Aerodynamics: Common Misconceptions

It looks so fast... So it must be very fast?

Adding some clarity to a few common technical misconceptions...

Basic aerodynamics for open-wheeled vehicles need not be so confusing. First, employ traditional approaches to minimize both form (pressure) and friction drag on the various frame surfaces for the primary vehicle operating condition. Then employ the new method taught by the invention to specifically minimize drag on only the critical uppermost wheel surfaces, while leaving lower wheel surfaces exposed whenever possible.

To minimize form drag, streamline exposed surfaces. To minimize friction drag, minimize the total surface area exposed to headwinds. Then to actually minimize overall vehicle drag, employ the minimal wheel fairings of the invention. The proper systems combination of all three approaches for the primary vehicle operating condition, then achieves optimal aerodynamic efficiency.

So many fast-looking very expensive bicycles often used in triathlon racing are actually inefficient in any substantial crosswind—having been overly-streamlined for use mostly in null wind conditions—as they often employ too large surface areas on the wheels and frames, thereby creating substantial form drag in crosswinds.

So the trick then is to determine what is the primary operating condition when configuring the bicycle for optimal aero efficiency. Real aero efficiency does not necessarily look so efficient. In strong crosswinds, a small-diameter round tube frame can actually be faster than some highly streamlined frames.

And for reference, since the inventions overturn a number of assumptions about how wheels actually affect overall vehicle drag, the new principals–as taught by the inventions—countering a number of the more commonly held technical beliefs are further summarized below.

1. Upper wheel surfaces always move forward, while lower wheel surfaces always move backward?

This is a very common belief based largely on a mistaken assumption that the various wheel surfaces are moving relative to the axle—which is itself moving at the vehicle speed. Actually, all wheel surfaces are moving to some degree in the forward direction relative to the ground—and thus the headwind—except where touching the ground. After all, the ground is not moving. And if the wheel is not sliding along the ground, it is then necessarily stationary where touching the ground.

2. Wind speed at the bottom of the wheel is the same as at the top of the wheel?

Since the wheel rotates about a central axle tied to the vehicle frame—which itself is moving forward in only one direction—the various surfaces of the wheel are moving simultaneously in many different directions at many different speeds. Moreover, these various surfaces are exposed to even more widely varying wind speeds, also depending largely on the external headwinds impinging thereon.

Furthermore, since the bottom of the wheel is stationary, and the axle is moving at the vehicle speed, simply geometry shows the top of the wheel to be moving forward at precisely twice the vehicle speed. Thus, the wind speed at the bottom of the wheel is null, while the wind speed on surfaces at the top of the wheel is at least twice the vehicle speed (absent a tailwind). (So when you see fancy CFD diagrams from leading aero wheel manufacturers depicting wind blowing across the bottom of the wheel, you can safely assume that their analysis employs faulty technical assumptions. But their expensive CFD plots sure look convincing!)

And if a ground headwind is also present, then the uppermost surfaces of the wheel are exposed to wind speeds significantly greater than twice the vehicle speed. For slower moving vehicles facing stronger headwinds—such as bicycles—wind speeds at the top of the wheel can easily exceed three times the vehicle speed. This greatly increases the power being dissipated on these same uppermost wheel surfaces over comparable slower-moving vehicle frame surfaces, since the power dissipation relation is a highly sensitive cubic function of wind speed.

3. Wheel-induced vehicle drag is centered about the level of the axle?

This assumption likely arises from the prevalence for measuring vehicle drag forces within wind tunnels—rather than the actual power being dissipated in drag—to determine vehicle aerodynamic efficiency. Measuring translational drag forces alone is largely adequate for comparing the relative efficiency of streamlining various vehicle frame surfaces. However, it is not a sufficient method if wheel efficiency is also to be included, since the wheel drag mechanics affecting vehicle propulsive efficiency is quite complicated mathematically. Our new patented method for measuring overall vehicle efficiency inside wind tunnels overcomes this drawback.

