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Home Aerodynamics Tires TriSpoke Tire Choice

TriSpoke Tire Choice

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TriSpoke: Old, Fast Wheel. Slow tires?
How the tire you choose can mean more aerodynamically than the wheel you have, or are considering purchasing

Significance

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 lose him the stage victory? Or, was it his front tire choice that may have cost him the top spot on the podium?

In the pages below, we’ll take a look at the importance of choosing not only the right wheel, but the right tire for your wheel from an aerodynamic perspective. Yes, it’s quite possible that the $70 tire you choose to use is more important aerodynamically than the $1000+ wheel you are considering purchasing or already own.

***Disclosure notice:all wheel/tire 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.

Background

The aerodynamics of bicycle wheels is not a new topic. Wheels have been studied by multiple, independent researchers from within the manufacturing community and also academia. It is well known that at around 30 mph, a typical OEM alloy rimmed wheel will produce, on average, around 200 grams of axial force over a 0-20 degree beta/crosswind range. A typical “aero” wheel will produce about half that axial force value over the same range.

The 100gram savings at 30mph mentioned above can also be represented in terms of CxA (axial force coefficient, sometimes confused with CdA) and seconds saved in a 20k time trial. It is no secret that an aero front wheel will typically save around 0.009 on CxA, or around 20-25 seconds over a 20 kilometer time trial.

What has not really been talked about with much fervor by manufacturers or researchers until recently is, “how much does the tire one chooses to install on an aero wheel impact the combined performance of the wheel system?” In caveman terms this can be translated into: “is the tire I choose going to turn my $1k wheel into an aerodynamic pig???”

The first experience I had that dealt with the topic of tires dates back to the late 90’s when I was testing at the wind tunnel on the campus of Texas A&M. I was a representative of Brand S wheels and we were doing developmental work for a new wheel, along with benchmarking the competition. Also testing their product that day was a representative from Trek.

As it turns out, we both wound up testing a Rolf Vector wheel during our runs, and the values we obtained were markedly different. However, there were two variables that changed; the wheel was not identical and the tire was a different brand and model. As a result of these two differences, it was decided to put tire B on wheel A, and another run was done. The data had spoken, and indeed, the tire was driving the original difference in axial force.

It was this original observation in Texas that intrigued me to follow up on the topic of tires during the testing I did in 2003 (see the report “The Importance of Tire Selection” in the "wheel data" wind tunnel package). This series of wind tunnel tests again reinforced that tires make a difference, and that the difference between narrow and wide tires was in the ballpark of around XX seconds per 20k. This time savings is roughly xxxxx the effect of going from a typical OEM wheel to a $1k aero wheel. Yikes.

While the A&M data was intriguing and got the trends correct, I decided to dig a bit deeper and investigate different brand tires that are approximately the same width using the more accurate and repeatable wind tunnel balance here in San Diego. And thus, the great “tire shootout” came into being.

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 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 SDASTC 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.

 

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.

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.

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.

Wheel

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

Tires

Five different foldable bead clinchers were tested on 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 the three spoke, 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. 

Results and Discussion

Due to the expenses involved with this testing, results are only available on a pay-to-view basis.

The results include translational axial force averaged over the two yaw angle ranges of 0-12.5 degrees and 7.5 to 20 degrees. The results and discussion also include "watts to spin" or aerodynamic torque over the same ranges and 4.5" aluminum roller based rolling resistance coefficients.

Last Updated on Tuesday, 23 February 2010 04:33  

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