effect of road grade on VAM

VAM is an easy to calculate proxy for power-to-mass ratio commonly reported for professional riders in European races, where television coverage plus well documented hill profiles facilitates its extraction. VAM is the rate of altitude gained per unit time, for example in meters per hour. I recently recorded 1402 metters per hour in a break-through ride up Old La Honda, breaking my previous PR. The excellent Science of Sport Blog recently posted the following plot of some VAMs from recent Tour history:


VAMs from the Tour de France, from The Science of Sport.
Michele Ferrari's numerical calculation of the effect of road grade on VAM are often used as a reference, by me included, but an analytic form is perhaps more useful.

I'll again assert a simplified, windless bike power-speed model, which is for "PowerTap power" neglecting drivetrain losses, but it is assumed drivetrain losses are a function of total power:

p = f_m m s + f_w s³,

where p is a fixed power, s is bike speed, and f_m and f_w are coefficients. This model neglects drivetrain losses, but I'll assume they depend primarily on total power p, and thus are fixed.

Differentiating with respect to speed s yields:

dp / dgrade = ∂p/∂s ∂s/∂grade + ∂p/∂grade

which since dp = 0 yields, keeping only non-zero partial derivatives:

0 = (f_m m + 3 f_w s²) ∂s/∂grade + m s ∂f_m/∂grade

or

∂s/∂grade = ‒ m s ∂f_m/∂grade / (f_m m + 3 f_w s²)

Using a definition for VAM with our linearized grade (sine = tangent):

VAM = grade × s

yields:

( grade / VAM ) (d VAM / d grade) = ( grade / VAM ) [ ∂VAM / ∂grade + (∂VAM / ∂s) (∂s / ∂grade) ]
      = 1 ‒ m grade ∂f_m/∂grade / (f_m m + 3 f_w s²).

The quantity on the left is the relative increase in VAM with the relative increase in road grade. For example, what is the ratio of the % increase in VAM with the % increase in grade? This can be simplified by defining δ_VAM as the fractional increase in VAM and δ_grade as the fractional increase in grade.

So given the following:

f_m = ( grade + Crr) g

then

∂f_m/∂grade = g

where g is the gravitational acceleration, and therefore

δ_VAM / δ_grade = 1 ‒ m g grade / (f_m m + 3 f_w s²).

If f is the fraction of power coming from wind resistance this yields:

δ_VAM / δ_grade = 1 ‒ ( m g grade s / p ) / ( 1 + 2 f ).

This can be simplified by defining α_RR = C_RR / (C_RR + grade), the fraction of mass-proportional power from wind resistance, yielding

δ_VAM / δ_grade = 1 ‒ (1 ‒ α_RR) (1 ‒ f) / (1 + 2 f)
      = [ 1 + 2 f ‒ 1 + α_RR + f ‒ α_RR f) ] / (1 + 2 f)

with a final result:

δVAM / δgrade = [ (1 ‒ f) αRR + 3 f ] / (1 + 2 f)

using the following definitions:

δVAM	fractional increase in VAM
δgrade	fractional increase in road grade
αRR	fraction of mass-proportional power from rolling resistance
f	fraction of total power from wind resistance

An alternate but equivalent representation would be (I prefer the previous one):

δ_VAM / δ_grade = [ α_RR + (3 ‒ α_RR) f ] / (1 + 2 f).

Or, if α_RR is close to 1, then this more symetric representation may be better, as the function of f on the right is the effect of relative decreases is mass on climbing speed, derived in an earlier post:

δ_VAM / δ_grade = 1 ‒ (1 ‒ α_RR) (1 ‒ f) / (1 + 2 f).

Consider the following example:

α_RR ≡ 1 / (C_RR / grade + 1) = 0.05,

f ≡ fraction of power to wind resistance = 0.12.

Then a 10% relative increase in grade (for example, from 7% to 7.7%) will result in a 3.2% increase in VAM (for example from 1400 m/hr to 1445 m/hr).

There are two limits for this result, each providing a nice check of the derivation. One is where f is essentially zero, in which case it simplifies to α_RR. So in this instance the VAM is reduced proportional to the fraction of power going into rolling resistance. In the other limit of α_RR → 0, it simplified to 3 f / (1 + 2 f), which represents the decreased power going into wind resistance with steeper grades. There is also the second-order term in the numerator ‒α_RR f which has negligible effect on steep relatively smooth roads. To the opposite extreme, where the road is almost flat and therefore f or α_RR are close to 1, the result will be close to 1, which implies for very shallow roads dominated by rolling resistance and/or wind resistance a fractional increase in road grade barely affects speed, and so the grade contributes directly to VAM. This is of course correct, so in each of these test limits the formula does the right thing.