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Tip size, camber and grooves

post #1 of 23
Quote:
Originally posted by PhysicsMan:
A return of the center groove would somewhat reduce the tendency of shaped skis to wander and wobble around when run flat.

Thoughts?

Tom / PM
How do you figure that?

Doesn't the tendency of shaped skis to wander when going straight happen as a result of the sidecut.
post #2 of 23
Thread Starter 
A center groove in the Ptex was standard on all skis until somewhere around the 80’s (?). When running flat & straight, it was widely held that it added considerable yaw stability (as if this was a big problem with old straight pencil skis ).

Although I have never personally done side-by-side comparisons of two skis, identical except for a center groove, I’m quite sure I have seen several (old) reports by researchers confirming this benefit. The way the center groove works is that if such a ski is running flat over snow that is even slightly compactable, the snow is forced up into the groove and forms something like a mating tongue-and-groove situation, so that it becomes quite difficult for any small yaw torques to perturb the path of the ski unless they are large enough to destroy the little ridge of snow that is formed. There is no doubt that such a center ridge of snow was formed. You could see it in the trails of almost every skier. Basically, in terms of yaw guidance, the center ridge of snow is in competition with the ridges in the snow made by the edges of the ski, but the latter are very diffuse on deeply sidecut skis running flat, so the effects of the center ridge on a modern ski (running flat) should be substantial.

Of course, as soon as you edge such a ski up by a couple of degrees, the only contact between the ski and snow is at the edge, so a modern ski with such a groove should completely revert back to its normal behavior.

I have also heard it said that the center groove in the Ptex also acted like a channel to funnel water to the rear and help reduce friction. I’m not so sure about the veracity of this possible benefit

Tom / PM
post #3 of 23
Thread Starter 

Tip size, camber and grooves

Anybody else wish they once again had decent sized tips on their skis, a return to a significant amount of camber, and the comeback of center grooves, except that now they would be on modern, shaped skis?

Big tips (probably not quite as large as those shown in http://www.turns-all-year.com/gmtrmuir/source/2.html ) would let you motor right over clumps of tracked up crud without the big decelerations, as well as prevent tip dive in powder without needing to soften up the entire length of the ski.

A decent amount of camber would provide a more constant downward force on the tip and tail for quicker engagement of the edges on groomers.

A return of the center groove would somewhat reduce the tendency of shaped skis to wander and wobble around when run flat.

Thoughts?

Tom / PM

[ October 25, 2003, 12:50 AM: Message edited by: PhysicsMan ]
post #4 of 23
I have two pair of old Head skis "living" out in my garage. Killy SL's @ 203 and an old pair of Standards @ 210.

Length alone would keep them running straighter, same principal in "freighter vs white-water canoes". The groove undoubtedly aids in that as does the deeper keel of the freighter. I know that the camber/rocker are factors too.

BUT ...... why do you need a ski that runs straight and true? You spend 99.97% of your time turning a ski, you are not trying to save time (like in the canoe), trying to get from point A to B.

Most importantly, speed in the skis comes from wax and structure to break the suction ..... and a fast ski ..... ask any racer, is a wobbly one that is not running straight and true. Just try this on a long schuss. Try to get very flat on the ski and it will have a slight wander ..... to get in it's state of least resistance is often the diference between a win or a loss ..... between that last gate and the finish. When you feel that slight wobble, your ski is running fast.

I never thought of yaw as a problem, a good wedeln was the ability to induce an occilation around that axis.

[ October 25, 2003, 05:20 PM: Message edited by: yuki ]
post #5 of 23
Thread Starter 
> ...Length alone would keep them running straighter...

True 'nuf, but I think we might just be able to have the best of both worlds, namely, deeply sidecut skis that run straight better than any of the current models, and most importantly, are no longer than the present models.
-----

> ...BUT ...... why do you need a ski that runs straight and true? You spend 99.97% of your time turning a ski, you are not
> trying to save time (like in the canoe), trying to get from point A to B...

