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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 Contents Handlebar Position Total Sizing Systems Different Cycling Disciplines

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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


April 2002



  • 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


  • 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%.


Major Cycling Muscles

Gluteus maximus (GM)

Biceps femoris (BF)

Vastus lateralis (VL)

Gastrocnemius (GA)

Tibialis anterior (TA)

Semimembranosus (SM)

Vastus medialis (VM)

Rectus femoris (RF)

  • As shown in the figure each muscle is used to some extent throughout the pedal cycle, but the maximum activity of each muscle occurs at a different point in the cycle.

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.







1. Crank Length


2. Saddle Height

3. Saddle Fore/Aft

4. Reach


5. Handlebar Drop


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.


Height to Top of Head

Height to Sternum Notch

Inseam Leg Length

Out-stretched Arm Length

Shoulder Width (bony protrusions)

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


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.


Saddle Position

There are a number of criteria to determine the optimal saddle position

The crank position allowing maximum force input (3 o’clock position) should coincide with the leg’s 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.


Andy Pruitt Method

25-30º knee flexion,

pedal BDC, normal foot position

John Howard Method

30º bend in knee when pedal BDC,

normal foot position

(goineometer on femur and tibia)

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 х 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


Knee Over Pedal Spindle (KOPS)

The widely excepted method for determining the horizontal position of the saddle in relation to the bottom bracket, is that when the crank is in the 3 o’clock position a plumb line dropped from the tibial tuberosity (bump below the knee) will intersect the pedal spindle.

The position of the CoM while seated then determined the fore/aft saddle position.

Bontrager’s recommends a method that uses the

position of the body’s centre of mass (CoM) while pedalling out of the saddle to determine the relative

The CoM should be 2-3 cm behind the pedal spindle in the 3 o’clock position.

position of the handlebars to the pedal.

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.


If handlebars are positioned in front of and below the saddle they can be more easily pulled up on to counter the forces on the pedals.

The lower and further away the handlebars the more aerodynamic the position assumed by the body.

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


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 - - Converts body measurements to set-up recommendations.

Serrota Cycles - - 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 - - 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 - - Advanced "measure the body and output a fit" system.  Popular in Europe.

Wobblenaught - - Uses body measurements and under foot wedges

Wrench Science - - On-line bike fit computer


Cross country Mountain Bike


Drafting, or slipstreaming, is banned in triathalon, so an aerodynamic position becomes the most important requirement.

Pedalling efficiency is still important, but slower speeds mean that aerodynamics have less effect. Handling requirements mean the body position is more

The reach is long

upright and and the handlebars are wider

and the

Downhill Mountain Bike and BMX

Emphasis moves completely from pedalling efficiency to handling. Handlebars are above the saddle, which is rarely used, and the

saddle is set further forward to put less strain on the muscles involved in running. Allowing them to be rested.

reach is very

short. The handlebars are very wide and in line with the forks to quicken steering.

Different Cycling Disciplines

Generally a recreational cyclist will put more emphasis on comfort and less on efficiency than a competitive one.



  • 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.

The relative positions of the pedals, saddle and handlebars, determine the position the body takes while cycling

The bike must be fitted to the measurements of the cyclist.

Crank length ~18-25% of inseam leg length (C),

Cycling uses many muscles

The saddle height should

be such that the knee remains slightly bent while pedalling

Try different methods

Set up is inexact, and depends on the intended use.

Handlebar position is a compromise between comfort and aerodynamics


Good bike fit is paramount for comfort, efficiency and performance


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.


Analytic Cycling -

Klein Direct Fit Philosophy -

How to Fit a Bicycle, Peter Jon White,

Human Power, The Technical Journal of the IHPVA -

Thanks To My Cranks -

The Myth of KOPS, Keith Bontrager -

Further References

Bicycle Crank Length: Some References -

Reference available in Sheffield Librararies -

Links to bike fit calculators -