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Midweighting, Part 4, Speed


Marco
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Love this clip with Sigi.

I've been watching it many times in order to remember it while carving.

I posted that video several days ago: http://www.bomberonline.com/vbulletin/showthread.php?t=31798&highlight=sigi

Many thanks!

That clip of Sigi is one of the really great clips to watch. I also really liked that clip someone posted here on BOL the other day of the Italian Team training. Really strong riding.

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Not to beat my favorite dead horse in every thread I can find, but if you watch

carefully you'll notice that about 1:15 in one slo-mo shot and at 2:52 in another are where the CM starts to change direction from the nearly ballistic trajectory it is travelling on during the transition. In both cases, the board is already outside the CM and being turned downhill. THe new edge is engaging and the board is starting to flex in response to the force being applied through the legs to push the body downhill into the next turn.

The board is doing the majority of the moving, NOT the CM. The rider is steering the board to where it can catch the body and toss it to the next turn, much like tossing a ball back and forth between the hands, or, for a more apt image, tossing a baseball bat back and forth while balancing it on its butt.

While riding on an edge you have a narrow and weak platform and very disadvantageous leverage to throw your body one way or another laterally. Try an experiment: Stand up and rock back and forth toe to heel to toe. How fast can you do it? Could you throw your body through an arc of 120 degrees (30 degrees off the slope each way) quickly enough to run gates? Try another experiment if this doesn't convince you: Lay your rock board down in the driveway and park your cars with a tire holding down each end of the board. Step into the bindings and try the toe-heel-toe rocking and see if your legs are strong enough to move your body even through - say- 45 degrees as quickly as you can carve Wedel turns.

If the board is not secured, you cannot apply more than a miniscule torque to it to move your CM. When it is locked into a carve you can apply a little more, but that's not when you want to be doing so. When the board is barely touching the snow or even flying between turns, you can do nothing to change the direction your CM is traveling. You can have some effect on your rate of rotation by pulling the board closer to the CM (like figure skaters and divers do) but you must have some rotation going already before you leave the ground. Angular momentum is conserved.

Try another experiment: Traverse across the hill on one edge and throw your body downhill across the board. To make your body rotate about the axis of the board, you have to apply a moment to it, by trying to rotate the board uphill (also about the long axis of the board). If the board is fixed to the snow, you will slooowly tip over downhill, but since it is not, the board will simply tilt more uphill, steer uphill, and move uphill faster than your body can follow, the net effect being that you will tip quickly downhill and your body will be hurtling across the slope and falling toward the snow. If you are moving fast across the hill and are clever about it, you will quickly reverse the input action and change edges, steer the board downhill and catch your falling body. You'll likely find the sensation very gratifying, and be impelled to start spending ridiculous amounts of time and money on repeating the experience, quickly omitting the traversing part and living for the toss and catch part. Repeat the experiment and recruit independent researchers to replicate it.

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. . . In both cases, the board is already outside the CM and being turned downhill. THe new edge is engaging and the board is starting to flex in response to the force being applied through the legs to push the body downhill into the next turn.

The board is doing the majority of the moving, NOT the CM. The rider is steering the board to where it can catch the body and toss it to the next turn, much like tossing a ball back and forth between the hands, or, for a more apt image, tossing a baseball bat back and forth while balancing it on its butt.

While riding on an edge you have a narrow and weak platform and very disadvantageous leverage to throw your body one way or another laterally.

If the board is not secured, you cannot apply more than a miniscule torque to it to move your CM . . .

That is one way to look at it.

The key thing is that people need to learn the movement patterns. The CM moves when you downunweight, or as it's also called, when you do the midweight sequence, into the next turn. You don't actually move your CM because the centrifugal force from the previous turn does that. Also, you don't throw your body, since you can use gravity and centrifugal force to do that.

(You can spot body throwers, as they tend to bobble, because they overpressure the board past the point of equilibrium in the turn by collapsing the body toward the knees on toeside and squatting too far back on heelside. This is why the movement is always started in the hips to flex the knees and ankles. If you don't, you end up bending your upper body to start the CM moving. Then you can bobble.)

With the midweighting technique, you can change edges really quickly and apply great lateral torque to the board in the process of changing edges without actually deliberately moving your CM as you might if you were standing on the floor. It might look like you are throwing your body, but you aren't.

On whether the board moves more than the CM, it depends on your speed, plus the type and shape of your turn sequence. In some turn sequences, the CM moves a great deal.