With all the catwalks and runouts around, I think it would be pretty nice to have a ski that could be skied flat without having to keep it a bit on edge and constantly pay attention to it. I remember distinctly having to learn to do this when shapes first came out. I also remember that this was one of the prime objections of a lot of people had with "shapes". So, if good straight running could be accomplished with essentially zero sacrifice in other areas of performance, why not do it? It costs hardly anything to groove the Ptex.
------

> ...speed in the skis comes from wax and structure to break the suction .....

True. That's why I said I wasn't quite sure of speed as a real side benefit of a grooved base. It is possible that a groove might improve the speed of a well waxed and structured base a bit, but this isn't the main reason I'm interested in the groove.
-------

> ...When you feel that slight wobble, your ski is running fast...

Two things:

(1) If you asked a racer from 1950 to 1980 the same question (ie, before the advent of deep sidecuts), would they have answered that they noticed a wobble when the ski is flat, and that they can use this as a cue to maximize flatness. I think it is possible that the wobble is really an artifact of not having a center groove on present race skis which have comparatively deep sidecuts, and that the wobble could be eliminated (or reduced) without any increase in friction simply by grooving the base. Put differently, the wobble might be a nice cue that the a groove-less ski truly is flat, but IMHO, wobble is not essential to running fast and true.

(2) Ultimate speed may be important to a racer, but for recreational skiers, I suspect that having a set of boards that don't feel like they might cross when running flat might be very appealing.

Just some of today's mental meanderings.

Tom / PM

[ October 25, 2003, 06:09 PM: Message edited by: PhysicsMan ]
post #6 of 23
Now how can we test this? OK! I'll bring the old Sears router. You can bring the skis. Long skis. About 198 would do?
post #7 of 23
PhysicsMan,

I would like a non-wobbly ski. I think skis with a lot of sidecut are the most fun, so that’s what I mostly ski. I’ve never gotten used to the wobbly business when they are flat. On straight skis, I used to find the run-outs and cat tracks very relaxing, whereas now I feel like I have to pay more attention to my skis.

Don’t you have a Bridgeport in your basement? I’m sure you could find a pair of skis in one of your ski sheds to experiment on! I would think a machine shop could mill the groove for you pretty cheaply.

As far as the big tips go, I seem to remember an old thread about making twin tips from regular skis. You could apply that technique to the fronts of another pair of old skis to try it out.

I’m not sure I would try both things on the same pair of skis. I think the groove would make the most sense with a short slalom ski so you could see how well it worked. The big tip would probably have to be done on a longer ski and you’d lose some of the sidecut when you moved the contact point back, making it a poor candidate for the groove experiment.
post #8 of 23
Thread Starter 
Quote:
Originally posted by yuki:
Now how can we test this? OK! I'll bring the old Sears router. You can bring the skis. Long skis. About 198 would do?
{southern_accent} Sirh, something makes me think you might be mocking the 198 StormRiders that I know you know that I still possess. Because, Sir, if you are impugning their reputation, I suggest that we meet at the first available opportunity at the bottom of Mahogany Ridge at K-mart, and I assure you that I will defend their honor to the end. {/southern_accent}

Quote:
Originally posted by mxp:
I would like a non-wobbly ski. I think skis with a lot of sidecut are the most fun, so that’s what I mostly ski. I’ve never gotten used to the wobbly business when they are flat. On straight skis, I used to find the run-outs and cat tracks very relaxing, whereas now I feel like I have to pay more attention to my skis.
This is exactly what I am talking about. I think that there are a lot of people who have completely forgotten how relaxing the flats used to be on straight skis. For example, you could ski alongside someone and chat with them, whereas on today's equipment, you better keep awake, keep your distance, and keep your skis always just a little bit on edge until you come to a complete stop.

