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Placement of the Mid Running Surface on a Directional Ski

 
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Cadman



Joined: 06 Apr 2010
Posts: 48
Location: Crystal Mountain, Washington

PostPosted: Thu Jul 20, 2017 1:07 pm    Post subject: Placement of the Mid Running Surface on a Directional Ski Reply with quote

http://s172.photobucket.com/user/CADMAN_2007/media/GENERAL%20SKI%20SHAPE.jpg.html?sort=3&o=3[img][/img]

I am working on a directional ski with longer blend curves at the front than at the tail. Since the tip is longer than the tail, the mid point of the effective edge is further back than the true center of the ski. I am trying to figure out how to place the mid running surface in relationship to the center of the ski.
I am not sure whether to place the MRS in relationship to the mid point of the sidecut radius (not considering the blend curves or the mid point of the true effective edge). I want to make sure that the ski "hooks up"or engages well during turn initiation. I attached a generic ski shape to try to explain my situation. Generally speaking the waist of the ski should be behind the the MRS and the length has to do with boot length so I measured a ski boot and found the difference between the boot length and the ball of the foot which is normally placed on the MRS.

Hopefully I am able to explain what I am trying to figure out. Any comments here?
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falls



Joined: 27 Oct 2009
Posts: 1426
Location: Wangaratta, Australia

PostPosted: Thu Jul 20, 2017 4:22 pm    Post subject: Reply with quote

In general in your design I would look at your effective edge centre point and place the boot centre mark 6.5cm behind this point. Highest point of camber at this mount point also if possible and narrowest part of ski.
Look at the relationship of this mount point in regards to true ski centre and the centre of the running length (I think what you are calling MRS) and if they are in a pretty similar area I think it will be OK.
I made a similar ski once and ended up moving the waist and mount point too far back in the ski. It was OK, but think would ski better if things were a bit more centred.
With some rocker in the tip and tail I usually look at what parts of the sidecut are actually likely to get engaged in a turn and use this as my "real effective edge" and then determine centre of effective edge and then mount point. Usually I position the sidecut on the ski looking at the rocker profile and try and make the effective edge longer than the actual running length when flat. This gives you easy turn initiation but still the feel of carving a longer ski.

At the end of the day on hard snow you are only skiing on your running length and true effective edge. But in powder the dynamic changes.
If the tip is markedly longer than the tail and you position everything based on hard snow performance it will ski well on hard snow, but the dynamic in soft snow may leave you feeling like you are way too far back.
If your design only has a slightly longer tip than tail (say within 5cm of each other) then you prob won't feel it.
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MontuckyMadman



Joined: 20 Jun 2008
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PostPosted: Thu Jul 20, 2017 7:32 pm    Post subject: Reply with quote

Wouldn't it be more advised to make the turning radius in the tip to be smaller than the tail so the ski engages easily initially and releases easily instead of being locked into a tighter radius at the end of the turn?
Has nothing to do with your question but the camber radius and apex play a role here as well
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Cadman



Joined: 06 Apr 2010
Posts: 48
Location: Crystal Mountain, Washington

PostPosted: Fri Jul 21, 2017 10:32 am    Post subject: Reply with quote

MontuckyMadman wrote:
Wouldn't it be more advised to make the turning radius in the tip to be smaller than the tail so the ski engages easily initially and releases easily instead of being locked into a tighter radius at the end of the turn?
Has nothing to do with your question but the camber radius and apex play a role here as well


For what it is worth,I did that on several skis a number of years ago and it didn't seem to work that great. Everyone that skied the skis thought it was not as smooth as a full radius placed in the right place.
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barnboy



Joined: 20 Oct 2010
Posts: 46
Location: Adirondacks (Westport, NY)

PostPosted: Wed Jul 26, 2017 11:53 am    Post subject: Reply with quote

There are a few things in your drawing that have me scratching my head just a bit (I like the look of the shape by the way, looks really nice). You have notations demarking; Running Surface, Length of Sidecut, and True Effective Edge. I'm imagining the Running Surface measurement is referencing your designed points of contact based on the forms the ski will be pressed with. Where I'm a bit cloudy is with the Length of Sidecut versus True Effective Edge measurements. It appears the arc of the turn does not meet it "end points" until the True Effective Edge marks... why is this not also the Length of Sidecut?

