Ski Flex Tester
lgs...@yahoo.com
nordic skis. I would prefer to make one which measures skis
individually not in pairs. I've given up trying to get my local shop
Reliable Racing/Inside Edge to flex test for me. They always have to
"locate and set up the tester" which never happens. I will not buy skis
anymore unless they are properly flexed...been burned to many times.
Nathan Schultz
For about $50 or less you can get a force measuring clamp and some
feeler gauges. For about $5,000 you can get a machine that has a perfectly
flat table and will measure camber and pressure distribution along the
length of a single ski. Unfortunately, it is a simple problem that takes
expensive, custom machinery to get the job done properly. That is why it is
best to find a dealer that has invested in the equipment and spent the time
researching what makes a good ski.
-Nathan
www.NatronNordic.com
<lgs...@yahoo.com> wrote in message
news:1129575715....@g47g2000cwa.googlegroups.com...
Gene Goldenfeld
GG
lgs...@yahoo.com
Craig Storey
mentioned, for $200-300 you can get a pressure gauge, a clamp and set of
standard thickness gauges. (Heck for $20 you can get a clamp from
HomeDepot that is really all one needs.) These tools along with a lot
of knowledge and experience are all you need to select excellent skis.
A digital ski tester with hundreds of transducers and high measurement
accuracy can still be used to select terrible skis! The whole secret
here is that knowledge and experience.
I suggest you start by reading Zach Caldwell's website and his various
articles on ski flex and selection. There's also the Eagle River Nordic
stuff. After that buy the $20 clamp from HomeDepot, find a thickness
feeler and test your skis to figure out their problems. Then find a
buddies skis, that you think are good and test them. Then test as many
skis as you can find. Compare the differences and similarities, learn
which work in what conditions and why. It's pretty interesting stuff.
Learning to build a scientific insturment to do ski flex testing is
really interesting too. But unless you have the backgorund in both
Mech. Eng. and ski selection it would be a long road to building a
useful system.
Craig
Bjorn A. Payne Diaz
Craig Storey wrote:
> Learning to build a scientific insturment to do ski flex testing is
> really interesting too. But unless you have the backgorund in both
> Mech. Eng. and ski selection it would be a long road to building a
> useful system.
A couple points. One is, do you need a flex testing "bed" like Eagle
River Nordic to select a good pair of skis? (IMHO, no.) The second
point is, how hard is it to build one? On this point, I don't think it
would be too hard to build one, you just have to find the information.
I'd start with pressure tranducers that can feed into a computer, and
figure out how to do that. (3 v input?) Then it's building a flat "bed"
with the tranducers, figuring out how many transducers you want, and
setting up the clamping device. I suppose there's a software issue too,
so how do you record the data.
After you get the data on a pair of skis, then what does it mean?
That's the kicker. I find it interesting to look at the ER Nordic data,
but I kind of doubt that data easily translates into picking good skis.
I have this feeling (read I don't know) that it probably takes a few
years of collecting data and skiing the skis to correlate the data to
fast skis in certain conditions.
Jay Wenner
Bjorn A. Payne Diaz
Craig Storey wrote:
> Learning to build a scientific insturment to do ski flex testing is
> really interesting too. But unless you have the backgorund in both
> Mech. Eng. and ski selection it would be a long road to building a
> useful system.
A couple points. One is, do you need a flex testing "bed" like Eagle
Mark Waechter
Good test equipment should be built so that it doesn't introduce any
unnecessary errors to the measurement.
A flex tester needs a flat platform (an 8 foot long piece of 3 inch
square extruded box-section aluminum will cost about $70). It will
also need a means of applying pressure precisely and accurately at a
specific location. And third, you need a means of measuring the
camber height and length.
Measuring the camber height and length is easy enough with feeler
gauges and a good digital calipers (under $100 ).
The tricky part is the application of pressure precisely and
accurately. Some people use pneumatic/hydraulic systems with a
pressure gauge. This has built-in errors (offset error, percentage
error, ppm/degree error, non-linearity error, etc). To get a really
good load cell (pressure transducer, sensor) you will spend a LOT of
money.
The flex tester that i use has a mass loading system that uses weights
loaded onto a press to push an exact amount of weight at an exact
point. This system avoids most of the error sources that are inherent
in load cell measurement schemes. It's built using a modified arbor
press and good quality weights (olympic free-weights). The resolution
is 0.5 kg, and the accuracy is quite good (within a pound).
Repeatability is exact.
The downside to this system is that it's a little slow, since you have
to physically move the weights on and off of the rack. If you're
using this at home or in a small shop then it's not a problem.
The whole setup is as precise and accurate as any i've seen anywhere.
