Patent Application
20150258847
1. Field
The present embodiment relates to vehicle wheels, and particularly to shields and devices used to reduce drag on rotating vehicle wheels.
2. Description of Prior Art
Inherently characteristic of rotating vehicle wheels, and particularly of spoked wheels, aerodynamic resistance, or parasitic drag, is an unwanted source of energy loss in propelling a vehicle. Parasitic drag on a wheel includes viscous drag components of form (or pressure) drag and frictional drag. Form drag on a wheel generally arises from the circular profile of a wheel moving though air at the velocity of the vehicle. The displacement of air around a moving object creates a difference in pressure between the forward and trailing surfaces, resulting in a drag force that is highly dependent on the relative wind speed acting thereon. Streamlining the wheel surfaces can reduce the pressure differential, reducing form drag.
Frictional drag forces also depend on the speed of wind impinging exposed surfaces, and arise from the contact of air moving over surfaces. Both of these types of drag forces arise generally in proportion to the square of the relative wind speed, per the drag equation. Streamlined design profiles are generally employed to reduce both of these components of drag force.
The unique geometry of a wheel used on a vehicle includes motion both in translation and in rotation; the entire circular outline of the wheel translates at the vehicle speed, and the wheel rotates about the axle at a rate consistent with the vehicle speed. Form drag forces arising from the moving outline are apparent, as the translational motion of the wheel rim must displace air immediately in front of the wheel (and replace air immediately behind it). These form drag forces arising across the entire vertical profile of the wheel are therefore generally related to the velocity of the vehicle.
As the forward profile of a wheel facing the direction of vehicle motion is generally symmetric in shape, and as the circular outline of a wheel rim moves forward at the speed of the vehicle, these form drag forces are often considered uniformly distributed across the entire forward facing profile of a moving wheel (although streamlined cycle rims can affect this distribution somewhat). This uniform distribution of pressure force is generally considered centered on the forward vertical wheel profile, and thereby in direct opposition to the propulsive force applied at the axle, as illustrated in
However, as will be shown, frictional drag forces are not uniformly distributed with elevation on the wheel, as they are not uniformly related to the speed of the moving outline of the wheel rim. Instead, frictional drag forces on the wheel surfaces are highly variable and depend on their elevation above the ground. Frictional drag must be considered separate from form drag forces, and can be more significant sources of overall drag on the wheel and, as will be shown, thereby on the vehicle.
The motion of wheel spokes through air creates considerable drag, especially at higher relative wind speeds. This energy loss is particularly critical in both bicycle locomotion and in high-speed vehicle locomotion. Previous efforts to reduce this energy loss in bicycle wheels have included bladed-spoke designs; the addition of various coverings attached directly to the wheel; and the use of deeper, stiffer, and heavier aerodynamic rims. As winds, and particularly headwinds, are a principal source of energy loss in bicycle locomotion, expensive aerodynamic wheel designs have become increasingly popular. However, these aerodynamic wheel designs have often been tuned to reduce form drag, rather than frictional drag. As a result, augmented frictional drag forces present on these larger-surfaced aerodynamic wheel designs tend to offset much of the gains from reduced form drag forces, thereby negating potential reductions in overall drag.
Bladed spokes, tapered in the direction of motion through the wind, are designed to reduce form drag. These streamlined spokes suffer from increased design complexity, increased weight and higher costs. In addition, such bladed designs are more susceptible to crosswind drag effects: The increased surface area of the bladed spoke can rapidly increase form drag in the presence of any crosswind; any crosswind directed upon the flat portion of the spoke quickly increases pressure drag upon the spoke.
Under low crosswinds, the bladed spoke presents a relatively small forward profile facing oncoming headwinds, minimizing form drag. Indeed, the thin profile of the blade generally minimizes form drag over that of round spoke profile. However, most external winds will not be precisely aligned co-directional with the forward motion of the wheel. Such winds cause a crosswind component to be exerted upon the wheel, leading to flow-separation and thus turbulence—behind the bladed spoke, and thereby generally negate the potential aerodynamic benefit of the bladed-spoke design. Under high crosswinds, the round spoke profile may even outperform the bladed spoke in terms of drag reduction. Perhaps a result of these conflicting factors, the bladed spoke has not become the common standard for use in all bicycle competitions.
