Tuesday, November 5, 2013

The Sum of Marginal Gains

Is it worth it, how much does it matter? Is paying attention to skewer alignment to save a tenth of a second really a useful way to spend your time? Do you really need to pace intelligently or can you just bike "all out"? Do these theoretical rolling resistance and aero time savings really occur in the real world?

I recently had the opportunity to compare two cases, from the same race, on the same day. I had access to the average power, height, weight, and equipment info of two riders who competed in the 2013 Austin 70.3 bike leg. The bike course at this race includes rolling hills, plenty of turns, and lots of imperfect pavement. It is very much a typical real world scenario. The difference was rather startling.

Rider 1
217 watts average
170 lbs
3 hours 10 minutes
17.6 mph

Rider 2
221 watts average
142 lbs
2 hours 21 minutes
23.8 mph

This is a massive 49 minute difference for the two athletes, on the same day, producing similar power.  How much of that can be explained by the difference in mass, size, and power alone?  We can plug this data into the equations of motion of a cyclist along with a reasonable approximation of the Austin 70.3 course and see that size, mass, and power account for only about 15 minutes of the difference at most. This leaves 34 minutes unaccounted for over the 56 mile course. Both riders were on entry level TT frames, and used their aerobars. 34 minutes.

Where does that time difference come from? A few minutes plus or minus could be attributed to power meter error perhaps. The rest comes from the sum of marginal gains, including some of the following, but not limited to, and in no particular order:

Rider 2 put a lot of effort into her bike setup, rode the course ahead of time, and had an intelligent pacing plan and this paid off.  Rider 1 is already well on his way to improving and I expect will surprise himself in the near future.


  1. I think it would be helpful to continue on with the equipment differences to see how much of that 33 minutes can be accounted for...you said you had that info, right? Let's start with tires and tubes :-)

    1. Indeed I had started speculating along those lines but felt it was adding speculation on top of speculation, so felt it would be simpler and more instructive to just illustrate the totality of 'missing time' that can actually occur. But some of the equipment differences - training wheels vs Jet6/disc, unknown tires with butyl tubes vs Conti 4000S w/ Latex tubes, torpedo bottle vs straw-based aerodrink, P2 with clean front end vs Fuji Aloha 2.0 with round base bar and exposed cables, aero helmet vs road helmet. Glued on number vs race-belt number. Omega brakes vs standard brakes,. Zipp aero skewers vs normal skewers. Aero chainring vs non aero chainring.

    2. My guess is that a vast majority of that is rider generated drag.

    3. Probably so. Note that the 15 minutes I account for does include the additional CdA expected from the taller and heavier rider, using the worst case of 3 different morphological models. If he was on gatorskin tires, or similar, that could account for an incredibly huge chunk of the time as well. Most likely, rider position is the biggest contributor. From race images, his position was not totally ridiculous. Unknown how well he held it, however.

  2. Once you get past the part of one rider being a male and the other a female, one rider being 28lbs heavier than the other rider and one rider being a newbie and the other being a well conditioned athlete.......you might be on to something where aerodynamics are concerned. Until that day comes you are posing an idiotic question.

    1. Hi Steve. Please note that I did account for the mass, and size differences using the equations of motions of a cyclist, including accounting for the innate difference in CdA expected from their size difference. This should account for 15 minutes at most. As for how gender affects this analysis, I have carefully reviewed my own source code and the literature and gender does not appear to be one of the variables that affect how fast a cyclist goes for a given mass, power, and CdA.

    2. Flapping numbers, flapping pony tails, and dare I suggest, drafting all come to mind as plausible culprits with significance. Weather conditions can also evolve throughout the day as well.

    3. Indeed, any comparison of bike splits in a crowded triathlon need to take drafting and traffic into account. Depending on the course the effect of constantly passing people can either help, or hinder progress a bit. In this case it is possible Rider 1 was slowed by the traffic more than Rider 2. Temperature, wind direction, and speed were stable during this period of the day. Effective (legal!) management of on course traffic, pony tails, and race numbers are all important marginal gains!

  3. You need to also look at NP/AP. At the same AP or NP, the higher that ratio, the slower the ride. Steady wins the day.

  4. Since there were hills on the course, optimum pacing would not quite be 1:1, but close. www.bestbikesplit.com is a neat resource for playing with this.

  5. BTW...hadn't seen bestbikesplit.com before now. I built the same model in 2005. ;-) Predicted mine and a few others' bike splits to within a minute or two. The model also spit out pacing strategies for hills, flats, wind, etc. I never thought to commercialize it!