While cycling on level terrain at racing speeds (≈40 km·h−1), approximately 90% of the resistance impeding forward motion is a result of aerodynamic resistance (R a ), with the remainder primarily a function of rolling resistance (R r ) (10,18,21,24,26). Consequently, to predict level cycling performance, the power output or energy-generating capacity of a given cyclist should be normalized to some measure of that cyclist's aerodynamic resistance. Investigators have used many different techniques (e.g., wind tunnel, towing, deceleration measures, estimates from frontal area) to assess aerodynamic resistance during cycling, yet no single method has been uniformly adopted. Candau et al. (4) have criticized previous measures of aerodynamic drag from wind tunnel, towing, and regression analysis because of their complexity and lack of specificity. Unfortunately, individual measures of aerodynamic resistance are not typically reported in studies evaluating cycling performance (4,5,7,9,10,17,20,21,23,26). Rather, aerodynamic resistance is simply assumed from previously established references or from estimates of projected frontal area, despite the large variability in aerodynamic resistance between different individuals, body positions, and equipment (1,22,24,25). Thus, an accessible and accurate technique to assess an individual's true aerodynamic resistance is needed. Here, we present such a technique using rear hub (Cycleops Power Tap®; Saris Cycling Group, Madison, WI) or crank- (SRM® Scientific Edition, Cologne, Germany) mounted power meters.

Recently, we have demonstrated the feasibility of using such tools in quantifying the magnitude of the drafting effect in road cycling (11). On the basis of the concept of tractive resistance and its relationship to velocity squared, we estimated aerodynamic and rolling resistances from field measures of power and speed.

If power output (W) and velocity (m·s−1) are known, then the total resistance to movement (R TOT ) in newtons (N) can be calculated as external power (P ext ) output divided by overground velocity (V) (equation 1).

For a cyclist riding at a constant velocity on flat terrain, with no wind, R TOT is equal to aerodynamic resistance plus rolling resistance (equation 2). Under these conditions, R TOT has been termed "tractive resistance" (R Tr ) (5,6,10,26,27).

Aerodynamic resistance is a function of velocity squared (m2·s−2), the air density (ρ, kg·m−3), the projected frontal area of the bicycle and rider (A P , m2), and a coefficient called the drag coefficient (C D , dimensionless) that is influenced by the shape of the bicycle and rider (equation 3).

If a constant (k) is used to represent the product of 0.5, air density, projected frontal area, and the drag coefficient, then aerodynamic resistance can simply be expressed as:

Substituting kV2 for R a in equation 2 yields the following equation for tractive resistance:

Equation 5 follows the form of a linear regression equation, where y = mx + b. Tractive resistance (which can be determined from field measures of power and velocity) is equal to the constant k (i.e., the slope of the linear regression) multiplied by V2 plus rolling resistance (i.e., the intercept of the linear regression). Accordingly, a plot of external power output versus velocity squared would yield a linear regression from which a cyclist's aerodynamic resistance (slope of the relationship) and rolling resistance (intercept of the relationship) can be calculated. The power and velocity measurements must be obtained for cycling on level terrain at a constant velocity with no external wind.

Because k (equation 6) represents the air density, projected frontal area, and drag coefficient of the bicycle and rider, the aerodynamic components of the bike/rider system can be determined once air density is calculated:

The product of the projected frontal area and drag coefficient of the bicycle and rider has been termed the effective frontal area or "drag area" (A d in equation 7) (7,14,15,17,21,26).

Accordingly, dividing k by one-half the air density yields the drag area (equation 8), which can be used to describe a cyclist's aerodynamic profile independent of the environment.

Rolling resistance is dependent on the mass of the bicycle and rider system (m), the acceleration of gravity (g), and a coefficient (C r ) describing the tire quality (e.g., size, casing construction, and tread material/pattern), road surface, and importantly tire inflation pressure (13). By dividing rolling resistance by mass and the acceleration of gravity, rolling resistance can be expressed as a rolling coefficient (dimensionless) that is independent of mass and gravity (equation 9). To facilitate an understanding of the concepts presented above, Figure 1 illustrates the concepts and their interrelationships.

