Steady-state model using typical road values: rolling resistance 0.005, effective frontal area 0.32 square metres, air density 1.225, drivetrain efficiency 97.5 percent. Assumes no wind.
Compare the model with reality WattLog.PRO records your actual power, speed and gradient together, so you can see where your real numbers sit against the physics and what your effective drag is really costing you.
Open WattLog.PROThe three forces
At steady speed your power fights exactly three things. Rolling resistance comes from tyre deformation and rises in direct proportion to speed. Aerodynamic drag rises with the cube of speed, which is why it dominates everything once you are moving quickly. Climbing power is your weight times gravity times the gradient, and it does not care about speed at all beyond the rate you gain height.
The cubic term is the one that shapes cycling. Going from 30 to 40 km/h on the flat is a 33 percent speed increase but demands well over twice the power. That is why aerodynamic gains matter so much at speed and so little on a steep climb, and why a rider can be transformed by a position change on flat roads while gaining nothing in the mountains.
What the model can and cannot tell you
On a climb the balance inverts completely. Above about five percent gradient, gravity accounts for the overwhelming majority of your power demand and aerodynamics becomes nearly irrelevant. Weight is what matters there - which is exactly why the power-to-weight ratio predicts climbing and raw watts predict flat speed.
Treat the output as a well-calibrated estimate rather than a measurement. The defaults describe a rider on the hoods on decent tarmac in still air; your real effective frontal area varies enormously with position, clothing and bike, and wind changes everything. As a tool for understanding how the forces trade off, though, the model is exactly right.
Frequently asked questions
Why does a little more speed cost so much more power?
Because aerodynamic drag scales with the cube of speed. Doubling your speed on the flat requires roughly eight times the power to overcome air resistance. This is the single most important fact in road cycling and the reason aerodynamics dominates flat-road performance.
How much does weight matter?
Almost nothing on the flat, where you are fighting air rather than gravity, and almost everything on a climb. At ten percent gradient nearly all of your power goes into lifting mass, so a few kilograms translate directly into time lost.
Why is my real speed different from this?
Wind, mainly, which the model ignores. Then your actual riding position, tyre choice and road surface, all of which shift the constants. Real drag varies from around 0.25 square metres in a good aero tuck to over 0.40 sitting upright - a difference worth many watts.