What is the Aero Testing Spectrum?
A Guide to Aerodynamic Testing
There is no single "best" aero test
Spend enough time researching aerodynamic testing and you'll quickly encounter strong opinions. Wind tunnels are described as the gold standard. Field testing is praised for its realism. Velodromes are championed for balancing both.
The reality is far less absolute.
Every aerodynamic testing methodology represents a different experimental design, balancing measurement confidence against riding realism. None is universally superior; each occupies a different position on what we refer to as the Aero Testing Spectrum.
The Aero Testing Spectrum provides a framework for understanding how different testing environments answer different questions.
Two competing priorities
Every aerodynamic test asks the same question: has this change made the rider faster? The difficulty lies in separating the effect of that change from everything else happening around it.
Two characteristics define how well a testing method can achieve this:
Measurement confidence describes how accurately and repeatably a test can resolve small aerodynamic differences. The fewer external variables present, the greater the confidence that any measured change is genuine.
Riding realism describes how closely the testing environment represents the conditions in which the rider ultimately races. Real roads, changing wind, rider movement and fatigue all influence aerodynamic performance, yet they also make accurate measurement more difficult.
These two priorities often pull in opposite directions; improving one frequently requires compromising the other.
Realism isn't always the objective
It is tempting to assume that the most realistic testing environment must also be the most valuable. In practice, the relationship is more nuanced.
The closer a test moves towards real riding conditions, the more variables enter the system. Wind strength and direction, road surface, rider pacing and environmental conditions all influence the result. These variables are not undesirable — they are part of racing — but they make it increasingly difficult to isolate the aerodynamic effect of a single change.
Conversely, increasing measurement confidence requires removing sources of variation. Laboratory environments deliberately simplify reality, allowing very small aerodynamic differences to be resolved with greater confidence. The compromise is that no laboratory environment can perfectly reproduce every aspect of riding outdoors.
The objective of aerodynamic testing is therefore not to recreate race day exactly. It is to create the conditions most appropriate for answering the question being asked.
The spectrum is a decision-making framework
The Aero Testing Spectrum should not be interpreted as a progression from "good" to "better". It is a framework for matching a testing methodology to the decision being made.
A rider investigating a subtle equipment change may require the measurement confidence of a wind tunnel. Another refining their race position may benefit from the balance of realism and repeatability offered by an indoor velodrome or Catesby Tunnel. Preparing for a specific event may justify validation through field testing.
Professional aerodynamic programmes routinely move along the spectrum, selecting each testing environment for its strengths rather than treating one methodology as universally superior. Equipment may be evaluated in one environment, rider position refined in another, and the final result validated under race-relevant conditions.
The objective is not to find the best test. It is to find the right test for the question being asked.
Applying the Aero Testing Spectrum
Every rider arrives with different objectives. Some are searching for the fastest race position. Others are validating equipment choices or preparing for a specific event.
The role of aerodynamic testing is not simply to produce a lower CdA. It is to provide sufficient confidence that every subsequent decision is based on measurement rather than assumption.
Understanding where your objectives sit on the Aero Testing Spectrum is the first step towards selecting the right testing approach.
Testing environments available through WattShop
WattShop delivers aerodynamic testing across a range of specialist environments, each occupying a different position on the Aero Testing Spectrum.
Highest measurement confidence · Lower riding realism
Available at Silverstone Sports Engineering Hub.
The wind tunnel occupies the highest point on the spectrum for measurement confidence. Aerodynamic drag is measured directly in smooth, controlled airflow, allowing exceptionally small differences between equipment and rider positions to be resolved with confidence. Controlled yaw sweeps also make it the most effective environment for evaluating crosswind performance.
The trade-off is riding realism. Although power output can be replicated, constrained riding cannot fully reproduce the dynamics of a rider supporting themselves on a freely moving bicycle.
Best suited to equipment development, detailed position refinement and resolving the smallest aerodynamic differences.
High measurement confidence · High riding realism
Available at Derby Arena and the Geraint Thomas National Velodrome.
An indoor velodrome provides a unique balance between controlled measurement and genuine riding dynamics. Testing is carried out at race intensity while the enclosed, windless environment removes many of the variables that complicate outdoor testing.
The result is highly repeatable aerodynamic data collected while the rider is moving naturally, making the velodrome particularly effective for optimising riding position and validating changes under realistic conditions.
Very high measurement confidence · Very high riding realism
Available at Catesby Tunnel.
Catesby tunnel occupies a unique position on the Aero Testing Spectrum, combining laboratory-stable atmospheric conditions with completely natural riding. Riders generate real power, ride dynamically and receive highly repeatable CdA measurements within an environment specifically suited to aerodynamic testing.
Few testing methodologies achieve such a high degree of both confidence and realism, making Catesby particularly valuable for validating rider position and equipment under race-representative conditions.
Highest riding realism · Moderate measurement confidence
Available worldwide
Field testing places aerodynamic optimisation in the environment where performance ultimately matters. Using the Streamlines Cirrus system, environmental variables including wind, gradient and speed are measured and accounted for, allowing meaningful aerodynamic data to be collected on real roads.
It is particularly valuable for validating race-specific positions, assessing performance in genuine riding conditions and monitoring aerodynamic development over time.
Discuss your objectives with a WattShop engineer to identify the testing methodology most appropriate for your goals.
The Aero Glossary
Drag
The air pushing back as you ride — 80–90% of your resistance at race speed.
CdA (drag area)
One number for how aero you are. Lower is more slippery.
Road ~0.30–0.40
TT ~0.20–0.25
Elite ~0.18.
Watts Saved
A CdA difference converted into power at your race speed — the number that decides whether a change is worth it.
Baseline
Your starting setup, re-run through the session to prove the data is stable.
Repeatability
Getting the same number twice for the same setup — the test of the test.
Yaw Sweep
Testing across wind angles, not just head-on, because that’s how courses are actually ridden.
Weighted average
Your CdA scored across the air speeds and wind angles your course actually produces, each weighted by how often it occurs – collapsed into one honest figure.