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unweight have any effect on speed?

post #1 of 20
Thread Starter 

assuming a straight decend, snowboard or skis.

post #2 of 20

Carver, the only situation I can imagine where it would make a difference is if you had a clock on you.  In racing, common belief is that staying on the ground is faster than going in the air.  And in that arena, no one ever unweights in a flat ski straight run, just for the purpose of unweighting.  Perhaps to cut down air time they will,,,, IE; prejump, suck up a bump, etc. 

post #3 of 20

If you think about it a little, you'll realize that when you are fully unweighted (ie. airborne), gravity pulls you straight down toward the center of the earth. That means that there is no force pulling you toward the finish line, or down the hill in general. So air time is slow time!

 

When you are on the snow, you can visualize gravity's force as split into two components--one perpendicular to the slope, pulling you onto the snow, and one parallel the the slope, pulling you downhill. On a flatter slope, most of the force is onto the snow, and only a small component pulls downhill, so we don't accelerate very fast. On a steeper slope, the reverse is true--most of gravity's force pulls down the hill, and only a small component pulls toward the snow. (The extreme is a vertical slope, in which all of gravity's force pulls toward the bottom, and none pulls toward the snow. Needless to say, you'd go just as fast as if you were airborne and free-falling, except this time, it would be toward the bottom of the hill!)

 

Does that answer the question? I'm sorry, my regular computer is in surgery right now, having its motherboard replaced. Otherwise, I'd draw a simple diagram that shows what I described above. Maybe later....

 

Best regards,

Bob

post #4 of 20

More....

 

Of course, there are many reasons to unweight--or not--that have nothing to do with maximizing speed. And, as Rick pointed out, there are occasions in which intentionally unweighting ("prejumping") can actually help minimize air time, resulting in faster speed overall.

 

Best regards,

Bob

post #5 of 20
Thread Starter 

Thanks both for the analysis. When I can understanding what you described. Yes, I believe no one will do that in a race. I was just curious if it make any speed difference at all. If we restrict the question to a very flat slope, where the maximum speed by straight decend is not affected by aerodynamic,  and look alone at friction. 

 

On unweight, not to the extend of airborne, we will have less friction, therefore more downhill pull?

after unweight we ll get back to the normal weight with a temporary more-weight = more friction= slowing down?

 

Does the two added together imply more speed or less speed? 

 

I m not so sure if that make sense. Just curious.

post #6 of 20

First of all, your unweighting and weighting will affect air resistance (because it alters your profile/shape from the ideal tuck), and in most cases air resistance will be the deciding factor.  If you want to minimise your energy loss you have to reduce air resistance, tuck all the way as the crazy Canucks so aptly demonstrated so many years ago.

 

Now let's just suppose for the sake of argument that we can set aside air resistance for the time being.  You want to go as fast as possible, so you have to minimise energy losses.  That means less friction, so if you have more weight on the snow, you have more friction and less weight on the snow you have less, so you are trying to figure out if it all balances out.  The way out of this quandry is to think in terms of work energy, not friction force.  Do less work on the snow.  If when you unweight and re-weight you crush the snow, you are doing work on the snow.  You should ski as if (not my original saying, but a very good one nevertheless) you are skiing on eggshells and trying not to break any (maybe ricepaper is more appropos?).  Don't bruise the snow!

post #7 of 20
Thread Starter 

Thanks Ghost. Great explanation.

post #8 of 20
Quote:
Originally Posted by carver_hk View Post

Thanks both for the analysis. When I can understanding what you described. Yes, I believe no one will do that in a race. I was just curious if it make any speed difference at all. If we restrict the question to a very flat slope, where the maximum speed by straight decend is not affected by aerodynamic,  and look alone at friction. 

 

On unweight, not to the extend of airborne, we will have less friction, therefore more downhill pull?

after unweight we ll get back to the normal weight with a temporary more-weight = more friction= slowing down?

 

Does the two added together imply more speed or less speed? 

 

I m not so sure if that make sense. Just curious.


 

Interesting question, now that you put it that way, Carver_hk.

 

Of course, reducing friction overall can make you go faster (or slow down less). I'm sure we've all hopped over rocks, sticky, sections of snow, bare spots, and other high-friction obstacles (snowboarders? I didn't say that...), for good reason. And we've all felt the slowing suction of wet snow on a warm day. Don't forget, though, that the momentarily decreased pressure from "unweighting" in these situations comes with moments of increased pressure as we leap off the snow, and as we land.

 

But less pressure does not necessarily mean less friction. Like ice skates, skis actually rely on pressure and, indeed, friction, to melt the snow a little, to make them slippery and fast. If you've ever skied on extremely cold, dry snow, you know how slow it is.