As far as mid running surface/mid point numbers... These numbers can be tortured in myriad ways, but I've found the easiest way to tackle Boot Center (herein "BC") location is as a percentage of the entire length of the ski, measured from the shape's tip. For years the industry average was 56%, recently with the advent of the "5D" shape style that number has crept towards 54%. Not everyone applies the shorter BC standards to the 5D shape. The K2 Pinnacle is a great example of a modern shaped ski that holds to an older design paradigm when it comes to BC. The ski is tons of fun by the way.

It does help to meter out your effective edge/running length and understand where your BC mark is in relation to all of these numbers, but at some point you (or someone) have to slap clamps on these things and see what's up, which is when you'll be able to hash out what role camber and turn apex, as MM mentioned above, are playing as well. I've found, as long as we're dealing with a small/natural camber pattern (say 7mm or less at apex) a 56% BC set-up lends itself more towards edging and front end speed control, and a 54% BC design lends itself more towards "BOOT SKIING" - which is still a blast, cuz it's skiing.

Some folks, myself included, keep the narrow point of the ski's turn and BC married, others allow these two design points to drift apart from one another... Most of your larger production houses do tend to keep the BC mark on the ski and the narrowest point of the ski coupled. This would place the ball of the foot of the skier just forward of the narrowest point of the ski.

As far as your blending theory goes (longer curve blends in the tip versus the tail), I'd say that's almost a must for most modern shapes. As you've pointed out, the portion of the ski's running length rear of BC is shorter than that forward of BC, and the ski's tail is also shorter than the tip so it's only "natural" that the blend should follow some sort of matching ratio... not sure which CAD package you're using, but I'm assuming you can use a parametric design scheme. You could start by echoing the parametrically controlled design scheme of the tip in the tail, then use your eye to tell you whether it's right for the shape/skier or not.

This is a lot of hot air, and I apologize if I've gone too far afield from your topic... Anyhow, the shape looks nice, I'd go with a 55% BC/narrow point and have at it.
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Cadman



Joined: 06 Apr 2010
Posts: 48
Location: Crystal Mountain, Washington

PostPosted: Thu Jul 27, 2017 8:56 pm    Post subject: Reply with quote

barnboy wrote:
Where I'm a bit cloudy is with the Length of Sidecut versus True Effective Edge measurements. It appears the arc of the turn does not meet it "end points" until the True Effective Edge marks... why is this not also the Length of Sidecut?


Thanks for all the feedback. As far as the sidecut length versus the effective edge, I drew in a sidecut radius and then added a blend curve in at each end of the radius so I noted the end points where the side cut ended and the blend curves started. The blend curves end up tangent to the curves that start into the tip and tail shape at the forward and aft effective edge points. (widest points at the tip and tail). That was pretty much all I was trying to convey. Technically, the combination of the sidecut radius and the blend curves up to the effective edge is probably the length of the sidecut.
If you put a point in at the exact center point between the end of the tip and the end of the tail ( say 900mm on an 1800mm long ski) I am guessing this is what they would call "True Center" of the ski. What significance this has on a direction ski is my question. Normally as you stated, 55% along the effective edge is a good "classic" point for the waist. Normally, the center of the boot is mounted on or about a cm ahead of the waist. In a perfect world, the ball of the foot would be on the mid point of the running length (1/2 way between forward and rear contact point). So depending on how much difference there is in tip length and tail length, the mid running surface moves away from the true center at different dimensions. I am not sure whether this change in distance has any true effect on how the ski turns. It may somewhat effect the balance point of the ski but I think that is about it. I would think that it has more to do with the relationship of waist to MRS than anything else since that is the area that you are riding on when initiating a turn on the pack. The tip and tail would appear to be more about getting the ski to float up off piste. On a true twin tip ski,where the tip and tail are the same, the center of the ski and the MRS would be in the same location.
So I see a number of skis that are designed with the waist pretty far back of the mid running surface and I am not convinced that this is a good design for a ski on the groomed runs. Maybe it works better out in the powder but no one seems to be able to clarify why it is done that way.,
I measured the foot from size 5 -13-1/2(US). I used a brannock device like you find in a shoe store and took the total length of the foot and divided it in half and then measured the distance that it placed the little sliding mechanism that fits on the ball of your foot. I was suprised that this distance did not vary much over 1.12cm over the range of sizes 5-13-1/2 Therefore as you mentioned, I think the distance from waist to MRS should not vary that much. The average size 10 boot measured 62.4mm from the center of the boot to the ball of the foot. If anything, I would think that the distance would only change by a few MM. I don't know if you agree but moving a binding 1cm makes a lot of difference in the way a ski initiates along with the quickness of the turns.