Wheel covers generally include a smooth covering material attached directly to the wheel over the outside of the spokes, generally covering a large portion of the wheel assembly, often extending from the wheel rim to the axle. Wheel covers add weight to the wheel assembly and can result in more wheel surface area being exposed to winds. The additional weight on the wheel is detrimental to wheel acceleration, while the large surface area of the cover can increase frictional drag. Although covering the wheel spokes can reduce form drag forces thereon, the increased frictional drag forces on the larger surface areas can largely offset any aerodynamic benefit. In addition, covering large portions of the wheel also increases bicycle susceptibility to crosswind forces, destabilizing the rider. For this reason, wheel covers are generally used only on the rear wheel of a bicycle, and generally only under low crosswind conditions. Perhaps as a result of these conflicting factors, wheel covers have not become the standard equipment for use in all bicycle competitions.
Recently developed for use on bicycles, deeper, stiffer and heavier aerodynamic wheel rims suffer several drawbacks: deeper (wider along the radial direction of the wheel) and streamlined rims are often used to reduce profile drag on high-performance bicycle wheels. As mentioned, these rims are generally designed to reduce profile drag under various crosswind conditions. However, these deeper rims—having generally larger rotating surface areas—can dramatically increase friction drag. As will be shown, friction drag is particularly increased on the expanded upper wheel surfaces, largely negating any potential benefit of the reduced profile drag. In addition, such deep wheel rims with minimal spokes must be made stronger and stiffer—typically with double-wall construction—than conventional single-wall, thin-rim designs. As a result, such deep rims often ride more harshly over bumpy terrain, and are generally heavier, adding weight to the bicycle, which becomes a drawback when the grade becomes even slightly uphill.
As a result of these and other countervailing factors, no single wheel design has emerged as the preferred choice for reducing drag on bicycle wheels over a wide range of operating conditions. Instead, a variety of wheel designs are often employed in modern racing bicycles. In the same competition, for example, some riders may choose to use bladed spokes, while others choose round spokes, while still others choose deep rims or wheel covers. The differences in performance between these various wheel designs appear to only marginal affect the outcome of most races.
In various cycles, fenders and mud-covers have been used to cover wheels for other purposes. However, these items are generally oriented on the cycle consistent with their intended purpose of shielding the rider from debris ejected from the wheel. As such, they are not necessarily designed to be either forwardly positioned, nor closely fitted to the tire and wheel for aerodynamic shielding purposes. On some bicycles, skirt guards have been employed specifically to prevent clothing of the rider from becoming entangled with the rotating wheel. However, these guards are often made of porous construction, and are generally employed on the rear-most wheel, rather than on the front-most wheel, where the potential aerodynamic benefit is generally greater.
Perhaps because aerodynamic devices are generally not allowed by rules governing many bicycle competitions, development of fairings for bicycles remains somewhat limited. Instead, fairings have been generally used to cover either the entire cycle, or the broad front area of the cycle, shielding both rider and cycle. Enclosing-type fairings typically have quite large surface areas, which augment frictional drag forces, largely negating any benefit in reducing form drag from streamlining the forward profile of the bicycle. Nevertheless, numerous bicycle speed records have been achieved using these larger fairings, validating their effectiveness. Frontal wind-deflecting fairings are typically used to reduce form drag on various components on a cycle; however, their greatly expanded surface areas can minimize their effectiveness by introducing greater frictional drag. The potential effectiveness of using smaller fairings—having minimal form and friction drag—for shielding specific, critical, drag-sensitive areas of moving wheel surfaces has not been properly recognized.
A study by Sunter and Sayers (2001), Aerodynamic Drag Mountain Bike Tyres, Sports Engineering, 4, 63-73, proposed and tested the use of a front-mounted wind-deflector fender for relatively low-speed, rough-surfaced, down-hill racing mountain bicycle front wheels. However, as will be shown, the tested fender was unnecessarily extensive; its extended design—covering the tire to well below the level of the axle—failed to focus properly on key sources of drag on a typical bicycle wheel. Instead, in this investigation, variations in drag were measured with differing tire tread patterns, and differing fender clearances, using knobby mountain bike tires, and were measured on the front wheel only. Moreover, sufficient fender clearances with the tire were investigated, with the aim of determining any potential benefit in reducing drag on the bicycle against the potential mud accumulation there-between.
