Aerodynamics of Cycling

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Aerodynamics of Cycling

• Stephen Kulju

http//www.mira.co.uk/Services/images/bike.jpg
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Outline

• Introduction
• Basic Fluid Mechanics
• Drag and Friction
• Bicycle Aerodynamics
• Position
• Velocity Power Output
• Reducing Drag
• Drafting
• Crosswind effects

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Introduction

• Aerodynamics, or wind resistance is an everyday
  experience to bicyclists. At average speeds
  aerodynamic drag is the largest resistive force
  aside from the gravity of a large hill
• Due to the fluidity of air.
• Composed of normal (Pressure) force and
  tangential (frictional) force.
• Extremely geometry dependent.

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Fluid Mechanics Dynamics

• Fluid a material that deforms continuously and
  permanently under the application of a shearing
  stress.
• Important properties
• Density (?)
• Specific weight (?)
• Specific Gravity (SG)
• Viscosity (µ)

Shear Force
Velocity Gradient
Velocity of fluid is zero along surface due to
friction . (No slip condition)
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Fluid Mechanics Dynamics

• Streamline
• Lines tangent to the velocity vector throughout
  the flow field

Figure from Fundamentals of Fluid Mechanics pg. 97
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Fluid Mechanics Dynamics

• Stagnation Point
• Largest pressure obtainable along a streamline
• Velocity is zero

Figure from Fundamentals of Fluid Mechanics pg.
108
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Fluid Mechanics Dynamics

• Air as a fluid
• When studying aerodynamics air is treated as a
  fluid.
• Follows all laws of motion and all laws of fluid
  mechanics

• SF mâ
• Conservation of Energy
• Conservation of Mass

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Fluid Mechanics Dynamics

• Continuity equation
• Mass is conserved
• V1A1V2A2
• Bernoulli equation
• P11/2 ?V12 ?z1 P21/2 ?V22 ?z2
• Relationship between Pressure, Velocity, and
  Elevation
• Based on conservation of linear momentum (Kinetic
  Energy)

V2
V1
A2
A1
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Aerodynamics

• Two effective forces
• Pressure
• Friction
• For cyclists, pressure effect is much larger than
  friction due to non-streamlined body.
• Streamlined bodies incorporate gradual tapering
  to minimize pressure effect and separation of
  fluid

(a) Normal pressure and friction forces (b)
Attached and separated flow around a cylinder (c)
Attached flow and pressure recovery along a
streamlined body
Figure from Bicycle Science pg. 174
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Aerodynamics

• Drag Coefficient
• CD drag/(area x dynamic pressure)
• Dynamic Pressure can be approximated for speeds
  under 100 mi/h as
• Dynamic pressure ?V2/2gc
• gc 32.174 lbm-ft/lbf-s2
• Drag
• The force in the direction of relative flow.
• Propulsion power to overcome drag
• W drag force x relative vehicle velocity

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Aerodynamics
Drag coefficients of various geometries
Figure from Bicycling Science pg. 191
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Aerodynamics

• Laminar Flow
• Layers of fluid flow slide smoothly over one
  another
• Turbulent Flow
• Boundary layer is composed of vortices that
  increase surface friction.
• Common at rear end of non-streamlined vehicle

Turbulent
Laminar
http//www.cheng.cam.ac.uk/research/groups/electro
chem/JAVA/electrochemistry/ELEC/l2fig/laminar.gif
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Bicycle Aerodynamics

• Bicycle is responsible for 20-35 of drag.
• Loose Clothing increases drag by up to 30.

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Bicycle Aerodynamics - Position

• Positions
• Goals reduce frontal area reduce drag
  coefficient

CD Frontal Area CDA Power to Overcome Drag
Tops 1.15 .55 m2 .632 m2 345 W
Hoods 1.0 .40 m2 .40 m2 220 W
Drops .88 .36 m2 .32 m2 176 W
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Bicycle Aerodynamics - Position

• Drag Coefficients

Figure from Bicycling Science pg. 188
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Bicycle Aerodynamics - Position

• Rearward vs. Forward position (23.57 22.28 N
  drag)

• Forward seat position decreases drag at the
  expense of comfort and pedaling mechanics.
• Union Cycliste Internationale limits the fore-aft
  position of the saddle requiring it be at least 5
  cm behind the bottom bracket spindle
• Injury preventive measure

Image and caption from Road Cycling Handbook
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Bicycle Aerodynamics

• Fairings
• Reduce Drag Coefficient up to 50

http//www.lightningbikes.com/sf40blu.jpg
Image from Bicycling Science pg. 191
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Bicycle Aerodynamics - Drafting

• Drafting

• Traveling close behind another rider
• Broken up air vortices propel second rider
• Offers advantage to both front and rear rider
• Riders in group expend 40 less energy than solo
  riders

http//pro.corbis.com/images/AX933548.jpg?size67
uid51D3B79C-B5D0-4A72-B318-B002D5C78EBC
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Bicycle Aerodynamics - Drafting

• Drafting

Negative drag propels object forward at close
distances( 1 diameter and under)
Image from Bicycling Science pg. 199
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Bicycle Aerodynamics - Drafting

• Drafting
• No advantage
• to side by side
• drafting.

Higher CD occurs at distances less than the of
the width of the strut (or rider)
Aerodynamic interference of two side by side
struts. Image from Bicycling Science pg. 201
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Bicycle Aerodynamics -Crosswinds

• Aerodynamic drag is usually calculated assuming
  calm weather
• Crosswinds create aerodynamic moments and
  instability.
• CP (point of action of aerodynamic forces) should
  be behind the CG for maximum stability.

Fcrosswind
CP
CG
Fcrosswind
CG
CP
stable
instable
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References

• "Efluids bicycle aerodynamics." EFluids. 04 Apr.
  09 lthttp//www.efluids.com/efluids/pages/bicycle.h
  tmgt.
• Gregor, Robert J. Road Cycling - Handbook of
  Sports Medicine and Science. Malden Oxford,
  2000.
• Munson, Bruce R., Donald F. Young, and Theodore
  H. Okiishi. Fundamentals of Fluid Mechanics. 5th
  ed. Jon Wiley Sons, 2006.
• Tamai, Goro. The Leading Edge - Aerodynamic
  Design of Ultra-streamlined Land Vehicles.
  Cambridge Robert Bentley Publihsers, 1999.
• Wilson, David G., and Jim Papadopoulos. Bicycling
  Science. 3rd ed. MIT P, 2004.