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Aerodynamics – a case study

Have you ever wondered how much of an advantage aero equipment is for a cyclist.  If you are a keen time trialler or pursuiter then I bet the answer is 'all the time!’.

Back in October 2011 I was working on a testing protocol to determine a riders aerodynamic profile (referred to as Coefficient of Drag or CDA for short) and came across some some interesting data collected by the Germans some prior to that.

The rider in question here is   Uwe Peschel , who was a noted time trialist.  The testing was done on a velodrome and as you can see from the results below they tried a variety of positions and equipment and recorded the power output (watts) required to sustain 45 kmh.

    • 'Standard' road bike, 32-spoke wheels, hands on hoods: 465 Watts
    • Same bike, hands down on the drops: 406 watts
    • Same bike, Easton Aeroforce aero bars: 369 Watts
    • Same bike Triathlon position (5.5 cm lower bar, saddle forwards): 360 Watts
    • Same as above, with 2 carbon Tri-spoke wheels: 345 Watts
    • Cervelo TT bike + tri-spoke wheels: 328 Watts
    • Cervelo TT bike + tri-spoke front + disk rear wheel : 320 watts
    • Same as above with Giro aero helmet: 317 watts
    • Same as above with speed suit: 307 watts

Its worth noting that all of these changes provide incremental power/time savings with the largest single reduction being moving from the hoods to the drops (~12.7% power saving).  However if you look at overall position changes the move from the hoods to the clip-on aero bars offers a whopping 20% saving.  Considering the overall decrease in power is ~34% that’s quite impressive and it proves that the single most important thing you can do to improve your time trial performance is to add a set of aero bars.

As you move down the list the savings get less and less and quite surprisingly the aero helmet was the least effective, although its worth noting several other tests since this one have proven helmets to be just as effective as an aerodynamic front wheel.  As this test was done some time ago I suspect the type of helmet used wasn't as streamlined as the newer models.  It’s also worth noting that the benefits of aero helmets varies from one rider to the next, meaning that a helmet that is shown to be the fastest for one rider might not be the best option for another.

It's also worth noting that changing from the two tri-spoke wheels to a rear disc didn't save much either (about 2.4%).   The reason for this is that the rear wheel is more sheltered than the front one, which plays a greater role in improving aerodynamics.  However, where a disc is really beneficial is on the open road when their is a crosswind.  The disc creates lift and actually reduces drag where most other wheels increase drag in crosswinds – the exception being tri-spokes (but wheel aerodynamics is a whole different topic). 

So, if you are looking to improve your aerodynamics in time trial or pursuit situations it is well worth considering what equipment will get you the best bang for your buck. Ideally you should ensure that you have a custom fit time trial bike (or at the minimum aero bars on your road bike that has been modified for an optimal time trial position).  Next up look at front wheel and a new aero helmet (generally speaking less or smaller vents are better).  If you can afford it then also go for a skin suit or speed suit with booties.

Footnote: It is possible to test your own CDA using a variety of techniques that don’t involve visiting a wind tunnel.  You will need a power meter and either flat 1km stretch of road, velodrome or loop course. Software such as Golden Cheetah which has a function called AeroLab that helps you determine your aerodynamic profile.

It’s worth remembering that improvements in aerodynamics are specific to each rider and different makes/models of equipment (esp. wheels) offer different levels of savings in different wind conditions.  Skin-suits and speed suits are also known to be specific to individuals with one suit being faster on one rider but slower on another.

Jason

Jason Mahoney Thursday 15 August 2013 at 09:16 am | | Rider Info
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