You’re riding well, down on your aerobars, when a pack of triathletes engulfs you, what do you do? Sit up and drop back the required 12m then do the same when the next pack comes along; work hard to go back round and ride off the front, only to blow up later in the ride; or do you sit in, maintain your pace, perhaps even take a turn on the front? Pictures from Ironman Melbourne (if you haven’t seen them, look here) would suggest this dilemma was faced by most age groupers at some point during their day. But it is a problem that extends beyond one race: with increasing numbers of athletes crowding courses the formation of packs seems almost unavoidable.
In simple numeric terms if you have a 12m drafting zone, as they did in Melbourne, then you can fit roughly 83 perfectly-spaced athletes in a 1km stretch of road, with a field of 2000 you need around 24km to correctly fit everyone in. The process of legally overtaking may mean exact distances vary, but basically it takes a lot of road to space out that many athletes. So when you shorten the swim element of a race effectively bunching the field closer together, a crowded bike course is guaranteed.
The impact of the shortened swim is quite obvious when you look at the number of athletes leaving T1 at minute intervals. In 2012 there were fewer competitors and a full distance swim, at no point were more than 50 athletes leaving transition during the same time frame. In 2013 though, with higher numbers and the reduced swim distance, transition peaks with 100 athlete all leaving in the same minute interval. It may have been necessary to shorten the swim, but the crowded bike course and formation of packs that followed was inevitable.
By the time the athletes return to transition the numbers have changed, as the graphs above show the rate is much lower (each column now represents a 5 minute period). The bunching that we saw at T1 has diminished and groups have generally split. The pattern is a little denser in 2013 than in 2012, so it appears they haven’t broken up as much, but this may be due to the larger field. Packs formed early in the race when the swim failed to separate the competitors, 180km later groups have tended to disperse and spread; there is little evidence of strong pelotons working together for the entire course.
There were timing mats every 45km in Melbourne and I can use that data to look a little more closely at how the groups dispersed. The graphs above allow easy comparison of each split. By 45km the density seen at the exit of T1 has significantly diminished with a peak of 208 athletes in a single 5 minute window; this continues to drop, peaking at 162 at 90km, 123 at 135km and 81 at T2. All good signs that packs were generally separating. I think it’s reasonable to say that the majority of those who found themselves caught in a bunch due to the density of competitors at T1 were likely riding clear by the end of the bike. Of course there will have been some who choose to draft, but this is a much harder contingent to assess. Timings of athletes at static points on the course reveal little of the overall dynamics of the race, simply being close to another athlete at a timing mat does not mean you were drafting.
I am experimenting, examining groupings of athletes over consecutive timing points and identifying those who maintain close proximity across the course. Too raw to give numbers from and it has its flaws: it can’t distinguish a legal pace line from a team time trial, nor could it spot an opportunistic drafter who jumps from athlete to athlete. Post hoc drafting identification will never be possible, but perhaps there is a way to compare groupings on different courses. With more work I may be able to suggest which courses see the highest rates of pack formation, I suspect it will correlate with number of athletes. In the meantime, for the curious, the chart below gives the athlete density at T1 for all the 2012 Ironman races.
Drafting will always be a problem, there are always those willing to cheat, but the density of athletes on a course amplifies this by drawing in those who might otherwise ride honestly. If you left T1 surrounded by dozens of other competitors, would you feel it was your obligation to drop back or theirs?