Design Considerations for underfoot stiffness

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petemorgan(pmoskico)
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Design Considerations for underfoot stiffness

Post by petemorgan(pmoskico) »

the basic formula for designing stiffness and flex is the thicker the core = a stiffer ski. But you also have to take into consideration the width of the ski and length of the ski. the formula is even more complicated when you start talking about materials!

lets simplify and just focus on stiffness underfoot. keep in mind that reducing weight is the other design constraint that i am striving for.


Binding considerations:

Should we increase the stiffness underfoot for a dynafit mount, but not worry as much for fritschis?

bindings like dyanfits add zero stiffness to the ski, versus bindings like fritschis which actually add a bit of stiffness underfoot, but since the heel does float a bit, it is probably minimal until you reach the end of the room allowed.

one concern of having too soft of a ski underfoot is that it can affect the performance and safety mechanism of the binding. ski bindings have room in the toe and heel (heel only for dynafits) so that the ski can flex and the boot wont be pinched inside the binding which would inhibit the DIN setting.


Add a layer of Fiberglass:

i have read that others have added a layer of fiberglass underfoot (maybe 2 feet in length). i am not sure how much stiffness this will add. one small concern is that you are increasing the distance the binding is away from the source of retention (which is the wood core). it would be fairly small distance but i imagine increasing distance can be exponential to the resulting torque created. what are people's thoughts on adding a layer of fiberglass underfoot?



Wood veneer talk:

One thing that i have noticed with experimenting with tons of different kinds of veneer topsheets is that the wood veneer has more of a impact on stiffness and weight of the ski than i originally thought. a hardwood veneer underfoot might actually add to the stiffness of that area. have people noticed veneer affecting stiffness?


cheers
skidesmond
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Post by skidesmond »

A binding that uses a plate or some form of plate will add stiffness for sure. The amount of stiffness depends on the size and material of the plate.

Adding a layer of FG will also add stiffness. The thickness of compressed FG is under 1mm. I've measured but forget what it was. I wouldn't worry too much about losing retention.

Veneer - I primarily use veneer. I never gave it much thought if veneer (usually 1/42inch or 1/32 in thickness) would add much stiffness. This has been discussed before and it will add stiffness. How much? Never compared it to anything else.
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vinman
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Post by vinman »

Yes veneer adds stiffness.

I use a biax binding mat 16-18 inches long. It does add some stiffness
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falls
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Post by falls »

I use a 45cm long piece of triax under the binding area.
It goes under the full length triax fibres.
I figure it helps with retention as to pull it away from the ski the whole length piece has to be pulled away also.
I think everything contributes to the retention not just the core.
The screw threads bite into the fibre/epoxy matrix as well as the core. I have also heard people say the topsheet aids screw retention too, but am less sure of this.

I'm not that much of a purist I don't think in terms of making the perfect ski for me to the degree of seeing what difference in flex certain bindings create. I like to just make the best of whatever I am riding on the day. There aren't any bad skis, just skiers bad at adapting to new gear? (yeah well that isn't quite true!). I think this is more of an issue for binding selection than ski selection. I know from experience skis feel different with different bindings as I have some pairs mounted for dukes and alpine bindings. I think for me it comes down to that I don't like what frame AT bindings do to a ski and I don't think changing the ski is going to greatly change it. I think really what I don't like about frame AT bindings is the increased stack height not the change in flex.

The default for me is for an alpine binding. I suppose if you know you are building a ski for a certain binding you might do something specific, but I think you are going to need to prototype and see how they feel. And then I'll come along or someone else and tell you they would like it a bit stiffer or softer.
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pmg
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Post by pmg »

Hi,

adding a layer of biax/triax under the binding area surely is the easiest way of specifically reinforcing the binding area. triax (or even uni) seems the more sensible option, as it stiffens the flex more, while biax does more for the torsion.

My skis are quite stiff anyway, so I don't add anything underfoot for dynafit bindings.
For any frame bindings, there should be no need to reinforce that area, as eighter the frame stiffens it or the binding is constructed to allow some forward/backward play as the ski is bent/unbent. Good alpine bindings all have that. If they dont, I doubt you can ride a ski so hard that the you reach the limits of the movement the heel of any DIN alpine binding has.
(Always given that the binding length is set correctly).
It should be the same for any frame AT bindings, as they are DIN certified as well.

The question it comes down to when talking about dynafit bindings is: How much has the ski to be bent in order to have the dynafit NOT working correctly any more? Unfortunately, I haven't found an answer to that question yet. But it could be tested with access to a binding testing machine:
The standard distance from the rear of the boot the the heel part of the binding is 5.5mm on Vertical/Radical bindings. So with a testing machine we could test:
- reduce the 5.5mm step by step, until the release forces start to increase a lot.
- then measure the distance (e.g. 3.2mm).
- take the ski with the mounted binding, set it correctly to 5.5mm distance, put the boot in, put an e.g. 3.2mm wide piece of something between the rear of the boot and the heel of the binding.
- Then bend the ski until the piece of something gets jammed. At this level of bending the binding will start to fail. Then you can jugde for yourself if that is a realistic scenario or not. If it seems realistic, then the binding section should definitely be stiffened if the ski is designed for a dynafit binding.


@falls:
I like your thought on the stand height with frame AT bindings, exactly the same here. That is why dynafits are my favourite, as they bring you very close to the ski (16/21mm). Fritschis are the worst, they add 39mm (older ones were worse afaik, they had about 50mm).

Can't wait for the Marker kingpin, unfortunately wasn't able to get one for this season.

Regards
Philipp
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falls
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Post by falls »

Tech bindings are the money, but they also cost money too! Lots of money :(
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jono
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Post by jono »

I have added triaxial glass, woven triaxial fiberglas and 1/16" maple skateboard veneer under foot. I also use aluminum window screen in the underfoot area, sometimes this is just under the bindings other times i use this for the entire underfoot area.

When I use fiberglass I taper it to a point and this avoids a rapid transition between thickness. and allows any extra stiffness to blend in smoothly. The extra glass layer bulges a little under the thin veneer top sheet.

Lately I have been using maple 1/16" skateboard veneers in a sort of leaf spring configuration in the underfoot area directly on top of the thin veneer top sheet. Usually I terminate these layers straight across the ski (to avoid delams). This adds quite a bit of stiffness. I don't put fiberglass under these layers but I could. Instead I use kevlar mat between these extra layers.

This approach allows me to use a regular but softer core that can then have a customized stiffness profile.

I got the maple skateboard veneers from
www.roarockit.com‎ (type it in the link is not working)
They have lots of colors.
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