This is email from Zach Caldwell at :
http://www.caldwellsport.com/ who explained to me, why some of the XC skate skis I used in the past, have base deformation and it makes them hard to scrape. After several scrape passes wax was still left out on places across the base surface because concaved base surface ( mostly close to the tip and back of the ski )
Zach has years of experience in cross country ski waxing, base grinding at the advanced and competition level. He goes through hundreds of skis every winter, waxing and grinding them and knows all the aspects of glide waxing.
Here is what he wrote:
In general, all skis start out quite flat when they come off the factory
finishing line. The factory equipment can’t make anything else. However, all
skis are composite objects, and are subject to some level of deformation over
time. Some of this can occur during a curing process between manufacturing and
when the ski is sold. In general, that should be minimal. But a brand-new,
never waxed, 2014 Fischer Carbonlite will for sure have a significant degree of
convexity in the tip and tail region (for example).
With
that as a starting point, it’s worth noting that about 95% of the deformation
we see in skis that come in for grinding is caused but the owner of the ski
during regular waxing. All of the propensity for deformation in every brand’s
construction is accelerated and exacerbated by heat.
Now
- a major point to keep in mind is that this is pretty much NEVER a problem
with the BASE of the ski. The Madshus have very heat-sensitive
core material and laminations. ALL of the serious deformation that we see is
caused by over-ironing the ski during waxing. Multiple iron passes put heat
into the core of the ski. The resins that hold the ski together are heat
activated, and when the material BELOW the base reaches temperatures in excess
of 100 degrees C, those resins soften, and the various composite parts of the
ski construction can start to move in relation to each-other. Because these
various parts have different levels of thermoplasticity, this almost always results
in deformation of some kind. And it’s almost always what we would call
“characteristic” deformation - it’s the same deformation that we frequently see
in a given brand or model.
The
waxing process is very widely misunderstood, and there is a huge amount of very
bad information out there about how it works. The way it ACTUALLY works is very
fast. Wax dissolves into solution in the amorphous material of the base as long
as the base reaches a critical temperature of just over 100C. We want to keep the
core of the ski well below 100C. So the temperature gradient through the base
is very important. If you have a 140 degree iron, working with CH6 (or
whatever), and a 20 degree (room temperature) base, then it’s easy to
understand that there is a steep temperature gradient, and it will take some
time for heat to penetrate from the iron, all the way through the base (which
is about 1mm thick). However, after that first pass, the heat dissipates in the
base, and now the whole base is much warmer, and the heat will penetrate
through much more quickly because there is less thermal resistance. It doesn’t
take too many passes with an iron - even when it’s set at a relatively low temp
- to get the core up to 100C. And that’s when you start causing the deformation
of the base.
And,
most important - it’s not necessary. You need the wax to dissolve into
the base near the surface, but you only ski on the surface. Really “soaking”
the base does NOT make it faster. It just ruins skis.
I
haven’t even mentioned actual base material damage, but this often occurs in
the same way. If you’re seeing black wax shavings, or your bases turn white
after a bit of skiing, then it’s due to one of two factors. 1 - high iron temp
for the wax - each wax can tolerate a certain amount of heat without
overheating. When the paraffin overheats it carries a lot of excess chemical
energy, and when it goes into solution in the base it will destabilize the
amorphous material. That is where the carbon-blacking live, is suspension. And
if you destabilize it, then you pull carbon-blacking out of the base when you
scrape wax, and the skis tend to slow down very quickly. 2 - “overcooking” -
long-term exposure to low heat will cause the base to lose its structure - like
overcooked pasta. It becomes “porridgy”. That frequently accompanies badly
warped skis, because it’s caused by the same type of waxing behavior.
So -
in short… no, ski manufacturers do not expect people to grind new skis. If you
do grind your new skis, it should be because you’re looking for optimal
performance from a better finish and structure pattern. Sometimes new skis that
have been on the shelf for a long time are “pre-deformed”. This will happen, to
a degree, depending on the construction. Generally, it’s not a deal breaker.
Flatness isn’t actually that important for the performance of the skis.
Final
note - when you’re seeing concavity in the base - low material in
the center, along the groove - that’s concerning. Pretty much the only way that
this occurs is when the ski has been subject to so much heat exposure that the
core has collapsed. That is very, very rare, and usually only occurs on skis
that are very old. It’s much, much more likely that you’re seeing wax remaining
in the center of the ski because you’re flexing the plastic scraper, or perhaps
using one of those Toko ceramic scraper sharpeners which, over time, but a nice
curve into your scraper. It would be a good idea to get your skis cleaned up,
and then examine them with a straight-edge or a true-bar. If you lay something
that you KNOW is straight across the base, and then look at it with a light
source behind the true-bar, you’ll be able to see whether the ski is actually
deformed.
Almost
all “normal” deformation causes the bases to go convex, not concave. The exception
was a couple of production series of Fischer skis from about 2005 when they
tried changing up some laminations to make their skis NOT go convex. It made
them terrible slow, so they stopped! But those skis had normal deformation that
actually made them concave. Beyond that, it’s pretty much only cores that have
been obliterated by heat that go concave.
Cheers,
Zach
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replacing this resistor completes the repair. 
