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Home Performance Demand The Pareto Principle and TT Aerodynamics

The Pareto Principle and TT Aerodynamics

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Words and images by:  Kraig Willett

(note from the author: this was originally written in late 2004 and posted on the BTR Insiders page. Since the original publishing of this article, the author and the BTR crew have worked with over 100 athletes at - the author didn't edit the content of this article based on those experiences.)

Around the turn of the 19th century, a clever Italian economist, William Pareto, made the observation that 80% of his country’s wealth was accounted for by 20% of the population.  This 80/20 rule, or the Pareto Principle, concisely states that there exists an uneven relationship between outputs and inputs. 

Although this basic concept appears benign at first, the 80/20 rule has evolved into a powerful philosophy in the world of business: to maximize gains, focus efforts on the few things that matter most.  When applying the Pareto Principle to time trialing aerodynamics, it can be argued that focusing efforts on improving rider trunk/torso angle and head position will result in a disparately large improvement of the system aerodynamics. 

Trunk Angle

Anyone who has tucked into a little ball while rippin’ down a fast descent has real-world experience with the aerodynamic effects of a decreased trunk angle (the angle that the torso makes with the ground).  When one tries to touch their chest to the top tube, the trunk angle approaches zero – in other words, the back approaches being “flat”.

Figure 1.  Trunk angle defined.

The scientific literature has done a thorough job of quantifying the relationship between trunk angle and frontal area of cyclists – but, what exactly is frontal area?   It is simply a more quantifiable way of measuring how large a hole a given TT position pokes in the wind.  The analysis in the literature predicts that when one tucks on a descent, the frontal area will be decreased due to a smaller trunk angle.  With this smaller angle, a smaller hole in the wind is made and ultimately one goes faster.  The crossover of the tucking concept to a time trial position should be transparent; and if not, the images below might help: 


Figure 2.  Rider position on the left has ~6% lower frontal area due to a 1 degree smaller trunk angle (note shoulder height) – this subtle difference has ~ 30% of the effect on performance as an aero front wheel upgrade.

Getting the trunk angle lower can have a significant impact on aerodynamics and performance as summarized in the “rule of thumb” table below (values generated using a mathematical model which uses an average power constraint to determine the optimal pacing strategy for a given TT course): 


Table 1.  Cherry Creek TT performance and how it is affected by trunk angle.

How does one go about decreasing trunk angle?  It’s as easy as dropping down the front end of the bike!  Try moving steer tube spacers that are underneath the stem to the top of the stem.  Also, if there are any risers under the elbow pads of the aero bars, try removing them.  For every centimeter the bars are lowered, the trunk angle can be decreased by ~1 degree.  Every little bit helps and it is probably a good idea to gradually progress towards a more aerodynamic, shallower trunk angle time trial position instead of just dropping the bars as low as they can go in one fell swoop.

Head Position

Another often times overlooked variable that can dramatically affect time trial aerodynamics is head position.  As the two frontal area animations below indicate, lowering the head can have a significant impact.


Figure 3.  Head up on the left.  Ducking the head on the right.


In the particular case shown above, the benefits of lowering the head are roughly equivalent to an aero front wheel upgrade, or ~25 seconds on the 10 mile Cherry Creek TT in Denver.

How does one get a lower head position?  It’s as simple as “ducking” – try it out.  A lowered head position will require some getting used to, though, as being able to look down the road with a ducked head means one has to look out the top of the eye sockets.  If this technique is done for extended periods of time, the ol’ eyeball muscles will tend to get pretty angry!  Work on it, and even if it’s not possible to do it for the entire event, just remember that every little bit helps.

Performance Isn’t Just About Aero

While the focus of this blurb has been TT aerodynamics, this topic is only a part of what determines overall TT performance.  Clearly, the optimum TT position will take both power production and aerodynamics into consideration (the best balance of these two factors is the ultimate goal), and any analysis that doesn't consider both of these is missing something.  However, with the recent proliferation of bicycle mounted power meters, the TT position fit process has finally been given the right tools to get the job done with confidence.

The aerodynamics of a TT position can be quantified easily in a wind tunnel (best results) and more difficultly with frontal area measurements and field tests (for trending/comparative purposes).  Combine either of these two aerodynamic tools with a power meter, and the best ratio of power and aerodynamics is achievable.  It might take time to get there, but coaxing every bit of performance out of the system is never easy; and it should also be mentioned that it takes experienced direction and application of the available performance measuring tools in order to truly optimize cycling performance.

In Closing

Just as William Pareto observed an uneven balance of outputs and inputs a century ago, keen time trialists have noticed that there are a few key body positioning factors that yield many benefits in the pursuit of speed.  If one initially focuses on getting the body trunk angle as close to zero as possible (i.e, “flat back”) and keeping the head down, good aerodynamics will be on the way.  After that is accomplished, then it’s time to start focusing on the “20” portion of the 80/20 rule; and this is when the finer details of time trial performance optimization begin to get interesting – and significantly more expensive!

Last Updated on Sunday, 27 December 2009 17:34