Quote:
Don’t you have a Bridgeport in your basement? I’m sure you could find a pair of skis in one of your ski sheds to experiment on! ... I think the groove would make the most sense with a short slalom ski so you could see how well it worked.
Err ... do I know you? How did you know about the Bridgeport? A few months ago, I just sold one of the Bridgeports from my old lab to the husband of one of my colleagues. We gave him a good price, and he offered to let me do hobby work on it whenever I wanted. So the machining part of the project is covered. The only remaining problem is finding a suitable set of skis. I agree that they should be short SL's that are in good enough shape to know if the groove makes any difference, but not so good that someone wouldn't mind losing them to science. I think the P-tex is thinner on today's skis, so to get enough depth in the groove, I probably would have to go just about all of the way through the Ptex to see a substantial effect, and that would likely shorten their life substantially. BTW, I would only do one ski at a time from a pair. That way, one could do an immediate L-R comparison between them. I know it's hard to believe, but I actually don't have a pair that meets these criteria. Maybe I should prowl the upcoming swaps and keep an eye out for a suitable candidate. If anyone has a pair they want to donate to the cause, I'll do the machining and initial testing and send them back to you for further testing.

Quote:
As far as the big tips go, I seem to remember an old thread about making twin tips from regular skis. You could apply that technique to the fronts of another pair of old skis to try it out. ... The big tip would probably have to be done on a longer ski ...
Exactly. The "Big Tip" experiment should probably be done to a substantially longer fatter ski, already pretty good in crud. Something with metal (so it would hold the new bend) but no wood. Again, I would do one ski (of a pair) at a time. The main problem with this experiment is that the required shop work is much less predictable than simply milling a groove, and the on-hill component of the experiment requires lots of soft crud, and that's not guaranteed around here till spring.

This sounds like fun. I hadn't thought about actually doing an experiment myself when I started this thread.

Tom / PM

[ October 25, 2003, 11:14 PM: Message edited by: PhysicsMan ]
post #9 of 23
You guys really are serious about putting in own bottom groove, aren't you. That's amazing. I don't have a pair of SL's that I would be willing to give up, but I think you might be on to something and I'm curious how it turns out. Good luck and post the results.
post #10 of 23
Several observations:

- The water groove went out with sintered bases (around '84-'86), and stayed out with foam core/topsheet trapezoid sidewall skis.

- Historically, some of the grooves have been very deep and U-shaped (older K2 blue-plastic base designs); trapezoidal-walled, shallow and broad (Volkl P20RS), shallow and square sidewalled (Elan Comprex RS- Ceramic K/Se base). Very much different shapes and profiles, so I am not at all sure these all all designed for "reverse keel effect"

- GS race skis to be used on extremely icy race courses still had water grooves.

- In "reverse keel" mode on loose snow, these would mean extra friction at all (four) corners of the grooves. Not the desired effect, surely.

Explanations might be that
a) the tooling was different enough with sintered bases and the benefit marginal they didn't bother.
b) sintered bases would keep factory structure far longer so the benefit was even less.
c) Weakening of a structural element (the base) in a foam-core ski was probably a bad idea in the days when the base would very likely develop a cavity in it when the foam cooled.

If I was going to do this, I would
-use an Electra base ski
-use a guide rod down the middle
-cut the groove by hand using engraving tools.

PM, you're looking at a lot of stabilisers these days?

