Ok, but why this frame?

Ok, but why this frame?

For the fellow aero nerds out there, I thought I’d go into a bit more detail on the aerodynamic design of our track frame.

We’re excited about the UCI’s expanded section profile rules (I’ll call it 8:1 vs. 3:1 here, even though the rules technically allow more than 8:1 in certain areas), both because of the drag reduction potential on the tubes, as well as what those new tube shapes allow us to do for the rider’s drag.  Imagine the air flowing over the rider and bike as your favorite stubborn friend/frenemy – you can only persuade them so much in a certain amount of time.  With a shorter 3:1 chance to influence the air, you mostly just have a chance to reduce tube drag, and you don’t have much remaining leeway to move the air in ways that reduce rider drag.  With a longer 8:1 (or higher) tube shape, not only do you have a chance to further reduce tube drag, often by a factor of 6(!), but you can now also do interesting things like make lift.

You might wonder why making lift is useful – after all, it’s a track bike, not a track airplane.  But lift is just another way of describing moving air, and moving air around the rider and wheels allows us to have a far bigger impact on overall speed.  Many are probably familiar with the guideline that the frame is responsible for only about 20-30% of the drag holding a rider back, with the rest being down to the rider, wheels, and other components.  With the ability to move more air efficiently, we can impact that other 70-80% much more effectively than we ever could before.

So how are we using all this new air-moving leeway we now have?

1)  Vortex creation: Making lift also makes vortices, and we can use those vortices to influence air quite far away from the tube that originally made the lift. Our top tube and upper seat tube especially, and the rest of the bike to a lesser extent, are designed to create a large scale vortical flow that encourages air to stay cleanly attached to the rider’s back, cutting rider drag.  Other bike manufacturers have done similar things before, but the relaxed rules allow us to do it much more efficiently.

2)  Propulsive lift generation: More airfoil-like bike tubes can make a lot more lift efficiently, and this lift is actually pointed slightly in the direction the bike is pointed.  With an efficient enough airfoil, the forward lift combined with the forces on your tires can produce a sail effect that literally pulls you forward.  But all this requires a yaw angle, and those are low on tracks in general, and very low on indoor tracks, so how can we take advantage for a track bike?  As the air moves around the rider, it changes direction – this produces a locally higher yaw angle.  By intelligently producing lift where the rider produces a local yaw angle even temporarily, we can create forward lifting pull, helping you ride faster.  We’re doing this primarily on the upper seat tube, where we can make plenty of lift under the influence of the rider’s legs.

3)  Flow shaping: With all of this new lift-generation capability, we can move the air out around the rider’s lower half with our fork, and back again at the rear using the seat stays.  This makes the entire lower half of the wheel-frame-rider system behave more like one huge airfoil, reducing drag.  The wide stance necessary to do this also has the side benefit of making our frame more wheel-agnostic, so you can use any modern aerodynamic wheelset you like without worrying about its impact on your frame’s performance.

4)  Better handling: Lift generation traditionally also has the downside of poor handling – everyone has probably ridden a deep wheel in high winds and been frustrated (or even terrified) by the handling effects.  When the wind changes angle or speed, the lift on a wheel or bike can change rapidly if it’s not well-designed, which is what causes those handling issues.  With longer airfoil shapes, we can much better control how our lift changes at higher yaw angles, making that sudden, handling-ruining lift drop into a smooth, predictable change that you can easily manage. Our stall occurs smoothly over roughly 5-10 degrees of yaw change, where prior shapes might achieve 3-5 degrees even for well-designed shapes.  It also occurs at much higher yaw angles, so you won't even have to worry about it until yaw angles that are unlikely even in outdoor velodromes.  All this means you don’t have to worry about whether your next race is inside or outside, and whether the performance-enhancing lift on your frame is going to handle strangely. 

5)  Fewer proprietary parts: As a bike designer, when you’re trying to reduce drag within 3:1 limits, you have to make some very tough choices about components and shaping.  For better or worse, recently that’s meant that faster bikes often have expensive, proprietary parts that make maintenance and travel a pain.  With more aerodynamic room to breathe, you can make a bike fast without resorting to proprietary parts, so that’s what we’ve done.  Use the stem, cranks, wheels, and bars of your choice, without penalty.

Thanks again for all the interest!  We’re excited and hopeful to be able to share this frame with the world, and we hope this helps explain what’s going into the aerodynamics of our bike.

-Ben, the Aero dude at Stromm.

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