Referencing an earlier study, Kyle (1985) Aerodynamic Wheels. Bicycling, December, 121-124, in this later study, Sunter and Sayers noted a 30% increase in drag on a wheel rotating with a speed equivalent to the exposed headwind, versus a stationary wheel exposed to the same headwind. As reported, this measurement seems to have represented the increase in torque needed to rotate the wheel about the axle. However, the change in torque measured about the axle on a fixed wheel mounted in an air-stream—as will be shown—cannot be considered an accurate representation of the change in drag force required to propel the bicycle. Torque measured this way is only an indirect factor needed to determine the effects on overall bicycle drag. As will be shown, the net drag force is generally not well centered on the rotating bicycle wheel, causing drag forces on the upper wheel to be magnified. Indeed, the offset drag force on the wheel contributes significantly more to overall bicycle drag than commonly understood.
A number of studies of bicycle wheel drag measured in wind tunnels also fail recognize the importance of drag forces on the upper wheel. Tests are typically conducted with the wheel suspended in the airstream, with the drag on the wheel measured via force gauges attached to the suspension arm. As will be shown, the magnification of upper wheel drag forces occurs when the wheel is in contact with the ground. Measuring drag on wheels suspended in an airstream will yield incomplete results, particularly for application to moving vehicles.
For example, an earlier study by Greenwell et al, Aerodynamic characteristics of low-drag bicycle wheels, Aeronautical Journal, 1995, 99, 109-120, measured translational drag on a wheel suspended from a torsion tube in a wind tunnel, where the wheel was driven by a motor and made no contact with a ground plane. They concluded that in this configuration—unexpectedly—rotational speed had little influence on the translational drag force directed upon the wheel assembly.
In a more recent study, Moore and Bloomfield, Translational and rotational aerodynamic drag of composite construction bicycle wheels, Proceedings of the Institution of Mechanical Engineers, Part P: Journal of Sports Engineering and Technology Jun. 1, 2008, vol. 222, no. 2, 91-102, the measured drag was extended to include rotational drag on the wheel. However, this study also failed to include a ground plane in contact with the wheel; the wheel remained suspended wind tunnel. As mentioned, this configuration does not accurately reflect the total retarding force upon a vehicle in motion caused by drag forces on the wheel.
Sunter and Sayers also failed to recognize the magnifying effect that an off-center net drag force on the wheel can have on overall bicycle drag. Instead, they concluded that with the modest improvement in drag torque measured upon the rotating wheel using the wind-deflecting fender, only corresponding modest improvement in overall bicycle drag could be expected. They further concluded that the use of extensive front-wheel wind-deflecting fenders—having a rather large forward profiles—might thus prove beneficial in the specific application of mountain bicycle downhill racing, where only modest reductions in overall drag might yield a winning advantage in higher speed races. This conclusion would be consistent with the faulty observation that total drag forces are generally well centered on the wheel.
Cycle spoke art includes a tapered spoke of U.S. Pat. No. 5,779,323 where the cross-sectional profile of the spoke changes from more highly elliptical near the wheel hub to more generally oval near the wheel rim. As will be shown, the spoke shown is tapered to minimize—rather than maximize—any aerodynamic benefit, especially when used in the presence of crosswinds.
An embodiment comprises a cycle wheel spoke tapered from a streamlined blade or highly elliptical cross-sectional profile nearest the wheel rim for reduced drag from higher speed headwinds, to a more circular cross-sectional profile nearest the wheel central hub where the relative crosswind components are higher, thereby minimizing potential drag-induced turbulent flow separation behind spoke surfaces along the entire length of the spoke, and thereby reducing the total drag-induced resistive forces upon the wheel assembly and minimizing needed vehicle propulsive counter-forces.
While one or more aspects pertain to most wheeled vehicles not otherwise having fully shielded wheels that are completely protected from oncoming headwinds, the embodiments can be best understood by referring to the following figures.
A reference embodiment from a parent application is first described in detail in order to present an operational description of how drag reduction specifically on the upper wheel surfaces dramatically improves vehicle propulsion. Through similar means, the drag reduction on the upper wheel surfaces provided by the embodiment of this application similarly improves vehicle propulsion under a variety of wind conditions.
