Finding Free Speed: Master an effective, accurate aero testing protocol

Getting it Right

When making adjustments to find the most aero position and equipment, there needs to be a systematic approach to the sequence in which each element is tested. There are interactions between the various components that you may be testing, for example skinsuits and helmets. There are multiple changes that can be made, and the permutations soon add up. That makes it important to track the variables being tested.

The amount of testing required can be optimised by testing the options in a logical order – one thing at a time.

Selecting a Test Venue

Selecting an appropriate venue is essential to accurate testing, in order to account for environmental factors and allow for proper calibration. The test venue can be one of the following:

• A flat “out and back” stretch of road, a minimum of three to five kilometers in length, with a consistent surface and minimal traffic.
• A dedicated cycle circuit with a consistent surface and minimal undulations.
• A velodrome.

Most on-bike aerometres like the Notio work best in low to moderate wind conditions with the wind in a near consistent direction. If the wind is gusting or at high yaw angles to the test direction, results will be less reliable. In general terms, the longer the duration of a test, the greater the accuracy.

Defining the starting position

There are two prerequisites that a rider should resolve before beginning aero testing. These are saddle position and the amount of “drop” that the rider is capable of sustaining for the duration of their target event. The hip angle is dependent on the drop between the saddle and the elbow pads, and also the fore and aft position of the saddle with respect to the axis of the bottom bracket.

The place to start is on the turbo trainer, using various combinations of saddle position and drop to find a position that feels comfortable and sustainable. The rider can then perform a CP20 test to get an average power for 20 minutes and compare that to their CP20 result from testing in a road bike position. Once the rider is satisfied with the saddle position and drop, ensuring that it is comfortable, sustainable and provides safe handling of the bike, they can then start thinking about optimising the front end and other aero factors to get the lowest possible CdA, and the highest ratio for the TT position CP20 power divided by the CdA.

One thing at a time

There are multiple potential changes in extensions, position, skinsuits, helmets and so on that can be evaluated for aero efficiency. The place to start is with the changes that are at the front end of the bike, those that the air meets first. The way the air behaves after it has passed the first obstruction affects how the air will behave when it meets the next obstruction further back. Unfortunately, downstream behavior can also have an effect on things upstream, so the testing does become a cyclical iterative process, and, with any luck, a converging process.

When adjusting position on the bike, it's important to measure virtually everything and record the measurements before starting the field testing. There is nothing worse than finding that you do not have the measurements to go back to a faster position you have tested previously. Every time you make a mechanical change, measure and record everything again. You can do this on paper to start with, but it is better to keep a record in a spreadsheet.

What to test / order of testing

Most riders won’t have the option of different base bars, but that would be the first thing to test if it is an option.

The obvious place to start, once the base bar is chosen, is with the extensions and riser stack. The hands and arms are the first thing that the air encounters. Already there are multiple variables with just the combination of extensions and riser stack, add to that the angling of the extensions and pads — now a standard feature on several market-leading systems — and the permutations multiply rapidly.

If you’ve already established the maximum sustainable drop on the trainer, start with that and look at extension styles and angles of attack. Start at the extremes – for example, S-bends and horizontal forearms compared to extensions with a larger angle, which produce a “high hands” position. Select the faster of the two for comparison with the next option, and so on.

Once settled on extension style, drop, pads and angle of attack, move on to testing that position with a range of helmets. In general, the gap between the rear of the helmet and the shoulders / back needs to be eliminated, and the uppermost part of the helmet is ideally no higher than the highest point of the shoulders / back. This is something that can also be evaluated on the trainer before field testing.

Once settled on a helmet choice, move on to tests skinsuits if you have the luxury of choice. Again, you need to keep a record of what you test so that you can always go back to the fastest combination after you have experimented with other options.

We are focusing here on the rider’s position and equipment, as that accounts by far for the largest portion of aero drag – up to 80% – as opposed to the bike itself. However, wheels are also frequently tested. Rear disk wheels tend to outperform deep section rear wheels. Often, though, most riders will only have one disk wheel to choose from.

For front wheels it comes down to two factors: speed (aero efficiency) and handling. High performance front wheels tend to be expensive, so the best option is to borrow them to test before making an investment decision.

Repeat tests to validate the results

One of the best ways to convince yourself that your testing results are accurate is to perform repeat tests. This can be testing two of three configurations on the day in A-B-C-A-B-C mode or A-B-C-C-B-A mode, or testing the same configuration on different days.

Another way to check consistency of results from test day to test day is to use a benchmark system. For example: start a test day with a known configuration from a previous testing day. Rather than have a single benchmark configuration you can choose the fastest configuration from the previous test day, and then try to better that by testing small incremental changes.

When you are seeing small differences in CdA from, for example, testing different front wheels or testing aero helmets, it is always better to use a repeating alternate testing protocol such as A-B-A-B-A-B. Changing wheels and changing helmets is a relatively quick process as no mechanical changes are involved.