The Physics of Drafting
by Richard Bowker
My interest in drafting was piqued when watching Stage 15 of the Vuelta a Espana (from Sabinaniga to Formegal) with my 9 year old son. You may recall the stage. Inexplicably, Team Sky started near the back of the group on the ride out from Sabinaniga. In doing so, they seemed to completely miss the fact that up front Tinkoff and Movistar were surrounding the Race Director’s car clearly desperate to get racing from the off.
That should have caused an urgent reassessment amongst the Team Sky riders but it appeared not to. Contador decided to attack and it was blistering, with even Quitana only just keeping up with the early pace. However, he did, and by 10km the front group were around 15 secs ahead of the chasing group with Chris Froome too far back to respond effectively. This was not least because most of his team mates were stranded even further back. By the end of the race, Chris Froome had lost around 2 mins 40 secs on Quintana, his Vuelta over barring a major disaster befalling Quintana.
Why not pedal faster?
Was it a mistake by Team Sky or just a stupendous ride by Contador, Quintana et al? “Both” said David Millar in his post race assessment. Mind you, he weighted it slightly in favour of the brilliant riding of Contador and Quintana rather than Team Sky’s mistake. But why, asked my 9 year old, when the gap was just 14 seconds and Froome clearly knew something was wrong did he not just “pedal faster for a bit and catch up”. Why not indeed?
Well, the answer is more complex than it might first appear. A large part of the reason why he couldn’t just “pedal faster” was that the lead group were working harder and more effectively as a team than the chasing group. Which begs the first question; if you work together as a team and by definition draft effectively, does it really make that much of a difference?
When an object moves through the air it pushes the air immediately in front of it to the sides and it flows round the back of the object. But it doesn’t enclose the object immediately. It takes time to flow round. Like an aircraft wing that is designed to create low pressure above the wing thus creating lift, a cyclist creates an area of low pressure immediately behind. And it is into this area low pressure, a vortex if you like, that another rider can slot and gain a benefit. The following rider gains a ‘pull’ from the low pressure vortex created by the lead rider and that is what we call drafting.
Tough on the lead rider?
Yes it is, because they are effectively giving the riders behind a tow. Interestingly, they get a slight boost as well by filling the space that would have otherwise have been filled by eddy currents. These eddy currents would otherwise slow a lone rider down. So the rider following behind reduces the drag that the lead rider would have otherwise felt from those eddy currents.
How is that possible you might say? Surely the laws of energy conservation say that you can’t create more energy with two riders than would have been created by those same two riders travelling along the road separately. Well, no you can’t. But by drafting as a pair you make the ‘system’ more efficient meaning you get more ‘work’ output as a drafting pair than you would as two riders travelling separately.
How much extra power?
This is where it gets really interesting. I found a fascinating blog post by Dave McCraw back in April 2013. You can read Dave’s excellent post by clicking here. In summary (and Dave does qualify the fact that his test was not carried out in perfect ‘lab conditions’) a rider leading a group requires 100W more power to keep a constant speed that the rider sitting in behind. (250W compared to 150W).
Since from basic physics we know:
P=FV (Power = Force x Velocity)
Then if Force remains constant (i.e. let’s assume it is the same rider in both the examples I am about to give) a rider travelling at 10 mile/h generating constant power of 150W will instead travel at just over 16.5 mile/h when generating constant power of 250W. Now that’s a massive difference (and now perhaps we can also see the obsession with power meters though that’s for another blog!)
Dave McCraw estimates that riding for an hour at 250W will consume 900 kCal (on average) whereas riding in a group of four drops that to 630 kCal per hour. I haven’t cross referenced these values from sources elsewhere as yet. But using them as a working assumption, 1800 kCal will get you 2 hours on your own or nearly 3 hours when working in a group of 4. So that’s one hour free for every 2 hours of work which is remarkable.
Quintana v Froome
So if Quintana and Contador could organise a group to work like this at the front, why couldn’t Froome do it further down the race? Well in theory, he could have done. But we now go back to the mistake made by Team Sky at the start. Froome got separated from his team mates early on and no-one else around him wanted to help.
In fact, it was worse than that. Valverde (Quintana’s team mate) was in the group with Froome and, with others, was positively disruptive. Froome’s adversaries chose to sit up and then attack regularly to prevent anyone getting into a rhythm rather than form a working ‘train’. That stopped Froome and Yates getting any sort of real response going until it was too late. Simple physics allied with old fashioned race cunning.
A chance blown?
So David Millar probably got it right; Quintana and Contador were incredibly organised when it mattered most. And if you watched the start of the race, they put in a tremendous effort at the begin of the race to create that gap. Actually, to be fair, Quintana put in a heck of a stint the entire stage both at the beginning and up the final climb, breaking Contador in the process. It was an incredible ride. And it left Froome rueing what might have been.