As shown in
With fairing 1 configured in this way, the spokes 5 positioned near the top of the wheel 4 are shielded from headwinds. Shielded in this way, the topmost spokes 5 are moving at an effective wind speed generally less than or equal to the ground speed of the cycle 3, rather than moving at an effective headwind speed of up to nearly twice the ground speed of cycle 3. As a result, the aerodynamic drag forces exerted upon the topmost spokes 5 are greatly reduced.
The reduction in drag force due to fairing 1 is generally greater near the top of the wheel 4, where the spokes 5 are moving fastest with respect to headwinds otherwise impinging thereupon. As uppermost spokes 5 rotate away from the topmost point to an intermediate position with respect to either of the two lateral mid-points at the height of the axle on the wheel 4, these headwind drag forces are greatly reduced.
The embodiment shown in
The shielding provided by fairing 1 is particularly effective since aerodynamic forces exerted upon exposed vehicle surfaces are generally proportional to the square of the effective wind speed impinging thereon. Moreover, the power required to overcome these drag forces is generally proportional to the cube of the effective wind speed. Thus, it can be shown that the additional power required to overcome these drag forces in propelling a vehicle twice as fast over a fixed distance, in half the time, increases by a factor of eight. And since this power requirement is analogous to rider effort—in the case of a bicycle rider—it becomes critical to shield the most critical drag-inducing surfaces on a vehicle from oncoming headwinds.
In any wheel used on a vehicle, and in the absence of any external headwinds, the effective horizontal wind speed at a point on the wheel at the height of the axle is equal to the ground speed of the vehicle. Indeed, the effective headwind speed upon any point of the rotating wheel depends on that point's current position with respect to the direction of motion of the vehicle.
Notably, a point on the moving wheel coming into direct contact with the ground is necessarily momentarily stationary, and therefore is not exposed to any relative wind speed, regardless of the speed of the vehicle. While the ground contact point can be rotating, it is not translating; the contact point is effectively stationary. And points on the wheel nearest the ground contact point are translating with only minimal forward speed. Hence, drag upon the surfaces of the wheel nearest the ground is generally negligible.
Contrarily, the topmost point of the wheel assembly (opposite the ground) is exposed to the highest relative wind speeds: generally at least twice that of the vehicle speed. And points nearest the top of the wheel are translating with forward speeds substantially exceeding the vehicle speed. Thus, drag upon the surfaces of the upper wheel can be quite substantial. Lower points on the wheel are exposed to lesser effective wind speeds, approaching a null effective wind speed—and thus negligible drag—for points nearest the ground.
Importantly, due to the rotating geometry of the wheel, it can be shown that the effective combined frictional drag force exerted upon the wheel is typically centered in closer proximity to the top of the wheel, rather than centered closer to the axle as has been commonly assumed in many past analyses of total wheel drag forces. While the net pressure (or form) drag (P) force on the forwardly facing profile of the wheel is generally centered with elevation and directed near the axle on the wheel (as shown in
Indeed, it is near the top of the wheel where the relative winds are both greatest in magnitude, and are generally oriented most directly opposed to the forward motion of rotating wheel surfaces. Moreover, in the absence of substantial external headwinds, the frictional drag exerted upon the lower wheel surfaces contributes relatively little to the net drag upon the wheel, especially when compared to the drag upon the upper surfaces. The combined horizontal drag forces (from pressure drag from headwinds deflected by both the leading and trailing wheel forwardly facing profiles, and from frictional drag from headwinds impinging upon the forwardly moving surfaces) are thus generally concentrated near the top of the wheel under typical operating conditions. Moreover, with the faster relative winds being directed against the uppermost wheel surfaces, total drag forces combine near the top to exert considerable retarding torque upon the wheel.
As mentioned, the horizontal drag forces are primarily due to both pressure drag forces generally distributed symmetrically across the forwardly facing vertical profiles of the wheel, and to winds in frictional contact with moving surfaces of the wheel. Pressure drag forces arise primarily from the displacement of air from around the advancing vertical profile of the wheel, whose circular outline moves at the speed at the vehicle. As discussed above, since the entire circular profile moves uniformly at the vehicle speed, the displacement of air from around the moving circular profile is generally uniformly distributed with elevation across the forwardly facing vertical profile of the wheel. Thus, these pressure drag forces (P, as shown in
Frictional drag forces (F, as shown
Significantly, both types of drag forces can be shown to exert moments of force pivoting about the point of ground contact. And as such, either type of drag force exerted upon the upper wheel retards vehicle motion considerably more than a similar force exerted upon a substantially lower surface of the wheel. Minimizing these upper wheel drag forces is therefore critical to improving propulsive efficiency of the vehicle.