4. Shielding more and more of the wheel automatically reduces more and more of the overall vehicle drag?

This is a technical assumption commonly displayed in the recent patent art, as well as in various racing vehicle applications, where wheel fairings typically are disposed extending well below the level of the axle. For example, the Bloodhound SSC rocket car is designed with a supersonic lower wheel fairing shielding a mechanically disadvantaged subsonic lower wheel surface. This is clearly illogical, as the lower wheel should remain exposed to headwinds in order to minimize vehicle drag, since the lower wheel more easily displaces the air than a frame shielding surface.

Further examples of applications using this faulty technical assumption can be found throughout the racecar community (lower front deflectors of IndyCar, etc.).

5. Vehicle frame-surface drag is largely equivalent to a similar wheel-surface drag?

Due to the magnified nature of upper wheel drag, an equivalent surface attached instead directly to the vehicle frame induces far less drag on the vehicle. Firstly, the uppermost wheel is moving forward against the wind at around twice the vehicle speed, greatly exacerbating the induced drag thereon. And secondly, the uppermost wheel drag is actually magnified through mechanical leveraging against propulsive counterforces that are necessarily applied at the lower-positioned axle.

This drag magnification occurs since both forces are leveraged against each other while pinned at the stationary ground contact point. Clearly the uppermost drag forces enjoy a nearly two-to-one mechanical advantage over propulsive counterforces directed at the axle. A small drag force directed near the top of the wheel must be countered by a much larger propulsive force applied at the axle.

For both these reasons, the magnified drag induced on an upper wheel surface retards vehicle motion much more than any drag force otherwise induced on a similar surface attached directly to the slower-moving vehicle frame. Shifting the higher drag otherwise induced on the upper wheel instead onto the vehicle frame can then actually reduce overall vehicle drag, thereby employing a systems mechanics principal taught by the invention.

AERODEFENDER wheel fairings are designed specifically to shift the highly magnified uppermost wheel drag onto an un-magnified surface of the vehicle frame (the fairing itself), thereby lowering the overall drag on the vehicle. However, any further increase in the size of the fairing to shield even more of the lower-drag-inducing lower wheel surfaces would then only minimize the effectiveness of this drag-reducing principal taught by the invention. Thus, AERODEFENDER wheel fairings are minimally sized, and disposed to shield only the most critical uppermost wheel surfaces.

6. Wheel drag is a relatively minor component of overall drag on a bicycle, regardless of vehicle speed?

Conventional wisdom assumes that wheel drag is a fairly minor component of overall drag on a bicycle, since the rider and bike frame have much larger surfaces. Moreover, this assumption is often further simplified by assuming that the ratio of drag on these various surfaces remains fixed, regardless of operating conditions.

However, the invention teaches—and our extensive downhill coasting tests only further confirm—that for open-wheeled vehicles this assumed fixed drag ratio is invalid. In fact, as either vehicle or ground headwind speeds increase, overall vehicle drag becomes increasingly more concentrated on the uppermost wheel surfaces. Thus, as headwinds rise it becomes increasingly more important to reduce drag specifically on the uppermost wheel surfaces over all other vehicle frame surfaces.

One major reason for this shifting drag concentration to the upper wheel is that the upper wheel is simply exposed to much higher wind speeds, causing a shift from laminar to turbulent flow conditions thereon to occur much more rapidly in rising wind speeds. While an equivalent surface on the vehicle frame may be under laminar flow conditions, the faster equivalent surface on the upper wheel most likely has already become turbulent.

Engineers know that a dramatic increase in drag occurs in the transition from laminar to turbulent flow, often increasing drag a factor two or more. So it becomes critical to maintain laminar flow as long as possible. Hence, the extreme effort employed by industry for streamlining various wheel and frame surfaces.

