How to optimize your position on the bike

Credit: Mike Powell /Allsport
Is the front of my bike too low to generate adequate power? Are my arms too far apart for optimal drag? Is my seat too far forward?

These are just a few of the questions youve probably asked yourself when setting up your bike. Even elite-level cyclists such as reigning U.S. national time trial champion Adam Sbeih have needed help answering such questions to optimize their performance.

What if it were now possible to set up your tri bike with the confidence that your position optimized power, efficiency and comfort? By using empirical methods and computer simulations, it most certainly is.

The cycling leg of a triathlon is unique in that it is the only discipline of the three that allows for an incredible mechanical advantage in propelling ones self. As such, speed can be obtained but with the limiting force of aerodynamic drag.

This presents an interesting situation; the harder the athlete works, the stronger the force acting against the athlete becomes. For example, it takes approximately 15 extra watts to go from 20 to 21 mph. However, it takes an additional 37 watts to go from 30 to 31 mph more than double the effort for the same increase in speed. This seems like an almost hopeless situation.

Fortunately, there are more variables at play than power. For example, if the rider who wants to go 31 mph decreases his frontal area (FA), or body surface area exposed to the wind, by 5 percent, which is done quite easily, the necessary increase in power is reduced to only 17 watts.

The problem with reducing FA is that it is usually done at a power penalty. To become more aerodynamic, the riders effective seat tube angle is often increased in an effort to preserve biomechanical angles when lowering the athlete on the bicycle. However, in doing so, mechanical leverage is lost. Dealing with such issues in properly fitting an athlete to a bicycle requires a procedure that provides a cost/benefit analysis.

This brief description of how solid aerodynamic fitting is analyzed shows the importance of the ratio of power to frontal area. It immediately debunks conjectural statements to the effect that it is never worth sacrificing one watt of power to gain any aerodynamic advantage that have pervaded cycling in the past.

If the same cyclist mentioned above was able to decrease his FA by 10 percent, he could afford to lose 2 watts and still increase his speed to 31 mph.

Power and FA are two very important measurable variables in determining the optimum fit for a rider; however, turbulent drag and the effect of the gravitational force when riding on hilly courses are also important.

Turbulent drag is a function of both the riders FA and shape. This includes details such as back shape, arm separation on the aerobars, and knee position while pedaling.

Turbulent drag is typically more expensive to measure because it is generally done in a wind tunnel. Results from wind tunnel tests can be extremely useful in maximizing benefits from proper positioning, but unless multiple tests are run to find a mean, data can be deceptive. All conditions in the tunnel must be regularly monitored, multiple samples must be taken (to obtain a statistically significant sampling), and the error of each of the measurements must be fully understood to ensure true data.

For example, the air density in a wind tunnel can vary 2 to 3 percent throughout the day. Thats enough variance to make the drag measurements between two similar positions appear different enough to make an erroneous decision on fit.

There are other problems with wind tunnel testing, including the issue of determining what actually is being tested. Wind tunnel measurements can be highly accurate when measuring an athlete sitting still in a tunnel, but testing an athlete who is pedaling creates different pedaling-induced forces and drag forces. Understanding the interaction of these forces requires additional monitoring, which can be done but is often ignored.

This is not to say wind tunnel data are not useful. It is just very difficult to obtain reliable data from a wind tunnel without comprehensive study. In addition, without frontal area measurements, it is difficult to understand why one position is better than another, and which changes in position actually improve performance.

Aerodynamic drag and power are not the only variables important to proper bike set-up. Moment of inertia is also important to consider, especially for a course that encourages an athlete to accelerate often.

The moment of inertia is particularly important for courses with turns, rolling hills or gusty wind conditions that can force the rider to continuously accelerate back up to speed. Moment of inertia also has a greater effect on less powerful riders, as it consumes a larger percent of their power to accelerate the same mass as that of a more powerful rider.

Having described a very small number of the important variables to proper bike fit, it should now be more apparent that a riders optimum position is highly dependent on the environment in which he will be racing as well as the riders characteristics. The position (and equipment) should be chosen depending on the length of the event, weather conditions (wind, temperature, relative humidity, etc.), course altitude, the profile of the terrain, the strength of the rider, the riders weight and the riders power, among other variables.

So how does a racer make all of these decisions? One way is to measure all of the variables mentioned above and plug them into a computer program (or model) that dynamically solves for a riders speed (and estimated time) on a given course. A parametric study can then be used to determine which position and equipment increases a riders speed.

Basically, this involves changing the variables about a rider and their bike, such as power, drag, wheels, etc., in order to find the lowest time for a given course. In the past, these types of models were reserved for expensive, high-level university studies (see Martin et al., 1998). Now, due in part to the development of the models by university studies and by small companies like PK Racing Technologies, testing can be done at a relatively low cost. As a result, the technology is available for use among a wide range of cyclists.

A parametric study to determine the best position and equipment requires that a database on the athlete be compiled. The proprietary protocol used by PK Racing Technologies works as follows: First, use a cycling ergometer (e.g. CompuTrainer) in conjunction with a fitting cycle to determine power output, pedaling efficiency, and comfort in various positions (i.e. differing seat tube angles, saddle heights, saddle setbacks, handlebar heights).

Then, using an FA measuring technique, the riders FA is calculated for these different positions and various wind angles in order to extrapolate drag numbers for each position.

In the past, measuring FA was very difficult and time-consuming because the measurement required carefully cutting out individual photos of a rider in different positions and weighing them against a control. The proprietary FA measurement technique developed by PK Racing Technologies allows for FA to be computed very quickly and accurately.

Once the FA is obtained, it is used in conjunction with an extrapolated drag coefficient based upon previous research. If a rider wishes, an experimentally-obtained on-road drag coefficient can be measured as well. This empirical information is then combined with a comprehensive aerodynamic database on various types of equipment and race course profiles. A course is chosen and an unbiased rider position is obtained.

This is not a position that looks powerful or looks aerodynamic, but rather a position, based on good science, that provides the best combination of power, aerodynamics, comfort and efficiency for the given conditions.

One factor that plays a major role in equipment selection and rider position, especially at the elite level, is sponsorship. Many triathlon bike manufacturers build machines with extremely forward seat tube angles, in the 78-degree range, that are not optimal for power production, comfort and general efficiency.

For example, during a recent fitting session with professional triathlete Maryellen Powers, it was found that her previous sponsors bike (with a 78-degree seat tube) sapped her of a large amount of power and caused pain in her hamstrings. Our fitting protocol showed that a 76-degree seat tube was optimal for her power development and comfort, and putting her in a more biomechanically efficient position did not increase her frontal area.

The moral of the story here is that a forward position does not always equate to a fast position, and that only by collecting a large amount of data on an athlete can you achieve proper bike fit.

Go to part two of this story: "bike set-up."

Christopher Kautz, M.A. and Eric R. Pardyjak, M.S. are co-founders of PK Racing Technologies. Kautz currently serves as tactical director of marketing for Nimble Bicycle Co.

Pardyjak, whose background is in aerodynamics and mechanical engineering, holds a turbulence research position at the National Laboratory at Los Alamos, N.M. and is finishing a Ph.D. in fluid mechanics at Arizona State University. Both are category 2 cyclists. They can be reached at www.pkracing.net.


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