Tom Gellie explains the virtual bump, Ron LeMaster's term for how the slope angle changes as you ski across the fall line. You must shift your weight forward when heading downhill and back when traversing to stay balanced perpendicular to the slope. An iPhone angle-meter shows the slope steepens and flattens throughout each turn.
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When we ski any slope, we need to realize that there is a relative change in slope angle depending on where we are in the turn. So we only experience the full pitch, steepness, or slope angle when our skis are in the fall line. Anytime we are turning back across the fall line, the slope angle actually gets relatively flatter for our fore-aft balance. We need to know this so we can make changes, predict, and be far more active with our fore-aft balance to stay centered in the ski and help that ski perform better. This relative slope angle change was first noted by professor of skiing Georges Dubert in the '70s. Ron LeMaster later called it the virtual bump in his books on skiing. I'd like you to pause the video here and just have a look at this diagram to help you understand the relationship of where you are in the turn relative to the slope angle and how that changes the slope angle and your fore and aft balance needs. Hopefully this diagram gives you a good visual of why the virtual bump term was given for this relative slope angle change. Even though you're on, say, a perfectly flat 40-degree slope, the forces experienced by you are like there is a virtual bump at the end of every turn. If we have a look at a point in a turn right here, we can easily see the angle or steepness of this slope. And when my skis are in the fall line right now, then I'd be experiencing whatever this slope angle is. Say let's call it 25 degrees. So my body needs to be relatively perpendicular to this slope like it is here in order to stay balanced and be right in the center of my outside ski. However, the more the turn is completed and it is coming across the fall line, say to this point, we're going to call that 45 degrees, then the relative slope angle actually reduces. So if it was 25 degrees right now, if we pulled out a slope angle measure and put it where my skis were right here, it would measure far less. So my balance needs to be different. It doesn't need to be as forward as it needed to be for this part of the turn. It needs to be back a little relative to this point we were at in the fall line. As we start a new turn, the slope angle we experience is going to get steeper and steeper. So right here I need to, and you can kind of almost see my body predicting and being more forward as I come closer to the fall line here, where I'm more right perpendicular to the slope angle. So I'm experiencing the full pitch of the slope at this point only, I start turning across the hill, the slope gets relatively less steep. Now as you ski, the flatter the run is, so for instance this little bench in the run, not being as steep, there's only going to be a smaller change in the slope angle. So say it was only 15 degrees, and my turn shape went across the hill, and so I was experiencing only 10 degrees, there might only be a change then of 5 degrees. There's not going to be as much of a fore and aft balance change. But as I get onto steeper parts of the run, like here, and my skis are across the hill and I enter this part, I'm going to need to be more dynamic in my fore and aft adjustment, getting forward so I can be perpendicular to that slope at this point. And then I'm going to make quicker movement, say by moving my feet in front of me, to stay centered as that slope angle gets flatter. To ensure you understand what I mean by the term perpendicular to the slope, take a look at this little experiment that you can try yourself. So imagine, pretend this is the slope angle, which looks very steep, Notice I have to place my pole 90 degrees or perpendicular to the slope angle in order for it to stay balanced as it slides down. Now if you changed the angle here and placed the pole not at this angle but anywhere behind it, what happens is the pole actually rotates more and more and you end up getting a sliding out. So the pole ends up going like this. Down, and you end up getting further and further behind. Notice it stays balanced all the way down until it hits my brake. So what you are trying to do, and what you need to realize, is as you come into the start of a turn and you've finished from across the slope, you're essentially needing to change your balance from, say, the slope angle being less to then being steeper and being perpendicular to it when you hit the fall line. So at this point, you are like my ski pole gliding down the hill, but as soon as you start coming across the hill, which happens quite quick unless you've got downhill 45-meter radius skis on, you're going to need to adjust your balance. So this slope angle is always changing, it's getting less and less and less., so your relative perpendicular angle you're going to need to keep your body at will change through the turn.
This is a little angle meter device on my iPhone, alright, so when you tilt it, it shows the degree of whatever thing you're measuring, so we're going to measure the slope, okay, so over here this little slope is 16 degrees. Okay, so when your skis are down the fall line on this slope, uh, to be in balance, your body needs to be— well, there we go, 17 here— perpendicular to this angle. Okay, now watch this. If I back up, here's my ski, back up, back up, back up to the top of the turn. Can you see on there Zoom in on that.
Zero.
Yes, same slope, I'm just facing across the hill. So if I'm a skier and I start a turn, the top of the turn now is 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 16, 15, 14, 13, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 0. So the slope changes, gets steeper, steeper, steeper, steepest there, 23 in that dip. Coming back across the fall line, you finish 90 degrees across the head of the fall line, it's zero. So you need to accommodate for this change when you are skiing. If you don't go so much down the fall line, if you just like here, so say at 9 Goes to 16 or 20 there, back to 14, back up to 20, 12, 11, right? So if you're more down the fall line, you experience this change in angle less, but if you go all the way across, there's a big change. Steeper at the start, steepest in the middle of the turn, and then it gets gradually flatter. And if you're all the way across, it goes to zero. So that is why that phenomenon is called the virtual bump.
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