Nevertheless, the geometry of the wheel means that the doubling wind speed at the top of the wheel can never be overcome, unless by employing fairing shielding! So shielding the upper wheel immediately drops the wind speeds on most upper wheel surfaces to well below the vehicle speed, while also effectively eliminating any crosswind turbulence induced thereon, to maintain laminar flow on upper wheel surfaces far longer than on exposed wheels.

As a result, AERODEFENDER wheel fairings are simply the most aero-efficient and likely cost-effective solution to minimizing vehicle drag on a bicycle.

7. Spoke drag is relatively minor component of wheel drag, regardless of vehicle speed?

Conventional wisdom also assumes that spoke drag is a minor component of overall drag on a bicycle wheel, since the wheel rim and tire generally have much larger surfaces. Still, recent industry efforts are geared toward both minimizing spoke count and the use of deeper rims, in order to reduce wheel drag as much as possible, even at the expense of increased crosswind instability and decreased cycling safety.

Most recently, this minimizing spoke count effort is complicated by the need to instead increase spoke count in stronger wheels for use with newer bikes using more effective disc brakes for additional safety. More spokes yields more wheel drag. Nevertheless, disc brake bicycles are the clear trend going forward, having even gained acceptance in bicycle racing.

However, drag coefficients are much greater on spoke surfaces than on the much smoother tire and rim surfaces. As such, it is clear that in rising wind speeds, the transition from laminar to turbulent flow occurs much earlier on spokes than on tire and rim surfaces. Thus, as wind speeds rise, overall wheel drag must shift to be more and more heavily concentrated on spoke and nipple surfaces. Thus, it becomes crucial to minimize drag on these critical spoke surfaces, especially nearest the rim where wind speeds are greatest, and therefore experience turbulent flow earliest.

(So switching to disc brakes shows that there must exist some industry recognition of the need to increase cycling safety? Why not also use simple and available aero enhancements for increased safety? Both AERODEFENDER wheel fairings and SPOKE FINS increase crosswind stability. Will convenient speed and safety enhancements remain unfashionable over the long term?)

8. Bladed aero spokes are more efficient in crosswinds that round spokes?

Bladed aero spokes are necessarily a compromised design. Aero spokes are streamlined for low crosswind conditions—as in the wind tunnel—being most effective against pure headwinds. But bladed spokes simply induce crosswind turbulence faster than even round spokes. While round spokes are more effective in crosswinds that bladed spokes, the round spoke also still suffers from developing higher turbulent drag at higher crosswind speeds, like the bladed aero spoke.

In fact, there is simply no avoiding the eventual transition from laminar to turbulent flow across any spoke in rising wind speeds. Deeper rims do help by minimizing wind speeds on the shorter spokes, but only at the expense of added crosswind instability and reduced cycling safety. The solution then is to optimally streamline the round spoke for any wind direction while used on a relatively shallow, more aerodynamically crosswind transparent rim, by using SPOKE FINS.

AEROCROSS wheels with SPOKE FINS are simply the most effective means to minimize spoke drag regardless of the wind speed or direction, being very effective in maintaining laminar flow conditions across the spoke even at higher wind speeds. SPOKE FINS reduce the drag coefficient of the round spoke by a factor of three to five, making the round spoke even more aero than bladed aero spokes at higher wind speeds and in crosswinds, where any aero spoke still suffers high drag from turbulent flow conditions.

9. Vehicle drag at a 50 mph vehicle speed is equivalent to the vehicle drag at a 25 mph vehicle speed while facing a 25 mph headwind?

Is vehicle drag actually ground-versus-vehicle headwind component independent, being only a function of total exposed headwind? So which is more difficult? Pedaling 1 mph against a 20 mph headwind, or pedaling 20 mph against a 1 mph headwind?

Clearly, the required pedaling effort for these different cycling conditions is not the same! And clearly the overall drag coefficient of a bicycle must then not be a constant with increasing ground headwinds. Instead, the vehicle drag coefficient varies considerably with changing ground headwind conditions.

