Endurance Analytics

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"Air pressure is everything"

The hour record is a hot topic at the moment and we have seen great interest in predicting how far both Alex Dowsett could go and how far Bradley Wiggins will go this Sunday June 7th. Based on comments in the media it is clear that Wiggins was hoping for unusually low air pressure this first week in June but just how much of a difference can this make?

Accompanying this article is a chart from the UK National Physics Laboratory (NPL) in Teddington, just across London from the Olympic Velodrome, which shows a time series of local air pressure during the last year. You can see that 1013mb, the typically accepted average global air pressure at sea level, is around about the average observation while 990mb and 1030mb could be considered towards the low and high ends of normal. By applying these numbers in our popular Power Calculator we see that, with some sensible estimates and at hour record pace, the difference between 990 and 1030 is very significant – at least 600 metres – and for less gifted riders might represent the difference between smashing the record and a near miss.

We can’t overemphasise the importance of air pressure when the goal is riding as fast as possible. It’s the very reason riders go to altitude – either for the physiological effects or, in the case of record attempts, to gain what could almost be considered an unfair advantage over sea level times. Our Effects of Altitude model demonstrates the extent of speed &/or power savings that can be had with increasing altitude, versus typical physiological costs. And if Wiggins does as predicted, setting an almost unacheivable mark for the hour, then just maybe the only way to beat it will be some future plan at altitude.

Turning back to the air pressure in London we have long used historical data from the NPL to consider the relative benefits of riding a time trial event based on forecast weather. One of the inputs to our Time Trial Sector Model is air pressure and we can show you just where a forecast value stands in the normal distribution of UK pressures. We display “faster days in the year”, “slower days in the year”, and relative speed and power advantages of the range of pressure percentiles.

At the time of writing the forecast for Wiggins’ ride is not looking favorable. Heat and possibly humidity can be determined by climate control at the velodrome, set at the preference of Wiggins’ team, but air pressure is forecast at a very high 1033mb. Just to recap – if that forecast comes to pass then Wiggins has been unlucky and could go 600m slower than in an atmospheric pressure at the luckier end of the scale. One final thought – track and field records in athletics have long been rectified for wind assistance...is it time cycling did the same for air pressure?

The Hour Record - Predict, Execute, Analyse

Sport science and engineering as applied to cycling is a double edged sword. On one hand the discipline uncovers countless truths about the nature of racing. We can make some really informed decisions about equipment choice, pacing strategies, and race execution in general. All of these things can deliver “free speed” and take us closer to “the perfect ride”. On the other hand some bike racing disciplines are now really predictable, a question of plugging rider’s power and other parameters into a mathematical model to calculate speed and race time. Not everybody likes that because sporting events were often designed to turn uncertainty into excitement.

On the spectrum of predictability the hour record is right up there, possibly the perfect example of preparing a rider to turn known knowns into a certain result. Hour record rides take place in a controlled environment, indoors, and recent attempts have been ridden with power meters. Preparing for the event boils down to making smart equipment choices then asking a simple question: how high can we get the ratio of this rider’s FTP (sustainable power output for one hour) to CdA (the aerodynamic drag metric). The answer determines the number we’re all waiting for...how many kilometres can this guy cover in an hour?

If modelling and predicting the hour record isn’t for you then browse away now. But if it is, and if you intend to take even the slightest interest in cycling during 2015, then you may be interested in our hour record calculator. The defaults are set to estimates of  Jens Voight’s parameters, indicating a distance of 51.11km, but you can tweak them endlessly to run your preferred scenario. We believe that when the big guns weigh in the record is going beyond 53km.  Our calculator will help reveal just what that takes in terms of power output. Want the quick answer? Quite a lot!