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Gearing Sizes?

crossedcrossed Posts: 213
edited May 2017 in Road general
I'm trying to get my head around this but can't figure out how it works.

If you take two bikes with a fixed gear, one running 42/14 and the other running 48/16 they both give the same gain ratio of 6.1 or 81 gear inches.
How can two gears of the same size have different characteristics, for example people say that one accelerates quicker and the other is easier to hold your cadence with?

Leading on from this is my question...
A roadbike running a compact chainset with an 11/32 cassette is 28.8 gear inches and a 1x11 system running a 44 up front and 42 at the back is 28.3 gear inches. Very similar ratios but would there be a noticeable difference in these gears when riding on a climb?

Posts

  • Simon MastersonSimon Masterson Posts: 2,740
    Two fixed gear transmissions will be the same. On geared bikes, the chainline varies. Is that what you are asking?
  • imposter2.0imposter2.0 Posts: 11,003
    There is some minor mechanical advantage in running big/big combos - as opposed to small/small - but other than that it's the same roll-out, so for the same cadence you will travel at the same speed regardless.
  • crossedcrossed Posts: 213
    I'm not sure I explained what I meant properly.

    I was reading something on the Velodrome Shop website which baffled me.
    An example of using the same/similar gear but with different Chainring/Sprocket combinations is 50x15 (90.0) and 47x14 (90.7), the 50x15 would suit Time Trial/Pursuit riders more due to the larger chainring, that is slower to accelerate but easier to maintain your average cadence (pedalling rpm), whereas your cadence will be obtained quicker with the smaller 47x14 chainring setup as the smaller chainring enables a much quicker acceleration hence this technique is better for short distance/sprinters. Further examples are 45x13 (93.5) instead of 49x14 (94.5), even though the 45x13 is smaller the benefits of using a similar gear with a smaller chainring are hugely advantageous for acceleration.

    And I couldn't understand how two gear with the same ratio could have different characteristics to make them easier to accelerate or to maintain cadence.

    With the example for 2x11 or 1x11 I can undersatand the slight effect of chainline but the fixed gear example above confuses me.
  • I might be wrong on this but I think the theory is the larger chainring means the chain has 'further' to travel at certain points of the pedal stroke (It doesn't actually travel any further of the two I guess it just travels less evenly than a chainring and sprocket combo that are more closely matched ie the 47x14 in the example).

    Comparing two similar size gears I use a lot for climbing, 52x23 and 36x16 I would say there is slightly different feel but neither is a harder gear, I can comfortably spin both when seated (depending on gradient of course!).

    Like I said I think that is the theory but happy to be proved wrong.
  • Forgot to add with regards acceleration I think they are therefore saying a larger chainring needs greater force at certain parts of the pedal stroke, meaning acceleration is harder than a smaller chainring.
  • wongataawongataa Posts: 884
    I think Velodrome Shop are talking rubbish. Gear ratio matters, not how you achieve that ratio. If you ever do calculations that involve gear rations (gearbox calcs/belt drive calcs) you input the ratio and you don't worry about the number of teeth on the gears when deciding what ratio to use. The number of teeth is only relevant when making sure you pick gears that are strong enough to take whatever load you are needing to transmit through the system.
  • lesfirthlesfirth Posts: 1,077
    I would like to know the theory behind the Velodrome Shop statement. It certainly sound like nonsense to me.
    The advantage of big/big over small/small versions of the same gear ratio are a bit complex but basically on smaller sprockets each chain link has to flex through a bigger angle .This results in higher power loss in the transmission. It might be a marginal gain as far as team GB as concerned but I don't think I need to think too much about it.
  • rower63rower63 Posts: 1,991
    Velodrome Shop stuff is total nonsense, but I can see where they've got it.

    If you're rotationally accelerating a ring about its axis then sure it takes more energy to accelerate a "heavy" ring than a "less heavy" ring*. Think of trying to spin up a bicycle tyre with your fingers vs trying to spin up a steamroller cylinder (assuming smooth bearings).

    But "heavy" in this context for it to make an appreciable difference you would need a ring, IMO, weighing at least many kilograms, maybe tens of kilos. Velodrome Shop are applying this thinking to comparisons of rings that weigh tens of GRAMMES, and any difference between the energy needed to accelerate a 48-tooth aluminium ring compared to a 44-tooth one is utterly negligible.

    * I'm substituting "heavy" here for the more strictly correct "moment of inertia"
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  • craigus89craigus89 Posts: 885
    Comparing two similar size gears I use a lot for climbing, 52x23 and 36x16 I would say there is slightly different feel but neither is a harder gear, I can comfortably spin both when seated (depending on gradient of course!).

    What a beast.
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