ratio.

    I say almost because the chain is less efficient when running on a really small rear sprocket - so it is actually better to use a bigger chainring at the front as you lose less energy in the chain.
  • TommyEssTommyEss Posts: 1,855
    Your efficiency for a given ratio comes from the straighter of the two chain lines. Smaller sprockets wear quicker than larger, which would be a sign of mechanical wear (i.e. less efficient)

    But you probably are overthinking it a little - it depends what kind of terrain you're in - if it's hilly and you're likely to be heading up hill soon after, then the smaller ring makes more sense, as your next gear change will be a couple of sprockets.
    Cannondale Synapse 105, Giant Defy 3, Giant Omnium, Giant Trance X2, EMC R1.0, Ridgeback Platinum, On One Il Pompino...
  • sub55sub55 Posts: 1,025
    If its transmission efficency you want , big chain ring, big sprocket , bigger the better with a straight chain line .
    This is why lots of testers out there ride with a 55 or 56 t chain ring. Not because they can push massive gears all day long. In a same vain , the bigger the chain ring , the smaller the percentage difference , one tooth on the sprocket makes. So its easier to maintain cadence.
    constantly reavalueating the situation and altering the perceived parameters accordingly
  • John.TJohn.T Posts: 3,698
    My question is for any given ratio is there an advantage in using the 34 tooth ring as the point of force application to the chain is further from the pedal? This means there should be a lever advantage compared to using the 50 ring.
    You then lose this advantage by driving a smaller sprocket to turn the same wheel. Using the small ring for the same ratio increases chain tension so may increase friction and wear. Using the bigger ring usually feels better unless the chain line is way out.
  • Stick8267 wrote:
    Is the effect significant?
    No.

    But you generally won't get exactly the same ratio from the two chain rings.

    The only difference may be in drive train efficiency losses when you consider large ring & large cog v small ring & small cog (which could be a percentage point or two of power). There is conflicting evidence about which is better combination from a drive train efficiency point of view, as well as being complicated by the amount of cross chaining going on.
  • cyco2cyco2 Posts: 593
    If you use a small ring at the front then you have to use a small cog at the back which means that the small cog is getting a lot of use and will wear out quickly. Which does mean that when you want some speed the little cog is not up to it. It's always a good thing not to settle in to the same gear ratio all the time. What does make a difference to how a gear ratio feels on the legs is the length of the cranks. Not where the chain is in relation to the cranks. Having said that a Physics Degree would help to work out the difference in chain/tooth resistance, which I guess is not a lot.
    ...................................................................................................

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  • smidsysmidsy Posts: 5,273
    Stick8267 wrote:
    My question is for any given ratio is there an advantage in using the 34 tooth ring as the point of force application to the chain is further from the pedal? This means there should be a lever advantage compared to using the 50 ring.

    The pedal is always the same distance from the centre of the BB (as the crankarm is a fixed length) which is where the rotation is so it will have no effect.
    Yellow is the new Black.
  • simon_esimon_e Posts: 1,676
    sub55 wrote:
    the bigger the chain ring , the smaller the percentage difference , one tooth on the sprocket makes. So its easier to maintain cadence.
    I'm not sure that's right.

    I thought that the percentage change between two cassette sprockets is constant regardless of the number of teeth on the chainring; what changes with chainring size is the overall gearing, the distance travelled by one rotation (and therefore velocity at a given pedalling cadence).

    Try http://www.sheldonbrown.com/gears/
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  • bristolpetebristolpete Posts: 2,255
    Ah, but what about crank length ?

    Throw that into the mix.

    Should a bike rider running two bikes have

    170mm arms with 53/39

    and a another bike with

    165mm arms with 53/39

    Should they run a smaller front chain ring to compensate for the loss of torque over the top of the pedal stroke ?

    Or again, is it all about the cassette ? Assuming that both are the same.
  • John.TJohn.T Posts: 3,698
    Simon E wrote:
    sub55 wrote:
    the bigger the chain ring , the smaller the percentage difference , one tooth on the sprocket makes. So its easier to maintain cadence.
    I'm not sure that's right.

    I thought that the percentage change between two cassette sprockets is constant regardless of the number of teeth on the chainring; what changes with chainring size is the overall gearing, the distance travelled by one rotation (and therefore velocity at a given pedalling cadence).

    Try http://www.sheldonbrown.com/gears/
    Sub55 is quite right. Using a bigger ring means you also use a bigger sprocket for the same gear ratio so one tooth difference in the sprocket size is a smaller percentage thus the ratios are closer.
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