Speed is in the Details

4/4/04

words and images by Kraig Willett

Brad McGee won the Tour de France prologue last year while riding some pretty questionable wheels (Shimano) and aero bars (ITM).  It sure was a good thing that Mr. McGee put out a fair amount of power and was lucky that day; because his two high profile equipment decisions might have otherwise cost him the victory.  The only thing that saved McGee during the effort was that David Millar wasn’t detail oriented enough to figure out that one should probably keep the front derailleur on the bike when riding on a bumpy course!  Clearly, more than high-zoot-factor aero equipment contributes to the success of a bike racer, but it is the attention to all the little details that separates good performances from great performances.

Wind Tunnel Test Facility
 
In a simple, discrete building just off the runway of San Diego ’s Lindbergh International Airport lays the Allied Aerospace Low Speed Wind Tunnel.  Judging from the exterior, one wouldn’t realize that this facility has stood there since the mid 1940’s and has been a part of the development of military (F-16, F111, B58, Global Hawk UAV, Tomahawk cruise missiles) and civilian (Boeing 7XX series, Cessna, etc.) aircraft ever since.
More recently, the folks at Allied have begun transferring some of their extensive wind tunnel knowledge gained over the last 60 years into measuring the aerodynamics of sports - cycling in particular.  Just last year, Allied designed and manufactured a dedicated wind tunnel balance (the aerodynamic force measurement system) and elevated ground plane from the ground up.  The highly accurate balance is one of the features that separates Allied’s facility from the rest of the more famous cycling wind tunnels in America (Texas A&M, UWAL, MIT, etc…).

The balance is the heart of a wind tunnel.  It is arguably the most important thing in making an expensive wind tunnel entry a success – especially when the force one is trying to measure is extremely small.  Most other facilities use the same balance that was designed to measure loads on 1 meter wing sections with the wind blowing at 160 kph – in other words, loads on the order of hundreds of pounds.  The use of this type of tunnel balance is not necessarily a problem for bike related testing since adequate results can be had at many facilities – someone had to raise the bar, though.  Trying to do experimental work at some of the existing cycling facilities has been described by people as similar to “Trying to weigh a dollar bill with a truck-scale”.

In an attempt to see if the balance was as good as claimed, the folks at Allied were challenged to weigh fifty cents with their tunnel balance during the entry on April 4th.  A lab quality scale measured the average weight of two quarters to be 0.0248 lbs.  The wind tunnel balance weighed the quarters to be 0.0265 lbs – a difference of 0.0017 pounds, or about 0.75 grams.  Don’t believe we tried it?  Here’s the proof:

Figure 1 .  Weighing a dollar bill with a truck scale…?  Allied Aerospace pony’s up to the fifty-cent challenge.

How’d they get a balance this sensitive?  Allied Aerospace has a dedicated in-house department that specializes in designing, fabricating, gaging, and calibrating precision force measurement systems for both internal use and for outside customers.  They have been doing this for years and have therefore become extremely competent – the end result of the project was an external wind tunnel balance calibrated to an accuracy of 0.02 lbs (less than 10 grams).

The walls of the Allied Aerospace tunnel are solid concrete, so not only are they extremely stable (insignificant dilation/vibration during tunnel operation) which creates an extremely low turbulence flow, but the tunnel is nearly sound-proof.  One can't hear the tunnel running when the power is on and the huge 20 foot diameter blades are spinning on the other side of the tunnel.

Figure 2 .  Lots of concrete and the 20-foot diameter wooden fan blades.

Figure 3 .  The prop tips spin less than a quarter of an inch from the tunnel walls.

 
Another feature of the facility is the elevated ground plane, or splitter plate.  This raised platform helps put the rider/test sample in the lowest turbulence and most uniform air flow of the tunnel – right in the middle of the section.

Figure 4 .  The elevated ground plane/splitter plate in the wind tunnel test section.

The control room at Allied is top-notch as well.  Using a custom developed LabView based data acquisition system, all the relevant tunnel and data monitoring parameters are displayed in real-time.

Figure 5 .  Real-time data display (right) and tunnel control  panel (left).

While these tunnel features may seem like inane details to some, it is these details that gives one the best chance of reliably and accurately documenting the aerodynamic differences between the most recent crop of aero bars.

Test Samples

Five bars were tunnel tested.  These included the Easton Attack, HED, Profile Carbon-x, Vision Tech Tri-Max Plus, and a garage tinkerer’s special (the “baseline” comparison standard): Look Ergostem/Scott clip-on/hack-sawed pursuit bar.

Figure 6 .  Top row (Left to right): Scott clip-on (baseline), Profile Carbon-X, Vision Tech Trimax Plus.  Bottom row (L/R):  Easton Attack, HED.

A few notes on the baseline bar are necessary.  This is a three piece bar consisting of a Look Ergostem, a set of old-school Scott clip ons, and a modified set of road bars.  The base pursuit bar is simply a set of road bars with the “drops” portion cut off using a hacksaw.  The remaining bar is then installed upside down in the stem and the clip ons are added.

Figure 7 .  Baseline aero bar - a garage tinkerer's special.

The bars that did not have integrated brake levers (Profile, VT, Scott), were tested with the same pair of Dia-compe 188 levers installed.

Figure 8 .  Profile bar with Dia compe brake levers installed.

 
Bars that did not incorporate an integrated stem ( Easton and HED), had the same TTT Forgie stem installed during the tests.

Figure 9 .  Easton Attack bar with the TTT Forgie stem installed.

  
Furthermore, all bars were set up such that they were in the narrowest and lowest position allowed.  For the Profile and HED bar, this meant that no elbow pad risers were installed.  The Easton bar, which has no elbow pad height adjustment, was set to the narrowest position.  The VT bar has no adjustability, and was tested “as is”.

Finally, since the Easton and HED bars were on loan from the Nytro bike shop, and needed to be returned in re-sellable condition, they were tested as they were originally packaged.  This meant that the HED bar extensions were not cut to length, and the Easton ’s clear, close-fitting plastic protective wrap that covers the yellow handlebar tape was not removed.  These details shouldn’t significantly impact the results, but out of completeness, are disclosed.

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