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Wind Tunnel verses reality
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Wind Tunnel Sessions
There are investigations that are quite difficult to do outdoors on demand. Looking at behaviour at yaw or looking for the smaller improvements in kit or position and the interaction between the two would require really very specific conditions. To measure yaw you need wind and wind makes outdoor testing less accurate so subtle differences can't be measured cleanly. Running a set protocol in the tunnel should theoretically give a set of data with more of the variables under control under the required conditions.
Improvements or not
Some of the variations tried produced no significant difference, some produced small changes and some were clear improvements with differences much bigger than the variation between repeats of the same setup. Repeating runs gave a clearer picture of the margin required to give high confidence in the measured differences. When the outcome of a measurement could be a change in kit and expenditure, real confidence in the measurement should be expected. A shift in saddle position for example could be larger than a measured difference between two helmets. Taking care to repeat as accurately as possible is key
Worth the cost?
The priority with all measurements is that they are good quality, otherwise they're guessing. After that it is necessary to test meaningful changes in a planned schedule to conserve time between runs. However the emphasis must always remain on quality rather than quantity.
For someone undertaking an aero fit for the first time I would expect to find similar gains without using the tunnel. First is to get the basics of the position sorted and validate them, this generally involves the most significant changes to the setup of the bike and takes time to implement that starts to feel quite expensive when paying an hourly cost. Once the basics are nailed down the search begins for smaller gains with differences between skinsuits, helmets and minor positional changes.
Post tunnel verification OR NOT
Once measurements have been made and changes applied to kit and bike it is necessary to validate the results in real world conditions. The performance improvement process is not complete until this has been done. This is where everything can change very quickly. My tunnel results showed stacking of improvements and repeated well which should in theory give reasonable confidence in the outcome. Scientific levels of confidence require a number of repeats that not many can afford in a tunnel and are still subject to being in a non-realistic environment. Once outside on a simple out and back course it was immediately apparent that rather than a 15W improvement I was seeing an increase in drag under race conditions. This required another set of measurements in a more controlled environment again, this time Reading velodrome. Some of the tunnel measurements were verified but the biggest were unfortunately reversed. This requires an explanation and is where the experience becomes useful despite being unproductive.