Numbers on Drafting

Two professional cyclists of similar size (185cm and 75kg) covered 1 km on flat course several times, at different speeds, alternating their positions: either in the front or drafting from the other. 

The graph below shows the difference in power output in watts between pushing at the front and drafting from a single rider: the saving at 30 km/h is 60w, increasing to 120w at 40 km/h and getting up to 220w at 50 km/h.

It is interesting to note that at 22-24 km/h the saving is about 40w: a quite considerable advantage also for speeds that are typical of climbing, corresponding to about 10% in power for a cyclist that pushes 400w. 
For a small sized cyclist, such advantage is even more significant, especially when drafting from a taller athlete: 50w for a rider climbing at 320w represent a 15% saving approximately. 
Therefore, smaller and lighter cyclists have an edge in mountain stages not only for the lower body weight, but also due to the fact that they definitely save more energy from drafting, compared to heavier competitors. 

Another cyclist covered alone the same 1 km stretch of flat road at 48 km/h, expressing an average power of 410w. 
He repeated the performance with a motorcycle 15m ahead of him: the average was 385w, a saving of 6%. 
Whenever we see the multitude of photographers motorbikes repeatedly hovering around the leading rider in a race, such considerable advantage is automatically given to him: it is as if they temporarily subjected him to an instantaneous transfusion of 1-2 units of blood...

25 Jan 2011