Speed Control

[rlspalding]

I forgot the example:

Lets say I know the following:
Turn radius = 15
Slope Pitch = 10

I look at the bottom axis labeled turn radius. I find the "15" has mark (Red Line). I follow this vertical line up until it intersects the second line from the slope pitch side (Pinkish colored line). (each line on the graph is a value of 5, so for the slope pitch side I need to find the second line). These two lines intersect right where the purple and and creme color touch. Follow this line all the way to the vertical axis and it corresponds to a final velocity of 10 m/s.

I cant figure out how to imbed a picture..I dont have a website to upload it too...sorry

[tdk6]

rlspalding, thanks. Im a naval architect so these 3 dimensional graphs are familiar to me but somehow I could not see the meaning of this. But now I do . As I mentioned before we never ski at slopes piched more than 45deg (cannot be gromed) and we do not carve in terms of arcing at piches above 25 or 30deg. Our modern shaped ski radius limits us to radius below 30m. This would give us a max speed of 25m/s and it very much goes to show that the chart works since the speed of 90km/h or 55mph is fast enough for any skier.

Tommy carving with his supershapes (r=40 estimate) on his favorit 85deg pich slope in cervinia comes flying out of his turn at 90mph but to his dissapointment that is not even a world record .

[tommy]

rlspalding,

that's a brilliant explanation of the graph, thanks! Couldn't have done it better myself...

just one additional comment, based on tdk's musings about me carving in Cervinia ( which btw is one of my favorite resorts...:


90 mph would probably be a world record because the graph measures just a single turn...., that is, the distance covered when reaching the "terminal velocity" is very limited...

Like I said, it's just theory, and no one would ski anything much above 50 degree slopes, not carving doing linked turns anyway, but the value (if any) of the graph, at least to me, was to clearly demonstrate why high caliber skiers like HH are able to carve even black slopes, while most recreational skiers are not. Leopold's excellent post above also IMO very clearly shows where in the turn most skiers fail...

[tdk6]

rlspalding,

that's a brilliant explanation of the graph, thanks! Couldn't have done it better myself...

just one additional comment, based on tdk's musings about me carving in Cervinia ( which btw is one of my favorite resorts...:


90 mph would probably be a world record because the graph measures just a single turn...., that is, the distance covered when reaching the "terminal velocity" is very limited...

Like I said, it's just theory, and no one would ski anything much above 50 degree slopes, not carving doing linked turns anyway, but the value (if any) of the graph, at least to me, was to clearly demonstrate why high caliber skiers like HH are able to carve even black slopes, while most recreational skiers are not. Leopold's excellent post above also IMO very clearly shows where in the turn most skiers fail...

How steep is that speed course in cervinia? Ive been in cervinia once, a long long time ago.

I am able to carve (arc) on wide flat groomed red (european) slopes and scandinavian blacks (if closed for training) but not in the alps. IMO the chart tells us very clearly why that is not possible. Short carved (arced) SL turns r10 down a black slope of 30deg gives us a speed of 30mph. The next turn would be a bit wider lets say r15 and that would give us a speed of 38mph. Lets say we got in the back seat and our turn radius grew to r30. Now we would be leaving the turn at 55mph. That would all happen in less than 10 seconds.

[h.harb]

We discussed the formula for speed control about a year ago. I think it?s still in the current posts.

Speed control is dependant on friction, friction of the ski?s side cut and tip engaged into the snow, from the beginning of the turn, to the end.

If you want speed control, you need to stay forward, if you are not forward, , you can make up for "some" speed control lost from less tip pressure, by increasing tipping angle. This is what Ligety does, he's back on his skis so he has to tip farther, to make up for less tip pressure. . It?s really about friction. Turn radius is dependant on friction, created by forward pressure on the tip in the snow or ice and ski angle.

Ultimately distance lost in transition has some impact on speed. The angle to the falline at the release, has a great deal of influence on speed, so finishing into the falline (not in the falline, into the falline) can not only save you from loss of control , but it can also give you time to rebalance and engage early for the next arc.
http://www.harbskisystems.com/hhsite/Image21.gif

At "Begin CB" you are into the falline

[tommy]

How steep is that speed course in cervinia? Ive been in cervinia once, a long long time ago.

Don't know, but "V?ggen" (name of slope) in S?len (swedish resort) is 45 deg, and "Chocken" in Idre (another swedish resort) is 48 deg. I've skied both, and was definitely unable to carve even a single turn - both were quite icy when I was there, and it was more a question of survival than skiing with "style and grace"....

45 deg is very steep - one of my kids on snowboard got scared in the middle of "V?ggen", and wanted to sit down to rest for a while... what happened next, once she lost her board edge by sitting down, was that she slid down all the way on her back screaming, unable to stop the "free fall"...