I just picked up a pair of vintage Renntigers with "Egbro" (sp?) stabilisers on the tips, just a rubber puck screwed into a metal plate that is glued onto the ski. Obviously, a mass-dampener, which brings me to my question:

Has anyone reused Deflex plates? With what adhesive?
post #11 of 23
Do you guys spend that much time on cat tracks? I usually dont have too much of a problem straight running my SL's and carrying on a conversation with a fellow skier on a cat track, or a slope for that matter. Would a grove help that much? The center of the base of my skis rarely touches the snow. When i race of course i try for smooth transitions, but when i free ski sometimes i let the bad habbits kick in and let the skis rebound from turn to turn w/o touching the snow at all. Only using about 7mm of the ski from the edge in on each side... i dont think a grove would make much difference on a slalom ski. I never have any trouble with skis with a longer radius... PM - your XP100's would be the ideal ski to try this with. They have a ton of shape, but are still considered an all mountain ski, so you end up running them flat a lot. I'm guessing that this is where the idea came from??
Later
GREG
post #12 of 23
In a further throwback to my pencil-skiing days, no doubt, I really do not prefer wide tips on skis to be used in random crud, specifically not in bumped-up tree runs where there is still a lot of loose material.

In my (lack of, eh?) style, the wider tips are more prone to getting knocked sideways, especially when a bump-unweight coincides with tip impact on a pile of loose aggregate.

Perhaps, PM, you are looking for purely more mass, not girth, in the shovel? Sort of like the (Volant?) perimeter weighting system of old?
post #13 of 23
Quote:
Originally posted by comprex:
Several observations:

- The water groove went out with sintered bases (around '84-'86), and stayed out with foam core/topsheet trapezoid sidewall skis.

- Historically, some of the grooves have been very deep and U-shaped (older K2 blue-plastic base designs); trapezoidal-walled, shallow and broad (Volkl P20RS), shallow and square sidewalled (Elan Comprex RS- Ceramic K/Se base). Very much different shapes and profiles, so I am not at all sure these all all designed for "reverse keel effect"

- GS race skis to be used on extremely icy race courses still had water grooves.

- In "reverse keel" mode on loose snow, these would mean extra friction at all (four) corners of the grooves. Not the desired effect, surely.

Explanations might be that
a) the tooling was different enough with sintered bases and the benefit marginal they didn't bother.
b) sintered bases would keep factory structure far longer so the benefit was even less.
c) Weakening of a structural element (the base) in a foam-core ski was probably a bad idea in the days when the base would very likely develop a cavity in it when the foam cooled.
I know I have skis with sintered bases and grooves (those lovely old teal Atomic SLs, for example). I remember those Kastles (I think!) with three grooves (one in front, two in back, none under foot), and that was in the mid-to-late 80s.

As I recall, the original reasons for removing the groove was to make pivoting easier. It's ironic to me that, now that most people can carve, the groovelessness is still there.

This is definitely worth an experiment, but I'm afraid I can contribute nothing practical to it, given that those '88 SLs are my most recent ski (yes, I've been demoing for the last 5 or 6 years!).

ssh

[ October 27, 2003, 03:52 PM: Message edited by: ssh ]
post #14 of 23
I was just doing some thinking... and doesnt the base structure take the place of a groove now?? Just something to throw out there.
Later
GREG
post #15 of 23
Less effort version of experiment:

Take two existing-groove straight skis. Mount to sled as runners so that skis form flat vee. Wax both skis with hard wax, say Swix CH04. Scrape groove out on one of them.

Launch down snow surface and see which way it turns, or not. Vary loading as desired.
post #16 of 23
Thread Starter 
Quote:
Originally posted by comprex:
...Historically, some of the grooves have been very deep and U-shaped (older K2 blue-plastic base designs); trapezoidal-walled, shallow and broad (Volkl P20RS), shallow and square sidewalled (Elan Comprex RS- Ceramic K/Se base). Very much different shapes and profiles, so I am not at all sure these all all designed for "reverse keel effect"... In "reverse keel" mode on loose snow, these would mean extra friction at all (four) corners of the grooves. Not the desired effect, surely...
That's right. I think some of these were "water grooves" and not intended to increase the yaw stability. Some were touted to increase the yaw stability, and others were supposed to do both. The only problem I'm trying to address is the relatively large yaw instability of modern, short, deeply sidecut skis for recreational skiers (ie, not racers), so I'm not all that concerned about reducing friction with water grooves, base structure, etc., just yaw.

