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This is a comparison of small vs large riders, comparing a 5 and 6 foot tall rider. I originally wrote this while taking post bac classes at Texas A&M.
First of all lets assume similar body composition
between a 5 foot tall cyclist, and a 6 foot tall rider( or 1.2 times as tall as the other rider).
Strength is largely dependent on a rider’s area. Area increases with the square of dimensions (1.2*1.2=
1.44 ) so the 6 footer has 1.44 times as much area when
Compared to the 5 footer. Physiologists use cross
Sectional area of a muscle to determine strength.
So the 6 footer is expected to be %44 stronger.
Volume increases with the cube of the linear
dimensions.(1.2*1.2*1.2=1.728) so the 6 footer has
about %73 more volume. If they are of similar body
composition, the 6 footer is %73 heavier.
So the 6 footer is %73 heavier, but only %44 stronger.
Does this put the heavier rider at a disadvantage?
Yes and no.
We all know that when pedaling at speed in the flats,
air/wind is the primary force the cyclist must
overcome. We also know rather well that as the road
turns uphill, and speeds slow down, the primary force
begins to change from air to gravity.
When pedaling uphill, the larger rider is obviously at
a disadvantage, as since his weight is %73 heavier,
but his engine is only %44 percent stronger. In a
hilly 98 mile stage of the 1992 Tour Dupont, the
average 140 pound cyclists finished 10 minutes in
front of the average 175 pound cyclist
When pedaling in the flats with no drafting, the
larger rider is at an advantage, because wind
resistance is in relation to surface area. Since
surface area squares with linear dimensions, and the
volume cubes with linear dimensions, the heavier rider
has less surface area compared to their weight. This
means that even though a larger rider requires more
energy to push through the wind, the energy costs
relative to weight are much less.
Research has indicated that larger cyclists require
%20 less oxygen per kilogram of body weight to ride in
the flats in a tucked aero position! (This is
partially because of the fact that the big guys were
able to reduce their wind resistance in an aero tuck
by a greater percentage of total when compared to the
small guys) This would predict that all things being equal, on a flat
time trial, the larger rider would win every time. Of
course this doesn’t always happen as riders have a
large variance in vo2max, LT, muscle fiber type,
motivation, pain threshold, etc. One study of elite
cyclists suggested that a 175 pound rider would finish
a flat 40k tt 4 minutes faster than a 145 pound rider.
Basically, smaller riders tend to have a more favorable strength to weight
ratio, while larger riders have a more favorable strength to wind resistance
ratio.
Why are some riders like Greg Lemond or Lance
Armstrong at the top of their game in tt and climbing?
They are pretty much just genetic anomalies with very high power outputs .
* Most of this info is plagiarized directly from Ed Burke's "High Tech Cyling" I'm not that smart. | 
