Why LE Matters
@rfa
Anything that moves the fin’s center of pressure matters, this includes both LE and DFT. In the book Fin Whispering, I went to great lengths to isolate individual ski behaviors so each behavior can be tuned without affecting the others. In this case, DFT is very important as LE-neutral DFT moves are essential when isolating smear changes.
With the WhisperFin, my focus is on making ski tuning as easy as possible by boiling fin adjustments down to two simple moves—back and forth, and in and out of the ski (no LE-neutral DFT moves here). The downside of this simplicity is that both of these fin movements affect multiple behaviors.
When I made the statement that "DFT isn't a measurement I worry much about. I'm more focused on how DFT affects the fin's leading edge location (LE) ...," it was in the context of WhisperFin tuning. And what I was implying was that of the two behaviors affected by a simple forward or backward fin move, I’m more concerned with how the move affects LE because the goal of these moves is to adjust yawed tip engagement to be as deep as possible without leading to tip-grab.
Let’s consider a forward fin move for illustration purposes. Moving the fin forward increases both LE and DFT. As LE moves forward, we’ll get more yawed tip engagement from our habitual inputs, which leads to more smear, tighter turns and more cross-course angle. As DFT moves forward, the fin loses some leverage (shorter moment arm between the bindings and fin) which also leads to more smear. It’s a happy coincidence that DFT’s affect on smear is complementary to the goal of getting more tip engagement with a longer LE.
Reducing fin area by moving the fin up into the ski also increases smear. In fact, reducing FD by .010” increases smear six times more than an equal .010 DFT increase (an updated topic in FinWhispering—2nd Edition). Accordingly, I consider fin area changes to be primarily aimed at adjusting smear, and back and forth fin movements to be primarily aimed at adjusting yawing tip engagement. This further explains the context surrounding my statement that "DFT isn't a measurement I worry much about. I'm more focused on how DFT affects the fin's leading edge location (LE) ...."
Finally, to help illustrate why LE impacts yawed tip engagement, let’s use a little exaggeration. The fin on a ski is like the feathers on an arrow. With the feathers at the very back of the arrow, it will be very stable in flight. The further forward you move the feathers up the shaft, the less stable the arrow will be (meaning the easier it will be to deflect the arrow from its path). If the feathers were to move forward past the halfway point, the front of the arrow would swing around and become the back.
Similarly, with the fin at the back of the ski, the ski is very stable and just wants to go straight down the lake. The skier has to force the ski to lay over on its side to turn or cut cross the course. The further forward the fin is on the ski, the less stable it is and the easier it is for the skier to deflect it from its natural path (force it to turn and cut). If the leading edge of the fin goes far enough forward, eventually the front of the ski will spin around and become the back of the ski, tip-grab in the extreme.
If LE is too far forward, the skier’s habitual inputs will yaw too much tip into the water and the tip will bite. If it’s too far back, it’s becomes too hard to keep enough tip in the water to complete turns well or to generate enough cross-course angle.
In short, the fin’s leading edge location (LE) matters a lot.