By Martin Ulloa
CdA — three letters that underpin nearly every conversation in time trial and long-course triathlon racing. Short for coefficient of drag area, CdA quantifies how much aerodynamic resistance a rider presents. In simple terms, the lower your CdA, the less power you need to hold speed. It's one of the most decisive metrics in race performance.
Three Ways to Measure CdA
There are three primary methods athletes use to test and refine their CdA:
1. Wind Tunnel Testing
The most technically sophisticated — and cost-intensive — option. Wind tunnels allow testing the largest number of variables across a range of yaw angles and wind speeds. They're ideal for benchmarking how specific equipment performs in different wind conditions. But there's a caveat: riders often adopt positions they can't realistically sustain in race conditions.
2. Track Aero Testing
Track testing offers a controlled environment while still requiring real-world handling and position stability. Measurement systems take a rider's power and speed, combined with atmospheric conditions, to calculate CdA. Once familiar with the rhythm of the velodrome, riders can focus purely on the run — no distractions, just data.
3. In-Field Aero Testing
Field-based aero testing is becoming the most accessible and race-relevant method for many athletes. Using on-bike sensors, the power meter, and speed sensor, field testing allows you to assess aerodynamic performance in the conditions you'll actually compete in — wind, gradient, temperature, and surface variability included.
Streamlines Background
The foundations of the Streamlines hardware started to take shape in 2016. It was 2021 when the Streamlines ecosystem really started to evolve for real-world aerodynamic measurement, emerging first with the Jumbo-Visma team. Since then, John and the team from Streamlines have amassed World Tour teams and Pro Teams to adopt the use of their technology.
Streamlines has developed the hardware and software that allow for accurate and reliable in-field measurement of aerodynamic efficiency. Combined with an accurate power meter and a speed sensor, Streamlines provides the equipment and protocol for actionable aero insights — outdoors, under real-world demands.
Take a look at our YouTube video to see how the session went.
Field Testing with Streamlines Aero
We partnered with Streamlines for our latest sessions to conduct in-field testing using the CIRRUS ecosystem. The Wind and Body devices attach directly to the bike and body position dynamically, while you ride. Speed, power, wind, environmental, and body data are all transmitted and logged in the NIMBUS data logger.
With Dylan and Martin, we tested various configurations — hydration, helmets, and clothing — while riding at target race intensities. The result? We gained clarity not just on what was fast, but on what each athlete should focus on for maximum benefit.
"We are happy with the initial work done in aero testing on the velodrome. We now want a more realistic picture of aero performance in the field. We are working through bottle placement and the development of accessories to support both aero performance and hydration requirements, and the Streamlines CIRRUS system can give us immediate, valuable feedback." — Ken Ballhause
Findings
For Martin, the main objective was water bottle positions. The findings were positive — there seemed not to be a penalty for bottles behind the saddle with this specific setup.
It was very interesting to cross reference the data from the velo to the real road. I think field testing is the real thing to be honest. In the velo I could get a crazy tuck going and not really have to look up, whereas on the road it felt more realistic with looking up the road, dodging rough patches, etc. Overall I was very happy with the field testing, I think this will be my preferred method of testing moving forward.— Martin Ulloa
Dylan found that the helmet played a huge role when playing with different fabrics on the sleeves. The helmet choice should be the first part of the puzzle when looking at upgrades on top of a good position.
Aero testing out in Doreen was a really successful day. I trialed different helmet, sock, and overshoe combinations and uncovered performance gains I wouldn't have otherwise found. The standout result was the integration of the Rudy Project helmet into my position. Its wide, flared shape helps smooth airflow across the top of my head and shoulders, reducing turbulence and improving efficiency. — Dylan Procker-Parker
At Sync, we leverage the CIRRUS system to evaluate athlete performance, optimize time trial positions, and refine equipment choices — from cockpit setup and garment selection to helmet placement. The system also plays a key role in the ongoing development of our products, such as the evolution of the Sync AB03 and its accessories. — Ken Ballhause
