Zipp 808 vs Specialized Tri-Spoke
Santiago Botero won the 2005 Dauphine Libere stage 3 ITT by one second over Levi Leipheimer. Each of these individuals chose to ride a traditionally spoked wheel, with Botero opting for the latest technologically advanced wheel offering from Zipp – the 808. Would the outcome have been different if Botero had chosen a different wheel for this TT? Since quite a few ProTour and US domestic pro’s (along with countless amateur TT specialists) ask me questions such as this, I decided to take a look at the aerodynamics of the Zipp 808 versus the less traditionally designed Specialized Tri-spoke/HED3.
In early May of 2006, I was fortunate enough to spend a day in the San Diego Air and Space Technology Center’s Low Speed Wind Tunnel (www.lswt.com) working with weekend warriors on their positioning and also investigating the aerodynamics of the Zipp 808 and the Specialized Tri-Spoke (currently known as the HED3). The wind tunnel time for the wheel testing and the samples were all purchased by me or my immediate family in order to eliminate any perceived financial obligations or connections.
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.
The 2006 Zipp 808 is one of their latest aero wheel offerings, and was purchased in early May from excelsports.com at their full retail price. All of the claims and details regarding the 808 wheel can be found on Zipp’s website:
The particular 808 wheel that was evaluated for this wind tunnel test measured 82mm deep in rim depth, 18.75 mm wide at the aluminum brake track, and had a peak rim cross sectional thickness of approximately 23.75mm. The 808 tested had 18 bladed spokes that were .95mm thick and 2.15mm wide. The Zipp stickers did not appear to be baked into the molded surface of the rim, and as a result did not appear to adhere to the bottom of the dimples.
The Specialized Tri-spoke was one of the original “heavy” wheels first offered in the early 90’s. The particular wheel tested has seen service in every year since its purchase during the first year it was offered for sale. 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:
Both wheels were tested with the exact same Continental Ultra 2000 700x20 folding clincher tire installed. In other words, one wheel was tested, the tire removed and then re-installed on the other wheel. The tire was pressurized to approximately 110psi in both cases.
This same width tire and brand/model has been used by me for the last two years during TT’s. It is not an unusually narrow tire.
Wheels were tested in isolation (wheel only) and rotated at ~18 mph due to motor constraints (note, however, that previous studies such as Greenwell et al, 1995 have shown the minimal impact rotational speed has on the aerodynamic characteristics of wheels). Wind Tunnel speed and axial force values (see below for illustration) were normalized to 30 mph. Axial force data was taken every 5 degrees over the range of 0 degrees to 30 degrees. As an internal check for repeatability, data was retaken at zero degree yaw after the 0-30 angle sweep was initially conducted. For the two samples tested, the average difference in zero yaw axial force (pre-sweep/post-sweep) was 2.5 grams of drag at 30 mph.
Figure 6. Force measurement coordinate systems.
It is also interesting to note that in 2005, nearly a year prior to this test, the same Specialized Tri-Spoke wheel was tested at 0 degree yaw during four separate trials. The average axial force for those four trials was 1.2 %, or less than 3 grams, different than the average axial force for the Specialized Tri-Spoke during this year’s (2006) trials. Three grams different in force measurement over a one year time span – that is pretty impressive repeatability.
Due to the expenses involved with this testing, results are only available on a pay-to-view basis.