 

Lighter skiers, exerting less pressure on the snow, do not usually glide faster than heavier skiers, and I don't think it's entirely due to air resistance. It would be an interesting experiment to do a straight gliding race with a heavy and a light skier, if we could eliminate air resistance. The laws of physics say that, if all else is equal, they should accelerate at the same rate, so any difference would be due to friction. I'm not sure how the skiing would be in a vacuum, though. (It might suck!)

 

In any case, other than avoiding high-friction areas, I don't think that unweighting strictly for the sake of reducing pressure and friction and maximizing speed is something to worry about much. But it is entertaining to ponder the possibility!

 

Best regards,

Bob

post #9 of 20

I should add that snow conditions play a role here too, along with surface area of our skis. On hard, average-temperature snow, narrow skis are typically faster than wider skis, even though the pressure that affects friction (force per area, pounds per square inch) is greater due to the smaller surface area. In deep, soft powder, needless to say, those same narrow skis tend to sink deeper and go much slower. But on sticky, wet snow or very cold, dry snow, again, the larger surface area of a wide ski will stick a lot more--unless it's able to break free from the stickiness and glide over it. At high enough speed, skis can even float on a cushion of air--which is pretty slippery (although they are not, strictly speaking, "unweighted").

 

Best regards,

Bob

post #10 of 20

This gives away my age.  I remember reading about Jean Claude Killy discussing his Olympic Gold Medals.  He said in one race he didn't have enough time to rewax his skis between runs. He didn't have much wax left so he intentionally tried to spend as much time in the air as to minimize the friction on the snow. I don't know if that was flawed logic, something missing in the French translation, or something he made up but I clearly remember that story.  I am also pretty sure he said "he" didn't have time to rewax instead of his ski technician. Boy that was a long time ago.

post #11 of 20
Thread Starter 

It surprise me getting some more insight. Skiing is so much fun because you never know that anything is probably conclusive or not. Thanks all for contribution again.

 

The origin of this thread was from a thread I read in a Chinese forum when someone claim he can get faster in snowboard with some up/down motion. I took that its hard proof and I attempted to give a reason. What m I seeing here is a proliferation of knowledge and experience. Now I am not so sure if it should be faster or not because so many factor creeped in. Its getting very deep.

post #12 of 20

>>>"can get faster in snowboard with some up/down motion"<<<

 

Well, there are times when you can do that, and techniques--ie. skating--that certainly incorporate some up-down motion. If you flex at the top of a mogul or rise, and extend forcefully down the back side, you can "pump" out some more speed. If there are repeated dips and rolls, extending in the downhill sections and flexing to absorb the uphill sections can generate significant speed. But this movement is different from unweighting. Indeed, it's the opposite--it involves adding pressure with just the right timing.

 

Best regards,

Bob

 

 

post #13 of 20

 my understanding is this, imagine you have a ball that you are going to drop.  Gravity pulls it to the ground, at an accelerated rate that will be whatever it is, but the same every time.  It will also take the same amount of time for the dropped ball to fall to the ground.  Now imagine that you have a 45 degree slop that you are going to roll the ball down from the same height to the ground.  Gravity is still trying to pull the ball straight down at the same accelerated rate, except that the slope is DISPLACING the ball to the side in such a way that the length of the path that it will take for the ball to reach the same height you dropped it from before, to the ground, is now a significantly longer path.  If gravity is able to pull the ball to the ground in the same amount of time as if it were free falling, but on a 45 degree slope, then the ball will actually have to be moving faster in order to cover that longer distance.  Of course there are other forces at play slowing it down, namely friction, but the theory here is that the speed of the ball will be greater on a 45 degree slope then it will be in freefall.  If you go to even steeper angles, the difference in speed would be less, and the influence of friction would be less.  On the other hand, go to a less steep slope and the displacement would become bigger and bigger, but the friction would also start to become a much more significant factor, even to overcome the displacement factor.

 

So bottom line is that without friction, on the snow will be faster then in the air.  However, with friction, then it depends.

 

post #14 of 20

 I want to add to that, from a more practical perspective, unweighting means a momentary loss of edge engagement.  If your edges are not engaged, then you are in as much control as a ball falling or rolling down the hill.  To most skiers this will probably feel like acceleration, because compared to what they are probably doing before and after the unweight...it probably is.

post #15 of 20

Good point, borntoski683, about the perception of going faster that many skiers might well feel when airborne, and the possibility that they really are accelerating, due to loss of braking when the (skidding) edges disengage. Since Carver_hk's original post specified a "straight descent," I have avoided discussing braking and skidding. But loss of edge engagement in any turn, for any reason (except intentionally releasing the edges to finish/start a turn) is likely to feel like a loss of control, which might well make it seem like you're going faster!