Also important—and due to the rotating geometry of the wheel—it can be shown that the vehicle propulsive force on the wheel applied horizontally at the axle must substantially exceed the net opposing drag force exerted near the top of the wheel. These forces on a wheel are actually leveraged against each other, both pivoting about the same point—the point on the wheel which is in stationary contact with the ground—and which is constantly changing lateral position with wheel rotation. Indeed, with the geometry of a rolling wheel momentarily pivoting about the stationary point of ground contact, the lateral drag and propulsive forces each exert opposing moments of force on the wheel centered about this same point in contact with the ground.
Furthermore, unless the wheel is accelerating, the net torque from these combined moments on the wheel must be null: The propulsive moment generated on the wheel from the applied force at the axle must substantially equal the opposing moment from drag forces centered near the top of the wheel (absent other resistive forces, such as bearing friction, etc.). And the propulsive moment generated from the applied force at the axle has a much shorter moment arm (equal to the wheel radius) than the opposing moment from the net drag force centered near the top of the wheel (with a moment arm substantially exceeding the wheel radius)—since both moment arms are pivoting about the same stationary ground contact point. Thus, for these opposing moments to precisely counterbalance each other, the propulsive force applied at the axle—with the shorter moment arm—must substantially exceed the net drag force near the top of the wheel.
In this way, the horizontal drag forces exerted upon the upper surfaces of the wheel are leveraged against opposing and substantially magnified forces at the axle. Hence, a relatively small frictional drag force centered near the top of the wheel can have a relatively high impact on the propulsive counterforce required at the axle. Shielding these upper wheel surfaces can divert much of these headwind-induced drag forces directly onto the vehicle body, thereby negating much of the retarding force amplification effects due to the pivoting wheel geometry.
Moreover, since the propulsive force applied at the axle exceeds the combined upper wheel drag forces, a lateral reaction force (R, as shown in
Given that the propulsive force (A) applied at the axle must overcome both the net wheel drag forces (F+P) and the countervailing lower reaction force (R) transmitted through the ground contact point, it can be shown that the net drag force upon the upper wheel can oppose vehicle motion with nearly twice the sensitivity as an equivalent drag force upon the static frame of the vehicle. Hence, shifting the impact of upper wheel drag forces to the static frame can significantly improve the propulsive efficiency of the vehicle.
Furthermore, as drag forces generally increase in proportion to the square of the effective wind speed, the more highly sensitive upper wheel drag forces increase far more rapidly with increasing headwind speeds than do vehicle frame drag forces. Thus, as the vehicle speed increases, upper wheel drag forces rapidly become an increasing component of the total drag forces retarding vehicle motion.
And given the greater sensitivity of speed-dependent upper wheel drag forces—as compared against vehicle frame drag forces—to the retarding of vehicle motion, considerable effort should first be given to minimizing upper wheel drag forces. And shielding the faster-moving uppermost surfaces of the wheel assembly from oncoming headwinds, by using the smallest effective fairing assembly, is an effective means to minimize upper wheel drag forces.
Contrarily, drag forces on the lower wheel generally oppose vehicle motion with reduced sensitivity compared to equivalent drag forces on the static frame of the vehicle. Propulsive forces applied at the axle are levered against lower wheel drag forces, magnifying their impact against these lower wheel forces. Shielding lower wheel surfaces can generally negate this mechanical advantage, and can actually increase overall drag on the vehicle.
Moreover, as discussed above, headwinds on the static frame generally exceed the speed of winds impinging the lower surfaces of the wheel. Hence, frictional drag forces on the lower wheel surfaces are greatly reduced. Thus, it is generally counterproductive to shield the wheel below the level of the axle. Drag on a vehicle is generally minimized with upper wheel surfaces shielded from headwinds and with lower wheel surfaces exposed to headwinds.
Wheel drag sensitivity to retarding vehicle motion becomes even more significant in the presence of external headwinds. With external headwinds, the effective wind speed impinging the critical upper wheel surfaces can well exceed twice the vehicle speed. Shielding protects the upper wheel surfaces both from external headwinds, and from headwinds due solely to vehicle motion.