[tdk6]

How steep is that speed course in cervinia? Ive been in cervinia once, a long long time ago.

Don't know, but "V?ggen" (name of slope) in S?len (swedish resort) is 45 deg, and "Chocken" in Idre (another swedish resort) is 48 deg. I've skied both, and was definitely unable to carve even a single turn - both were quite icy when I was there, and it was more a question of survival than skiing with "style and grace"....

45 deg is very steep - one of my kids on snowboard got scared in the middle of "V?ggen", and wanted to sit down to rest for a while... what happened next, once she lost her board edge by sitting down, was that she slid down all the way on her back screaming, unable to stop the "free fall"...

Ive never been to V?ggen (The Wall) in S?len but Ive been to Trysil not far away in Norway. They have this 45deg black slope (dropp off) that is for experts only and its fun to ski but there are people sliding down in various ways everytime Ive been there. The WC slope in Levi Finland is very wide and when I carve down the steep part at the bottom I go from side to side and manage to crank in 3 turns on SL skis. The WC guys turn at least 15 times.

[rlspalding]

At "Begin CB" you are into the falline

What does CB stand for? Im assuming not counter-balance because you need counter balance in the arc too...?

[rlspalding]

I have re-read and re-analyzed the topics of BPSRT in the "history" forum.

From i think it was JBotti and HH's posts, the way to do this is to continue aggressively tipping the free foot (uphill foot), and to relax or flex the stance foot (downhill foot) so that the stance ski has a higher edge angle then the stance foot.

THis brings me to my question...would this action not cause you to do the splits?? Im having trouble visualizing this concept and really dont want to try it until I know it wont end up in the splits (very uncomfortable for me).

What I guess Im having trouble with is that the free foot is going to be at an agressive edge angle meanwhile the stance foot has less of an edge angle. Even if "brushing" does start, how do your skis not natural spread way apart? I know your stance foot should be bearing most of your weight, but your free foot ski is still on the ground so it edge will engage and "force" the turn? Or is 100% of your weight on the stance ski and essentially the brush turn can be done on one ski? and you are only doing the tipping of the free foot to stay in a consistent motion with PMTS and using this motion to turn your stance foot so you do not resort to twisting to cause the brushing?

[Max_501]

Book/Video 2 does a great job of explaining how to create a BPST.

The short answer to your question is that tipping the inside ski leads the movements and creates the angles. The outside ski follows that tipping action and matches the angle created by the inside ski. The point is to be active with the inside and passive with the outside.

[François]

tdk,

the diagram is showing the increase in kinetic energy, gained over the actual vertical distance covered in a single turn, that is, the transformation of potential enegy to kinetic. This vertical distance depends on 2 parameters, the turn radius and the slope pitch.

The x axis shows the radius, the z axis the slope pitch, and the y axis the resulting velocity in meter/sec.

Obviously this is very theoretical stuff, ignoring a lot of real world params like friction and air resistance, but still IMO it displays the importance of keeping the turn radius small for speed control.

Btw, I'm not following your statement that slope pitch can't exceed 45 deg - even if few skiers would actually ski anything substantially steeper - but to me the limit is 90 deg, that is, free fall.

cheers,
T1

It is a great graph, and seems to match my experience, thanks.

I'm having a little trouble with the math. It seems if you move 10 m down hill using one C half circle of 10m diameter (5 meter radius) you travel a distance of pi*5 m, and if you make two circles of 5 m diameter S you still travel 2 * (pi * 5/2)= pi * 5 m. It seems to me that you convert the same amount of potential to kinetic energy while traveling exactly the same on-snow distance. Could you help me out here ?

[rlspalding]

The outside ski follows that tipping action and matches the angle created by the inside ski.

I dont have my books with me, but Im getting the first book next week. However, I understand the point of tipping and the fact that by tipping my free foot, the stance foot willl follow with the same edge angle.

However in the other threads where people describe the brushed carve, they say that you have to tip aggressively still, yet the stance foot does not follow the same edge angle...this is not the same that ive learned in PMTS and not the same to what you mention above. The way it was described was the brush carve requires aggressive tip of the free foot but the stance foot has less of an edge angle, which to me seems like would cause your two legs to spread apart?

[François]

I think it's as Harold says, a=V^2/r. Shorter radius means more acceleration means more force between the base and the snow means more friction. I would still love to see the math behind the graphs though.

[trtaylor]

RL,

See Jay's post on brushing.

[tdk6]

The way it was described was the brush carve requires aggressive tip of the free foot but the stance foot has less of an edge angle, which to me seems like would cause your two legs to spread apart?

You are correct, your two legs spread apart. BTW, from a functional point of vew it really doesent matter if the turn is carved (arced) or brushed (skidded).