 

I suspect that your ball-on-a-ramp analogy is somewhat flawed. While it is true that, if the ball lost elevation as quickly on the ramp as in freefall, it would need to travel faster on the ramp (the moreso the shallower the angle), the reality is that it will not lose elevation as quickly on the ramp. And it won't be entirely due to friction. It should be quite obvious that the shallower the angle of the ramp, the more slowly the ball will accelerate and roll down it--just as the shallower the slope angle, the slower we'll glide on skis (all else being equal). On a very shallow ramp (which must also become very long, in order to get the ball all the way to the "floor"), it will take a long, long time for that ball reach the end. Meanwhile, the free-falling ball will be gathering dust on the floor.

 

Nevertheless, the ball-on-a-ramp image does illustrate the role of the slope in dividing gravity's downward pull into the two components I described earlier--one perpendicular to the slope--holding the ball to the ramp, and the other parallel to the slope--accelerating the ball along the ramp. Without the ramp, the ball in freefall will experience no force whatsoever in the direction of the slope--ie. downhill (unless, of course, the ramp itself is vertical).

 

Best regards,

Bob

post #16 of 20

Bob, was not meaning to counter you, only trying to provide a different way of illustrating essentially the same point.  

post #17 of 20

No countering detected, BTS. (And I wouldn't have minded if you were!) I think that your ball-on-a-ramp image is a good one, as I said, to illustrate the effect of gravity, slope angle, and acceleration. I just wanted to suggest that the ball would not actually behave quite the way you described it. Of course, there's another variable in the rolling ball, too, that does not affect the skier--the rolling itself. The ball must accelerate not only linearly--along the ramp--but also angularly (rotationally), as it rolls about an axis as well.

 

Best regards,

Bob

post #18 of 20

This is, I think the best and simplest explanation for why skis on the snow create speed:

 

Quote:

Originally Posted by Bob Barnes View Post

 

Nevertheless, the ball-on-a-ramp image does illustrate the role of the slope in dividing gravity's downward pull into the two components I described earlier--one perpendicular to the slope--holding the ball to the ramp, and the other parallel to the slope--accelerating the ball along the ramp. Without the ramp, the ball in freefall will experience no force whatsoever in the direction of the slope--ie. downhill (unless, of course, the ramp itself is vertical).

 

 

 

post #19 of 20

Good explanation by Bob.

We don't really need to get into the intimate details of the ramp.  Suffice it to say, the steeper the ramp, the more the ball's acceleration approaches 9.8 m/s/s.

 

I just want to add, that while you are in the air, after you stop going up, you are increasing your speed, but the acceleration is all in a downward direction.  When you make contact with the hill, and you will, that velocity will be changed to a direction parallel to the slope, said change being noticeable to you as your skis rocket away.  Be careful your skis don't leave you behind.  It is exceeding hard on the abs to negotiate high speed carves off the tail of your ski when you are in a prone position from the knees back.

 


Edited by Ghost - 2/9/2009 at 11:42 pm
post #20 of 20

I would amend the 'prejumping' advantage described by others above to include another element. 

 

When straight-lining down a slope you may come to a roll in the hill. Extending aggressively to prejump (done properly) while still on the down-slope will jettison the skier slightly forward as well as upward.

 

Also, while in the air (or at least unweighted) the skis will have less friction against the upward-slope on the leading side of the rise.  Moreover, less force will be exerted against the skier's body Mass by that brief  up-slope that would otherwise slow the forward momentum of the skier's CM.

 

In effect, with a prejump the skier's CM follows a shorter, straighter and less resisted ballistic curve - and on average doesn't need to travel higher as when launched further upward by just taking the jump normally. They also get back to traveling toward the finish (downslope) sooner.

 

Yes, landing from higher up will produce a brief spurt of forward thrust - but at the costs of briefly increased friction, potential balance problems and if the ski tips go up at all - *much* greater air friction - especially close to the snow due to the 'ground effect' as magnified by the much higher air density (generally colder and more humid) right next to the surface.

 

Can't say it's always better to prejump since each situation depends on the speed, relative angles and size of the roll, course terrain and curvature both above and below the roll, etc.

 

Might be fun to experiment on a controlled course though...

 

The more I think about it... the more I think we should form an "EpicSki Investigations Team" and solicit Research Grants for just this kind of thing.

 

.ma

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