Indeed, wheel surfaces covered by the shield are exposed to winds due solely to wheel rotation; headwinds are deflected. The effective drag winds beneath the shield are generally directed tangentially to rotating wheel surfaces, and vary in proportion to radial distance from the axle, reaching a maximum speed at the wheel rim equal to the vehicle speed, regardless of external headwinds. Since drag forces vary generally in proportion to the square of the wind speed, the frictional drag forces are considerably reduced on shielded upper wheel surfaces. Using these wind shields, shielded wheel surfaces are exposed to substantially reduced effective wind speeds—and to generally much less than half of the drag forces without shielding.
Diminished drag forces from external headwinds impinging the slower moving lower surfaces of a rolling wheel generally oppose wheel motion with much less retarding torque than drag forces from winds impinging the faster upper surfaces. Indeed, tests demonstrate that with upper shields installed on a suspended bicycle wheel, the wheel will spin naturally in the forward direction when exposed to headwinds. Without the shields installed, the same wheel remains stationary when exposed to headwinds, regardless of the speed of the headwind. And an unshielded spinning wheel will tend to stop spinning when suddenly exposed to a headwind. This simple test offers an explanation for the unexpected result achieved from Greenwell—mentioned above—and demonstrates that by minimally shielding only the upper wheel surfaces from external headwinds, the overall drag upon the rotating wheel can be substantially reduced.
Furthermore, as external headwinds upon a forwardly rotating vehicle wheel add relatively little frictional drag to the lower wheel surfaces—which move forward at less than the vehicle speed—but add far more significant drag to the upper wheel surfaces, which move forward faster than the vehicle speed and which can more significantly retard vehicle motion, shielding the upper wheel surfaces against headwinds is particularly beneficial. Since drag forces upon the wheel are generally proportional to the square of the effective wind speed thereon, and the additional drag on the wheel—and thereby on the vehicle—increases rapidly with headwinds, shielding these upper surfaces greatly reduces the power required to propel the vehicle. Moreover, the relative effectiveness of shielding upper wheel surfaces generally increases with increasing headwinds.
An examination of the retarding wind vectors on a rotating wheel can reveal the large magnitude of drag retarding moments upon the uppermost wheel surfaces, relative to the lower wheel surfaces. And an estimate of the frictional drag torque on the wheel can be determined by first calculating the average moments due to drag force vectors at various points—all pivoting about the ground contact point—on the wheel (results shown plotted in
In order to determine the relationship between this torque and elevation on the wheel, the magnitudes of the drag wind vectors that are orthogonal to their corresponding moment arms pivoting about the point of ground contact must first be determined. These orthogonal vector components can be squared and then multiplied by the length of their corresponding moment arms, in order to determine the relative moments due to drag at various points along the wheel rim.
The orthogonal components of these wind vectors tend to increase linearly with elevation for points on the rim of the wheel, and also for points along the vertical mid-line of the wheel. Calculating the moments along the vertical mid-line of the wheel can yield the minimum relative drag moments at each elevation. Calculating an average of the maximum drag moment at the rim combined with the minimum drag moment along the mid-line can then yield the approximate average drag moment exerted at each elevation upon the wheel. Multiplying this average drag moment by the horizontal rim-to-rim chord length can yield an estimate of the drag torque exerted upon the wheel at each elevation level (
From the resulting plots (
These calculations—generally confirmed by tests—indicate a substantial reduction in retarding drag torque upon the shielded upper wheel surfaces. In the absence of external headwinds, the plots of
As discussed above, since upper wheel drag forces are leveraged against the axle—thereby magnifying the propulsive counterforce required at the axle—an increase in drag force on the wheels generally retards vehicle motion much more rapidly than does an increase in other vehicle drag forces. And while under external headwind conditions, the total drag on a vehicle with wheels exposed directly to headwinds increases still more rapidly with increasing vehicle speed.
Shielding upper wheel surfaces effectively lowers the elevation of the point on the wheel where the effective net drag force is exerted, thereby diminishing the magnifying effect of the propulsive counterforce required at the axle, as discussed above. As a result, the reduction in drag force upon the vehicle achieved by shielding the upper wheel surfaces is comparatively even more significant with increasing external headwinds. Shielding these upper wheel surfaces can thereby improve relative vehicle propulsion efficiency under headwinds by an even greater margin than under null wind conditions.
