Human Factors in Bicycle Design

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Human Factors in Bicycle Design
Determination of Optimum Values for Bicycle
Sizing Parameter
( or How Big Should My Bike Be?)
Human Factors in Engineering Design - Case
History
Adam Lorimer April 2002
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Contents

• Handlebar Position
• Total Sizing Systems
• Different Cycling Disciplines
• Conclusions
• Summary
• References

• Introduction
• Mechanics of pedalling
• Which Bike Measurements?
• Which Body Measurements?
• Crank Length
• Saddle Position
• Saddle Height
• Fore/Aft Saddle Position

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Introduction

• For the cyclist interested in performance, good
  bike fit is paramount.
• A properly fitted rider will feel efficient and
  comfortable on the bike.
• An efficient position is one that enables the
  rider to produce-
• More power for a given muscular effort
• That power without working any muscle groups
  excessively or needlessly
• Comfort and efficiency may be at odds in certain
  riding conditions
• Position may have to be modified to favour one
  quality over the other.
• In a time trial efficiency matters most, so a
  position allowing the best aerodynamics is
  desirable.
• In a long road race, an uncomfortable bike may
  ultimately be more fatiguing than a small loss in
  efficiency

Some studies have shown that oxygen uptake (VO2)
can be optimised by having the correct saddle
height. Setting-up a Time Trial bike properly can
result in improvements as much as 4.5.
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Mechanics of Pedalling
Electromyography (EMG) measures the electrical
pulses generated by contracting muscles. EMG
has been used to identify the operation of the
major cycling muscles of the leg.
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Which Bike Measurements ?
There are three points at which the cyclist comes
into contact with a bicycle
It is therefore the relative position of these
points with which we are concerned.
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Which Body Measurements ?
These distances considered with respect to the
anthropometric measurements of the cyclist affect
the position the body takes while cycling.
Most systems use the following body measurements
as a basis for calculations
Some systems take extra measurement such as
Thigh Length Hip Flexibility Forearm
Length Body Weight Foot Size
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Crank Length
Cranks tend to be supplied in standard lengths,
170mm for road bikes, 175mm for mountain bikes A
longer crank allows more torque to be applied to
power the bicycle, but the foot must travel a
greater distance for each pedal rotation. This
means a longer crank effectively lowers the gear
ratio. The gear ratio can be easily changed so
this is not a factor in determining the optimal
crank length.
Optimal crank length is some function of overall
leg length or thigh length. If the crank is too
long the leg must develop power outside its most
efficient range of motion. If the crank is too
short, pedalling cadence must be very high. This
is limited to the speed at which muscles can
switch on and off. Efficiency is also reduced
at very high cadences.
There are no accurate methods available for
determining crank length. Crank length should
normally be 18-25 of inseam leg length (C), but
this does not hold true for cyclists with very
long or very short legs. Beyond this,
fine-tuning should be based on what feels right.
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Saddle Position
There are a number of criteria to determine the
optimal saddle position
The crank position allowing maximum force input
(3 oclock position) should coincide with the
legs natural position (maximum power output from
cycling muscles)
The pelvic girdle should remain horizontal and
the knee slightly bent throughout the pedalling
circle for comfort, efficiency and to prevent
injury.
Optimum Saddle Position is a COMPROMISE
The lower the saddle the smaller the front
profile of the cyclist, so therefore the lower
the aerodynamic resistance
A lower saddle lowers the centre of gravity of
the cyclist, thereby improving handling.
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Saddle Height

• Traditional Methods
• Knee just bent with heel on pedal
• Leg straight with foot under pedal

Greg LeMond Method Saddle height (2) 0.883 ?
Inseam leg length (C) (- 3mm for clipless
pedals)
?These methods take no account of a cyclists
tendency to pedal with the foot angled up or down
? These methods address the first requirement for
saddle position directly
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Fore / Aft Saddle Position
Keith Bontrager The Myth of KOPS Bontrager
claims this method is ungrounded in biomechanics.
It assumes, falsely, that the direction of
gravity has some bearing on a cyclist ability to
pedal and only addresses the cyclists position
when seated.
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Handle Bar Position
The position of the handlebars is limited by the
length of the upper body (B-C), the length of the
arms (D) and the flexibility of the hips and neck.

