Beyond the Numbers
Ski Sidecut
by Jason Petek
PSIA-NW Tech Team

This edition of ‘From the Garage’will examne the effect of ski sidecut variation on turn transition direction. Specifically, the terms ‘sidecut’ and ski ‘turn radius’ will be defined and described. Necessary assumptions will also be described concerning turn transition direction in order to isolate the affect of sidecut variation. Popular notions about transition direction will reviewed, and finally a more accurate description will be offered.

To begin, just in case there is still life in the horse, I will review what the terms sidecut and turn radius mean and describe the way in which they are calculated. Before getting caught in the idea that the numbers explicitly define ski performance, it is important to realize that they are only guidelines to ski shape. The idea of confining ski performance to a single turn shape and size, because of the skis shape and size, is dangerous not only for our movement pattern versatility but also for our wallet. It is also important to note that ski experts and manufacturers don’t always agree about the exact calculations used to derive these terms. I have seen many different ways of calculating the turn radius value for a ski, and without specifically contacting every ski manufacturer I can’t positively provide the exact right equation. Fortunately, the purposes of this paper don’t require an exact definition, so a simple description will be adequate. Ski sidecut is a shape description that focuses purely on the difference in width between the ski tip and tail, and the skis’ waist. Specifically, if the ski is put on edge, p e r - pendicular to the ground, the sidecut is the greatest distance between the ground and the waist of the ski. Typical sidecut values range from 12mm for giant slalom skis to over 20mm for modern carving skis.

Ski turn radius initially appears to simply be the size (radius) of the turn the ski will make in meters. Since skis don’t only make one turn size, clarifications must be made. Some basic calculations for turn radius involve lying the ski flat, tracing the edge, and using the sidecut value combined with the distance between the widest points of the tip and tail to extrapolate the size of the constant arc that passes along the edge. More sophisticated turn radius calculations accommodate for an inconsistent arc along the ski (e.g. different arc in front of waist then behind the waist), and require the location of the ski waist with reference to the tip and tail. Other sophisticated calculations i n - volve the ski placed on edge forty-five degrees and flexed until edgeground contact is made, the flexed ski then defining the size of the skis’ turn radius. Specific turn radius values range from 21 meters for a giant slalom ski to 12 meters for modern carving ski. Regardless of the specific methods used to calculate these ski characteristics, it is clear that skis with more sidecut and a smaller turn radius will bend more for a given edge angle and produce more turning power than skis with less sidecut and a larger turn radius. This simple point is the basis for the following analysis of transition direction.

Before examining the effect of changing ski shape on transition direction it is important to review current philosophy concerning transition movements. Transition movements are focused on the specific intended direction of our Center of Mass (CM). The CM can be thought to exist ‘around the navel’. Often the word ‘diagonal’ is used to described transition direction. ‘Diagonal’ captures the idea of a lateral component of movement downhill and a longitudinal movement along the ski. The specific combination of these two components dictates the size and shape of the next turn. The transition movement is often described as being directed toward the fall line (roughly the turn midpoint) of the next turn. The actual amount of directed movement along the ‘diagonal’ line varies because the desired point along the arc at which the Center of Mass reaches its’ intended offset from the ski path varies. Simply put, you move in the ‘diagonal’ direction until you have created the required edge angle for the intended turn shape, and because edge angle and intended turn shape vary, the blend of lateral and longitudinal movements varies. The reality is that the ‘diagonal’ transition movement depends on many variables, in addition to the ability to predict the future. It turns out that the actual direction (based on the desired endpoint) of the Center of Mass through the transition changes depending on many variables, of which ski shape is an important one.

The affect of ski shape on transition direction becomes clearer if we remember that the basic rule that more shape (sidecut) will produce more turning power (turn easier and tighter) for a given edge angle. Take the example of a short radius turn, using both a large sidecut short radius ski and a small sidecut large radius ski in Figure 1. It is assumed that the turn is of a consistent arc and is directed down the fall line. The dashed ‘diagonal’ line represents the transition direction based on the midpoint, or fall line, of the next turn. The solid curved lines represent the different paths of the actual Center of Mass along the turn based on ski ‘sidecut’. The figure uses the example of the small radius turn to illustrate the idea that small sidecut skis require more movement directed along the ski at turn initiation than large sidecut skis. This is due to the fact that the large sidecut ski will produce more ski flex simply by increasing edge angle, which as a result will produce more turning power. A small sidecut ski will require more pressure forward along the length of the ski at turn initiation in order to produce the desired ski flex. The small sidecut ski Center of Mass path varies in distance from the ski compared to large sidecut ski path due to the increased edge angle required by the small sidecut ski to create enough ski flex to complete the same turn. This illustrates the blend of lateral and longitudinal movements of the Center of Mass changing along the turn.

The most important thing to realize is that large sidecut; small turn radius skis require less movement longitudinally along the length of the ski for a given turn compared to a small sidecut, large radius turn ski. Conversely this implies more movement laterally or downhill with the large sidecut ski. This ‘move inside’ is an unfortunate result of the shift to carving or large sidecut skis by the majority of the ski population. The active balancing and pressure control movements that take place when moving more longitudinally along the ski during transition are de-emphasized.

It is important to remember that the specific transition direction is a varying blend of movement downhill and movement along the ski. The blend is based on ski shape and turn size, snow condition, ski flex pattern and construction, ski tune and geometry, amount of ski camber, boot size and mounting position, boot stiffness and forward lean, fall line direction, slope angle, and on and on and on… Of course, the grand realization is that perfect skiing, given the number of variables to constantly calculate, should be impossible. A wise man once said “You must consistently create inconsistent movement patterns in order to create consistent outcomes”. In skiing, everything depends on everything. Good luck.