NOVEMBER 2009
RZR Drag, Lift and Inviscid Flow - By Paul Lew
Drag and Lift
The VP of Sales and Marketing for a bicycle wheel company once said, “80 grams of negative drag translates to 10 watts
of power. It’s like an airplane, you get air under the wing and you get lift.”
There are a lot of things wrong with this statement. First, there is no such thing as negative drag, and to confuse it with lift
represents a lack of understanding of aerodynamic forces. Lift is an aerodynamic component that cannot exist without drag, they
are not the same, nor does creating lift minimize drag. Second, air under a wing (the high pressure side) does not create lift.
Lift is created on top of a wing (the low pressure side). Lift is a result of low pressure on top of a wing which creates a vacuum
pulling the wing toward the low pressure, not high pressure pushing it. Additionally, where there is lift, there is always drag. Drag
is a component of lift. The goal for optimal bicycle wheel performance is not to create lift, and as a result not to create drag.
Rotating components can auto-rotate generating no apparent drag, or even what some manufacturers call “negative drag”.
This is not a new aerodynamic phenomenon. It is not something that only one bicycle component manufacturer can claim that
pertains to wheel performance. The fact is, that there are naturally occurring (leaves that fall from trees), and man-made
components, including bicycle wheels that will auto-rotate, but they will not create what some manufacturers may
term “negative drag.”
Drag (or negative drag) should not be confused with lift. The term negative drag is used by some bicycle component
manufacturers to describe a force in the opposite direction of drag. This is very different from lift. Lift is a force that is typically
perpendicular to the plane of the wing, tube, or wheel. The term drag, as applied to cycling is the force that is retarding forward
progress. To describe a component as generating negative drag is confusing, and misleading. And it is not generating negative
drag. It is a clever manipulation of the wind tunnel balance which can only be simulated by the illustration below. It is not possible
to generate forward propulsion from a bicycle component. It is not possible to have zero drag, meaning zero effort to move you
forward. It is possible to register low drag, and low drag is the goal of high-performance component manufacturers.
“The term negative drag is used by some bicycle component
manufacturers to describe a force in the opposite direction of
drag. This is very different from lift.”
Sailboats move through the water using lift, not negative
drag because in aerodynamics “negative drag” is not a valid
term, nor (as it described by some cycling manufacturers)
is it a useful force. A sail does not generate lift that will
move the boat through the water unless two things happen
simultaneously. First, the sail must be aligned at an angle
that is not parallel to the hull, or the wind. In fact this parallel
alignment is exactly what sailors do when they do not want
to make forward progress. If the sailor wants to make
forward progress, while the sail is “lifting” (with the sail
off-axis to the longitudinal plane of the boat hull), the boat
must “slip” sideways. Racing boats use sophisticated keels
to minimize drag, and to use lift to off-set or balance the lift
of the sail so that the boat will not capsize.
Fig.1
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A publication of the Reynolds Composites Studio ©2009 All Rights Reserved
Light Up Your Ride.™
NOVEMBER 2009
RZR Drag, Lift and Inviscid Flow - By Paul Lew
A bicycle is not a sail boat. If a bicycle were able to yaw
its wheels at an angle other than in the plane of the bicycle
(Fig.1) turning both wheels left or right while riding a straight
line, and slip to the side (perpendicular to the direction of
travel), all at the same time, the force from a strong wind
could generate a very small force (bicycle wheels are
inefficient lifting mechanisms) in the direction of travel.
This, clearly, is not possible. Fig.1 describes how a bicycle
wheel can generate lift and forward momentum.
Examination of the illustration demonstrates that a bicycle
and its wheels cannot function in the manner, and therefore
they cannot generate forward force.
Textured Aerodynamic Surfaces
Surface treatment of bicycle wheels intended to reduce
drag by the creation of an inviscid (air with no viscosity/ no
resistance) layer of air is also confusing and misleading.
Applying blunt body theory to streamlined shapes is invalid.
A streamlined body refers to an airfoil or flat plate shape.
A blunt body refers to primarily round (spherical) shapes.