Moreover, shielding these upper wheel surfaces can be particularly beneficial to spoked wheels, as round spokes can have drag sensitivities many times greater than that of more streamlined surfaces. As round spokes—in some configurations—can have drag coefficients ranging from one to two orders of magnitude greater than corresponding smooth, streamlined surfaces, shielding the spokes of the upper wheel from external wind becomes particularly crucial in reducing overall drag upon the wheel.
Accordingly—given these multiple factors—a relatively small streamlined fairing attached to the vehicle structure and oriented to shield the upper surfaces of the wheel assembly from oncoming headwinds substantially reduces drag upon the wheel, while minimizing total drag upon the vehicle. Consequently, an embodiment includes the addition of such a fairing to any wheeled vehicle—including vehicles having spoked wheels, where the potential drag reduction can be even more significant.
The addition of such minimal fairings to each side of a traditional spoked bicycle wheel, for example, reduces windage losses and improves propulsive efficiency of the bicycle, particularly at higher cycle speeds or in the presence of headwinds, while minimizing cycle instability due to crosswind forces. Since crosswinds are a significant factor restricting the use of larger wheel covers, minimizing the fairing size is also an important design consideration. And minimizing form drag induced by the forward-facing profile of the fairing also will influence the fairing design. The preferred fairing size will likely substantially cover the upper section of the exposed wheel, and be placed closely adjacent to the wheel surfaces, consistent with general use in bicycles. In heavier or powered cycles, design considerations may permit somewhat larger fairings, covering even more of the wheel surfaces.
As shielding upper wheel surfaces can reduce overall drag on the vehicle, while simultaneously augmenting the total frontal profile area of the vehicle exposed to headwinds, a natural design constraint emerges from these competing factors: Shields should be designed sufficiently streamlined and positioned sufficiently close to wheel surfaces to provide reduced overall vehicle drag. And as shielding effectiveness potentially increases under headwind conditions, shields designed with larger surface areas and larger frontal profiles may still provide reduced overall vehicle drag under headwind conditions, if not under null wind conditions. Thus, a range of design criteria may be applied to selecting the best configuration and arrangement of the fairing, and will likely depend on the particular application. In any particular application, however, the embodiment will include a combination of design factors discussed above that will provide a reduction in overall vehicle drag.
In a cycle application, for example, fairings positioned within the width of the fork assembly will likely provide the most streamlined design which both shields spokes from headwinds but also minimizes any additional form drag profile area to the vehicle frame assembly. In other applications, insufficient clearances may preclude positioning the fairings immediately adjacent to moving wheel surfaces. In such situations, headwinds may be sufficient in magnitude to cause a reduction in overall vehicle drag to justify the use of wider upper wheel fairings—positioned largely outside the width of the fork assembly—with extended forward profile areas.
Furthermore, from the previous analysis a consideration the drag torque curves wholly above the level of the axle, it becomes apparent that shielding the wheel is best centered about an elevation likely between 75 and 80 percent of the diameter of the wheel, or near the center of the area under the unshielded torque curve shown in
As discussed, the precise elevation about which the major upper drag-inducing surfaces are centered, as well as the precise extent to which surfaces in the forward quadrant and in the upper half of the wheel central structure are included in the major upper drag-inducing surfaces, will depend on the particular application and operating conditions. Certain wheel surfaces with higher drag sensitivities, such as wheel spokes, generally need to be shielded when positioned within the region of the major upper drag-inducing surfaces. Other surfaces such as smooth tire surfaces having lower drag sensitivities may also benefit from shielding if their surface areas are extensive, are positioned near the critical level in elevation, or are the primary upper wheel surfaces exposed to headwinds. In the example analysis of
A similar analysis can be performed for form drag forces on the moving forward vertical profiles of the wheel rim or tire. The results obtained are generally similar in form, though may differ somewhat in magnitudes as the effective wind speeds on the moving profiles are generally lower on the upper wheel—equal to the vehicle speed—and will depend on the particular application, including the total area of the wheel forward profile exposed to headwinds, and to headwind and vehicle speeds. Nevertheless, the net pressure drag torque caused by the moving outline of the wheel is also centered above the level of the axle, and thereby merits consideration in determining the particular height of the critical elevation level, and in the ultimate configuration of the fairing.
Hence, the fairing shown in
In consideration of further embodiments described below, the operating principles described above will generally apply, and may be referred thereto.
