The question of the utility of "free-flex" bindings comes up quite often, and I have some actual data that speaks to this issue. Specifically, I did some actual measurements of the change in flex due to inserting a boot in the binding of a ski.
I set the experiment up on the bed of a Bridgeport "mill" to provide an absolutely rigid support for the ski under test. This is essential because of the need to measure very small deflections in some of the tests. I used a machinist’s “dial indicator” to read the flex of the ski to within 0.0005 inch (for small deflections) and a machinist's "vernier" (for large deflections). I flexed the ski with anywhere from two to six 25 lb lead bricks (formerly used for radiation shielding - grin) hung by a rope threaded around a stiff metal plate perpendicular to the ski, under the boot, at the exact fore-aft center of the binding.
What I found was that on a 193 cm Elan SCX monoblock with Look turntable bindings, when I clicked my boot into the binding, the flex of the combined boot-ski system changed by a maximum of 25% when measured over a one foot span directly under the binding. Because of the extremely small deflections seen over such a short span, this change in flex could only be measured under very heavy loading (150 lbs/ski). Under light loading, the deflections were too small (ie, a few mils or less - 1 mil = 0.001") to provide accurate data.
This observed 25% change in flex must be put in perspective. We are talking about deflections (and changes in them) so small as to be utterly undetectable without instruments. The only real importance of this experiment is that it refutes the oft-heard statement that the boot “negates any flexing of that area of the ski”. It reduced the flex by 25%, it certainly didn't reduce it to zero.
Of much more relevance to normal skiing are the measurements I made of the flex of the center half of the ski. For the ski I was measuring, this was a span of roughly 1 meter, starting roughly one half meter back from the tip. It is the part of the ski which exerts maximum downward force on the snow. If I remember correctly, the average flex of the ski over this span changed only by around 5% upon insertion of the boot. This number is much less than the 25% number in the previous experiment because roughly two thirds of this central one meter span is outside the binding area and is completely unaffected by the boot. In addition, these areas are thinner and hence more flexible than the short region directly underfoot. A 5% change in flex over the center half of the ski is probably at or under the limit of detectability for most people.
Finally, I measured the average flex over the entire length of the ski (i.e., from the tip to tail of the ski). Over this even larger span, the change in average flex upon insertion of the boot was almost unmeasurable, even with loadings that produced large deflections (reverse camber).
My measurements were limited in that they only were on one ski, one binding, and one boot, but it is clear that the magnitude of the change in flex upon insertion of the boot is quite small for this particular ski and binding. The red SCX monoblock that I used is an early shaped ski, and is quite stiff, so one can expect to see larger effects on softer skis. If one assumes that the softest skis might have (say) at most 3x the flex of these SCX's, then the change in mid-ski flex upon inserting a boot might be 15% instead of 5%. This would be a noticeable, but not huge change. One would also expect somewhat larger numbers with shorter skis where the binding occupies a larger fraction of the center of the ski.
My conclusion is that there is a indeed a reduction in flex produced by the boot-binding-ski interaction, but it is at *very* most a modest (tens of percent) effect and is clearly never going to completely negate the flex of the ski as some people suggested.
Hope these hard numbers help.
Tom / PM
PS (in edit) - I forgot to mention that I first posted this data about a year ago. This post is essentially the same as my earlier message on powdermag.com
, just edited a bit for clarity, and I inserted the comment about the red SCX's being unusually stiff and some other details.[ October 10, 2002, 08:26 AM: Message edited by: PhysicsMan ]