Modify/stiffen your own skis?

Has anyone ever tried to modify or stiffen their own skis?

I didn't do any head to head tests but it seemed to me that my new
touring skis (Fischer Country) were too soft in the tail and/or
torsionally too soft. They would tip and squirm with every change in a
soft or uneven trail. Also, when doing hard turns the tails would
often give out on me. I suppose it's all relative but I've had a hard
time adapting to the 2 kinds of softness that this ski has. I suppose
my old touring skis were stiffer---they had a double camber in
contrast to the single camber of this new ski.

Anyway: I was thinking of sanding the top of the skis for a few inches
in front then maybe 8" in back of the binding. Then I'd soak a thin
sheet of carbon fiber cloth in epoxy and lay it into the roughed-up
area and smooth it in and let it dry. This would add only a thin
"paint" layer of epoxy plus a very thin layer of carbon to the top
plate.

I wonder if such a mod would stiffen up a ski.

In a good way.

I really should do more baseline testing first. I should mount up the
old unbroken "great" touring ski that I have left and ski with both
kinds of ski on and see how they differ in a more objective way.

Still---if I wanted to stiffen a ski---both longitudinally and
torsionally---I wonder if my idea might work. Anyone try such a mod?

--JP

In article , jeff potter writes:
Has anyone ever tried to modify or stiffen their own skis?

No, but I have successfully repaired an RCS skate ski
which cracked on the top (I work with composites a lot).

Anyway: I was thinking of sanding the top of the skis for a few inches
in front then maybe 8" in back of the binding. Then I'd soak a thin
sheet of carbon fiber cloth in epoxy and lay it into the roughed-up
area and smooth it in and let it dry. This would add only a thin
"paint" layer of epoxy plus a very thin layer of carbon to the top
plate.
I wonder if such a mod would stiffen up a ski.

Yes, this will definitely stiffen the ski. But you want to use
the same material as what the ski uses. My RCS was all fiberglass
on top, and I'll be that's what your ski has. So you want to use
fiberglass rather than carbon. Also, I'd definitely use
_unidirectional_ fiberglass rather than cloth. The uni glass
is 2x stiffer and 3-4x stronger than the cloth.


BTW,
The reason you don't want to mix fiberglass and carbon is because
of the mismatch in maximum strain. When the ski is flexed,
the carbon add-on would take most of the load and fail prematurely.
In contrast, a fiberglass add-on will share the load well
with the existing fiberglass ski material.

One other thing... for max strength, it's important to
compact the wet fiberglass during curing. Cover it with
non-stick polyethylene, then hard foam, then pile on
a ton of weights -- at least 10 psi.

On Mar 29, 4:33 pm, (Mark Drela) wrote:
In article , jeff potter writes:

Has anyone ever tried to modify or stiffen their own skis?

No, but I have successfully repaired an RCS skate ski
which cracked on the top (I work with composites a lot).

Anyway: I was thinking of sanding the top of the skis for a few inches
in front then maybe 8" in back of the binding. Then I'd soak a thin
sheet of carbon fiber cloth in epoxy and lay it into the roughed-up
area and smooth it in and let it dry. This would add only a thin
"paint" layer of epoxy plus a very thin layer of carbon to the top
plate.
I wonder if such a mod would stiffen up a ski.

Yes, this will definitely stiffen the ski. But you want to use
the same material as what the ski uses. My RCS was all fiberglass
on top, and I'll be that's what your ski has. So you want to use
fiberglass rather than carbon. Also, I'd definitely use
_unidirectional_ fiberglass rather than cloth. The uni glass
is 2x stiffer and 3-4x stronger than the cloth.



most skis have wood or honeycomb structure inside, and it's the
honeyomb/wood what stiffens the skis the most. I doubt if a single
layer of kevlar/carbon cloth can stiffen the ski, unless it's thick
(and too heavy).

On Mar 30, 8:10*pm, "
wrote:
On Mar 29, 4:33 pm, (Mark Drela) wrote:





In article , jeff potter writes:

Has anyone ever tried to modify or stiffen their own skis?

No, but I have successfully repaired an RCS skate ski
which cracked on the top (I work with composites a lot).

Anyway: I was thinking of sanding the top of the skis for a few inches
in front then maybe 8" in back of the binding. Then I'd soak a thin
sheet of carbon fiber cloth in epoxy and lay it into the roughed-up
area and smooth it in and let it dry. This would add only a thin
"paint" layer of epoxy plus a very thin layer of carbon to the top
plate.
I wonder if such a mod would stiffen up a ski.