Quote:
...GS race skis to be used on extremely icy race courses still had water grooves...
I suspect that this wasn't even at the level of ski design voodoo, just a holdover artifact of other skis being produced in those days.

Quote:
...Explanations might be that
a) the tooling was different enough with sintered bases and the benefit marginal they didn't bother.
b) sintered bases would keep factory structure far longer so the benefit was even less.
c) Weakening of a structural element (the base) in a foam-core ski was probably a bad idea in the days when the base would very likely develop a cavity in it when the foam cooled. ...
I agree with all of your explanations.

Quote:
...If I was going to do this, I would
-use an Electra base ski
-use a guide rod down the middle
-cut the groove by hand using engraving tools.
The only reason I like using a mill is that I have one available, and it makes the transverse locating absolutely idiot proof, once the ski is set up on the bed. A depth guide would also be easy to set up, and the feed certainly is handy. You're right, tho, a Dremel with appropriate guides for depth and transverse location would probably also do just fine and is certainly a lot more available to people.

Quote:
...PM, you're looking at a lot of stabilisers these days? I just picked up a pair of vintage Renntigers with "Egbro" (sp?) stabilisers on the tips, just a rubber puck screwed into a metal plate that is glued onto the ski. Obviously, a mass-dampener...
So, you noticed . It's not that any of my skis are giving me particular problems, it's just that I think current short ski design could be improved even further with a few simple additions. BTW, I still have my old pair of 207-ish Zebras out in the garage, and I can't say that I ever felt that they needed to be more stable.

Quote:
......which brings me to my question: Has anyone reused Deflex plates? With what adhesive?
Not me. Sorry.

Tom / PM

[ October 28, 2003, 11:02 AM: Message edited by: PhysicsMan ]
post #17 of 23
Thread Starter 
Quote:
Originally posted by HeluvaSkier:
...I usually dont have too much of a problem straight running my SL's and carrying on a conversation with a fellow skier on a cat track, or a slope for that matter...
True 'nuff, but it still requires more concentration that it ever did on straight skis, so if one could get back the old levels of this aspect of ski performance at essentially no cost or other performance penalty, why not do it?

Quote:
... The center of the base of my skis rarely touches the snow. ... Only using about 7mm of the ski from the edge in on each side... i dont think a grove would make much difference on a slalom ski... I never have any trouble with skis with a longer radius...
That's absolutely right. From the wax wear patterns, its easy to see that only the outboard bits of ptex and the edges are the only parts of the ski that ever get used in "real" skiing on packed snow. However, there are still lots of places I know I would prefer to just lay 'em flat and just turn my mind off (like the old days ). One place that comes to mind is trying to make the schlep from one end of Sunday River to the other.

So again, it comes back to "if there is no other cost or performance penalty, why not put in a groove (if it would work)?"

Quote:
... PM - your XP100's would be the ideal ski to try this with. They have a ton of shape, but are still considered an all mountain ski, so you end up running them flat a lot. I'm guessing that this is where the idea came from??
Good thought, but actually, the idea wasn't inspired by them. In spite of their extreme shape (123-68-106), they are surprisingly stable, probably because of their high swing weight at 184. My perception of the need is from short skis I've been on and tried skiing flat. Even if they don't have all that much sidecut, if they are under 165-170, I think they all leave something lacking in yaw stability when flat, so, as I said above, if a simple groove would help, why not. They could then stay short and reap the benefits of that.

Tom / PM

[ October 28, 2003, 11:05 AM: Message edited by: PhysicsMan ]
post #18 of 23
Thread Starter 
Quote:
Originally posted by ssh:
...As I recall, the original reasons for removing the groove was to make pivoting easier. It's ironic to me that, now that most people can carve, the groovelessness is still there ...
... and now, here comes this guy that wants to go back make it (a bit) harder again .

