Zipp was not the world’s preeminent race-wheel brand in the mid-’90s, before Poertner and Ording attempted to crack European cycling. Only a few companies were making deep-rim carbon wheels at the time, but Zipp needed to take a technological leap forward before it was able to sign Team CSC and become a fixture in triathletes’ bike shop fantasies. That technology was rim shape. Although spoke count, spoke shape, hub construction and weight are all important characteristics of a race wheel, the aerodynamic performance of the rim is the dominant trait—and on this point Zipp had a problem. Steve Hed, the namesake of Hed Cycling, and his then business partner, Robert Haug, had a patent protecting the fastest rim shape of the time, which still closely resembles today’s best aero wheels. Hed and Haug called the shape “toroidal.”
The toroidal rim has an elliptical profile when cross-sectioned that is “about 1.05 to about 1.25” times wider than the tire, according to the patent, with the broadest point partway between the tire bed and the spokes. Instead of building aero wheels with a V-shaped rim, which was the dominant shape at the time, Hed and Haug asserted in U.S. Patent 5,061,013 that a bulbous rim with a football-like cross-section would create a “wheel having good aerodynamic properties and good crosswind stability,” and this approved patent gave the pair exclusive rights to that rim shape.
In 1999, eight years after Hed filed that original patent, Zipp filed a patent on the rim shape they call hybrid toroidal and used this design to protect the 404 wheelset they were already building. Instead of blending the tire with the rim to create a continuous aerodynamic profile as listed in the Hed/Haug patent, a hybrid toroidal rim has a bulbous deep section of the rim and a narrow brake track. The thin brake track sandwiched between a wide tire and even wider rim created a figure-eight shape, not a continuously toroidal shape. Wind tunnel testing confirmed these wheels are fast, but Zipp and most other companies have since abandoned narrow brake tracks in favor of the wider design listed in the document Hed and Haug filed. Zipp had a good wheel, but Hed seemingly owned a monopoly on the shape that would dominate aero wheel design for the next decade, and this seminal piece of intellectual property prevented Zipp from designing a rim that smoothly integrates the brake track with the rim’s deep section. Hed Cycling seemed to have secured a major technological advantage, but the relationship between Hed and Haug deteriorated, and Haug offered to sell his portion of the patent to Ording and Zipp. Ording eagerly accepted. Buying Haug’s share of the patent granted Zipp the rights, shared only with Steve Hed, to sell wheels with a fully toroidal rim. Owning a share of this patent gave Zipp the ability to build the fastest rim shapes they knew of without legal hindrance. In 2004, Zipp used these rights to make the 808, a fully toroidal rim 34mm deeper than the 404. It had an exceptionally broad rim—27mm at the widest point—and a sloping brake track that continued the toroidal shape through the entire rim. The release of the 808 coincided with Zipp’s explosion as a company and the widespread adoption of aero wheels by age-group triathletes.
At first glance, the toroidal shape fails the “eye wind tunnel” test. It doesn’t look very aerodynamic. But the toroidal shape is so powerful because it helps reduce drag when riding in moderate crosswinds, the condition most commonly faced by cyclists. Although rim depth has become the characteristic most frequently associated with an aerodynamic wheel, rim shape is equally or perhaps even more important than depth.
The majority of the wind resistance faced by a cyclist comes from pressure drag, the dominant component of a rider’s total aerodynamic resistance. Put your hand out the window of a car with your palm facing forward and you can immediately feel the pressure against the front of your hand, but the less-perceptible low-pressure zone behind your hand is an equally important component of the fluid drag you are fighting. If you were to increase the pressure behind your hand without changing the pressure in front of it, the total drag on your hand would plummet. This is because pressure drag is the difference between the pressure at the front of your hand and the back, and the bigger the difference, the greater the drag.
When the air approaches your hand, it collides with the surface, which increases the pressure at the front of the palm. After fighting its way around your hand, air immediately rejoins the surrounding airflow and it flutters violently. This uncontrolled flapping reduces the pressure behind your hand, increasing pressure drag. To reduce pressure drag, an object must allow air to smoothly pass around its leading edge while guiding it off its trailing edge. As Zipp design engineer Michael Hall puts it, “when designing for low drag, you want to give the air as smooth a surface to attach to as possible” to allow air to easily pass off the back of the object rather than fluttering and creating that dramatic low-pressure zone behind the object. Aero wheels do exactly that by creating a surface—the deep-section rim—that controls the airflow around and off the object. But not all rims of equal depth are equally effective at guiding the air.
Even a subtle crosswind dramatically affects the angle between the rider and the wind, known as yaw angle. The bulbous toroidal rim shape is effective in wind conditions riders face most frequently, crosswinds—not just direct headwinds, which are quite rare. As air wraps around the rim from an angle other than head on, it needs a place to re-attach to the rim or pressure drag spikes. Unlike toroidal rims, deep V-shaped rims are narrow where Zipp believes they should be broad—somewhere around the center of the rim—so the passing air cannot easily reattach. This simple change to fatter rims provided the foundation of Zipp’s growth from underdog to behemoth. As a result of the shared intellectual property granted by Hed and Haug’s patent, Zipp and Hed Cycling were able to dominate the race for aerodynamic performance through the last decade, and Zipp in particular turned that into a business explosion. Zipp won the Kona wheel count in 2005. In 2007 and every year since, more Zipp wheels have been ridden in Ironman Hawaii than all other wheels combined. Cycling component giant Sram bought Zipp that same year and gave them the resources to continue to grow at a torrid pace.
When Zipp and Hed’s toroidal rim patent expired in October 2009, many observers expected a deluge of toroidal wheels to hit the market on that very day, and that didn’t happen—they are only now beginning to arrive en masse. But the expiration of Patent #5,061,013 forced the two pioneers of aero wheel design to immediately venture beyond the shelter of the patent and create rim shapes that are faster than the toroidal rims in anticipation of the rest of the market catching up to their 2009 technology. But these new designs are not their exclusive property. Ironically, Zipp has innovated themselves into a more competitive landscape. Zipp’s new rim technology called Firecrest—it’s actually much more bulbous than the original toroidal wheels—and Hed’s Stinger wheels, with an equally broad design, reveal some of the key attributes of aerodynamic design that neither company has yet been able to protect with a patent, although both are pursuing patent protection for their designs. When they designed a faster yet unprotected rim shape, the technological monopoly Zipp and Hed enjoyed for years evaporated. And the list of wheel makers fighting to convince triathletes that they, not Zipp or any other company, make the best race wheel has mushroomed.
Although there wasn’t an explosion of toroidal wheels the day the patent expired, many wheel makers recently have incorporated wide rims into their aero wheels, and several serious wheel companies have overhauled their rim design with characteristics resembling Zipp’s Firecrest. For example, Bontrager recently unveiled a complete line of extremely broad carbon race wheels; Enve commissioned the assistance of a former Formula 1 aerodynamicist to design its newest aero rim shape, which is similar to Zipp’s; Reynolds Cycling produces shockingly expensive wheels constructed almost entirely from carbon with a unique rim shape based on a very broad rim; and, as they have been for decades, Hed Cycling is still at the upper limit of aero wheel performance. And many more brands are aiming squarely at the Indiana-based company.