FIGURE 1-Dia: gram outlining the calculation of aerodynamic resistance (Ra, N) and rolling resistance (Rr, N) from measures of external power (Pext, W) and overground velocity (V, m·s−1). By taking measures of Pext and V at the rear wheel while cycling at a steady speed onlevel terrain in windless conditions, a cyclist's aerodynamics, represented as k, can be calculated as the slope of the relationship between tractive resistance (y, Pext/V) and velocity squared (x, V2), whereas Rr can be calculated as the intercept of this relationship. In this case, k represents all of the primary determinants of aerodynamic resistance independent of the velocity and is equivalent to a force per velocity squared (N·m−2·s2). If air density is known, then k can be further reduced to the drag area (Ad, m2), a value that describes aerodynamics independent of the environment. Finally, if Rr is divided by the total mass of the bicycle and rider system (mass) and the acceleration of gravity (g), a rolling coefficient describing the quality of the tire and road interface can be calculated.

Using this technique, Grappe et al. (14) assessed aerodynamic resistance during actual field cycling using the MaxOne® (Look Cycle International, France) rear hub power meter. The MaxOne was capable of discerning distinct body positions. Unfortunately, trials were only performed on a single individual. In addition, the MaxOne is no longer manufactured, and although its measure of power did correlate with their reference measure of power, the slope and intercept of those values did not agree. A similar method combined with a new computerized digitizing determination of frontal surface area was also used by Debraux et al. (8) to calculate an estimate of C D . Therefore, it seems that a reliable and practical means of assessing an individual's aerodynamic resistance can be developed using today's new generation of portable power meters. Such a technique could be extremely valuable in helping athletes, coaches, and scientists to predict and improve cycling performance.

The purpose of the present study was to use Power Tap and SRM power meters to measure aerodynamic resistance (k) per velocity squared, drag area (A d ), rolling resistance (R r ), and the rolling coefficient (C r ) of individual cyclists riding on level terrain in distinct body positions at different tire pressures. We hypothesized that: 1) at a constant tire pressure, a change in body position would alter measures of aerodynamic resistance but not rolling resistance (change in k and A d but not in R r or C r ); and 2) in a single body position, alterations in tire pressure would alter measures of rolling resistance but not aerodynamic resistance (change in R r and C r but not in k or A d ).

METHODS

Subjects.

Ten subjects, male (n = 7) and female (n = 3), were recruited for this study. All were experienced road cyclists actively competing as triathletes (n = 3) or road cyclists (n = 7), at the professional (n = 7) or elite amateur (n = 3) level. At the end of the study, however, data from only eight subjects met the appropriate criteria for inclusion in data analysis with only six of those eight completing all trials. The Human Research Committee at the University of Colorado at Boulder approved the protocol used for this study. Informed written consent was obtained from all subjects before data collection.

General protocol.

All subjects rode a standard road bike during the trials that was used during their training and racing sessions. Each cyclist used his/her own personal helmet throughout the experiment. Every recorded trial was conducted with the same bicycle, helmet, and clothing. The cyclists were required to use a standard racing kit. The outer layer was limited to a racing jersey (short or long sleeved), arm and/or leg warmers, cycling shorts, gloves, and fabric or Lycra shoe covers. The same outfit was used for all trials. Jackets, vests, and neoprene shoe covers were not allowed.

Subjects cycled over a level section of road extending along an east-west axis, termed the collection trap (CT). The CT consisted of a section of road stretching 200 m, linked at either end by "acceleration distances" of 400-500 m. The surface of the CT was composed entirely of smooth black asphalt. The gradient of the CT was determined by standard surveying techniques to be flat with a slight roll over the course that did not vary by more than 0.135° or 0.03%.

Four experimental conditions were studied based on a combination of two body positions and two tire pressures. The two body positions were determined by the location of the subject's hands and included the following: 1) hoods-seated with hands on the brake hoods and 2) drops-seated with hands on the bottom section of the handlebars. On the basis of each subject's personal preference and riding experience, the body positions for the two hand locations were determined. Once established, each individual was required to hold the same body position throughout all conditions. The two tire pressures used were 120 psi (828 kPa) and 60 psi (414 kPa). These two tire pressures were chosen to elicit a substantial difference in power required to ride to measure the sensitivity of the method. All tires were inflated using a Topeak® floor pump with pressure gauge (Topeak, Inc., Taichung, Taiwan) to the required pressure with athletes sitting on their bicycles to remove the influence of individual body mass on tire pressure. Accordingly, the four experimental conditions were as follows: 1) hoods at 120 psi, 2) drops at 120 psi, 3) hoods at 60 psi, and 4) drops at 60 psi.

