Drafting - how far is close enough?
Is 10m offset enough to prevent unfair drafting in triathlon?
Drafting is a topic of hot discussion in the triathlon world at the moment, particularly amongst the pros where the field is getting closer and closer together and every watt and every second counts! Triathlon is intended to be an individual’s race where there is no team support, no external help, just the athlete against themselves and the elements. In this respect, drafting another rider or any vehicle for that matter does not belong within the DNA of triathlon competition. In most professional and amateur triathlon races, the 10m rule applies on the bike course, where riders have to keep at least 10m between themselves and the rider in front. If overtaken, the overtaken athlete is required to immediately drop back to re-establish the gap. In the ‘Challenge’ triathlon series, this offset rule is increased to 20m. But how much is enough to prevent an athlete gaining an unfair advantage from drafting?
The Swiss Side aerodynamics team recently conducted a CFD study and a wind tunnel test to quantify the effects of drafting in triathlon. For the CFD study, we put the super computer cluster to work, creating some massive models to simulate two riders with different offsets between them, from 0.1m to 20m. The target was to quantify not only the drag reduction on the drafting rider, but also on the lead rider who is punching the hole in the air. In the wind tunnel, we put two riders with 0.1m offset to each other and in addition. We measured the aero drag on the drafting rider and also with the use of the Swiss Side Aero Pod, we were able to measure effective airspeed which the drafting rider felt. The results were very enlightening:
Flow field of riders drafting at 0.1m offset distance
Drag savings with drafting distance
Flow field of riders drafting at 10m offset distance
Wind tunnel drafting simulation
Swiss Side Aero Pod for air velocity measurement on the drafting rider.
The drafting effects are very significant. In CFD, the pure aerodynamic drag of bike and rider at 45km/h is 250W (not including other resistances such as rolling resistance and drivetrain losses). In close drafting with two identical riders only separated by 10cm, the advantage is huge, with a 90.1W (39.5%) drag saving. Even the lead rider’s drag is reduced by the presence of the drafting rider, with a drag reduction of 10.9W (4.4%). The leading rider continues to benefit from a drag reduction due to the drafting rider up to 5m offset.
10m offset distance is definitely not enough to prevent a drafting advantage in triathlon. At 10m a significant drafting effect is still present with drag reduction of 33.5W (13.4%). The simulations were conducted for wind still conditions but nonetheless, even with light winds, drafting advantages at 10m offset will still be measurable. This is confirmed by many professional athletes. In Germany’s Triathlon Magazine from this month (Sept. 2017), many leading pro athletes including Andreas Dreitz, Boris Stein, Maurice Clavel, Nils Frommhold, Lionel Sanders, amongst others, were interviewed about the effects of drafting. They all confirm that at 10m the drafting effect can be felt.
The CFD study indicates that drafting effects are still measurable up to 20m offset distance, however this is with wind still conditions only. In this range, the wind will play major role and it can be assumed that in the real world, by 20m the effect will be minimal.
Flow field of riders drafting at 20m offset distance
For further reference and correlation, the effect of the two riders separated by 10cm was simulated in the wind tunnel. In this case, the leading rider was a large (tall) rider and the drafting rider was a medium rider. Also the leading bike was positioned using a home-trainer base. The drafting riders’ drag was decreasing by an astounding 65% at 45km/h, significantly more than seen in CFD. This is because the leading rider was a lot larger than the drafting rider and in addition, the home-trainer provide extra additional artificial blockage. These two things clearly enhanced the drafting effect. The Aero Pod measured a local airspeed of the drafting rider of 26km/h, which perfectly correlates with the measured drag reduction.
So in summary, drafting effects are hugely significant. In close proximity, drag reduction of over 50% is possible for the trailing rider. A 10m offset is clearly not enough to prevent drafting advantages with a drag reduction still in the order of 13%. 20m is however enough and in the range from 10m to 20m, it will depend on local wind conditions.
For more information on drafting, also check out the recent triathlon magazine (September issue). They feature a very informative article with lots of interesting opinions on the topic from pro athletes to event organizers. Aero Expert Jean-Paul Ballard from Swiss Side explains his research findings from the CFD studies, which are in line with what the athletes feel on the road. The magazine is available at the magazine stores right now or for download here. This is what you would look for:
Images: Copyright by Swiss Side