Tom / PM
post #19 of 23
Thread Starter 
Quote:
Originally posted by comprex:
Less effort version of experiment: Take two existing-groove straight skis. Mount to sled as runners so that skis form flat vee. Wax both skis with hard wax, say Swix CH04. Scrape groove out on one of them. Launch down snow surface and see which way it turns, or not. Vary loading as desired.
Interesting thought. It might give further support for the "reverse keel" effect (great name, BTW), but I think that effect is already quite well established, at least in the old days (eg, see SSH's post). Unfortunately, I think it would eventually still come down to having to perform the real experiment on short, highly shaped skis. I do like the idea of waxing in the groove to allow one to go back and forth between grooveless and grooved configurations.

Tom / PM
post #20 of 23
Thread Starter 
Quote:
Originally posted by HeluvaSkier:
I was just doing some thinking... and doesn't the base structure take the place of a groove now??
Yes, with respect to breakup and transport of the melt water layer; No, with respect to the yaw stability issue.

Tom / PM
post #21 of 23
Thread Starter 
Quote:
Originally posted by comprex:
...Perhaps, PM, you are looking for purely more mass, not girth, in the shovel? Sort of like the (Volant?) perimeter weighting system of old?...
That's exactly the sort of question I'm pondering. I'm not familiar with that Volant system, but did you see my thread, Fritzmyer (?) Skis with changeable tip and tail weights .

Tom / PM
post #22 of 23
Quote:
Originally posted by PhysicsMan:
</font><blockquote>quote:</font><hr />Originally posted by comprex:
...Perhaps, PM, you are looking for purely more mass, not girth, in the shovel? Sort of like the (Volant?) perimeter weighting system of old?...
That's exactly the sort of question I'm pondering. I'm not familiar with that Volant system, but did you see my thread, Fritzmyer (?) Skis with changeable tip and tail weights .

Tom / PM
</font>[/quote]Yes, and I aesthetically prefer that method because it is not explicitly dependent on factors external to the ski, like snow quality and skier weight.

I mentioned the Egbra(s) above as a low-duty alternate to the Flow Packs mentioned in that thread, because it appeared to be a significantly massive chunk of rubber and steel as compared to VAS plates and tip-cross prevention loops.

Laboratory simile: Imagine a #14 flask stopper screwed onto a 2mm steel plate that is then adhered to the ski with some 1.5mm more of viscoelastic goo. Directly behind the shovel curve.
post #23 of 23
Thread Starter 
Quote:
Originally posted by comprex:
...Imagine a #14 flask stopper screwed onto a 2mm steel plate that is then adhered to the ski with some 1.5mm more of viscoelastic goo...
I suspect you weren't really suggesting the same material that stoppers are made from (too firm), just something of that approx size and shape.

For a material, I would guess that something like the blue stuff sold everwhere as insoles for street shoes ("Dr. Scholes") would probably have about the right softness and energy dissipation. My guess is that it should be quite thick, perhaps as much as 5 - 10 mm. This would allow it to remain compliant for large amplitude vibrations (ie, the lower freq ones).

Having it in the shape of an thin ring (ie, annulus with a cut out center) perhaps 5 cm O.D. would prevent it from being too stiff. Having the O.D. that large would allow it to catch not only gross up-down motion but slope changes (ie different vibrational modes) as well. The 2 mm steel that you suggested would probably do a nice job on the higher frequencies, and could always be increased if one wanted chew into lower parts of the spectrum as well as provide extra swing weight.

For the prototype, I think that it would be fine to use velcro between the ski and the "rubber", as well as between the rubber and the steel to allow one to easily substitute different materials, thickness, etc..

Obviously, such a damping system would only benefit skis that are too lively to start with.

Don't be surprised if you hear about some wierdo with Dr. Scholes insoles on an old pr of skis this season. It sounds like a easy, non-destructive experiment to have some fun with this season.

Tom / PM
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