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Home Aerodynamics Wheels 1080 vs TriSpoke

1080 vs TriSpoke

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1080 vs TriSpoke

Stage six of the 2009 Tour of California in Solvang saw Levi Leipheimer snag victory from David Zabriskie by a scant 8 seconds during the violent 30 minute effort. During this time trial stage, Leipheimer chose the time-proven three spoke for his front wheel, while Zabriskie chose the new offering from Zipp: the 1080. Did Zabriskie’s wheel choice cost him the stage victory? Or did Leipheimer win despite his front wheel selection?

 

In the pages below, we’ll present the methodology used to explore which wheel is faster. We will also present the results as watts to spin and watts to translate at 30mph for the crosswind ranges of 0-20 degrees, 0-12.5 degrees, and 7.5 – 20 degrees. As a slightly new twist, the results will be a blended average of three tires that were tested on both wheels (Continental Ultra 2000, Bontrager Aero TT, Veloflex Record).

Finally, if the reader is looking for a full-blown discussion of the results, this is not the report for you. The results will be presented after the discussion of the methodology and that’s about it.

***Disclosure notice: the wheel samples and tunnel time were purchased independently by BikeTechReview.com. We buy it. We test it. We sell the results to whoever values it.  Period.

Wind Tunnel Test Facility

In a simple, discrete building just off the runway of San Diego’s Lindbergh International Airport lays the San Diego 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 wind tunnel folks in San Diego 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.  In 2003 they designed and manufactured a dedicated wind tunnel balance (the aerodynamic force measurement system) and elevated ground plane from the ground up.  The highly accurate and repeatable balance is one of the features that separate San Diego’s facility from the rest of the 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 in San Diego were challenged to weigh fifty cents with their tunnel balance during an entry on April 4th, 2004.  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 I tried it?  Here’s the proof:

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

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

The walls of the San Diego Low Speed Wind 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 the San Diego Low Speed Wind Tunnel 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 two aero wheels in question.         

Test Samples

Wheels

The Specialized Tri-spoke that was tested is one of the original “heavy”, matte black wheels first offered in the early 90’s.  The particular wheel tested had a brake track that was 18.8 mm wide, and has seen service in every year since it was purchased in the early 1990’s.  A few years ago, the Specialized Tri-Spoke was re-badged as the HED3.  HED currently sells this product and more details can be found on their website:

http://www.hedcycling.com/wheels/hed3.php

The 2009 clincher Zipp 1080 is one of their latest aero wheel offerings, and was purchased in late March from excelsports.com at their full retail price.  All of the claims and details regarding the 1080 wheel can be found on Zipp’s website:

http://www.zipp.com/wheels/detail.php?ID=32

The particular 1080 wheel that was evaluated for this wind tunnel test measured 108mm deep in rim depth, 18.7 mm wide at the aluminum brake track.  The calipers that I have were unable to measure the peak rim cross sectional thickness, so I must rely on the claims of Zipp and report that this measurement is approximately 24.2mm.  The 1080 tested had 16 bladed spokes that were 1.0 mm thick and 2.24mm wide.  The Zipp stickers did not appear to be baked into the molded surface of the rim; however, they did appear to adhere to the bottom of the dimples that are molded into the surface of the 1080.

Tires

Three different foldable bead clinchers were tested on the Zipp 1080 and the three spoke and they are tabulated below:

Test Protocol

Wheels were tested in isolation (wheel only) and were spinning at ~30 mph ground speed.  Wind Tunnel speed and axial force values (see below for nomenclature illustration) were normalized to 30 mph and corrected for beta/crosswind.  Axial force data was taken according to the beta/crosswind schedule (in degrees): {0, 5, 7.5, 10, 12.5, 15, 20}.  As an internal check for repeatability, data was retaken at zero degree beta after the 0-20 angle sweep was initially conducted.  For the samples tested using these wheel/tire combinations, the average difference in zero beta/crosswind axial force (pre-sweep/post-sweep) was approximately 9 grams of axial force at 30 mph, or around 1.2 watts.

Figure 6.  Force measurement coordinate systems.

In addition, a tare run was conducted where the wheel was removed but the wind tunnel axle and upright supports remained installed.  The forces that occurred during this set of runs was subtracted from the wheel+support forces;  i.e – the logic at play here is essentially:

 (Wheel+Support)force – Supportforce = Wheelforce

This methodology might not be the best way to get an accurate value of the forces on the wheel itself, as there might be unaccounted for interactions between the support structure and the wheel. 

Watts to spin

There has been very little published on the topic of aerodynamic torque.  There are several sources on the internet that have been dug up…

http://www.recumbents.com/WISIL/MartinDocs/Validation%20of%20a%20mathematical%20model%20for%20road%20cycling.pdf

 

http://www.soton.ac.uk/~aijf197/Wheel%20Aerodynamics/Results%20and%20Discussion.htm

 

http://journals.pepublishing.com/content/g1463815454723l0/

http://ir.canterbury.ac.nz/handle/10092/1800

…that discuss the topic, and the various results have been mixed in terms of magnitude of the reported aerodynamic torque.  For this report, a pretty straightforward data reduction method that uses the highly repeatable six component balance installed in the wind tunnel here in San Diego was developed by the author.  As a result, it was possible to calculate the rotational performance properties of these two wheels as a blended average of the three tires listed above. 

Results

The tunnel derived translational and rotational watt requirements at 30mph are presented below as a blended three tire average over the beta/crosswind ranges of 0-20 degrees, 0-12.5 degrees (representative of the range of crosswind angles seen during calm conditions) and 7.5 to 20 degrees (range seen during windy conditions).  All values have been calculated based on atmospheric conditions of 70 degrees Fahrenheit, and one atmosphere of pressure.

Results are available on a pay to view basis.

Last Updated on Tuesday, 23 February 2010 04:35  

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