In
The profile is shaped to maintain a generally constant total area in cross-section, in order to retain a relatively constant tensile strength along the full length of the spoke. The spoke profile may include cross-sectional areas varying somewhat along the spoke length, typically with larger areas nearest the ends of the spoke to enhance strength near attachment points. Nearest the wheel rim, the streamlined profile is more elliptical (or flat like a blade or thin wing), as shown in
Streamlined spokes reduce drag upon the wheel in the presence of direct headwinds. A crosswind directed upon the wheel can cause turbulence across the broad face of the streamlined spoke, quickly increasing drag thereon. Eliminating crosswind turbulence upon the streamlined profile is essential to minimizing drag under crosswind conditions. The design challenge becomes to minimize drag through spoke streamlining over the widest range of crosswind conditions; too wide a blade design can exacerbate drag under even minimal crosswinds, thereby negating any advantage of the streamlined spoke profile.
Notably, the relative crosswind-to-headwind vector component varies significantly depending on the relative location on the wheel. Near the top of the wheel, headwinds are strongest, and any relative crosswinds are less significant. Near the bottom of the wheel, headwinds are minimized and crosswinds are more significant. Thus, crosswinds can be a more significant relative source of drag on wheel surfaces closer to the ground.
The broader width of the bladed spoke provides greater streamlining for the higher speed headwinds near the top of the wheel, thereby minimizing drag on these critical drag-inducing surfaces. Any turbulence from the relatively smaller crosswind components directed upon the faster moving uppermost portion of the bladed spokes is generally minimized. The same crosswinds directed upon slower moving spoke surfaces near the center of the wheel are a more significant relative component of the total wind vector thereon, and thus have a greater potential to induce turbulence—and thereby to increase drag.
And as lower surfaces of the wheel are exposed to substantially reduced headwinds, and also contribute much less resistive torque upon the wheel, crosswind-induced turbulence on the lower spokes is a relatively insignificant factor contributing to overall vehicle drag when compared to the upper wheel surfaces. Thus, spoke profiles are best tapered for optimum reduced drag on upper wheel surfaces, rather than for lower wheel surfaces.
As a circular spoke profile generally produces far less drag-inducing turbulence than a flat blade profile when obliquely facing the wind, the portion of the spoke most sensitive to crosswinds should be closer to circular in profile, while the portion of the spoke less sensitive to crosswinds should be closer to a streamlined wing shape in profile. Thus, a tapered spoke—whose profile gradually transitions from thin and streamlined near the rim of the wheel, to more oval or circular near the central hub of the wheel—can reduce the drag on the spoke over a wider range of crosswind conditions than traditional generally constant cross-sectional profile—either bladed or circular—spoke designs.
Exposed wheels can generate considerable drag forces on a moving vehicle. These forces are directed principally near the top of the wheel, rather than being more evenly distributed across the entire profile of the wheel. Moreover, these upper-wheel drag forces are levered against the axle, thereby magnifying the counterforce required to propel the vehicle. As a result, a reduction in drag upon the upper wheel generally enhances propulsive efficiency significantly more than a corresponding drag reduction on other parts of the vehicle.
With the net drag forces being offset and directed near the top of the wheel, nearly equivalent countervailing reaction forces—also opposing vehicle motion—are necessarily transmitted to the wheel at the ground. These reaction forces necessitate augmented down-forces to be applied in higher speed vehicles, in order to maintain static frictional ground contact and, thereby, vehicle traction and directional stability. As wings and other means typically used to augment these down-forces in such vehicles can add significant drag, it becomes evident that substantial effort should be made to reduce the upper wheel drag forces on most high-speed vehicles.
Drag can be reduced on the bladed spokes of bicycle wheels, by simply by tapering the blade of the spoke at the rim to a more round profile at the wheel hub. By tailoring the profile of the spoke to accommodate the range of crosswind components—which vary in magnitude from the rim to the hub—drag can be minimized on the spoke over a variety of crosswind conditions. Moreover, the tailfin could also be adapted with a similar variable cross-sectional profile along its length in order to minimize drag on the spoke under crosswind conditions.
The embodiment should not be limited to the specific examples illustrated and described above, but rather to the appended claims and their legal equivalents.
This is a division of application Ser. No. 13/799,005, filed Mar. 13, 2013, currently pending, by the present inventor.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13799005 | Mar 2013 | US |
| Child | 14727823 | US |
