Bumpy frame, smooth air: Is this the next big thing in bike aerodynamics?

by Dave Rome


When Australian sprinter Matthew Glaetzer took to the velodrome at the Rio Olympics he did so on a bike with a difference. On the front of his seatpost were a series of ridges designed to keep air flow close to the bike, and away from his legs. Few, if any, noticed the novel design, but those bumps might well represent a significant step forward for bike aerodynamics.

Could it be that a series of bumps on the front edge of airfoil-shaped tubes is the next big thing in fighting the wind? To find out we spoke to Associate Professor Richard Kelso, the person responsible for overseeing the research project at the University of Adelaide’s Faculty of Engineering, Computer and Mathematical Sciences. The university owns a newly published patent for the technology and looks set to apply it to the next generation of aero bikes.

What it is

Remember the spiel Zipp gave about its 454 NSW wheels? How the wheels imitated the tubercles (or small bumps) on a humpback whale’s flippers for better aerodynamics?

Yes, it sounded ridiculous, but it wasn’t BS. In fact, Zipp based its airflow-stabilising design on academic papers from many years earlier; research that has since led to a whole new wave of aerodynamic design. For example, back in 2006, Richard Kelso and a number of academic researchers began working with tubercles and similar drag-reduction devices on wings, with a particular focus on industrial fans.

“We are in the process of commercialising some of this work now,” Kelso told CyclingTips. “Companies such as General Electric and Ziehl-Abegg have been working on similar technologies for gas-turbine engines and industrial fans, respectively. Our work has led to 20 research papers and three students achieving PhD degrees, and a significant improvement in our knowledge of drag reduction.”

According to Zipp, a sawtooth-like rim shape both reduces aerodynamic drag and improves handling stability in crosswinds relative to more conventional profiles.

Fast forward to today and Kelso’s latest patent — “Improvements to the Aerodynamics of Bicycles” — is effectively a progression of his earlier studies, adapted and refined specifically for bicycle design. “The bumps are designed to generate vortices that pass around both sides of the component and delay the separation of the boundary layers in yawed flow,” explained Kelso. “This, in turn, leads to a decrease in drag.”

As a cyclist, the drag you’re attempting to overcome is almost never in a straight path (head-on). As a result, “yawed flow” or “yaw angle” is a hot topic in modern cycling aerodynamics, and is the angle between the rider’s direction of motion and the ‘relative wind vector’. What’s the relative wind vector? Think of that as the combination of the direction the wind’s blowing in, and the speed of the rider.

To visualise yaw angle, imagine a balloon strung above you. If you rode dead straight and into a perfect headwind, that balloon would trail in a straight line behind you. However, add natural sideways wind forces and the direction of the trailing balloon changes accordingly. That balloon won’t go exactly in the direction of the wind, but will rather find a balance between the forces of the wind and the wind you’re creating by moving forwards. The angle between your direction of motion and the string that holds the balloon is the yaw angle.

So how does that yaw angle affect how the bumps on the frame work?

“The mechanism of the bumps is similar to the mechanisms of dimples on golf balls and turbulators on aircraft wings, but the effect is greater,” Kelso said. “At yaw angles above 4º, when the flow around most airfoil-shaped bicycle components begins to separate, the bumps prevent or delay boundary layer separation, thereby reducing the drag. However, unlike conventional turbulence generators, they don’t increase the drag at lower yaw angles when they are not actually needed.

“It is worthwhile noting that the aerofoil shapes used in (most) seat tubes, downtubes, etc. are usually quite thick, and when combined with the low-speed flow, this can lead to boundary layer separation at angles of attack as low as 5º, something that directly leads to increased drag.”

So just how much of a speed difference can a few bumps make? According to Kelso, “[For] a state-of-the-art time trial bicycle travelling at 54 km/h, well-designed bumps on the seat post and seat tube alone can decrease the bicycle drag by 6% at 10-15º yaw.”

Taken from the patent drawings, here’s one iteration of the bumps design that sit on the frontal profile of an aerofoil.

In 2013, Mavic measured the yaw angles experienced across the whole Kona Ironman course, finding that yaw angles relative to the bicycle exceeded 4º for 68% of the time and exceeded 10º for 30% of the time, with maximum yaw values exceeding 20º. Using Mavic’s yaw distribution model from Kona, Kelso suggests adding specific bumps on the seat post and seat tube would reduce the drag of a best-in-class triathlon bike by 4%. However, that’s for an area of the bike that’s already quite turbulent, and Kelso suggests that the ongoing development into downtube and fork designs would result in even greater drag reductions again.

“The design works best when the yaw angle exceeds 4º,” he said. “At lower angles there is no benefit, but at yaw angles above 4º the benefits progressively increase. On a velodrome the yaw angle is mostly below 4º so there is no measurable benefit. However, for road, time trial and triathlon, the concept is very effective due to the influence of environmental wind.”

Faster in the wind it may be, but adding such external bumps does introduce other engineering challenges, such as balancing weight, stiffness and strength. According to Kelso, advances in 3D-printing should eliminate such concerns, and existing methods of carbon fibre manufacturing should allow for the designs to be implemented without significant weight gain. Of course, such structural additions will undoubtedly come with a substantial cost increase.

Australia’s Matthew Glaetzer raced at the Rio 2016 Olympic Games with the patented technology on his seatpost. It would seem that the design offers the greatest benefit in gusty conditions (i.e. outside).

Speculative speculation

Richard Kelso has been pretty open in explaining where the bump-based designs offer the most benefit, and in explaining that he and his team are in discussions with some industry partners. When asked whether he fears that the UCI could stamp on their fun, Kelso explained that he’s not too concerned, explaining that “the University of Adelaide is actively seeking industry partners to work with and will consider licensing the technology to industry partners. In particular, we are interested in using the technology on triathlon bicycles, where the technical rules allow greater scope for innovation.”

A quick look at Kelso’s previous contracted work reveals he’s long worked with Cycling Australia, he’s worked with Scott Sports to help design the brand’s existing line of aero helmets, and he’s currently got a mystery research and development contract with Cycling Australia for the development of track bikes, almost certainly to be used at the Tokyo Olympic Games next year.

That last element is quite interesting. Even if these tubercle-inspired bumps don’t offer a measurable difference on the track, it will be exciting to see if the technology is adopted on the road.

Given Kelso’s hinting at triathlon products, his past relationships, and the relationship Cycling Australia has with Canadian bike company Argon 18, my bet is that Argon 18 and Scott Sports are likely just two companies in conversation with the University of Adelaide about licensing the bump-based aero concept. And like so many aero-focussed products, we’ll likely see it proven in the triathlon world before it hits road markets.

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