Since Greg LeMond used them in the 1989 Tour de France to secure an 8 second overall win over Laurent Fignon at the final time trial, aerobars have become a staple of time trials and triathlons for both pros and amateurs. Using aerobars allows you to ride tucked in, reducing the surface area that you expose against the wind, and lets you achieve higher speeds with the same effort.
The aerodynamic advantage a cyclist gains depends on the type of aerobars he/she is using, their proper installation on the bike, the proper riding posture, and many other factors. Extensive testing with professional cyclists using accurately adjusted equipment in wind tunnels have shown a reduction of about 10% in the wind resistance, when their bikes were outfitted with aerobars. On a flat road and at high speeds (more than 20 mph), the gain in speed is about one third of the reduction in the wind resistance. In other words, an aerobar that helps you reduce the wind resistance by 10% will allow you to reach a speed that is about 3% larger at the same power level (as you can readily convince yourself by using a cycling power meter). This is a substantial advantage during a race, as it shaves off about 2 minutes for every hour of cycling!
The situation is very different, however, when the race has substantial inclines. As you ride up a hill (even a moderately steep one), a larger fraction of your effort goes into ascending the hill and fighting against the friction of the tires with the road surface rather than into dealing with the wind resistance. As the grade of the incline increases, the relative aerodynamic gain with the aerobars decreases. Moreover, add-on aerobars may weigh as much as 1-2 lb (500-1000 gr), forcing you to spend more effort carrying them up the incline. If the grade is large, the diminishing aerodynamic advantage of the aerobars is overcome by the work you need to put in order to carry them up the hill to the point that you actually ride slower with the aerobars than without them.
What is then the break-even grade, beyond which it is a disadvantage to ride with add-on aerobars? The actual number depends on your weight, your bicycle, the weight of the aerobars, and the average power at which you are attacking the hill. In most cases, however, the break-even grade is about 3-5%. The graph below shows the relative gain (or loss) in speed with and without aerobars, in a number of typical scenaria for a cyclist weighing 150 lb (about 68 kg), riding at three different power levels (150-250 Watts), along roads with different grades, from -3% (downhill) to 10% (uphill). On a flat road (0% grade), the cyclist gains about 3% in speed when using add-on aerobars. However, as the grade of the road increases, the gain is reduced and eventually the cyclist actually loses speed by using the aerobars. The break-even grade increases with the power output of the cyclist. It is about 3% at 150 Watts and 5% at 250 Watts.
What if the race is along a closed loop, as is the case in triathlons, where the bike leg starts and ends at the transition area? Wouldn’t the gains during the downhill segment always outweigh the losses during the uphills, if the up- and down-grades are more or less the same? Unfortunately, it is the slowest segment of a race that mostly affects the overall time and, therefore, the loss during the uphill weighs more than the gain during the downhill. For an out-and-back race, with similar uphill and downhill grades, the break-even grade is 6-8%. The relative gain (or loss) in overall time for an out-and-back race with and without aerobars is shown in the plot below, for a number of different cases.
It is worth emphasizing here that the numbers shown in the figures are based entirely on the change in weight and in the aerodynamic properties of the cyclist and bike, under the assumption that the cyclist can maintain the same power output both in the normal position (without aerobars) and tucked in (with aerobars). Besides anecdotal evidence that riding with aerobars is uncomfortable for most people, recent research shows that mildly trained cyclist cannot maintain their regular power output when using aerobars and ride tucked in. (Of course pros who train countless hours on aerobars can overcome these difficulties). If we consider the change in the cycling efficiency and in other physiological variables, aerobars will hamper rather than aid performance even at smaller grades.
The punch line? If you have light-weight aerobars (less than 1 lb), which are properly fitted on your bike, and you have trained using them, then put them on, as long as you’re riding a relatively flat race (with grades less than about 3%).