Now that I’ve got a few races under my belt with power data, I wanted to take some time to go over what I’ve learned and how useful I think it is. Some of the analysis is for power data only, but I’ll also go over what other telemetry (GPS, heart rate) can mean for racers.
The most important data derived from GPS is lap times. Lap times should be consistent overall for optimal pacing. Striking a balance between a good start and good pacing is key. On a technical course with difficult passing, you could easily lose 30 seconds in the first half lap if you’re stuck in traffic. However, those 30 seconds may only be the difference between a couple places at the end of the race. A few seconds may be lost waiting to pass each rider as the race progresses, but overall it becomes increasingly easy to pass as the race goes on.
As far as pacing, I’d suggest trying to slot into the place you intend to finish at the start of the race. Racing too far ahead can lead to poor pacing later in the race due to fatigue. Cyclocross lap times are surprisingly consistent. In this example from last year, the leader’s times varied less than 10 seconds over each of 9 laps!
Above is a tracing from a recent race which shows pretty good pacing. One of this issues I had, which is reflected in the tracing, was death by bobbles. I was able to stay with the lead group for the first two laps, but I had a few bobbles a near crashes that left me surging to regain the group. This lead to my heart rate spiking a few times until I eventually lost contact with a large group of about eight riders.
I still rode smart enough to maintain my pace, as can be seen in my consisten heart rate at threshold for the majority of the race. My lap times were consistent, slowing from 7:35 on the first lap to 7:46 on the second to last. On the last lap I wasn’t within reach of anyone else so I eased off quite a bit as can also be seen in my heart rate. Throughout the race as riders fell off the lead pace, I passed about one rider per lap and I managed to finish a respectable fifth place.
Above is a power tracing from the first lap of the same race. It looks almost identical to an eight minute segment of a criterium. The power numbers for the first lap are a bit higher than the subsequent laps by about 10%, but actually average the same if the initial 30 second sprint is taken out.
Easily the most important metric will be Normalized Power (NP). NP is calculated to account for the increasing physiologic demand of higher power outputs. In a hard hour long criterium, I expect to have a NP very close to my threshold power. So far this cyclocross season, I’ve been seeing numbers about 5% lower than threshold (320w for this race). I believe this to be due to the time off the bike. Four, six second running sections per lap will take 24 seconds, or 5% of an 8 minute lap, thus decreasing power by 5%. Of course, the power meter doesn’t measure anything when not pedaling, and the running sections also have high physiologic demand, so a short but critical part of the race isn’t being measured.
Another important metric is Average Power, and how it is compared to Normalized Power, which is sometimes referred to as VI (Variability Index). This shows how even the effort was. Some courses will naturally have higher variability than others, so I’d take a look at VI as the race progresses. In the opening laps of this race, my VI was highest (1.15) and lowest (1.08) in the middle of the race when I was riding smoothly and setting my own pace. I’d suggest that riding the corners at maximum speed will limit the power required to get back up to speed, lowering VI, NP and reducing physiologic demand without increasing lap times.
The last sentence above alludes to a major point of racing cyclocross: technical skill is a major part of the equation. Inefficient body mechanics will unnecessarily increase fatigue, slowing excessively in the corners will cause higher power output, but not faster speed. Higher power may not always be a sign of a good race, also the case in road racing.
I’m not sure how helpful it is, but I made a quick spreadsheet with a few possible metrics for ‘efficiency’, comparing speed, power, NP, and VI. I’m certain it won’t be comparable across courses (it can’t account for more technical courses or slower course conditions), but I think the data shows which lap I rode most efficiently.
I’ve colorized the data at the extremes in each set of data. Clearly, something happened on the fourth lap (Fifth row, above). From memory, I’d say that I’d given up on sprinting to regain the lead group and was riding as fast as I could at my own pace. My average HR was lowest (ever so slightly), but my speed was second fastest despite my second lowest power numbers. During this lap I likely had the intense yet relaxed focus that is the essential midset of a successful cyclocross race.
The MPH/VI metric seems to select this lap as well, showing the highest value. Interestingly MPH/VI is independent of actual power output and could probably be compared among different riders on the same course and may be an indicator of smoothness and/or carrying speed through the corners.