For each of the four experimental conditions, subjects rode in both directions (east and west) along the CT at power outputs in 50-W increments that ranged from 100 to 300 W for women and from 100 to 400 W for men. These power ranges represented the expected power ranges in racing/training for the gender populations used in this protocol. At the end of a given pass in one direction, subjects abruptly ceased pedaling for 20 to 50 m after the end of the CT. This denoted the end of the CT in the data downloaded from the onboard computer associated with each subject's power meter. A given power output in a single direction was considered a trial, whereas the average of both directions was considered a trial doublet. Men performed seven trial doublets for each experimental condition, whereas women performed five trial doublets for each experimental condition.

Devices and calibration.

Subjects used either the Power Tap (n = 8) or SRM (n = 2) power-measuring devices, both of which record concurrent measures of the subject's external power output and ground velocity. Two subjects already owned and outfitted their bicycles with the SRM measuring device. Power Tap devices were mounted on the bicycles of those who did not own a power-measuring device. These data were stored on an onboard microprocessor unit and downloaded after the completion of all experimental conditions for each subject. Before commencing trials, all units used during testing were calibrated against a zero torque reference, while pedals were stationary and unloaded as directed by the manufacturer.

For the calculation of ground velocity, wheel circumference was initially measured directly with riders atop inflated tires but later standardized to a value of 2096 mm for the corresponding combination of rim diameter (622 mm) and tire height (≈23 mm) found with the 700 C rims and 23 mm "clincher" tires used by all subjects. This standardization stemmed from the finding that the maximum discrepancy between the circumference value of 2096 mm and that measured was only 10 mm (∼0.5%)-a range that was within the sensitivity of our ability to measure velocity.

It has been demonstrated using an external torque dynamometer that the Power Tap and SRM are reliable and valid measures of power output (2,12,18,27). On the basis of comparison to a first-principles mechanical system of the units used in our protocol, the offset between the two units, attributable to frictional losses in the drive train, remained constant at −7.6 ± 2.5 W throughout a power range of 100 to 500 W (19,28). This value was used to correct the SRM measurements to a power output representative of the power at the rear hub.

Environmental conditions.

To calculate (16) air density (ρ), continuous measures of ambient temperature (T), station (barometric) pressure (P S ), and relative humidity (H r ) were collected via a Vantage Pro model weather instrument (Digital Instruments, Enterprise, OR). In addition, wind velocity in the middle of the CT was measured at discrete 1-s intervals throughout all testing sessions using a hotwire anemometer (Extech Products, Inc., Melrose, MA). To minimize the effect of environmental conditions, each subject was required to complete all four experimental conditions in one test period-a period of 1-1.5 h. All trials were completed in the early morning between 6 and 8 a.m. when wind conditions are generally the calmest in Boulder, CO.

Data reduction and analysis.

P ext and V were analyzed using Power Coach™ (Kochli Sport, Sonvilier, Switzerland) software operated on an Apple G4 computer. This software presents data (V, P ext , time, distance) as discrete points representing averages of the recording interval specific to the device in use, Power Tap (3 s) and SRM (2 s). In analyzing data from each trial, all points contributing to the 200-m distance preceding the point at which subjects ceased pedaling (P ext = 0) were averaged for both V and P ext . In addition, the final data point (directly preceding P ext = 0) was always excluded so that trials in which subjects ceased pedaling at a time before the finish of the final 3-s average were not contaminated by an underestimate of the true P ext value.

The P ext and V values achieved through this process were then averaged for the east and west directions to give a mean external power output and mean system velocity for each prescribed power output. No difference, however, was found between the mean measures for east trials compared with west trials. These mean values for P ext and V from the east and west trials comprised the data points used to assess tractive resistance.

Tractive resistance was calculated as power divided by velocity (equation 2). A linear regression of tractive resistance versus velocity squared yielded a slope equal to aerodynamic resistance (k) per velocity squared and an intercept equal to rolling resistance (equation 5). Drag area was calculated by dividing the aerodynamic drag by (0.5 × air density) (equation 8). Finally, a coefficient for rolling resistance was calculated by dividing rolling resistance by the product of subject + bicycle mass and the acceleration of gravity. Mass was measured for each subject and their bicycle using a scale (Detecto Scales, Webb City, MO).

Exclusion criteria.