Attention to the surface texture of aerodynamic components
is not new. For example wings and airfoils (streamlined
bodies) are typically smooth, while golf balls (blunt bodies)
are typically dimpled. Wings and airfoils are typically smooth
because the smooth surface improves laminar flow and, as a
result, increases lift and/or reduces drag.
“Surface treatment of bicycle
wheels intended to reduce drag by
the creation of an inviscid (air with no
viscosity/no resistance) layer of air is
confusing and misleading. Applying
blunt body theory to streamlined
shapes is misleading.”
Laminar flow, also known as streamline flow, occurs when a
fluid flows in parallel layers, with no disruption between
the layers.
In fluid dynamics, laminar flow is a flow regime
characterized by high momentum diffusion, low momentum
convection, pressure and velocity independent from time. It is
the opposite of turbulent flow. In nonscientific terms laminar
flow is “smooth,” while turbulent flow is “rough.”
For a streamlined body, such as an airfoil, turbulence
typically reduces lift and increases drag.
Airflow over a bicycle wheel cannot be laminar because the
wheel is rotating, and turbulence is created as a result of
the dynamic nature of its rotation. The airflow over a bicycle
wheel is always turbulent unless the wheel is not rotating.
Because the airflow on the surface of a bicycle wheel is
turbulent, adding a surface treatment that encourages
turbulence will have no effect on decreasing drag, and little
effect on increasing drag. A rough surface treatment on a
bicycle wheel, which is much more of a streamlined body,
than a blunt body will increase drag, and has been proven
to do so if the wheel is not rotating. When the wheel rotates
the effect of the increased drag due to the rough surface is
largely lost in the turbulence generated from the rotation of
the wheel. Rough surfaces and dimples do have an effect
on streamlined objects, they increase drag. The decreased
drag, and high-performance from wheels & rims with dimples
or other surface textures is not the result of the surface
treatment, but is a result of a low drag well-designed rim
cross-section shape.
If dimples reduce the drag of golf balls, doesn’t it make sense
that they would also decrease drag on bicycle wheels?
Dimples on golf balls work to reduce drag because the object
is blunt, not streamlined, and because the dimples that
actually do the work to reduce the drag are the dimples on a
leading patch described by an annular ring on the front
of the golf ball.
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“Rough surfaces and dimples do have an effect on
streamlined objects: they increase drag.”
A publication of the Reynolds Composites Studio ©2009 All Rights Reserved
Light Up Your Ride.™
NOVEMBER 2009
RZR Drag, Lift and Inviscid Flow - By Paul Lew
The dimples on the side of the golf ball actually increase
drag, but the net gain from the effect of the dimples on
the leading patch off-sets the additional drag from the
dimples on the sides of the ball. Because golf balls spin,
there is no way to predict what region of the surface of
the ball will be the leading patch. Dimpling the entire ball
ensures that the leading patch will always have dimples
presented to the high-pressure (leading) region. In a perfect
world, a golf ball would not rotate and the dimples would
only be present in one region defined by a small circle on the
leading portion of the ball. The reason for adding dimples to
the front of the ball is to create turbulence which travels rearward and fills in the vacuous region trailing the ball. This is
blunt body theory, which is not valid for stream-lined objects.
Bicycle wheel manufacturers who claim that a textured
surface reduces drag claim that they are creating an inviscid
layer of air. Inviscid means a layer of air that has no viscosity (no resistance) so that the air above it can flow over the
surface with less friction due to the fact that the inviscid layer
creates a slippery surface. As described above, a rotating
wheel by nature creates an inviscid layer of air. In order for
the layer of air on the surface of a wheel to not be inviscid,
the wheel must not rotate. This further invalidates the notion
that dimpled or textured surfaces on a rim improve
the performance.
A dimpled or textured streamlined surface applied to the
leading edge of the tube in a round-tube bicycle would
potentially produce lower drag numbers than a tube without
dimples or texture. This cross-sectional shape is a blunt
body, and therefore would benefit from dimples or texture
in the same way a golf ball benefits from dimples. An airfoil
shaped tube is a streamlined body, and dimples or texture
applied to the tube in any location would increase the
drag of the airfoil.
A publication of the Reynolds Composites Studio ©2009 All Rights Reserved
Light Up Your Ride.™