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If two riders of different build

  • rider 1 - shorter, lighter, thinner
  • rider 2 - taller, heavier, stockier

Are riding a rolling terrain. No notable climbs. A few short climbs lasting no more than a couple of minutes maximum - but otherwise a flat to undulating terrain.

Both riders having the exact same power-to-weight. And apply their power in equal measure ie. same.

  • rider 1 - 60kg - 270 watts 4.5watts/kg FTP
  • rider 2 - 80kg - 360 watts 4.5watts/kg FTP

Is rider 2 actually faster than rider 1? (because he is applying his 360watts on flattish terrain)

  • Do you think the 80kg rider has 1/3 more frontal area? – paparazzo Feb 25 '16 at 13:42
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    Good point. Can we keep it simple - and say any difference in frontal area is not significant to consider? – OraNob Feb 25 '16 at 14:57
  • But frontal area is a factor. Watts went up by 1/3. Do you think frontal area went up by less than 1/3, exactly 1/3, or more than 1/3? – paparazzo Feb 25 '16 at 15:01
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    As a rough rule of thumb, CdA scales roughly with the 2/3rd power of mass, so although the larger rider has more area, it's less than proportionally more. An additional but smaller effect is that if they ride a bike meeting UCI rules, bikes have a minimum weight (currently, 6.8 kg) so the lighter rider bears a larger percentage increase in total mass compared to body mass. – R. Chung Feb 25 '16 at 15:05
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    Here are some links to references that show the relationship between size and CdA. The short summary is as I said above: as a reasonable rule of thumb, CdA scales less than proportionally with size. So the larger rider will have the same power/kg but higher power/CdA. – R. Chung Feb 25 '16 at 16:19
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  • On flat terrain raw power is what is more important than Power to weight ratio.
  • On rolling terrain P/W is important, but the larger rider still has the advantage.
  • On mountainous terrain P/W is the determining factor.

You can see this from races in pro cycling. Flat races are won by large strong riders and sprinters. Rolling races are won by mid size riders with punchy acceleration, but not the highest FTP or P/W ratio. Only in the mountains do we see the riders with the best P/W ratios winning.

So yes, in this case rider 2 will be significantly stronger, as he can match the lighter rider on the climbs and has a huge 90W extra on flat sections.

Speaking from personal experience as a 60kg rider, even riders with a lower P/W ratio than me can put me in big trouble on the flats even if I can destroy them on a long climb.

  • Thanks for the reply! With Strava being prolific these days - you can't help but notice averages on other peoples rides. And then it got me wondering - hence the question. And I guess with all the variables and factors in cycling - its what makes it such a great sport - where a climber can dominate in one race but look mortal in another! – OraNob Feb 26 '16 at 11:44
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Weight is a factor of a 3 dimensional change in body size. Frontal area is a factor of only 2 of those 3 dimensions. Therefore I would say the larger rider would be faster. The frontal area should only be about 20% more which is about 6/5ths but the power output is 4/3rds. That would put the larger rider at a 20 to 18 advantage. (20 to 20 would be even)

  • But one cannot assume that the frontal area of the heavier rider is bigger, the lighter rider could have broad shoulders and the other the opposite, for instance – gaurwraith Feb 26 '16 at 8:58
  • It was stated that rider 1 is shorter, therefore, it is a reasonable assumption that rider 1 also has less frontal area in a real world scenario. However in this example it was stated in a comment that we can assume similar frontal area. – David Feb 26 '16 at 9:44
  • Yes. The original question was not to do with aerodynamics - although they do have a role. But that would have opened up more questions. – OraNob Feb 26 '16 at 11:49
  • @OraNob And you don't think aerodynamics the causality of the empirical data in the accepted answer? – paparazzo Feb 27 '16 at 19:45

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