There are two key components to the perfect time trial or multi-sport bike leg:
There are no short-cuts to increasing your power (or perhaps more critically your power : weight ratio) - this can only be built up through the application of a smart training program and many hours of good honest toil.
However, the rider's aerodynamic position can be carefully honed to gain free speed. Finding an optimised aerodynamic position which allows the best application of that hard earned power is critical to your success as a tester, multi-sporter or even road bike racer.
Many athletes invest a considerable amount of cash purchasing cutting edge equipment and even more time tinkering with their position. Yet, it is extremely difficult to quantify the potential benefit of these changes - how many times have you overheard the classic phrase?:
"Does this look aero?"
Of course the pro teams now routinely have access to wind tunnel technology, but this can cost upwards of £1,000 for just a few hours analysis - putting it out of reach for many amateurs.
Basic Principles of Bike Aerodynamics
Aerodynamic quality is measured in terms of CdA - the product of a coefficient of drag (Cd) and the frontal area (A). The lower the CdA value, the more aerodynamic the bike and rider. The more aerodynamic, the less power required to propel the rider at any given speed - or, in other words, the faster a rider can go for a given number of watts of power.
It is always important to note that the impact of aerodynamics increases the faster the bike is travelling - so the better you become as an athlete, the more significant your aerodynamic position will be.
A reduction of Cd is achieved through the careful selection of technically advanced equipment, fabrics and setup to ensure that air can pass as cleanly as possible. These gains are important, especially when aggregated together (the principle of marginal gains), but looking to improve your aerodynamics by focusing on drag alone may not be the most effective approach.
However, it is often possible for a rider to make a significant reduction in their frontal area by making changes to the position or equipment - for example, lowering the head, narrowing the shoulders, adjusting bar positions or changing helmets.
Frontal Area and Aero Score
So frontal area is a key part of the aerodynamic efficiency of a rider. But this figure may be difficult to interpret in itself, especially if we consider that inevitably a taller rider will naturally represent a larger frontal area than a smaller rider. However, you can normalise the frontal area as a ratio to the rider's height to create a comparable Aero score. This can be used as a benchmark by which one rider's frontal area can be compared to that of other riders of different heights, or to the same rider in different positions.
Of course, it should always be noted that the most aerodynamic position is not always the best - the rider must be comfortable (especially in multi-sport, where it is necessary to run off the bike) and able to apply power effectively. In many respects the power : CdA ratio is just as important to the time trialist as power : weight is to a climber.
For more information I strongly recommend reading the excellent published work of Dr Robert Chung who has pioneered this method of estimating CdA from power data (the "Chung Method"):