**Power/weight**
argylesocks said:

here are 2 different riders:

80kg person with 320watts at LT (4watts/kg)

60kg person with 240watts at LT (4watts/kg)

riding at LT, who would be faster

a) on a flat 40km TT

b) in a rolling race

c) on a 8mile hillclimb

lets assume same bike, same body shape, same aerodynamics, etc... other than weight & power, all things are equal.

Assuming all else being equal (or at least in proportion to body weight), the larger rider would be faster under all conditions. Aerodynamic drag is proportional to frontal area, and frontal area does not increase proportionally with mass, so the larger rider will have a smaller frontal area/mass ratio. Since these two riders have the same power/mass ratio, the larger rider will have a smaller frontal area/power ratio, and proportionately less drag for their power. Therefore, in a flat TT, the larger rider has the advantage.

For climbing, the larger rider in this example still has an advantage. The weight of the bike will be a smaller proportion of the total weight for the larger rider, so if you add in the weight of the bike, the larger rider will have a larger power/weight ratio than the smaller rider, so the larger rider will climb faster.

So, you might ask, why do smaller riders usually climb faster than larger riders? Because typically, smaller riders have a higher power/weight ratio than large riders. Muscle strength might increase proportionally to size, but aerobic capacity does not. If you were to compare the best time triallers in the peloton to the best climbers, you would find that the time triallers typically are heavier and generate more power than the climbers - but that though the climbers power output was smaller, their weight was much smaller, so they end up with a higher overall power/weight ratio.