Yes, this will definitely stiffen the ski. *But you want to use
the same material as what the ski uses. *My RCS was all fiberglass
on top, and I'll be that's what your ski has. *So you want to use
fiberglass rather than carbon. *Also, I'd definitely use
_unidirectional_ fiberglass rather than cloth. *The uni glass
is 2x stiffer and 3-4x stronger than the cloth.

most skis have wood or honeycomb structure inside, and it's the
honeyomb/wood what stiffens the skis the most. I doubt if a single
layer of kevlar/carbon cloth can stiffen the ski, unless it's thick
(and too heavy).- Hide quoted text -

- Show quoted text -

The stiffening effect one would get with this method is called "Stress
Skin" a totally different approach to the original internal design of
the ski. Done right it will stiffen the ski. They make wall panels
for houses with a method like this that covers styrofoam with a glued
skin that results in incredibly stiff panels

The stiffening effect one would get with this method is called "Stress
Skin" a totally different approach to the original internal design of
the ski. Done right it will stiffen the ski. They make wall panels
for houses with a method like this that covers styrofoam with a glued
skin that results in incredibly stiff panels

Right, so the styrofoam is the same idea as the honeycomb.

Just curious - how thick is that styryfoam layer, and how does the
stiffness of those panels compare to the stiffness of a honeycomb
panel of similar thickness? I am thinking of making a backpack frame
of that sort, which I would have to glue together myself (since a
plain flat panel would not work).

On Mar 30, 10:52*pm, "
wrote:
The stiffening effect one would get with this method is called "Stress
Skin" *a totally different approach to the original internal design of
the ski. *Done right it will stiffen the ski. *They make wall panels
for houses with a method like this that covers styrofoam with a glued
skin that results in incredibly stiff panels

Right, so the styrofoam is the same idea as the honeycomb.

Just curious - how thick is that styryfoam layer, and how does the
stiffness of those panels compare to the stiffness of a honeycomb
panel of similar thickness? I am thinking of making a backpack frame
of that sort, which I would have to glue together myself (since a
plain flat panel would not work).

My imprsssion was that he glued skin was the magic that made the panel
stiff. There's no way you could make a structural wall for a house
out of styroform, but with the skin the wall panel was stiff enough to
replace a typical stud wall. This is done on Post and Beam
construction. The wall isn't weight bearing, just inflexible enough
to work as a house wall

In article , " writes:

most skis have wood or honeycomb structure inside, and it's the
honeyomb/wood what stiffens the skis the most. I doubt if a single
layer of kevlar/carbon cloth can stiffen the ski, unless it's thick
(and too heavy).

Huh?
The stiffness of a ski is determined _entirely_ by the composite
outer shell. The internal honeycomb adds no significant stiffness
by itself --- it's there only to hold the outer shell in place.
If you double the thickness of the outer shell with
the same material, you will double the stiffness.

In article , " writes:

Just curious - how thick is that styryfoam layer, and how does the
stiffness of those panels compare to the stiffness of a honeycomb
panel of similar thickness?

The thickness of the composite shell typically varies long the ski.
Halfway between the tip and binding, the top fiberglass on my busted RCS
ski was about 1mm thick. The rest of the ~15mm thickness was honeycomb.
The sidewalls are also thin -- 1.5mm plywood, covered by bias fiberglass cloth
for shear strength. So the ski is mostly air inside.

On Apr 1, 1:15*pm, (Mark Drela) wrote:
In article , " writes:

most skis have wood or honeycomb structure inside, and it's the
honeyomb/wood what stiffens the skis the most. I doubt if a single
layer of kevlar/carbon cloth can stiffen the ski, unless it's thick
(and too heavy).

Huh? *
The stiffness of a ski is determined _entirely_ by the composite
outer shell. *The internal honeycomb adds no significant stiffness
by itself --- it's there only to hold the outer shell in place. *
If you double the thickness of the outer shell with
the same material, you will double the stiffness.

Re the honeycomb. If you double the thickness of the honeycomb (or
other core material), you will increase the sections flexural
(bending) stiffness by a factor of 4, assuming that the sidewalls do
not contribute to the cross sectional thickness.

I would think if you wanted to soften a ski functionally, you could
broaden the flex range in the middle toward the tip and tail by some
careful scraping.

rm

(Mark Drela) wrote:

In article , " writes:

most skis have wood or honeycomb structure inside, and it's the
honeyomb/wood what stiffens the skis the most. I doubt if a single
layer of kevlar/carbon cloth can stiffen the ski, unless it's thick
(and too heavy).

Huh?
The stiffness of a ski is determined _entirely_ by the composite
outer shell. The internal honeycomb adds no significant stiffness
by itself --- it's there only to hold the outer shell in place.
If you double the thickness of the outer shell with
the same material, you will double the stiffness.