09 July 2013

Headwinds Dominate Stage 10!

German Marcel Kittel just edged fellow countryman André Greipel at the finish line of a very windy Stage 10.  Below is a comparison between Kittel's time and our prediction.
  • Stage 10:  4h 53' 25" (actual), 4h 34' 42" (prediction), 18' 43" fast (-6.38% error)
Had we not been following today's action and seen only the winning time, we would have been quite disappointed.  What we noticed while following the race is some wild wind.  There were crosswinds of 30 kph (19 mph) with headwind components dominating the entire stage.  We simply knew our prediction was going to be too fast if cyclists had to battle headwinds all day!

Below is a table showing what a small horizontal headwind can do to our prediction for Stage 10.

vhead (kph) Prediction
1 4h 41' 17"
2 4h 48' 13"
3 4h 55' 30"
4 5h 03' 11"
5 5h 11' 16"

Note that what is assumed in making the above table is that riders feel a constant headwind of a given magnitude for the entire race, which is a gross oversimplification of reality.  What we noticed today were gusts of winds that reached 30 kph with directions mostly perpendicular to the riders' motion (i.e. crosswinds) with headwind components.  That means that cyclists felt only a small piece of the wind as a headwind on average.  There were times when the headwind was strong, and there were a few times late in the stage when the wind actually gave a tailwind component.  What the above table shows us is that the effect of today's wind was equivalent to riders feeling a constant headwind of about 3 kph.

Below is Kittel's average speed.
  • Stage 10:  11.19 m/s (40.28 kph or 25.03 mph)
Stage 11 is an individual time trial.  Beginning in the northwest French commune of Avranches, the 33.0-km (20.5-mi) time trail ends on the tidal island of Mont Saint-Michel in the English Channel.  Riders should go all out with a flat stage to follow on the next day.  Below is our prediction.
  • Stage 11:  39' 23" (prediction)
Look for top speeds tomorrow!

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