Bike Fit Position Measurements

by | Aug 9, 2023

Bike fit position measurements are a way for a fitter or cyclist to capture the parameters that determine the relationship between the primary contact points on a bicycle, as these serve to reflect the cyclist’s position on a bicycle.  Measuring bike parts is easier than measuring humans on bikes unless you have a motion capture system.  Plus, the bike is a largely static object, whereas the human is a sack of organic matter prone to shape shifting, making it hard to recreate the same position.

A full bike fit report is also likely to include some underlying frame measurements like frame reach and stack as well as an inventory of the key fit-related components.  These include information on saddle make, model, and width; pedal system and cleats; handlebar width and reach.  These are also important variables that influence how a cyclist feels and performs on a bicycle.

Bike fit position measurements provide a reference point and are useful for:

  • Checking to see that nothing has moved or slipped, like a seat post
  • Resetting a bike to a known position if someone (bike shop mechanic, family member) has moved the contact points on YOUR bike to ride it or work on it, and not returned them to your position
  • Renting a bike for a bike travel vacation and wanting it to be set up similarly to your regular ride.
  • Ordering a new bike, especially a custom bike
  • Help determine the selection of a new production bike. As in – will it fit?

Measurements are a language of communication used to describe both frames and fit positions. Like any language, there are variations in dialect and meaning – which can lead to some confusion and misunderstanding.  In addition, some measurements can be done with a simple tape measure–metric, please! While others require a few home handyman tools like self-leveling lasers and levels, and yet others require specific tools.

As there are different methods of measurement let’s explain those first.

Linear Measurements

Linear measurements are point-to-point measurements.  These are the most common measurements and the simplest to take, but also the least accurate.

There are two types:

  • Actual – refers the just that: the actual distance between two points, like saddle nose to center of handlebar.
  • Effective or Virtual – uses a reference point at the end and assumes a horizontal line to the other end.  This is most commonly used for frame measurements rather than fit position measurements.  The classic example is for frame top tubes, which are rarely horizontal these days. Effective top tube length is a handy way to compare apples to apples, not actual top tube length.

Linear measurements can be taken using a tape measure, rigid level, or a tool like the Fit Stick.

Linear Measurement image

XY Measurements

XY measurements use what is known as a Cartesian or rectangular coordinate system to define a point in a plane, in relation to a starting point.

Our starting point is the center of the bottom bracket.  This is 0 0

X is a horizontal measurement and Y is a vertical measurement.

For example, frame reach and stack numbers are XY measurements, where Reach is the X coordinate and Stack is the Y coordinate.  This is the primary frame geometry use of XY Measurements.

In bike fitting, we also use XY measurements to:

  • Describe the location of the handlebar at the stem clamp, known as HX/HY
  • Define the position of the center of the saddle rail in the saddle clamp, known as SX/SY, although this is mainly used on an adjustable fit bike.
  • Define the location of the arm pads on a TT/Tri bikes.

So the main Fit Position XY measurements are going to be either Handlebar X and Y (on a flat bar or drop bar bike), or Pad X/Y on a TT/Tri bike.

XY measurements require either the use of a sliding XY measurement tool; a sophisticated laser XY plotter; careful use of a laser level and a physical level or tape measure; or some software to convert polar measurements to XY measurements.  What are polar measurements?  Read on….

XY Bike Measurements

XY Bike Measurements Defined–Most bikes list “stack” and “reach” as main measurements

Polar Measurements

Polar Measurements are another type of coordinate system used to define the relationships between different points.  The Pole is Zero, and on a bike is the center of the bottom bracket, the same as for XY measurements.  The measurement from the Pole to somewhere is called a Ray and is comprised of both a length and an angle from the horizontal.

Using the Veloangle to perform Polar Measurements

You won’t find any polar measurements used in bike geometry charts or diagrams, but you may encounter them in measuring and defining a bike fit position.

Polar measurements are the most accurate form of measurement, but the accuracy requires the use of a precision instrument.  For bike fitting, that instrument is the VeloAngle.  The VeloAngle can be used to measure from the bottom bracket to the saddle and from the bottom bracket to the handelbars, as well as any other user-defined point, like the rear of aero arm pads.  Polar measurements can also be converted to XY measurements using the dedicated Veloangle App.