Trials were excluded if mean wind velocity exceeded 1.0 m·s−1 or mean acceleration exceeded 0.5 m·s−2. All exclusions were bidirectional, i.e., if either trial (east or west) was determined unfit for further analysis, so too was the second trial of that pair. In the circumstance that three or more east-west data points were deleted from a single condition, the entire condition was removed from further statistical operations. If a subject did not complete all experimental conditions on a single test date, that subject's data were also eliminated from analysis. Of the 512 trial doublets scheduled for completion by 10 subjects (7 men and 3 women), 102 trial doublets (19.9%) were excluded or not completed. Six subjects completed all test conditions, two completed only the 120-psi condition in the hoods and drops, and two others were eliminated because of a combination of excessive wind and an inability to complete all test conditions.

Statistical analyses.

A 2 × 2 repeated-measures ANOVA was performed on a within-subjects (n = 6) basis to distinguish primary effects of body position and tire pressure on measures of k, drag area, and rolling resistance. The primary effects were further compared by paired, one-tailed, Student's t-tests for the effect of tire pressure (n = 6), and body position (n = 8). Significance for all statistical analyses was set at P < 0.05.

RESULTS

Individual, mean, and SD values for aerodynamic resistance (k) per velocity squared, and rolling resistance (R r ) are presented in Table 1. Drag area (A d ), rolling coefficient (C r ), air density, and system mass (rider plus bicycle) are presented in Table 2.

TABLE 1: Individual and mean ± SD values fork (N·m−2·s2) and rolling resistance (N) plus associated correlation coefficients while riding in the hoods or drops at 120 and 60 psi. TABLE 2: Individual and mean ± SD drag area (m2), rolling coefficient, air density, and system mass while riding in the hoods or drops at 120 and 60 psi.

For each subject and experimental condition, the relationship between tractive resistance and velocity squared was linear and significant (P < 0.05). Table 1 presents the means ± SD and individual correlation coefficients for the plots of tractive resistance versus velocity squared across conditions. All of these values exceeded r = 0.9951, indicating that our field procedures were able to very accurately determine this relationship. At a given tire pressure, the slope of this relationship decreased significantly (P < 0.05) from the hoods to the drops, indicating a decrease in k. The intercept, however, did not change significantly at a given tire pressure between the hoods and drops, indicating a constant rolling resistance. In a given body position, slope did not change significantly, indicating a constant k. The intercept, however, increased significantly (P < 0.05) from 120 to 60 psi in a given position, demonstrating an increase in rolling resistance.

Changing position from the drops to the hoods increased aerodynamic drag from 0.1560 ± 0.0232 to 0.1750 ± 0.0258 N·V−2 at a tire pressure of 120 psi and from 0.1574 ± 0.0280 to 0.1765 ± 0.0290 N·V−2 at a tire pressure of 60 psi. Aerodynamic drag was significantly different (P < 0.05) between the hoods and the drops but not different between the hoods at 60 versus 120 psi or between the drops at 60 versus 120 psi (Fig. 2).

FIGURE 2-D: rag area for individual subjects and mean for all subjects while riding in the hoods and drops at 120 versus 60 psi. Drag area decreased significantly from the hoods to the drops at both 120 and 60psi with no difference in drag area in a given position at either tire pressure.

Decreasing tire pressure from 120 to 60 psi increased rolling resistance while in the hoods and the drops. Likewise, this decrease in tire pressure increased the rolling coefficient while in the hoods and the drops, an increase of 24.82% ± 7.02% and 24.83% ± 6.14%, respectively (Fig. 3). The rolling resistance and rolling coefficient were significantly greater (P < 0.05) at 60 psi compared with 120 psi while in the hoods and drops position. No significant difference, however, was found between rolling resistance and rolling coefficient between body positions at 60 psi or at 120 psi.

FIGURE 3: Rolling coefficient for individual subjects and mean for allsubjects while riding in the hoods and drops at 120 versus 60 psi. Nosignificant difference was found between the hoods and drops at a giventire pressure. Significant difference, however, was found between 120 and 60 psi in both positions.

DISCUSSION

Our results were consistent with our hypotheses that, at a constant tire pressure, a change in body position would alter measures of k and A d but not R r or C r and that, in a single body position, alterations in tire pressure would alter measures of R r and C r but not affect k or A d . Although it may seem obvious that a change in tire pressure and body position would alter rolling resistance and a cyclist's aerodynamic resistance, we have established that the combination of our protocol and cycle-mounted power meters had the sensitivity to independently discern between minor changes in body position and major changes in tire pressure.