Finally, what is a "5D" shape? Thanks for joining in!

By the way, I made up a spread sheet with my findings of the distance to the ball of the foot from the center of the foot. If anyone is interested, please PM me with an email address and I will send it to you.
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barnboy



Joined: 20 Oct 2010
Posts: 46
Location: Adirondacks (Westport, NY)

PostPosted: Fri Jul 28, 2017 10:07 am    Post subject: Reply with quote

Hey Cadman... love the obsessiveness!

5D is one of the ridiculous industry names for modern shapes. Like an S7, classic DPS, etc... shapes with shorter running lengths, and longer tips and/or tails.

I know that back in the early-mid stages of the telemark movement there was a lot of time spent playing around with things like "cord center" (sometimes chord center) and balance points, etc... and how they should relate to where the binding is placed on the ski (this was especially true when the skier intended on doing a fair amount of uphill traveling on the skis). To this day the most (if any) direction I receive when folks drop off their gear for mounting is from tele-heads. Yes, I do absolutely agree that a poorly mounted ski can alter and/or challenge the riders ability to have fun, and also that slight variations in the location of the mount will indeed affect the performance.

On a slightly related side note, you've probably noticed that Asym builds are catching on again (K2 / Praxis). The Scotty Bob sight has a lot to say about all of this stuff, and why they believed asym design was the answer... you can peep it here: http://www.scottybob.com/skiworks/design/bobtail-design

Anyhow... remember that Boot Center is not actually the center of the boot, but just a mark delineating where the jig/boot should be placed when preparing to drill holes in the ski for mounting the binding. I think the ball of the foot being just ahead of a 55% located waist is perfect, but I firmly believe the best place to control your skis/have a great time is out near the nose... by generally adhering to a 55%-ish design scheme (building the turn of the ski to find its apex at that 55% mark) I think it allows for the right amount of real estate in front of the skier to impose their will via the ski, and not just ski their boots. I've always felt like when the action starts taking place under neath your boot you're kinda "behind", but it's definitely a personal choice, and fun can definitely be had in many many ways.

In response to your design theories, I agree with you that finding the right home for the apex of your turn (i.e. narrowest point of the ski), will most heavily influence the ski's performance. I think the reason you are seeing skis with "deep" waists has to do with their use of "new" design elements. For example, in the design you posted the ski's tip is close to 60% longer than the tail. On many modern designs, the difference between the two is even greater. As the location or zone of the running length becomes less symmetrically located within the ski's shape it's only natural that things will start to appear a bit "out of place". I guess I'm suggesting that the ski designs you're seeing with "waist pretty far back of the mid running surface" may be mild optical illusions due in-part to an overall "unbalanced" design scheme?

I'm super psyched to read about your attention to the detail of skier position and how it relates to your design of the ski's turn (inspiring!). I know a lot of small houses (and I'm sure larger ones too) will slap demo binders on "new" designs and ski them in multiple positions under multiple pilots and seek a consensus opinion on where the best mount point is for the shape. When designing and building one-offs this is not always an option, so having a trusted design standard is huge. Even if it just means you believe what you're doing is the right thing for the ski/skier (rather than actually KNOWING it), it goes a long way towards helping you sleep at night. So keep digging. I agree with you though, that getting the turn designed properly within the ski's silhouette will go the farthest towards ensuring a good time.

Looking forward to hearing the results!
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Cadman



Joined: 06 Apr 2010
Posts: 48
Location: Crystal Mountain, Washington

PostPosted: Tue Sep 26, 2017 8:53 am    Post subject: Sort of the same thought here but now on a Rocker ski Reply with quote

So I am working on a rocker ski right now with about a 30m rocker. It would appear to me that the true center of a rocker ski and the waist now have a pretty important relationship. I am thinking that where the tip and tail begin to turn up would be pretty close to level and the waist should be behind the true center of the ski by the same amount that you would have on a standard directional cambered ski. The waist would be on the center of the rocker.

Any comments on this design or am I out to lunch? Cool
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