Tag: Wind

Strava – Tour de Richmond Park Clockwise

Screenshot 2019-05-22 at 15.24.51

Following my recent update on the Tour de Richmond Park leaderboard, a friend asked about the ideal weather conditions for a reverse lap, clockwise around the park. This is a less popular direction, because it involves turning right at each mini-roundabout, including Cancellara corner, where the great Swiss rouleur crashed in the 2012 London Olympics, costing him a chance of a medal.

An earlier analysis suggested that apart from choosing a warm day and avoiding traffic, the optimal wind direction for a conventional anticlockwise lap was a moderate easterly, offering a tailwind up Sawyers Hill. It does not immediately follow that a westerly wind would be best for a clockwise lap, because trees, buildings and the profile of the course affect the extent to which the wind helps or hinders a rider.

Currently there are over 280,000 clockwise laps recorded by nearly 35,000 riders, compared with more than a million anticlockwise laps by almost 55,000 riders. As before, I downloaded the top 1,000 entries from the leaderboard and then looked up the wind conditions when each time was set on a clockwise lap.

In the previous analysis, I took account of the prevailing wind direction in London. If wind had no impact, we would expect the distribution of wind directions for leaderboard entries to match the average distribution of winds over the year. I defined the wind direction advantage to be the difference between these two distributions and checked if it was statistically significant. These are the results for the clockwise lap.

RoseSegmentBarSegmentclockwise

The wind direction advantage was significant (at p=1.3%). Two directions stand out. A westerly provides a tailwind on the more exposed section of the park between Richmond Gate and Roehampton, which seems to be a help, even though it is largely downhill. A wind blowing from the NNW would be beneficial between Roehampton and Robin Hood Gate, but apparently does not provide much hindrance on the drag from Kingston Gate up to Richmond, perhaps because this section of the park is more sheltered. The prevailing southwesterly wind was generally unfavourable to riders setting PBs on a clockwise lap.

The excellent mywindsock web site provides very good analysis for avid wind dopers. This confirms that the wind was blowing predominantly from the west for the top ten riders on the leaderboard, including the KOM, though the wind strength was generally light.

The interesting thing about this exercise is that it demonstrates a convergence between our online and our offline lives, as increasing volumes of data are uploaded from mobile sensors. A detailed analysis of each section of the million laps riders have recorded for Richmond Park could reveal many subtleties about how the wind flows across the terrain, depending on strength and direction. This could be extended across the country or globally, potentially identifying local areas where funnelling effects might make a wind turbine economically viable.

References

Jupyter notebook for calculations

The best wind for a KOM on Strava

Two key aspects of the weather influence the time to complete a Strava segment: the wind and the air density. This blog considers the direction and speed of the wind. The following blog will examine how aerodynamic drag is affected by changes in air density.

Clearly, on an exposed, arrow-straight segment, the most favourable weather would be a hurricane tailwind. Like other KOM hunters, I have searched for segments that align with the predicted wind direction when a gale is forecast, though I’ve usually ended up going kitesurfing instead.

When the segment is a loop, such as the Tour de Richmond Park, discussed in the previous blog, the question becomes more interesting. Consider a light aircraft flying above the Richmond Park segment at an altitude of 300m. Any constant wind, regardless of direction, will result in a slower time than completing the circuit in still air. Why? Since any headwind slows down the plane, it hinders the pilot for more time than the tailwind provides assistance, resulting in a net increase in the total time.

However, cyclists do not ride in constant winds. Trees, buildings and the terrain all affect the wind’s speed and direction. Variability is so strong that it is recommended that multiple anemometers should be positioned at intervals alongside the 100m track at important athletics meetings.

All this means that it is quite likely that there are optimal wind conditions for all Strava segments. Most people suggest that a tailwind up Sawyers Hill is best for Richmond Park, as this part of the segment is an uphill drag that is exposed to the wind, whereas other sections of the route are much more sheltered. The bearing of a tailwind would be from just North of Easterly.  Historically, this is not a very common wind direction for London. The following charts shows the prevailing wind direction over the year is Southwesterly.

roseyear

Easterly winds are even rarer in July and August, when many PBs have been set, though in September they have been a little more frequent. (An interactive version of the chart can be found on this site.)

rosesummer

Now, if the wind had no effect on the Strava segment, we would expect the distribution of wind directions on which riders set their PBs to be similar to the historic distribution. So we are interested in the difference between the distribution of wind directions on the dates derived from the leaderboard relative the background average. The following chart compares the segment against the historic average annual average. The compass rose clearly shows a much higher frequency (13%) of the PBs of the top 1000 riders were set when the wind was blowing from the East and a relatively lower incidence in the opposite direction.

rosesegment

The next hand chart “unwraps” the two curves to show the relative difference, which is statistically highly significant (p<0.01). A forensic analysis of the data confirms that the best wind direction for a PB around Richmond Park is indeed an Easterly tailwind up Sawyers Hill.

barsegment

So far we have not considered the strength of the wind. The next chart shows the average windspeed on the days that PBs were set, according to the direction of the wind. This shows a bias towards stronger winds from the East, consistent with the frequency of PBs.

windspeedbydirn

Combining this with the results of the previous blog, the following conclusions may be drawn. However good a cyclist you are, your best chance of achieving a high ranking on the Tour de Richmond Park leaderboard is to choose the evening or morning of one of the rare summer days when the wind is blowing strongly from the East. And, you guessed it, on the evening of August 2015 when Rob Sharland achieved his KOM, the wind was blowing at 11mph on a bearing of 80° .

The next blog will examine how temperature, pressure and humidity, as well as altitude, change the air’s density. This is the principal environmental factor affecting your aerodynamic drag, when you are going for a KOM.