The accuracy of our protocol depended on a number of factors. These factors included 1) the ability of the rider to hold a constant velocity to eliminate inertial resistance, 2) the use of a level road to eliminate gravitational resistance, 3) the absence of an external wind so that ground velocity would equal relative air velocity, 4) a sufficiently long trap to achieve a constant velocity and to minimize the variation in power output resulting from torque or cadence fluctuations between individual pedal strokes, 5) the ability of a rider to hold consistent body positions between different power outputs and experimental conditions, and 6) the ability of cycle-mounted power meters to be reliable and valid. Of these factors, the only two that created issues were wind and accelerations within the collection trap. Accordingly, trials were eliminated when the mean wind velocity exceeded 1.0 m·s−1 or when there was a measured acceleration through the collection trap that was greater than 0.5 m·s−2. Still, only 20% of the data needed to be excluded, and only data from two of the original subjects needed to be eliminated because of excessive winds.

Despite these potential sources of error, the lowest correlation coefficient obtained between tractive resistance and velocity squared for all of our subjects and experimental conditions using only five (women) or seven (men) trial doublets in each regression was 0.9951 with a mean ± SD of 0.9959 ± 0.0028. These values were significantly greater than the range reported by Grappe et al. (14) using the MaxOne rear hub power meter (r = 0.90-0.95, n = 12 trials × 4 conditions) and also higher than the values reported by di Prampero et al. (10) (r = 0.98, n = 33 trials) and Capelli et al. (5) (r = 0.97, n = 40 trials and r = 0.96, n = 19 trials) during measurements of tractive resistance while towing at distinct velocities. Although our strong, linear correlations do not imply that our data are more accurate, they do suggest that, for any given increase in velocity, the protocol and power meters used were extraordinarily reliable in their ability to measure an appropriate increase in aerodynamic resistance.

Outside reassessing each subject's aerodynamic resistance with another technique, the most practical way of understanding the potential accuracy of our aerodynamic measures is to simply compare them with those reported previously for road cyclists. The aerodynamic resistance of cyclists, however, can differ dramatically depending on the body position, clothing, wheels, and bicycle frame used (3-5,14,20). Thus, a direct comparison of our values with others is limited by the unique equipment, body position, and morphology of our subjects.

Notwithstanding, our mean values do compare favorably with those reported in the literature for similar body positions and equipment. Using cyclists on standard road bicycles, with spoked wheels, and in a dropped position, Pugh (26) has reported a mean A d value of 0.33 m2 (n = 4), whereas Davies (7) and di Prampero et al. (10) have reported mean values of 0.28 m2 (n = 15) and 0.32 m2 (n = 2), respectively. Although these values are similar to our A d value of 0.32 ± 0.05 m2 for the dropped position, the techniques used in these previous studies required more complex procedures to obtain their A d estimates. For example, Pugh (26), after assessing the relationship between oxygen consumption and power on a laboratory ergometer, measured the metabolic cost of cycling along a level road at different velocities to determine k and, subsequently, A d . Likewise, Davies (7) also determined the relationship between tractive resistance and velocity squared to calculate k but regressed power after first measuring the metabolic cost associated with cycling against distinct wind velocities on a treadmill placed in a wind tunnel. Although both protocols are limited by many of the same factors that affect our study, the additional sources of error introduced by the metabolic measuring equipment and the accuracy of these measures raise questions about the precision of these techniques. In contrast, di Prampero et al. (10) determined A d from k in two subjects by directly measuring tractive resistance while towing cyclists behind a motorcycle at several velocities. The correlation between tractive resistance and velocity squared, however, was not as strong as that determined with our protocol and may have been adversely affected by turbulence generated by the motorcycle.

Some authors have suggested that the most reliable technique for assessing aerodynamic resistance is a coast down or deceleration test performed in a large and enclosed hallway (4,9). Although this technique may not be specific to actual cycling in the field, it is promoted as being more specific than wind tunnel measures and is highly reproducible and sensitive to slight changes in body position (4,9). Using this technique in what could be considered a similar body position as our dropped position, de Groot et al. (9) evaluated A d in seven subjects riding standard road bicycles with their hands on the brake hoods in a position they characterized as "racing." In this position, a mean ± SD A d of 0.32 ± 0.04 m2 was found with a range of 0.28-0.38 m2. Not only is this mean identical with our own (0.32 ± 0.05 m2), the variability and range were also comparable with a broader range for A d of 0.27 to 0.44 m2 found in the present study. This range is equivalent to a 39% difference in A d between our least and most aerodynamic subject, whereas the range found by de Groot et al. (9) was equal to 26%.

