Gearing efficiency question

billysan
billysan Posts: 575
edited April 2010 in Road buying advice
Assuming on a road bike you can acheive exactly the same gear ratio on both the big ring and the small ring, which is the most efficient gear to use? And why?

My reason for asking is that with my current gear setup (39/52 - 11-23 9 speed) I can get a 85.4 inch gear in two positions. I waswondering which was the best to use.

For the purposes of this please ignore chainline related issues, I know you shouldnt make the chain sit on the smallest gears at both ends simultaniously!

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Comments

  • sungod
    sungod Posts: 17,505
    afaik the big ring is more efficient mechanically

    but which is more efficient for a human to *pedal* depends on speed/cadence

    if you have to mash the pedals on the big ring, but can spin more easily on the smaller one, then the smaller one will be more efficient - in that you'll get further before your legs give up

    if you can spin the big one ok but can't keep up on the small ring then the situation is reversed
    my bike - faster than god's and twice as shiny
  • dennisn
    dennisn Posts: 10,601
    I think that as far as the math goes it doesn't make any difference what gear you're in.
    To go any given distance in a given gear takes a certain amount of energy. To go the same distance in another gear(either bigger or smaller) takes the same amount of energy as the 1st. one does. This does not take into account any variables(wind, etc.). Same bike, same rider, same distance, same route, will, in theory, require the same amount of total work for the rider, irregardless of gearing and irregardless of time taken.
    However, cadences of 85-95 are generally accepted to make the task SEEM easier for experienced riders.
  • If you are getting the same gear inches then it's the same gear and neither is any more or less efficient surely?
  • dennisn
    dennisn Posts: 10,601
    If you are getting the same gear inches then it's the same gear and neither is any more or less efficient surely?

    Sorry, I sort of missed the gist of his question. Yes, the same gear inches should make no difference.
  • If you get the same ratio, then you will pedal with the same cadence in both, achieve the same speed for a given power output etc. I don't think therefore that it has any effect on your pedalling efficiency whatsoever, but if the chain line is not ideal (big to big, or small to small) , or the deraileur's not set up properly then you could generate more or less friction in the drivetrain. I would guess most of us ride the same ratio in either front chainsets, it all depends on where the road goes next - uphill or downhill and whether you expect to change up or down.
  • Technically the most efficient would be using the two gears that have the least amount of inertia. In practice the difference is likely to be minimal.

    I would say you should base your gearing on what you are likely to do next. E.g. if you know you will be encountering a climb soon, use the small ring combo. If you are going up and down a few gears on rolling terrain, stay in the larger ring. If you ride a compact, the larger ring is more favourable also. But this depends on where you two equivalent gearings "meet".
  • sungod
    sungod Posts: 17,505
    If you get the same ratio, then you will pedal with the same cadence in both....

    oops, yes i completely misinterpreted the question!
    my bike - faster than god's and twice as shiny
  • billysan
    billysan Posts: 575
    Cheers for the comments thus far.

    I was wondering because of a conversation I was having about this the other day you see. A friend was argueing that the use of the smaller chainring would ALWAYS be more efficient IF the chainline was not considdered.

    He reasoned this because for one rotation of the crank, there is less chain pulled round. Therefore there is less friction within the chain due to each link bending and straightening again as they go round the gears.

    Is there any substance to this do you think?

    For my bike the big ring would be the gear of choice due to the better chainline, which probably has more of an effect.
  • billysan
    billysan Posts: 575
    Or look at my question a difference way;

    Track bike, so no chainline issues.

    2 gear options 45/13 or 52/15.

    Very similar gears, but which is more efficient (forget the rider, Im just talking mechanical efficiency)
  • neeb
    neeb Posts: 4,473
    In theory, there may be a slight difference. But it's incredibly technical and by no means clear cut:

    http://djconnel.blogspot.com/2010/01/dr ... model.html

    In practice, if you are in the same absolute gear it will make no detectable difference.. As navier_stokes implied, you won't be selecting one particular gear on the big ring or the small ring just for that reason, you will be on the big ring or on the small ring depending on which provides the best range of gears for the situation you are in. Changing chainring is always more hassle than changing a cog at the back, because the shifting is more clunky and it puts you in a very different gear, probably requiring compensatory cog changes as well.
  • dennisn
    dennisn Posts: 10,601
    neeb wrote:
    In theory, there may be a slight difference. But it's incredibly technical and by no means clear cut:

    http://djconnel.blogspot.com/2010/01/dr ... model.html

    In practice, if you are in the same absolute gear it will make no detectable difference..

