Field-Based Position Optimisation
When speed is the ultimate outcome, performance comes down to Watts per coefficient of drag (CdA). Time trial disciplines are the perfect example and the seasoned athlete knows that achieving results comes down to tuning both aspects of performance.
In the lab, we aim to develop a robust base time trial position that allows the athlete to achieve the expected power/duration targets. While we look for indicators of aerodynamic efficiency in the lab, on the track we put these to the test and determine the fastest combination of variables. This is exactly what we did with Nick from Saint Cloud, one of Adaptive HP's retail partners.
The time period between the baseline fit and the aero testing session (4 weeks) allowed Nick sufficient time to familiarize himself with the base position and work out his sustainable power output. We always recommend a short period of time following the initial bike fit session, allowing the athlete enough time to adapt to positional changes and establish their power/duration targets. Consistency in position is only something that comes with time on the bike and time in position, at a range of exercise intensities. By the time we go to the velodrome we want the athlete to be 100% confident with their base position.
Image by Steve Whiting (@squidsteve)
Testing Process
Just like any scientific investigation, the implementation of controls is vital in the aero testing process. First and foremost, track aero testing relies on the quality of power and speed data.
The wheel and tyre combination are vital in emphasizing the contribution of the rider in the overall system (we want to test the rider) and stability of tyre pressure is vital for ensuring the accuracy of wheel speed data. We use a control wheel set (HED Stinger Disc and HED GT3) in combination with control tyres, eliminating this as a source of discrepancy between subjects, runs and sessions.
Image by Kane Watters (@kwaichang)
Like almost all performance measures in cycling, power is a key input metric and in this case, it is used to calculate the system coefficient of drag (CdA). The Track Aero System (TAS) collects a live data stream from the bike (power and speed) and in combination with the input of the environmental parameters of the velodrome, calculates the system CdA. The TAS allows us to measure and evaluate the aerodynamic efficiency of equipment and positional variables, in real-world performance conditions.
When testing with Nick, we had the luxury of using the Verve InfoCrank. The InfoCrank is now well accepted as the gold standard for power meter accuracy and is the first choice when an athlete wants to invest in a high-quality measurement tool. While not every athlete will have a device of this caliber, a functioning power meter is a prerequisite for aero testing.
Image by Steve Whiting (@squidsteve)
Image by Kane Watters (@kwaichang)
Image by Blake Norrish (@blakenorrish)
Testing Positional Techniques
Each test run consists of multiple laps, allowing the athlete to get up to speed and settle into a rhythm. We like to include around 10 laps of data in our analysis, providing an accurate representation of position consistency. With each variable (i.e. helmet change, hand position) we like to collect data from a number of runs, ensuring repeatability of measures.
For Nick, the goal of this initial aero testing was to establish the impact of positional techniques. The head tuck and the shoulder shrug are two of the most useful techniques that an athlete can employ to reduce their frontal area and having measures for each is often an important part of rider education. The head tuck was the first variable tested and the head tuck plus shoulder shrug was the second variable tested.
Three test runs were used to assess Nick's baseline CdA value. Two test runs were used to evaluate each of the position variables.
Nick’s CdA at baseline was 0.225 m^2, showing we have a good platform to work with and the potential for some significant improvements. The head tuck was the first variable tested, with an average CdA of 0.217 m^2. The second variable tested was the combination of the head tuck with the shoulder shrug, resulting in an average CdA of 0.209 m^2.
It is not hard to visualize how these techniques reduce frontal area, but it is sometimes hard to understand how significant the gains are. While CdA values are quite foreign to most people, these can quite easily be expressed as equivalent power gains or time savings.
Saving Time
Nick’s CdA was reduced by 3.6% when performing the head tuck. At 300 Watts, this is the equivalent to an 11 Watt gain in power output, or a time saving of 39 seconds over a 40 km race distance. When successfully performing the combination of the head tuck and shoulder shrug, his CdA was reduced by 6.9% over baseline. At 300 Watts, this is equivalent to a 21 Watt gain in power output, or a 75 second time saving over a 40 km race distance.
Interpretation
The gains seen with these positional techniques are not uncommon, but what is interesting in Nick’s case is that each technique provided equal gain. The split is not always so even and there are always individual differences to consider in the refinement of an individuals position.
Whats Next?
Stay tuned as we aim to take Nick's CdA sub-0.200 with further equipment and position optimisation strategies.