These ranges demonstrate that even in a single body position, there are large differences between individuals in their aerodynamic resistance-differences that can have significant performance consequences. For example, at a speed of 40 km·h−1, the power required to overcome aerodynamic resistance would be equal to 214 W for our most aerodynamic subject and 350 W for our least aerodynamic subject at sea level (ρ = 1.16 kg·m−3). Thus, it is important to realize that although our mean value for A d in the dropped position is similar to the mean values reported in the literature, it is unlikely that mean values adequately represent the performance requirements of a given individual.

From the hoods to the drops position, our subjects decreased their drag area from a mean ± SD of 0.36 ± 0.05 to 0.32 ± 0.05 m2, which represents an average decline of 10.8% ± 3.5%. The minimum decrease from hoods to drops was 5.7%, whereas the maximum decrease observed was 17.8%. Like our results, Grappe et al. (14), also using a rear hub power meter, demonstrated an 8.3% decrease in drag area from 0.299 to 0.276 m2 in a single individual when moving from the hoods to the drops, whereas Candau et al. (4), using a deceleration test, reported a 6.6% decrease from 0.355 to 0.333 m2 in a single individual when moving from a trunk angle of 40°-35°. Similarly, Jeukendrup and Martin (15), using modeled data from wind tunnel measures, predicted a 14% decline in A d from 0.358 m2 to 0.307 m2 when moving from an upright to crouched position. Thus, not only are our data within the range reported on an absolute scale but also the relative change in A d associated with a change in body position matches previous reports. Practically speaking, the relative decline from the hoods to the drops results in a proportional decrease in the power required to overcome aerodynamic resistance. For our subjects, this results in an average decrease from 290 to 260 W at 40 km·h−1 at sea level when moving from the hoods to the drops. Assuming the same metabolic power output, that would translate to an average time saving of 2 min 6 s in a 40-km flat time trial.

In addition to aerodynamic measures, the tractive resistance protocol also allows the elucidation of rolling resistance and, consequently, a coefficient for rolling resistance. Like our values for A d , our C r values compared well on both an absolute and relative basis compared with those modeled and measured in the literature. For example, Grappe et al. (13), using a deceleration technique, found that increments in tire pressure (P T ) from 150 to 1200 kPa elicited an exponential C r response, described by the following equation:

When this equation is applied to the two tire pressures used during this investigation, the predicted C r values are 0.0044 (828 kPa) and 0.0061 (414 kPa). The actual mean values measured were 0.0047 and 0.0066 at 828 and 414 kPa, respectively. This difference would result in a loss of 1 min 32 s on average in a 40-km flat time trial at an equivalent power output. On an absolute scale, our actual values are only 6% greater at 828 kPa and 8% greater at 414 kPa. On a relative scale, the predicted difference between the two tire pressures was 38.6%, whereas the actual measured difference was 32%. Although the absolute and relative values compare well, the small differences found between predicted and actual may be explained by the use of clincher tires in our study and the use of tubular tires by Grappe et al. (13) because tubular tires generally have a lower rolling resistance (18). In addition, the deceleration technique used by Grappe et al. (13) may simply give lower values for C r owing to the smoother floors often associated with indoor hallways. As an example, Candau et al. (4) and de Groot et al. (9) also measured lower mean values of 0.0041 and 0.0038 for C r at tire pressure ranges of 600 to 1000 kPa during deceleration tests on a linoleum floor. In contrast, di Prampero et al. (10), during tractive resistance measures while towing cyclists on road surfaces, found a mean value for C r of 0.0047-a value equivalent to our own for 828 kPa.

In conclusion, the techniques described here when used with the Power Tap and SRM power meters are sufficiently precise to distinguish the affects of body position and tire inflation pressure on measures of aerodynamic and rolling characteristics, giving drag area and rolling coefficient values that compare well with values reported in the literature on both relative and absolute scales. Compared with other techniques for assessing aerodynamic and rolling resistances, this technique is inexpensive, relatively easy to control, and the most specific technique for a given individual and their equipment for road cycling. Because of the important performance consequences associated with changes in aerodynamic and rolling resistance, this protocol is an important technique for better profiling individual cyclists and in conjunction with physiological measures should help coaches, athletes, and scientists to better predict road cycling performance.

This research was funded by Cycleops Power Tap. The views expressed are those of the authors and do not reflect those of Cycleops Power Tap.

The results of the present study do not constitute endorsement by the American College of Sports Medicine.