You might have heard a lot about VAM or vertical ascent in meters per hour to give it its full name. VAM is a measurement coined by Dr Michele Ferrari as a means of comparison between riders and climbs giving us an ability theorize and attempt calculate relative power output.

It is important to understand that VAM is affected by the gradient. The greater the steepness, the greater the VAM at a given power output. The relationship between VAM and relative power output is expressed as follows:

Relative power (Watts/kg) = VAM (meters/hour) / (Gradient factor x 100)

This gradient factor ranges between 2.6 for a gradient of 6% and 3.1 for a gradient of 11%

To work out the gradient factor take 2 + (% grade/10)

Let's see this in action.
If we take Astana rider Alberto Contador's decisive climb up the Verbier in the 2009 edition of the Tour de France and feed the figures into the equation what do we come out with?

8.7km climb at 7.5%. Ferrari quotes power figures of 6.73W/kg

This has implications for Contador's climb. For Contador, the relative power output can be calculated by taking the VAM (1864m/h) and dividing by 275 (the constant for 7.5%). This gives a relative power output of 6.78 W/kg. (Ferrari calculates 6.73W/kg, but he uses a VAM of 1852m/h, because he measures the ascent as 638 meters, not 640 meters. This shows again how careful you have to be in measuring the climbs)

Due to the way VAM is calculated, on a steeper climb, the same power output would produce an even higher VAM. Therefore, Contador's record VAM would have been higher on any of the other slopes - eg Alpe d'Huez with its gradient of 8.1%, he would produce higher VAMS. For example, had Contador been on an 8.5% slope, producing the same power output, his VAM would increase to 1932m/hour.

I hope this has helped you to understand what VAM is and how you can compare climbing performances given the relevant data.

Power Meter

Power Meter

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