Thursday, May 6, 2010

Which is faster: the Cervelo P3C or the Cervelo P2T?

by Andrew R. Coggan, Ph.D.

Over on another forum, I mentioned in passing that I had previously field-tested both a Cervelo P2T (Cervelo's track version of their original P2k) and Cervelo P3C (also the track version), and found that the latter was measurably faster. Another poster expressed a bit of surprise at this result, so I thought I would share the data here.

To compare the two frames, I used exactly the same procedures as described in this prior article:

except that I used a different saddle, front brake, helmet, wheels, and tires in this more recent round of experiments. The P2T and P3C were, however, fitted with the same components, i.e., the only thing that differed was the frameset. Furthermore, my position on the two bikes was identical (facilitated, again, by their equivalent geometries).

The results of this testing are shown in Figure 1 below, which illustrates my power vs. speed relationship (corrected for slight differences in air density) when riding the two bikes. (Note that for the sake of clarity, I have chosen to start both the X and the Y axis at a positive value, and not at zero.)

Figure 1. Power versus speed when riding a Cervelo P2T vs. a Cervelo P3C.

As can be seen in the figure, I required slightly, but nonetheless measurably, less power when riding the P3C, especially at higher speeds where wind resistance becomes progressively more important. For example, to ride at 13.89 m/s (50 km/h) on the P3C I would need to produce "only" 395 W, versus 402 W when riding the P2T, corresponding to a reduction in my CdA from 0.220 to 0.212 m^2. While this 7 W (1.7%) difference in power requirement/0.008 m^2 difference in CdA may seem small, anyone who has trained/raced with a power meter and/or done any field testing using one will realize that it is not. In terms of a time differential, using the P3C instead of a P2T would save me 0.85 s/km, or 2.55 s in a 3 km pursuit or 34 s in a 40 km TT.

Note that the above measurements were made under very low wind conditions, i.e., at/near 0 deg of yaw. Since modern aero frames are designed to especially effective at the yaw angles typically encountered in competition, the above is likely an underestimate of the difference that would be observed under non-calm conditions. Indeed, Tom Anhalt has previously reported a slightly larger difference than the above when comparing identically-equipped Cervelo P2k and P3C time trial bikes in mildy breezy weather.

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