Sunday, February 24, 2013

Energy saving while drafting (IM Focus)

So it's now less than 4 weeks to IM Melbourne in Australia, and just in time for a somewhat light hearted look at drafting, or conserving energy legally, whichever you prefer to be the correct term.

It's been widely accepted that there is benefit drafting behind another rider, in fact Kyle(1) published data as early as 1979 showing a 47% reduction in drag at 0 m behind another cyclist, and 27% reduction at 2 m.

Some time ago our local triathlon forum had two schools of thought emerge on "pacelining" in IM, where to be paceline "legal" a cyclist has to be 12m from the cyclist in front; the first being there was no benefit at 12m, and the second that there was benefit but that it was fairly minimal.  Knowing that there would be some benefit, but unsure how much, I designed an experiment that would try to test the reduction.

This involved mounting a laser pointer on the bike (Cervelo P3) pointing at the ground 12m away from the front of my bike.  When following another rider, all I had to do was make the laser dot on the ground match the cyclist in front's back wheel and bingo accurate measure of the correct drafting distance.  Since this test was done on a velodrome, it was pretty easy to concentrate enough (and safely) to ensure that I was pretty accurate with the trailing distance (perhaps +/- 0.2 m error over the interval durations).

But I was concerned that I would possibly be biased in the way that I extrapolated the results; I believed the benefit to be there, so I might unwittingly find what I was looking for without knowing that I had somehow biased the analysis.  The solution was to get friend and previous coach Alex Simmons to do a blind analysis of my power file WITHOUT telling him what I was doing, only that I'd done a number of intervals and that sometimes I was in front, and sometimes not. Each interval had no intermediate or lap markers to denote changes from in front to drafting, and Alex did not know whether I started in front or behind on any of the intervals.

My quick analysis of the results was enough to confirm what I thought, notably an approximate 10% reduction in power required at 12m separation (front wheel to front wheel), which is pretty darn significant, and could mean the difference between running the IM marathon of your life, and lying huddled on the side of the road somewhere imbibing flat coke in a desperate attempt to get to the finish.... but I digress.

Alex, however, went to town on my file; presented with a challenge he left no scientific stone(2) unturned in the quest for knowledge....

Results

He found (to cut to the chase):

So in summary, the gain by drafting the other rider was a reduction in apparent-CdA of:

Interval 1: 0.035m^2
Interval 2: 0.033m^2
Interval 3: 0.026m^2

In terms of energy benefit for for Rob when drafting over leading, when riding at 40km/h this equates to wattage savings of:

Interval 1: 29W
Interval 2: 27W
Interval 3: 21W

The intervals ranged around 240, 260, 280 watts average in respective intervals 1, 2, and 3.  So a very handy saving - roughly 7.5 - 12%.

How significant is that you ask? Well if you're typical triathlete drag (CdA 0.285) at fairly typical triathlete race watts for IM (190) and you got 10% free speed (209 effective watts) your speed would increase from 9.71 m/s to 10.08 m/s, sending you 0.37m (1 ft 3 inches) further down the road for EVERY SECOND you are on the bike!  Your bike time would drop from a hypothetical 5:08:58 to 4:57:37 (3).  A whopping 11 minutes and 20 seconds saved. 

OK, so after all this nerdy work what would be great is if we could get some practical advice on how to use this to our advantage in an IM cycle race. Assuming you have a power meter, and you know what average watts you intend to target for the entire race, I've scribbled up some cheat sheets give you a pictorial and rule based guide to using the ~10% energy saving you'd get from drafting at 12m as you interact with other riders.  It's available in PDF in both right hand drive (Aus, NZ, UK, Japan etc) and left hand drive (Euro, USA & Can etc) versions.

Have fun out there!

Big thank you to Alex Simmons for taking the time to write up the results with proper analysis and detail.
You can read the full analysis on Alex's blog.

References:

(1):

Kyle, C.R. (1979) Reduction of wind resistance and power
output of racing cyclists and runners travelling in
groups. Ergonomics, 22 (4), 387–397.

in


R.A. Lukes*, S.B. Chin† and S.J. Haake*


The understanding and development of cycling aerodynamics,
ISEA Sports Engineering (2005) 8, 59–74


* Sports Engineering Research Group, Department of Mechanical Engineering, University of Sheffield, UK
† Department of Mechanical Engineering, University of Sheffield, UK


Note: Lukes et. al. is a great read (not overly scientific) and a copy is available on this site.

(2) Science: it works b*tches.

(3) Yes it is a hypothetical calculation, and no you can't hold exactly the same aerodynamics for an entire IM bike leg. Despite this, of the relatively few predictions  for IM based bike times where I've known the rider characteristics (3 total), I've been within a minute or so of the riders actual time.

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