Veloangle Polar to XY conversions for bike fit measurments

Using the Veloangle App to convert polar measurements to XY

Angles

An aspect of Polar Measurements is an angle in degrees.  Angle measurements can also be used independently to measure things like saddle tilt, lever position, handlebar angle, and hood position.  Bike fitters and cycling enthusiasts alike may go about measuring the angle of a piece of equipment in different ways, hence the method of taking the measurement should be defined to ensure that it can be repeated with some semblance of accuracy.  Our Saddle Tilt Tool and Fit Stick #3 offer angle measurement capabilities, as does the VeloAngle.

Bike Fit Position Measurements

The list of measurements can be limited or extensive, depending on who is taking the measurements, what measurement tools are available, and what the end use is.

A common list of bike fit position measurements is as follows:

  • Saddle height
    • From bottom bracket (BB) to saddle. The big question then is “where to on the saddle”
    • From pedal spindle to saddle. Fit Kit Systems have two tools for taking this measurement: The Seat Height Tool and the Fit Stick
  • Saddle setback (either nose or rear of the saddle from vertical line off the BB)
  • Saddle tilt or angle.  We use our Saddle Tilt Tool for this measurement.
  • Saddle XY (to center of rail)
  • Saddle nose to stem clamp center (saddle model needs to be referenced)
  • Saddle-to hood trough (drop bars) or center of grips (flat bars).  The saddle model needs to be referenced.
  • Handlebar XY (center of bar)
  • Saddle to handlebar drop (to top of bar or center of hand grips on a flat bar bike). Note that this may be a negative or positive number depending on whether the bars/ grips are higher or lower than the top of the saddle.
  • Saddle to hood trough (where in the hood trough?)
Bike Fitting Systems - Measuring handlebar drop

Measuring handlebar drop on a triathlon/TT bike with the Fit Stick

Use Cases

Rental Bike

Bicycle travel organizations that also rent and supply bikes will typically ask you for a set of measurements from your current bike to help select a size and set up your rental bike.  For example, someone who is going on a Trek Travel vacation and renting a bike through that organization is asked for 3 measurements:

  • A Center of crank arm to the top of the saddle
  • B Nose of the saddle to the center of the handlebars
  • C Top of handlebars to the center of the wheel

Trek Bike Fit Measurements for Trek Travel Vacations

Tri Bike Purchase

The key measurements needed for a tri bike purchase is the position of the aero pads in relation to the bottom bracket, based on your current position or a pre purchase fit session to determine these.

  • Pad X  (to rear and/or center of pad)
  • Pad Y
  • Saddle height from BB

Many, but not all brands selling tri and TT bikes offer either a visual calculator or chart to reference these numbers against their range of frame sizes.

Cervelo PX Bike Size and Setup Calculator

Cervelo PX bike size and setup calculator web app

Custom Frame

This depends on the frame builder, and many will have their own list of measurements they would like provided.  The 4 key measurements are:

  • Saddle height (BB to saddle)
  • Saddle setback (need to define saddle model as well)
  • Saddle nose to handlebar center
  • Saddle to bar drop

Different Bike fit position measurements

Documentation Software

Many bike fitters will use some sort of program, form, or spreadsheet to capture details about a bike fit, including the bike fit position measurements.  We have two offerings in this arena:

  • Fit Kit Studio is the main digital tool for professional bike fitters
  • The VeloAngle App is for both personal and professional users who have the VeloAngle tool and wish to record information and measurements for one or more bikes online.

Whether you use a keyboard or a notepad and pen, it can be useful to take at least a few key measurements and record these as a reference.  You may never need to use them, but if you do you will be glad you have them.

John Higgins

John Higgins

John is an elite level bike fitter who works with non-elite cyclists - although a few have won races! Many don't race at all, but ride for fun, fitness, or to compete against themselves. John has worked with 18-80 year olds (and younger and older), novices, age groupers, masters racers and all levels of weekend warrior. These include road riders, mountain bikers, triathletes, tandem riders, tourers, commuters, bike packers and gravel riders and racers. All share a love of cycling and just wanted to ride more comfortably, and in many cases faster. John is the owner of Fit Kit Systems, and provides bike fitting services through BikeFitr (bikefitr.com)

0 Comments

Submit a Comment

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Recent Posts