Examining the Aerodynamic Performance of

Commercial Bicycle Racing Wheels using CFD

Matthew N. Godo, Ph.D.

FieldView Product Manager, Intelligent Light

David Corson,

Program Manager - AcuSolve, Altair Engineering

Steve M. Legensky

General Manager, Intelligent Light

Yves-Marie Lefebvre

Sales & Support Engineer, Intelligent Light

Intelligent Light

• Established in 1984

27 years in July 2011!

Global Customer Base

• Two components to our business:

– FieldView Software

– Applied Research Group • Customer-driven R&D

• CFD & Post-processing Research

• CFD for Wind Energy

Our Mission

To help our customers using CFD

to do more with less and make

better decisions

How we accomplish our mission:

• CFD post-processing products & methods

• Workflow automation

• Development of new CFD

methodologies

The Right Wheel?

Background • Wind Tunnel testing used extensively in

cycling for over 20 years

– Typical for Zipp, 85h at $850/h,

run 3 or 4 times per year

• Benefits to cyclists from Wind Tunnels

– Improved rider positioning for lower drag

– Significant performance improvements in

equipment design

• Current status

– Still considerable component variations

– UCI rule changes & enforcement can be

rapid & unpredictable

– Wind Tunnel reaching its limit today

– Interpretation of results ‘controversial’ Zinn, L., “Spoked aero’ wheels catching up with discs”, Inside Triathlon, 1995, 10(4), p 36-37

Advertisement ca 2007

From Greenwell et.al.

Wheel drag is responsible for 10% to

15% of total aerodynamic drag Rider makes up the drag majority

Improvements in wheel design can

reduce drag between wheels by as

much as 25%

Overall reduction in drag can be on

the order of 2% to 3%

0 1 2 3 4 5

Percentage Time Difference

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

Fin

ish

Po

sit

ion

Tour de France 2008

Stage 20 Individual Time Trial

3.0%

0 1 2 3 4 5 6 7 8

Percentage Time Difference

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

Fin

ish

Po

sit

ion

IronManTM Lake Placid Triathlon 2008

Male 45-49 Age Group

3.3% Q Q Q Q Q

How much does it matter?

outer wheel

(incl. tire)

inner wheel

(incl. spokes)

non-conformal interface

ground plane

hub

mesh for inner wheel

is created separately

Boundary Conditions

Moving Reference Frame

Mesh Displacement

(unsteady)

Realistic spoke rotation

Boundary Conditions

yaw angle

Surrounding

domain

ground plane

top tube

down tube

head tube

ground plane

Ground plane no-slip surface

translational speed of 20 or 30mph

Far Field uniform velocity profile

yaw angles from 0o to 20o

Fork, frame, caliper, brake pads no slip surface

zero relative velocity

Postprocessing Objectives Performance Metrics

• Resolved Forces

• Turning Moments

• Aerodynamic Torque

• Power to Overcome Aero Resistance

Requirements

• Quantitative & Qualitative

• Easily automated & scalable

• Batch compatible on remote clusters

Side View

Axial Drag Force

Vertical Force

Direction of Wheel Rotation

Wind Velocity (effective)

Top View

Axial Drag Force

Side (Lift) Force

Bike Velocity (relative)

Wind Velocity (effective)

Turning Moment

P = 𝐹𝐷 𝑉 + 𝑀𝜔 𝑏𝑜𝑡ℎ 𝑠𝑖𝑑𝑒𝑠

𝑀 = 𝑟 ∙ 𝑇𝜃𝑥𝑑𝐴

𝑅

0

CFD Results vs Wind Tunnel Data

Circumferential Variation, Side Force

D i r e c t i o n o f F l o w

Turning Moments, All Wheels

0 2 4 6 8 10 12 14 16 18 20

Yaw Angle [degrees]

-0.4

0

0.4

Mo

men

t [N

·m]

Rolf Sestriere

Zipp 404

Zipp 808

Zipp 1080

HED TriSpoke

Zipp Sub9 Disc

(right axis)-4

-2

0

2

Turning Moment vs. Yaw Angle at 30mph

0 2 4 6 8 10 12 14 16 18 20

Yaw Angle [degrees]

-0.4

-0.2

0

0.2

Mo

men

t [N

·m]

