Bike Sizing

​Bike Sizing is a process to determine if a particular bicycle is of a suitable frame size and geometry to accommodate the rider in a position that:

  1. is bio-mechanically safe
  2. comfortable, given the riders physical mobility and stability
  3. does not adversely affect the control and handling of the bicycle
  4. allows for fine tuning of the seat and handlebar position to optimize comfort, efficiency and performance. i.e the contact points are not at “end of range”

Sizing is ideally performed:

  1. prior to purchasing a bicycle
  2. in the early phase of fitting an existing bike, to confirm that a good fit is achievable

Pre-Purchase Sizing

Approaches to pre-purchase sizing can be as basic as checking for stand over clearance or using the rider’s height to select a bike size, or as sophisticated as using a software powered size cycle to simulate different final fit positions and potential and recommended bike makes, models, components and setups. Fit Kit Systems offers an intermediate, measurement-based approach that is fast, affordable and effective.

using the Bicycle Fit Kit
using the Bicycle Fit Kit

Pre-Fit Sizing

To attempt to fit a bike to a cyclist that is fundamentally the wrong size or shape for them is a futile exercise resulting in frustration and dissatisfaction for both the fitter and the cyclist.  By first confirming that a bike is an appropriate size for a rider the fitter can proceed to fit with the confidence that they will be able to adjust the bike to the benefit and satisfaction of the rider.  If the bike is the wrong size to allow for a good fit, then a discussion can take place about what course of action is best pursued, before time is wasted attempting a fit.

Two Approaches to Sizing

There are two primary data-driven approaches to projecting a bike size for a cyclist:

1/ using anatomical proportions obtained from body measurements.
Bike recommendations are usually expressed as top tube and stem length, and seat tube angle, together with predicted and initial set up guidelines for seat height and seat to bar drop.

Examples include our own Fit Kit System, and a similar approach by Cyclefit in Europe.

Advantages: simple, quick, effective, low cost to the operator, systematic, easily taught and learnt, low or no cost to the cyclist.  Far superior in projecting a frame size than using a rider’s height or stand over clearance, or the sales associate’s guesstimate.

Disadvantages: not seeing the cyclist ride, not predicting a final fit position before selecting a frame size and component mix.

2/ using normative range joint angle measurements, derived from observation and measurement of a cyclist on a bike, along with overall postural and positional assessment and refinement.
Bike recommendations are usually expressed as mathematical co-ordinates using either frame co-ordinates (stack and reach) and/or fit co-ordinates (XY positions for seat and handlebars).

Examples would include fitters using the full Trek Precision Fit system (which at this point has been mostly abandoned by Trek), or Retul technology, amongst others.

Advantages: more accurately predicts a final fit position, and can be used to specify component selections including crank arm length, seat post set back, saddle, stem and bars.  Can be used in conjunction with frame finder software to identify suitable specific makes and models.

Disadvantages: requires the use of an adjustable size cycle, typically with embedded software of with motion or video capture, higher investment cost for the operator, high level of training and skill required, more time consuming, typically has a cost to the customer.

Both approaches should be used in conjunction with an appropriate level of rider interview and physical assessment to ascertain riding history and goals, riding style, relevant physical constraints and abilities, injury history, and other factors that would influence a recommendation.

Given the ready availability and proven effectiveness of both approaches, anyone still using stand over clearance or total height as a sizing guide is not only providing a lack of expected service to their customers, they are providing a dis-service.

Fit Kit for bike sizing
bike fitting
measuring inseam length

The Fit Kit Systems approach to Bike Sizing

The “Fit Kit System” of sizing uses anatomical proportions (body measurements), modified by responses to a simple rider interview.

The recommendations that come out of the proprietary data set embedded in our sizing calculator are derived from extensive measurement of real riders on real bikes.

The information is most effectively used in the hands of an experienced bike technician (sales, mechanic, coach or fitter) who can qualify the outputs, identify any red flags, and interpret the results and recommendations for the riders understanding and benefit.

The information gained from using the Fit Kit System can be used to:

  • recommend a bike model and size to a purchaser
  • contribute to the frame design inputs used by a custom frame builder
  • select a suitable bike for renting to a customer
  • confirm or deny that a rider is on an appropriately sized bike
  • provide initial bike set up parameters on either a bike or a size cycle.

This is obtained using our handcrafted, durable and inexpensive Fit Kit, comprised of our main body measurement tools and sizing calculator.

Use of the Fit Kit System does not constitute completing a bike fit, but the information can be used to establish a starting fit position.

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