10. Vehicle drag coefficient is independent of headwind speed?

Indeed, the overall vehicle drag coefficient for an open-wheeled vehicle varies with either vehicle speed or external (ground) headwind speed, while actually being most sensitive to ground headwind speed. The vehicle drag coefficient is not fixed, being instead a variable. This variable drag coefficient effect is well-documented in our various videos of our side-by-side downhill coasting tests, where relative speed gains of the AERODEFENDER bicycle over the un-faired stock bicycle vary considerably with changing ground headwinds.

11. Form (pressure) drag dominates any frictional drag on an open wheel, especially on a racecar wheel with wider tires?

Due to magnified drag being instead concentrated on the upper wheel, and due to the lower wheel drag actually being de-magnified with respect to propulsive counterforces directed at the axle—as discussed above—form drag on the wheel is then necessarily also concentrated on the uppermost portion of the wheel. As it retards vehicle motion, wheel form drag is not actually centered at the axle level, as commonly assumed in the racecar community.

And since the uppermost wheel surfaces are exposed to wind speeds approaching twice the vehicle speed, friction drag on these uppermost wheel surfaces is also far greater than generally appreciated. So both form and friction drag on the wheel are major components of vehicle drag on wider racecar tires, as well as on wider bicycle or motorcycle tires while operating at higher vehicle speeds.

12. Shielding the lower front wheel with a forward deflector is required in order to create needed downforce on open-wheel racecars?

Certainly, racecars need downforce to maintain adequate cornering traction. But why also increase the drag on the critically drag-magnified upper wheel while simultaneously increasing overall vehicle drag by shielding the de-magnified, decidedly non-critical, slower-moving, lower wheel by using a lower forward deflector wing?

The front wing is moving at the full vehicle speed, while the lower wheel is moving much slower than the vehicle speed, and whose surfaces more easily displace the otherwise still air around the sides of the wheel anyway, than does a faster fairing fixed to the vehicle frame. It is the outline of the wheel that only appears to move at the vehicle speed. Frontal lower wheel surfaces are actually generally moving much faster downward than forward. And form drag resistive pressure on these lower wheel surfaces is generally directed radially toward the axle.

Moreover, deflecting wind upward to increase the drag on the upper wheel actually decreases cornering traction! Drag on the top of the wheel produces a ground slip counterforce that causes the wheel to slip backwards without an adequate downforce needed at higher vehicle speeds.

Such is the sad state of racecar aero engineering. The approach may be governed by adherence to strict equipment rules, but to rules then designed to decrease both traction and thereby vehicle safety!

Instead, any needed downforce could be easily generated by moving the deflector upward a few inches to shield instead the critical upper wheel, minimizing overall vehicle drag while simultaneously maximizing cornering traction, and thereby racecar safety. Very likely, by minimizing drag instead on the upper wheel, the racecar community will find that the assumed high drag induced on the lower wheel and loss of traction from any aero-planing under the tire is vastly overstated.

In 2019, the new Formula E electric racecar has now totally enclosed the previous open-wheel design with large fenders, thereby enhancing performance considerably by shielding the uppermost wheel. Shielding the entire wheel in order to shield the uppermost surfaces certainly is somewhat effective at the higher racecar speeds. However, this design could be much further optimized if the principals for vehicle drag minimization using a minimal fairing—as taught by the present inventions—were adopted instead.

13. Measuring translational drag force on an open-wheeled vehicle inside a wind tunnel yields an accurate assessment of overall propulsive efficiency?

Not if wheel mechanics affecting overall vehicle drag of an open-wheel vehicle are to be measured! Simply spinning the wheels of the vehicle while being pedestal-mounted are not enough. However, this is still how Formula One measures drag in a wind tunnel, thereby having to apply a bit of guesswork to the results. Better is to measure power transferred through the bottom of the wheels while the vehicle is self-propelled and unrestrained, as it is while driving on the highway. This method is taught in our new wind tunnel patent.

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