Rolf Sestriere

Zipp 404

Zipp 808

Zipp 1080

HED TriSpoke

Zipp Sub9 Disc

(right axis)

-1

0

1

Turning Moment vs. Yaw Angle at 20mph

0 2 4 6 8 10 12 14 16 18 20

Yaw Angle [degrees]

-0.4

0

0.4

Mo

men

t [N

·m]

Rolf Sestriere

Zipp 404

Zipp 808

Zipp 1080

HED TriSpoke

Zipp Sub9 Disc

(right axis)-4

-2

0

2

Turning Moment vs. Yaw Angle at 30mph

0 2 4 6 8 10 12 14 16 18 20

Yaw Angle [degrees]

-0.4

-0.2

0

0.2

Mo

men

t [N

·m]

Rolf Sestriere

Zipp 404

Zipp 808

Zipp 1080

HED TriSpoke

Zipp Sub9 Disc

(right axis)

-1

0

1

Turning Moment vs. Yaw Angle at 20mph

Wheel Only Studies

• Configurations: 6

• Speeds : 1

– 20mph

• Yaw Angles: 1

– 10o

• Design Points: 6

• Time steps: 256

– For each design point

• Total steps: 1536

• ~1.2TB of data

– ~200GB per wheel

Zipp 404

Zipp 1080 HED TriSpoke

Rolf Sestriere Zipp 808

Zipp Sub9

Streaklines revealed strong periodic shedding,

distinctive for each wheel studied

1.0

2.0

3.0

5.0

6.0

Str

ou

hal

No

.

Strouhal range obtained from resolved drag, side, vertical forces and moments

Strouhal No., All Wheels (20mph, 10 degrees yaw)

Expanding the Scope… • Configurations: 9

• Speeds : 2

– 20mph, 30mph

• Yaw Angles: 5

– 0o, 5o, 10o, 15o, 20o

• Design Points: 90

• Time steps: 256

– For each design point

• Total steps: 23040

• Numbers of merit (for each step)

– Drag & Side Force

– Turning Moment

– Aerodynamic Torque

– Total Power

Wh

ee

l o

nly

R

eyn

old

s C

arb

on

B

lac

kw

ell

Ban

dit

Zipp 404 Zipp 1080 HED TriSpoke

Solver & Coprocessing

Socket Communication

Python Script

User Requests (variables, elements, etc.)

Parallel AcuSolve Processes

Volume Mesh

FieldView UNS file mini-grids

Prism layers

AcuSolveTM

Based on stabilized

Galerkin/Least Squares

Second order accuracy

Time and space

Equal order interpolation

for all variables

Globally & locally

conservative

Fully coupled pressure/

velocity iterative solver

Fully parallelized for

shared mem & clusters

Batch Postprocessing Workflow cluster or cloud system

Batch

mini-

grids

forces forces

XDB XDB

forces

Solver

Parallel, 54 cores

4-6h elapsed per design

point

Only mini-grids saved

Batch Postprocessing

FVXTM scripts used for all

performance metrics

Concurrent, typically 40

jobs in queue

Less than 1h per job

Batch Postprocessing Workflow cluster or cloud system

Batch

Solver

runs

XDB Data Reduction

46X smaller files

Full numerical fidelity

FTP to local desktops for

interactive postprocessing

XDB XDB

FTP

Solver

Parallel, 54 cores

4-6h elapsed per design

point

Only mini-grids saved

Batch Postprocessing

FVXTM scripts used for all

performance metrics

Concurrent, typically 40

jobs in queue

Less than 1h per job

Power vs Time vs Yaw Angle TriSpoke

Industrial Relevance “For Zipp, working with Matt on this paper [AIAA-2010-

1431] was largely what spurred the Firecrest rim shape

development on the handling side. Before this, we had

some super fast shape concepts, but realized from the data

that there was just so much more to be done on the

handling side, that we spent a few extra months in

development chasing favorable handling characteristics

(rearward center of pressure and shedding behavior).

Ultimately we still can't replace the wind tunnel with CFD,

but the ability to understand and predict so many aspects of

performance and handling is pretty awesome!

And that's just the beginning...”

Josh Poertner, Category Manager,

Zipp Speed Weaponry, Indiana

Zipp 404 Firecrest

cross section profile