Why test bike wheels at 30 mph if I only ever ride 20??

I’ve heard this more then a few times:

“How many people ride at 30 mph? I’m not riding in the Pro-Tour, why do I care if a wheel is faster at 30 mph if I’m riding 20 mph?”

Because you care about aerodynamics, that’s why. And 30 mph is how you determine what is Aerodynamic.

In all of the analysis we’ve done CFD domain run speeds were set at 30 mph. Not only is this the de-facto industry benchmark but it gives you a nice representation of the developed flow structures.  Reynolds Number, which is a non-dimensional indicator of how a flow behaves (mainly laminar flow vs. Turbulent) is the indicator for which kind of flows you should expect for a given object flying through the air. For bicycle wheels the flow ranges from 11,000-650,000 (20mm rim @ 20mph and Full Disc at 30mph). Flows (of any kind, water, air, gas, etc) transition from smooth/mathematically predictable structures, called laminar, to unpredictable Turbulent flows around Re’s of 2000-4000.  Reynolds Number Turbulence Developement

This is sufficiently high to place wheel aerodynamics in the fully developed turbulent region of air flow (i.e. Re>4000). This means that flow structures remain relatively constant throughout interested flow speeds and Drag will scale directly with a square relation to velocity. This means that in the drag equation F_d=\frac{1}{2}\ \rho\ C_d A\ v^2.

The 1/2 *rho *Cd * A term will remain constant for a range of velocities.

At low speeds the spread between different wheel manufacturers is very small, meaning experimental or computational error noise would make it difficult to determine which wheel design is in fact better. A higher wind speed amplifies these differences in order to actually see what wheel is faster.

The other takeaway from this to be aware of is that at low speeds a lot of wheels are ESSENTIALLY the same, Aero Performance Wise. This is why we took a multi-disciplinary approach to designing wheels; including features such as tubeless technology and wider rims.

VeloNews Wheel Drag Plot

The plot above is from actually a pretty good VeloNews test on wheel performance (usually they’re fairly half ass-ed). Notice that the largest discrepancies among top Aero wheel brands is 25 grams at best (8 pennies weight worth of force for those keeping track). At lower AoA’s (where you spend most your time riding) this difference is smaller, additionally at 20 mph that drag spread drops down to 4 pennies worth of drag (~12 grams). At which point wind tunnel measurement inaccuracies start becoming a significant part of your delta. Not to mention things like atmospheric conditions (in an open wind tunnel like A2), manufacturing imperfections, and tire wear can all play a pretty big role in affecting your measurements.

In the end performance always comes at a premium. You CAN  drop $2500 on a wheelset that is sometimes 3 grams faster than the competition, but as an informed cyclist you shouldn’t feel obliged to. It’s not going to hold ANYONE back (unless your last name’s Froome). Instead, maybe just go out and ride a little more.


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