by Andrew R. Coggan, Ph.D. - Faced with the question of which of these two frames to use for the pursuit at master track nationals in 2004, I opted for the Cervelo. I did so because 1) unlike the Javelin, it is a true track frame with horizontal fork ends, thus making gear selection much easier, and 2) this particular P2T had a winning “pedigree”. However, after an unexpectedly poor performance at nationals due in part to higher-than-expected aerodynamic drag, I began to wonder if perhaps I had made the wrong choice. In the fall of that year I therefore conducted a series of field experiments using my powermeter to see if I could discern any difference in aerodynamic drag characteristics between the two frames. The results of this study are described in this report.
To compare the two frames, I used an SRM Professional track crank to measure the power required to propel them at steady speeds ranging from ~20 to ~50 km/h. I used these data, along with measurements of barometric pressure and air temperature (to calculate air density), to determine my effective frontal area (i.e., CdA in m^2; product of coefficient of drag, Cd, and frontal area, A) on each bicycle (see Data Analysis). These tests were performed on a ~1 km segment of a very flat, smooth, asphalt road (i.e., Centaur Road in Wildwood, MO). The influence of wind was minimized by 1) using the westernmost portion of this road, which is sheltered by dense woods, 2) collecting data immediately after sunrise and only on days when wind speeds were minimal (i.e., less than 0.5 m/s), and 3) performing 6-9 “runs” in both the easterly and westerly directions and in a random order. With this approach, I was able to estimate my CdA to within ±2%, or with approximately the same degree of precision as can be achieved when testing a pedaling rider in a wind tunnel.
I tested both the Cervelo and the Javelin using two different positions, i.e., once with a 17 cm drop from the saddle to the elbow pads of the aerobars, and once with a 20 cm drop. These distances, along with saddle height, saddle setback, distance from saddle to end of elbow bars, etc., were all confirmed by careful measurement. Fortunately, the two frames had essentially the same “reach” and “stack” (i.e., length of top tube forward of, and height of the top of the headset above, the bottom bracket, respectively). Thus, after positioning my saddle in the same location relative to the bottom bracket on each, all that I needed to do to ensure that my position was the same on both of them was to simply transfer the same handlebars and stem from one to the other.
During all trials, I wore the same technical fabric T-shirt, skinsuit, socks, shoes, shoe covers, and Troxel Radius II helmet. Furthermore, in addition to using the same handlebars (Oval Concept A700) and stem on each bike, the following components were also kept the same:
Fork: Cervelo Chord
Front brake/brake lever: Shimano Ultegra/Tektro 4.0
Front wheel: Zipp 404 with Veloflex Record tubular inflated to 125 psi
Rear wheel: Hed track disk with Tufo S3 tubular inflated to 135 psi
Crank/bottom bracket: SRM Professional track model/generic square taper sealed bearing
Pedals: Speedplay X-2Saddle: Avocet O2 Air
Thus, the only things that differed between tests were 1) the frame, 2) the seatpost, and 3) the chainring and cog used (i.e., 53x13 on Cervelo, 50x14 on Javelin). Different seatposts had to be used because of the Cervelo’s proprietary aerodynamic design, whereas different gearing was used to keep the distance between the trailing edge of the seat tube cut-out and the rear tire the same (i.e., 1.0 cm) on both bikes. Although in theory this could have resulted in a difference between the two bicycles in drivetrain efficiency, any such difference was considered likely to be insignificant and less important than standardizing the frame-tire gap. Finally, to make the comparison the two bikes as easy to interpret as possible, I used electrician’s tape to seal over openings in the Javelin frame normally used for internal routing of brake and shift cables (however, I chose not to saw off the front derailleur hanger!).
Following completion of each set of trials, data were downloaded from the SRM handlebar computer into TrainingPeaks WKO+ (http://home.trainingpeaks.com/wko-desktop-software/analysis-software-for-training-files.aspx) for subsequent analysis. The average speed and power during each run was first determined, taking care to ensure that the speed was identical at the starting and ending points of each run (to eliminate variations in stored kinetic energy). These directly-measured power data were then adjusted downward by 2.5% to account for frictional losses in the drivetrain. This value was assumed based on the results of published scientific studies as well as extensive comparisons of this specific SRM crank to other power-measuring devices that sense power at the rear wheel (i.e., PowerTap, Velodyne). The power and speed data were then analyzed by fitting them to a curvilinear regression of the form:
where Y is the power (in W) and X is the speed (in m/s). As such, the constants a and b represent rolling resistance (in N) and the product of one-half times the air density (in g/mL) times CdA (in m^2), respectively. Air density was calculated based on air temperature and barometric pressure at the time of each trial as reported online from the nearby (~2 km) Spirit of St. Louis Airport in Chesterfield, MO. Furthermore, the absence of any significant local variations in temperature was confirmed by comparing those reported from the airport to those measured on-site using the SRM handlebar computer. To check for possible gradient- or wind-effects, data from easterly and westerly trials were first analyzed separately; however, no significant trends in the data were apparent. Data from both easterly and westerly trials were therefore pooled for further analysis, yielding a single estimate of CdA per day/condition (bicycle set-up).
Data from a representative series of runs are shown in Figure 1, whereas the overall results are shown in Table 1, both of which are shown below.
Table 1. Complete results.
As displayed graphically in the figure and as demonstrated by the low standard errors of the estimate shown in the table, the model provided a very close fit to the experimental results. Furthermore, in both positions my CdA was lower when riding the Cervelo than when riding the Javelin, although the magnitude of this difference was only slightly greater than the uncertainty of the measurement.
It must be emphasized from the outset that the difference in CdA between the Javelin and Cervelo was so small that it could have been entirely due to chance alone. On the other hand, the fact that the magnitude of the difference was consistent across the two positions, as well as the fact that it was apparently possible to detect a difference between the two positions in the first place, suggests that the difference in CdA between the two bikes may in fact be real. Although small, a difference of the magnitude observed (i.e., ~0.005 m^2) is potentially quite significant in competition, as at normal racing speeds it would result in a time differential of ~0.5 s/km, e.g., ~1.5 s in a 3 km pursuit or ~20 s in a 40k TT.
While the data suggest (but do not prove) that the Cervelo has less drag than the Javelin, the reason why there might be such a difference is not immediately clear. Obviously, however, the difference must lie in the frame (and/or seatpost) itself, since all of the other components were the same. It is therefore interesting to compare and contrast the specific design features of the two frames, even though it is impossible to draw any definitive conclusions.