Back in 1980, before the days of power meters, Dr Ferrari was looking for a way to gauge a rider’s performance that wasn’t affected by factors like wind, temperature or drafting. Although these factors will always have some influence, their effect is reduced enough on climbs, particularly those with gradients of 8% or more, to provide a fairly reliable measure.
That measure is VAM and to work it out you need just two figures: the time it took to ride the climb (in minutes) and the elevation gained in the process (in metres). Then you plug them into the following equation:
VAM = (metres of elevation gained x 60) ÷ by minutes taken to ride the climb
Suppose, for example, you ride up Alpe d’Huez (average gradient 7.9%). In the course of doing so you gain 1,090m of elevation and it takes you 37mins. So…
(1,090m x 60) ÷ 37mins = 1,768
Therefore, your VAM for Alpe d’Huez would be 1,768 – you’d also be one of the favourites to win the Tour de France!
However, while this initial figure provides a marker for you to work on and improve, VAM becomes more useful when you know the gradient of the climb. This is because you can then get an idea of your relative power (watts per kg). How? Follow this…
Watts per kg = VAM ÷ by (gradient factor x 100)
Note: gradient factor =
2 + (% gradient ÷ 10)
Now, if Alpe d’Huez’s average gradient is 7.9% then…
2 + (7.9 ÷ 10) = 2.79
So watts per kg = 1,768 ÷
(2.79 x 100)
Or 1,768 ÷ 279, which means your relative power works out to be 6.34 watts per kg.
While wind, temperature and drafting have a negligible effect on VAM, altitude will skew the figures. After all, climbing a 10% incline starting at sea level is a different matter to climbing one at 2,000m.
In short, VAM is a decent enough measure of power but it’s only a rough guide