    +1...... I think it's a little over the top to expect to NOTICE anything different and would bet money that no one could tell the difference blindfolded. :wink::wink:
  • rake
    rake Posts: 3,204
    the best option is the big ring. it puts less tension in the chain for a given ratio, its kinder to the chain and sprockets because less force os applied. probably less friction loss as well. you wont notice any power increase in practise but whats kindest to the bike wins for me.
  • Escargot
    Escargot Posts: 361
    :lol: Neglecting the chain line both gears result in identical ratios so neither will be more/less efficient.

    Any gain from one chainwheel will be negated by the corresponding cog so the torque required to propell you forwards will be the same.

    You can do the calcs yourself to satisfy your mind if you know the diameters of each gear.

    Moment, M = Force, F, x distance, d, so just draw a diagram and work out the moments and forces generated by the force application at the pedals.
  • rake
    rake Posts: 3,204
    yes but the big ring puts less tension in the chain so is kinder to the drivetrain regarding wear. he knows the torque and power will be the same as he stated equal gear inches. use your maths escargot and calculate the tension in the chain which also puts load on the rear hub bearings.
  • rake wrote:
    yes but the big ring puts less tension in the chain so is kinder to the drivetrain regarding wear. he knows the torque and power will be the same as he stated equal gear inches. use your maths escargot and calculate the tension in the chain which also puts load on the rear hub bearings.

    How on earth can that be? Same ratio at same cadence = same speed = same force applied through the chain, assuming same efficiency (which we all agree is most affected by chain line, and this is probably too small to be noticeble when riding).
  • Escargot
    Escargot Posts: 361
    rake wrote:
    the best option is the big ring. it puts less tension in the chain for a given ratio, its kinder to the chain and sprockets because less force os applied. probably less friction loss as well. you wont notice any power increase in practise but whats kindest to the bike wins for me.

    I'd agree with that

    In addition you would distribute the force on the chain over more of the chainwheel/cog too, thus stressing the chain less.
  • Escargot wrote:

    I'd agree with that

    In addition you would distribute the force on the chain over more of the chainwheel/cog too, thus stressing the chain less.
    That bit makes sense to me.
  • rake
    rake Posts: 3,204

    How on earth can that be? Same ratio at same cadence = same speed = same force applied through the chain, assuming same efficiency (which we all agree is most affected by chain line, and this is probably too small to be noticeble when riding).

    its because the sprocket sizes are different. the end result is the same but the bigger ring puts less tension on the drive chain because its pulling at a bigger rear sprocket, which requires the chain to have less tension but moving faster. power= force x speed.
    so lower speed chain x big force= higher speed chain x smaller force.
  • Escargot
    Escargot Posts: 361

    How on earth can that be? Same ratio at same cadence = same speed = same force applied through the chain, assuming same efficiency (which we all agree is most affected by chain line, and this is probably too small to be noticeble when riding).

    Do the math mate. Draw two circles, one larger than the other (representing each chainwheel). Then draw the crank and a tangential force at the pedal.

    Next use the equation; Moment, M = Force, F x distance, d.

    What you should see is that a force at the pedal will result in a moment at the bottom bracket.

    As M = Fd then F = M/d. Thus the larger chainwheel will result in a lower force as the distance i.e. the radius, is larger.

    Hope this helps
  • rake wrote:

    its because the sprocket sizes are different. the end result is the same but the bigger ring puts less tension on the drive chain because its pulling at a bigger rear sprocket, which requires the chain to have less tension but moving faster. power= force x speed.
    so lower speed chain x big force= higher speed chain x smaller force.

    If the ratio is the same, then the speed of the chain is the same!
    Your cadence does not change -> the speed of the wheel does not change -> therefore the chain speed is the same in both examples, it cannot possibly be different! a gear ratio of 84" means just that no matter what the combinations are.
  • Escargot
    Escargot Posts: 361
    Steve, whilst rake's explanation is quite suspect (just pulling your pecker rake :wink: ) I think you're making a simple mistake of thinking that the forces along the drive train should be equal.

