Twin brothers Jon and Chris Thornham create FLO Cycling to make wheels more aerodynamic and cheaper.

The specifications of a wheel would be short if created by a layperson, calling for little more than it being round. But for two mechanical engineers studying their bicycles, just being round was insufficient. Why can’t a bike wheel be strong, aerodynamic, light—and cheap?
Cheap being a relative term. A pair of FLO Cycling’s wheels start at around $1,500. Competitive aero wheels can be thousands of dollars each.
The twin brothers, Jon and Chris Thornham, founded FLO Cycling to reinvent the bike wheel. Chris has left FLO Cycling and competes in triathlons, including the toughest ones, the Ironman events, which have a 2.4-mile swim, a 100-mile bike ride and a marathon. Brother Jon now runs FLO Cycling on his own.
Who Needs a Wind Tunnel?
The brothers had purchased a set of expensive aero wheels assuming quality was directly related to price. It wasn’t. He assumed the design had been tested in a wind tunnel. It wasn’t. Convinced that a good aerodynamic bike wheel could be cheaper, they formed a company to prove it. How hard could it be? They left their engineering jobs (both were working in Nevada) and founded FLO Cycling to find out.
To a modern engineer, a wind tunnel must seem like a remnant of a bygone age, when Chuck Yeager was breaking the sound barrier or carbon-emitting vehicles were trying to meet yesterday’s gas mileage standards. Today’s engineers have software that can take the place of wind tunnels—at least for the most part.
Not all bike parts manufacturers can afford or are located near wind tunnel testing facilities. Thornham was able to find one that charged $600 an hour. Given the cost, he understands how only a few designs can be tested in wind tunnels. But with simulation, many design variations can be analyzed. With generative design, even more designs can be tested. The wind tunnel can be reserved to validate the design variations with the best results.

Thornham uses CFD and generative design to get hundreds of variations of a wheel design. He allocates $10,000 in simulation for each model of a wheel. Autodesk Inventor is used to model the wheel and CD-adapco’s1 STAR-CCM+ is used for flow simulation and generative design.
The CFD used to be done locally, but a model with millions of volumetric cells would take 28 straight days to run, according to Thornham. With a cloud-based solution, results are obtained in minutes.
Wheel Power
After the rider, the biggest drag force on a bike is not the bike frame but the wheel. Thornham uses power expended by a cyclist as a total measure of the performance of a wheel as it accounts for rolling resistance as well as aerodynamic drag. The popular Mavic Pro wheel is used as a standard.
“If the Mavic Pro wheel uses 100W of power, our wheels take only 40W,” said Thornham.
The streamlined shape of a FLO Cycling wheel is created by bonding a carbon-fiber fairing to an aluminum rim. Spokes pass through the carbon fiber fairing and are screwed into the aluminum rim. The fairing is nonstructural, providing an aerodynamic benefit. As triathletes know, the extra weight of the fairing is more than compensated for by the reduction in aerodynamic drag—by far the biggest force against them.
A section through the tire, rim and fairing is roughly the shape of a raindrop, with a bulbous leading edge and a tapering trailing edge. The conventional wisdom of aero wheel design has been the deeper the rim (or in this case, rim/fairing assembly), the better. Rims with over 60mm in depth are popular with aero-obsessed riders. These rims can taper to a sharp edge in keeping with wing sections.
We’ve seen the wind tunnel tests with smoke trails over wings and the laminar flow neatly separating on the leading edge and converging past a sharp trailing edge without a trace of turbulence. We know wing shapes undergo extensive analysis and testing. But can we assume that what works for wings also works for wheels?
Thornham was not convinced that the same principles that applied to aircraft wings applied to bike wheels. In fact, the two were fundamentally opposite. Aircraft wings are shaped to produce lift from lower pressure on the top surface of the wing. Uneven pressure on a bike wheel, on the other hand, can only cause problems.
Going Sideways
Tri bikes force the rider into a lower, more aerodynamic position and have more aerodynamic frames and wheels than road bikes. The conventional aerodynamic design of these specialized bikes has focused entirely on the bike being ridden with airflow in line with forward motion. And no crosswind whatsoever.
But riders often encounter crosswinds. In a crosswind, riders find that their aerodynamic bikes are no longer aerodynamic, that their projected area has now increased many times. The sides of deep rims, blade spokes and teardrop or oval frame sections now face the wind. A gust across the road, or a sudden crosswind, such as that produced by passing trucks, can blow a rider off the road.
Uneven aerodynamic forces generated in the front and back of a bike’s front wheel can twist the wheel and destabilize the bike. We found no definitive study of the exact nature and effect of these dynamic forces, but there are riders who are only too well aware of them. The greater the side projected area, the greater the effect. At the limit, a disk wheel, opaque to a crosswind, cannot be used as a front wheel in an outdoor race.
In addition, a deep rim tapering into a sharp edge causes separation and turbulent flow in a crosswind, which saps power from the rider.

Thornham had a hunch that crosswinds were more of a factor than bike designers were accounting for. To prove it, he mounted wind speed and direction sensors along with a computer to the front of his bike and took over a hundred thousand measurements while riding over the course of Ironman competitions. His hunch was spot on. He found that 98 percent of the course had crosswinds.
“We found that a more shallow, round edge on the fairing was the best all around in terms of energy loss and control,” said Thornton. The more rounded rim made for less power-absorbing separation as the flow was able to curl around the curve.
FLO Cycling’s aero wheels may not look as aerodynamic as the more expensive aero wheels on the market, but Thornham’s simulation and wind tunnel tests have proven that they are. The wheels represent a victory of generative design over design limited by preconception.
We credit Jon Thornham with confirming that crosswinds were more the rule than the exception and STAR-CCM+ for an optimization that worked with fluids to achieve a more aerodynamic shape that accounts for crosswinds.
1. CD-adapco was purchased by Siemens Digital Industries Software in 2016.