• The position must be comfortable and be
  maintainable for long periods of time.

• If the position is too low, the power that can be
  developed as the pedal passes the top of its
  cycle is reduced. This depends on the cyclists
  hip flexibility.

• The position must be sufficiently upright to
  enable good visibility and the head to be held in
  this position for long periods of time.

There are no simple, generic techniques for
determining handlebar position.
Trial and error must be used to determine the
longest, lowest position that meets the cyclists
comfort requirements.
Total sizing systems estimate reach and handlebar
drop in relation to all other sizing parameters
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Total Sizing Systems
Custom frame builders often have there own
systems for fitting a bicycle to a customer The
system is often a collection of rules of thumb,
based on years of experience as well as some
biomechanical analysis. Some systems are
available commercially as computer packages.
Others are available to use on-line, free of
charge. There follows a selection of companies
and websites offering these services.
Fit Kit systems - http//www.bikefitkit.com/ -
Converts body measurements to set-up
recommendations. Serrota Cycles -
http//www.serotta.com/sizing.htm - This system
concentrates on flexibility, strength, and
biometrics rather than body measurements. Can be
done on a bicycle, or a Serotta "Size
Cycle".  Ergo Fit - http//www.bicyclefit.com/ -
Concentrates on knee (femur/tibia) angle and
wedges (Big Meat) under the foot to correct
problems and maximize performance. Widely
available in the USA.  Bioracer -
http//www.bikefitting.com/English/SiteInfo.html
- Advanced "measure the body and output a fit"
system.  Popular in Europe. Wobblenaught -
http//www.wobblenaught.com/ - Uses body
measurements and under foot wedges Wrench Science
- http//www.wrenchscience.com/WS1/default.asp -
On-line bike fit computer
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Different Cycling Disciplines
Generally a recreational cyclist will put more
emphasis on comfort and less on efficiency than a
competitive one.
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Conclusions

• Bicycle fitting, as with all human factors
  engineering, is an inexact science. It requires
  trial and error to determine what feels right
• There is no consensus on how to fit a bicycle or
  even what approach to take.
• Each method will give a similar, but slightly
  different set up. If set-up has to be precise,
  it is prudent to try a number of different
  methods and determine which one gives the best
  cycling position.
• Intuition, trial and error can be used to
  optimise the position.
• Once the perfect fit has been obtained, the
  measurements should be accurately recorded, to be
  applied to a new bicycle or if the set up is
  changed.
• Ultimately, the way a bicycle is set up and how
  critical this is, depends on the purpose it will
  have. An elite road racer will ride a very
  different and more more accurately set up bicycle
  than an occasional bicycle commuter.

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Summary
Good bike fit is paramount for comfort,
efficiency and performance
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Selected References
Books and Articles Baker, A. Bicycling
Medicine, Simon Schuster 1998. Burke, E. R.
Serious Cycling, Human Kinetics. Doughty, T.
The Complete Book of Long Distance and
Competitive Cycling, Simon Schuster. Metcalfe,
J. Mountain Bike Fitness Training, Mainstream
Publishing. Whitt, F. R., Wilson, D. G.
Bicycling Science, 2nd Edition, MIT
Press. Websites Analytic Cycling -
http//www.analyticcycling.com/PedalModelConcept_P
age.html Klein Direct Fit Philosophy -
http//www.kleinbikes.com/tech_guide/fit.asp How
to Fit a Bicycle, Peter Jon White,
http//www.peterwhitecycles.com/fitting.htm Human
Power, The Technical Journal of the IHPVA -
http//www.ihpva.org/pubs/hpindex.html Thanks To
My Cranks - http//www.thankstomycranks.com/ The
Myth of KOPS, Keith Bontrager -
http//www.bsn.com/Cycling/articles/kops.html Fu
rther References Bicycle Crank Length Some
References - http//www.nettally.com/palmk/crref.h
tml Reference available in Sheffield Librararies
- http//www.thankstomycranks.com/refs.htm Links
to bike fit calculators - http//www.cyclemetrics.
com/Pages/FitLinks/bike_fit_links.htm