    Remember it is the torque that is constant here but it is the forces that vary due to the different cog sizes.
  • rake
    rake Posts: 3,204
    to quote steeve 100. thats not true. every time a 50 tooth chainring rotates once 50 teeth have past a given point.
    so for each rpm 50 teeth have gone round, but at the same rpm(cadence) on the small ring only 34 teeth have gone round. each tooth is 0.5 inches so 50 teeth is 25 inches gone round versus 17 inches gone round at the cicumference of the small ring. so the chain has only travelled 34 teeth on the small ring instead of 50, so its moving slower.
    one could say the 50 tooth ring put 17/25 less tension on the chain and sprockets and also less linear pull on the rear hub bearings and frame rear triangle so less flex as well.
  • keef66
    keef66 Posts: 13,123
    I have a triple so I can actually do these combinations which keeps the maths simple:
    50 front x 25 rear vs 30 front x 15 rear

    In each case for every revolution of the chainring the rear wheel goes round twice, so mechanically it's exactly the same, so presumably the same effort at the pedals for the same result.

    Ignoring the extreme cross-chaining issue, I suppose the bigger chainring + sprocket could be kinder to the chain because more of it is supported on teeth at any one time, although you could also argue that the tension is shared by fewer links between sprocket and chainwheel.

    I'm fascinated by the observation that the chain does move slower with the smaller chainring; it's counter-intuitive, but it must be the case.
  • neeb
    neeb Posts: 4,473
    to quote steeve 100. thats not true. every time a 50 tooth chainring rotates once 50 teeth have past a given point.
    so for each rpm 50 teeth have gone round, but at the same rpm(cadence) on the small ring only 34 teeth have gone round. each tooth is 0.5 inches so 50 teeth is 25 inches gone round versus 17 inches gone round at the cicumference of the small ring. so the chain has only travelled 34 teeth on the small ring instead of 50, so its moving slower.
    Yes, this is SO counter-intuitive at first but it must be right. You can visualise it from the back too - when you are using the same gear ratio on the big ring, you are using both a larger chainwheel at the front and a larger sprocket at the back, so the chain is of course moving faster at the back too (the sprocket set is doing the same rpm at the same gear ratio, just as the cranks are). Basically, on the large ring and a large sprocket, the rpm of both the cranks and the sprockets are the same as on the small chainring and a small sprocket, but the chain itself is doing more rpm.

    So it follows that the chain is under less tension on the big ring because it is moving faster... but it is claimed that the greater tension when on the small ring (or when using a compact setup vs. standard) leads to greater efficiency in power transfer. On the other hand (if I have understood the jist of the article I linked to above), this may be counteracted by the greater amount of bending of each link...

    So in terms of power transfer efficiency (as opposed to wear) there are at least two variables that may or may not cancel each other out, depending...

    And in practice it's still not doing to make a blind iota of difference!! :wink:
  • sheffsimon
    sheffsimon Posts: 1,282
    Real men ride in the big ring - always.

    End of.

    :wink:
  • neeb
    neeb Posts: 4,473
    Real men ride in the big ring - always.

    End of.
    While metrosexual types change down to the small ring to save wearing the titanium sprockets on their dura-ace & super record cassettes... :)
  • 165
    165 Posts: 35
    neeb wrote:
    In theory, there may be a slight difference. But it's incredibly technical and by no means clear cut:

    http://djconnel.blogspot.com/2010/01/dr ... model.html

    In practice, if you are in the same absolute gear it will make no detectable difference.. As navier_stokes implied, you won't be selecting one particular gear on the big ring or the small ring just for that reason, you will be on the big ring or on the small ring depending on which provides the best range of gears for the situation you are in. Changing chainring is always more hassle than changing a cog at the back, because the shifting is more clunky and it puts you in a very different gear, probably requiring compensatory cog changes as well.

    navier stokes??? who he?
  • neeb
    neeb Posts: 4,473
    navier stokes??? who he?
    One of the previous posters... but I'll grant you it does sound a bit like the sort of name the author of a seminal 19th century paper on mechanical engineering might have...
  • neeb
    neeb Posts: 4,473
    Sh*t, just googled navier stokes... he, or rather they, WERE 19th century engineers!
  • rake
    rake Posts: 3,204
    neeb wrote:
    While metrosexual types change down to the small ring to save wearing the titanium sprockets on their dura-ace & super record cassettes... :)
    thats why they only use titanium on the larger sprockets because less chain tension doesnt wear them as fast. the smaller ones are steel but the chain will give them more agro.

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