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That is one way to look at it.

...You don't actually move your CM because the centrifugal force from the previous turn does that. Also, you don't throw your body, since you can use gravity and centrifugal force to do that.

You do move your CM, but you move the board more. Pretty much all the gross movement of the CM is done by pushing with the legs to resist the acceleration (centrifugal force) of each turn. The CM does move a little bit in reaction to moment applied to the board to change the tilt or edge angle, but so little that it is negligible.

.....

It might look like you are throwing your body, but you aren't.

It feels like you are because the turn starts with relatively modest force applied to the board, so there is something of a sensation of weightless flying. From some angles it looks like you are because the board and feet are not as obvious and aren't doing something dramatically different from what we expect them to do. In my humble and offhand opinion, we aren't accustomed to seeing the human body flying that low, so when we see the body moving dramatically downhill relative to the feet it kind of looks like it's been tossed. One of the coolest illusions going, IMHO...

On whether the board moves more than the CM, it depends on your speed, plus the type and shape of your turn sequence. In some turn sequences, the CM moves a great deal.

I have to disagree with you on this, Marco. The board always takes the longer path, further outside than the body in every turn, even dead-slow skidded turns. Even on a smooth slope one cannot initiate a slow skidded turn out of a traverse without getting the board to the outside (uphill) at least a little. Gravity acts straight down. The CM only gets pushed downhill by a component of force acting on it in that direction, and the only force available is provided by the legs acting through the edge of the board (or the portion of the base contacting the snow, if you prefer). If the board is suddenly moved from edging to flat relative to the slope, without either steering uphill a bit and/or tipping the body to move the CM downhill slightly, the board will simply squirt downhill out from under the CM. With the board flat, the normal force on the base can be resolved into a component straight up through the CM, and one horizontally that pushes the board downhill. The second component does not act through the CM, so it rotates the system about the CM. The rotational inertia of the system resists the rotation, so the angular acceleration is finite, but until one gets the balance right it can be pretty quick. We've all seen this while learning or teaching sideslipping - too flat and the board gets ahead of you so you fall uphill. Until you get the hang of it, you tend to oscillate between too flat and too angulated.

To generate turning force with a board that can only create a compressive force pretty much perpendicular to the base of the board (I am ignoring the drag due to friction and any force on the edge acting parallel to the base), that force has to be applied from the outside of the turn. The board cannot pull you - all it can do is push. (Technically, it can pull you, but if your board is pulling you, you aren't going to be very happy with how it turns out ... ). You can no more turn with the board nearer the apex of the turn than the body than you can turn a bicycle with your wheel contact patches nearer the apex than is your body. You can change the orientation of the board, but not the path of the CM (think flat spin) until you push the CM into the turn by applying force to the board with your legs.

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  • 4 weeks later...

In all the video examples, the lengthening of the body (extension) is happening before the edge change, not after.

if anything, Sigi is staying fairly static with regards to joint flexion during execution and Tyler Jewell is compressing during the top 2/3rd's.

I only mention this because you say that you were inspired by Phil saying that the extension comes after the switch. It could, but none of the riders in this video are doing that.

Rad video, just the same.

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Good to raise this point, Rob. You may be right and it's good to have discussions, because no single explanation ever covers it all.

This is complicated and I wouldn't want my videos to be taken dogmatically. Riders should experiment and figure out what works for them. It's a mistake to go out and say to yourself, "I'm going to cross-over here, and cross-under there." Better to let the basic movement patterns determine how you ride and not attach names to things.

I think that the body can start to elongate up before the new edge is reached if someone is going really fast and if they are generating a lot of centrifugal force, like a WC rider. If you're generating 3 G's of force and you downunweight, you are going to move across the board pretty quick. You could begin to extend while the board is still releasing the old edge, but only pressure the board after you've reached the new edge. It may be partly speed, it may be partly timing, it may be partly style. It might depend on the rider. Everyone is different.

On the other hand, I think we have to be careful attributing extension as the cause of the higher body in transition, since the body is going to appear higher, hence extended, just because it's more upright as it travels across the board engaging the new edge.

Sigi does these movements, but he's really subtle about them, so he may not appear to be doing them. I think for a higher level rider, and I even mean the good amateurs who post on this site, the movements can often have more to do with directions of pressure--down, static or up--than clearly visible motions, as their bodies may not appear to flex and extend much at all.

Thanks for the compliment on the vids. I hope they help. :)

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