Straight to the point

A background in metrology — the study of measurement — means Dave Archer, President of UK-based Arch Design Works and designer of the Velo Angle system, knows more than most about what points are of interest when measuring a bike.

Within Cycling, point definition is not trivial, as there isn’t universal agreement as to what locations should serve as references.

Using the BB-saddle-handlebar/arm pad triangle as an example, the bottom bracket is the only location that is universally defined. Unfortunately, as it is not a point of rider contact, it is the least functional of the three. This illustrates the common situation where history, convenience or other considerations influence convention.

The bottom bracket is the logical origin for dimensions convenient for frame selection. However, the majority of riders seeking professional fits will apply the results to existing bikes. The ideal reference would be the surface of the shoe insole. While this may seem impractical, a compilation of pedal/cleat and shoe stack heights would allow simple translation from the BB.

The saddle reference is the most problematic corner of the triangle, as the appropriate spot is both difficult to reliably locate and difficult to agree upon where it’s located. Early conventions like the saddle tip or in-line with the seat tube were driven more by convenience than function. More recent suggestions, such as a distance from the tip as a proportion of overall saddle length, are improvements, but not directly tied to anatomical features that determine position.

Latest suggestions of referencing the saddle at a particular width, (most published suggestions are between 70 and 80 mm) appear a logical attempt at setting a more effective convention for rider position, particularly if saddle changes are contemplated. Figure 1 (below) depicts a design for establishing and documenting a reference point anywhere on the saddle surface.

Fig. 1

As riders are very sensitive to slight changes in saddle tilt, accurate documentation is important. While the typical ‘Panini Press’ across saddle length of is fine for a given saddle, the option of isolating the rider’s contact as shown in figure 1 makes dialling in saddle changes much easier.

Locating points of interest during measurement

Locating is not redundant with defining, as the lack of positive location of the reference point is a common source of error. Positive location can be defined as basing the measurement precisely on the point intended and being able to do that consistently.

Figure 2 (below) provides a good illustration of these concepts with terms that are commonly used, but often not fully understood. Note that the terms precision and repeatability can be used interchangeably.

Fig. 2

Inaccurate but repeatable measurements are usually the result of improper tool calibration or test setup. For example, if a seatpost diameter was measured with a set of digital callipers that wasn’t zeroed properly, or if measurements were made on a bike that wasn’t plumb and level.

The source of compromised accuracy and repeatability is usually some combination of the measurement system and user technique. Readings taken with tape measures or levels are highly technique dependent. The combination of cross-line lasers and fixed scales provide a stable measurement system, but location of the origin and reference features are still visual and not positively located. The validation studies of multiple measurements of a known standard that are common in industry are rare in activities like bike fitting.

Because the rider that gets a professional fit is likely conscious of small variations, assessing your current technique will be time well spent. A common source of measurement error in fitting is parallax — the difference in apparent location when a feature is viewed from different lines of sight. An example is shown in Figure 3a (below).

Fig. 3a

Fig. 3b

The potential for error is reduced as the distance between the point of interest and the reference is reduced. Camera-based systems can also have significant parallax errors depending on sensor design and location. Figure 3b (above) shows an example where the point of interest is in contact with the scale, as was also the case in figure 1. Digital readings, such as the angle measurement in figure 3b, are inherently free from parallax.

Apply a measurement system to capture the relative location of reference points

Fixed systems (cameras, cross level lasers, jigs) whether optical or mechanical rely on the bike being aligned with the system. To get an idea of how much minor misalignment impacts measurement accuracy, a CAD study was performed on the author’s primary road bike (56 frame) assuming a 1 degree levelling error.

As the saddle is not located at the midpoint of the wheelbase, error varies depending on which wheel is high. In this case, each degree of levelling error results in an average measurement error of 13.3 mm in X and 4.8 mm in Y.

Fits performed in trainers often have this level of error because establishing a true level plane is not simple. The angle-based system discussed can eliminate the need to level the bike by autozeroing the level plane to the bike’s orientation at the points of wheel contact or the QR nuts.

This direct point-to-point measurement based on length and angle (polar coordinates) has significant time and accuracy advantages over any approach that requires separate measurement of X and Y.

While defining bike setup using angles is uncommon, this approach should not seem foreign to bike fitters as this is the primary parameter for establishing rider position. The X-Y approach became a bike measurement standard largely because it was consistent with readily available gravity-based hand tools like levels and plumb bobs. However, it seems logical that the format used to describe the rider’s position on the bike should be consistent with that used to define where they contact the bike.

For example, the author’s 106 mm handlebar drop is of limited usefulness without knowing the associated length. On the other hand, expressing drop by angle (in this case -8.1°) is much more universal. Figure 4 (below) illustrates how expressing the relative location of bike contact points by length and angle can provide a more intuitive means of correlating bike and rider data in addition to providing the benefits of direct point-to-point measurement.

Fig. 4

Note from the author: My goal was to present a cycling metrologist’s view of how bike setup might be more effectively captured and replicated and to provide some food for thought. The product images are of a tool and app we developed to enhance accuracy and flexibility at an affordable price.

darcher@archwerx.com