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Home Aerodynamics Wheels Tubular Rears: H3C vs. Lightweight Disc vs. Zipp Sub9

Tubular Rears: H3C vs. Lightweight Disc vs. Zipp Sub9

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Significance

Stage 18 of the 2009 Tour de France around Lake Annecy saw Alberto Contador snag victor from Swiss TT king Fabian Cancellara by a slim 3 seconds in what was around a forty eight and a half minute effort for both riders. The generally flat course around the lake detoured up the category 3 Col de Bluffy, making climbing prowess more of an issue than in many time trials.  During this stage Contador reportedly used a rebadged Lightweight Disc, which is known for its low mass and has a lenticular shape. Cancellara appeared to use Zipp’s Sub9, which is not quite as feathery as the Lightweight, but has a more complex shape that bulges at the rim and then becomes flat near the hub. Did Cancellara’s wheel choice cost him the stage victory?  Or on this somewhat hilly course would both riders have been better off with the H3c, which Lance Armstrong had often ridden to victory in previous Tours de France but abandoned in 2009 for a Lightweight Disc like Contador’s?

In the pages below, we’ll present the methodology used to explore which wheel is faster. We will then examine how each wheel performs in its intended habitat—installed on the back of a time trial bicycle. This contrasts with standard rear wheel tests, which typically focus on the performance of rear wheels in isolation by mounting them in standalone wheel struts. We will present the results as average watts to translate (also in grams of axial force) at 18 and 30mph for the crosswind ranges of 0-20 degrees, 0-12.5 degrees, and 7.5 – 20 degrees. We will also present a figure plotting the watts required to translate each rear wheel and bike combination at crosswind angles of 0,5,7.5,10,12.5,15, and 20 degrees at speeds of 18 and 30mph. All three wheels were tested with a 700x20c Vittoria Crono Tubular tire installed.

***Disclosure notice: the wheel samples and tunnel time were purchased and/or independently acquired by a third party not directly associated with BikeTechReview.com or the wheel manufacturers of the samples tested. BikeTechReview.com witnessed the testing and reduced the data. The third party would like to share their data in an effort to recoup a portion of their wind tunnel time investment.

Wind Tunnel Test Facility

In a simple, discrete building just off the runway of San Diego’s Lindbergh International Airport lays the San Diego Air and Space Technology Center’s 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, combined with the easy and quick adjustability of beta/crosswind angle are some of the features that separate San Diego’s facility from the rest of the cycling wind tunnels in America (Texas A&M, UWAL, MIT, A2, Colorado, 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 Hed 3 tubular that was tested was the all carbon variety. With no tire or cassette the tested wheel weighed 904 grams. HED currently sells this product and more details can be found on their website:

http://www.hedcycling.com/wheels/H3_tubular.asp

The 2009 tubular Zipp Sub9 is a shaped disc that bulges at the rim and then runs flat to the hub.   With no tire or cassette the tested wheel weighed 1030 grams.  All of the claims and details regarding the 1080 wheel can be found on Zipp’s website:

http://www.zipp.com/wheels/sub-9-disc-tubular

The Lightweight Disc that was tested was a 2008 model. It has a lenticular shape that is narrowest at the rim and wider near the hub.  With no tire or cassette the tested wheel weighed 890 grams.  More information about the nearly identical but even lighter 2011 version of this wheel can be found on the Lightweight website:

http://www.lightweight.info/index.php?id=29&L=1%2F

Tires

The tire used on all three wheels was a Vittoria Crono 700x20c. Installed, the Crono measured 20.0 mm in width.

Bike

The bicycle on which the wheels were tested is the one pictured below. It was tested without pedals, with the chain in the big chainring and small rear cog.  The non-driveside crank arm was fixed parallel to the chainstay by a zip-tie running though the pedal hole.

Bike details

Frame: Trek TTX, size small

Fork: Trek TTX

Front wheel:  H3 Clincher, Bontrager Aerowing TT

FD: Shimano Dura-Ace 7800, K-edge chain-catcher installed

RD: Shimano Dura-Ace 7900

Cassette: Shimano Dura-Ace, 11-23

Crankset: SRM, Shimano Dura-Ace 7800, 170mm

Chainrings: Shimano Dura-Ace 55x42

Front Brake: Simkins Design Egg Brake, Nokon cable housing

Rear Brake: Shimano Dura-Ace 7800, Shimano cable housing

Stem: 3T Zepp XL, 120mm, 80 degrees

Base Bar: USE Tula, 38cm, Aero Pods

Brake Levers: USE Tula inline

Extensions: HED Lazy S-bend

Shifters: Shimano Dura-Ace 7800, Shimano housing routed inside extensions

Saddle: ISM Adamo, Racing

Seatpost: Trek TTX

 

Bicycle measurements

Saddle height: 73 cm (center of bb to top of saddle at clamp)

Saddle setback: 5cm (nose of saddle to center of bb)

Extension reach: 75cm (bb to pivot of shifters)

Pad reach: 46cm (bb to center of pads)

Pad width:  Inside of pads touching each other.

Pad drop: 9.5cm (top of saddle at clamp to top of pads)

Extension width: 6cm (c-c at shifters)

 

Test Protocol

Wheels were tested in a built up Trek TTX frame 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, and also had the strut tares removed. Axial force data was taken according to the beta/crosswind schedule (in degrees): {0, 5, 7.5, 10, 12.5, 15, 20}.  For beta greater than 0, the crosswind was always blowing on the drivetrain side of the bicycle. It is unclear whether relative performance of the rear wheels tested for this report would differ substantially if the crosswind was blowing on the non-drivetrain side of the bicycle. However, none of the wheels tested have a noticeably asymmetrical shape like some discs wheels on the market.

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 6 grams of axial force at 30 mph, or around 0.6 watts. In a historical context, the average difference in zero to zero repeatability is slightly larger than this, or approximately 9 grams (1.0 watts)

Figure 6. Force measurement coordinate systems.

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

 

(Bicycle+Support)force – Supportforce = Bicycleforce

 

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

 

Results

The tunnel derived translational watt requirements at 18 and 30mph are presented below as an 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).  In addition, the grams of axial force are also presented for each wheel.  Finally, figures are presented that plot the watts required to translate each rear wheel and bike combination at the individual crosswind angles of 0,5,7.5,10,12.5,15, and 20 degrees at speeds of 18 and 30mph. All values have been calculated based on an air density of 1.225 kg/m^3.

Results are available on a pay to view basis

Last Updated on Monday, 01 August 2011 02:02  

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