It cost a little under $500 to build. (I did my own welding and
cutting, but you could have it done at a machine shop relatively
cheaply). Oddly, the biggest single expense was buying high-quality
weights!
Yes, I've got an engineering degree. And background in test equipment
design (aerospace and medical), so that helps.
If you want specific info on building something like this, go to
www.ultratune.net and see info on flex testing - there's an email link
there if you have questions you'd like to ask.
--
Mark Waechter
Zach Caldwell
sounds as though he's got the cheapest and most practical solution out
there that can offer better results than the old c-clamp trick. I can
attest that, inded, my really good force sensor did cost a bundle. I
found it on e-bay and paid less than half the going rate, but it's
still an expensive way to go. I like having the ability to measure the
load applied right at the ski, so there is no concern about frictional
loss, etc. And the error specs that Mark outlined are well within the
margin of error of any flex test that we conduct. So my extra money
gains me flexibility and saves me time, especially when I'm plotting a
force-response curve. It would be a long day of moving weights to load
each ski with forty or so different weights. Bottom-line - if you're
looking to set up something for home use you'd be hard-pressed to do
better than what Mark has described.
The really critical thing is determining what you're actually trying to
measure. Flex can be measured and described in lots of different ways.
For our purposes we like to think of measuring two distinct (but
related) types of characteristics: the interaction of the ski with the
snow (pressure distribution, wheelbase, etc) and the interaction of the
ski with the skier (camber action, feedback, etc). There are many
different measurements that can be made in both of these areas so the
trick is trying to determine which measurements are significant, and
what they tell you about the skis.
We've got a really good opportunity to learn a lot of this through
reverse engineering of a sort. Every summer we bring in full fleets of
race-proven skis from a bunch of national level skiers. Kris Freeman,
for example, will bring us everything he has raced in the past season,
and we'll make notes on his experience with the skis. Then we take our
time and measure everything we can about the skis. In this way we can
start to figure out what really great skis have in common with
each-other, in addition to what types of characteristics Kris likes in
his skis. We do the same thing with about half a dozen athletes who
carry enough skis to actually have meaningful preferences, and we can,
over time, start to learn quite a lot about what works, what doesn't
and what significant measurements we can make on the skis.
Because of this approach our flex testing procedure has evolved quite a
lot over the past several years. We're getting to the point where we've
taken most of the measurements that we're capable of quantifying on the
set-up that we've got. Meanwhile, for the past two years we've had a
new flex tester in development. By the time this thing is done we'll
have quite a few thousands of dollars invested. Much of the investment
is in automation. It doesn't end up doing us much good to learn of a
really important measurement that is not practical to repeat for
customers because it's too time consuming. We end up evaluating many
hundreds of pairs of skis every year, so it's important to get good
measurements quickly under the circumstances.
At least one person mentioned the direct measurement of pressure
distribution using a series of transducers under the ski. Bert Kleerup
has really been the pioneer in this approach, at least on this
continent. When I first started working on my (current) flex tester I
set-up an elaborate system to measure pressure distribution using a
series of polyurethane tubes which ran under the ski at 5cm intervals.
We filled the tubes with extremely light viscosity silicon fluid and
measured displacement in capillary tubes (for vertical exageration of
the measurements. The system was cumbersome and a total pain in the
butt to callibrate. But it was enough to convince me that measuring
every five cm was not sufficient resolution to tell me anything really
interesting about the shape of the pressure distribution. With a lot of
the "hotter" skis being built today we're looking at extremely rapid
transitions from very light pressure to very high pressure over just a
few cm. The shape of this transition can be critical and we're going to
miss it measuring just every five or ten cm. I felt that I could tell
more using tactile feedback with feeler guages than with the
fluid-displacement system. But the whole issue of resolution has been
resolved in the next flex tester.
The other area that isn't considered often enough is the vibration
response of the ski - sympathetic frequencies and dampening
characteristics. We're working on that end of things as well. But I
expect it'll be a long time before we can make any "definitive" claims
about what is desireable and what isn't. The problem is that you don't
know what you need to measure until you've got the ability to measure
it and determine its significance...
Zach Caldwell
http://www.engineeredtuning.net/
Zach Caldwell
Vladimir
lgs...@yahoo.com писал(а):
Hi from Russia.
Bathroom scale and two pieces of wood can substitute a flex tester in
some ways. Wooden bricks should be wide enough to stand on them and
narrow enough to put the pressure on a specific area - about 10-20 cm.
Tie the skis at their ends, place one ski on one piece of wood, put
another wooden brick on the upper ski, put a piece of paper or a gauge
between the skis. Stand on the upper brick with one leg, and on the
bathroom scale with another leg, so that the piece of paper sticks
between the skis. Try to pull the paper, slowly transfering your weight
from the skis to the scale, looking at the numbers on it. Try to note
the weight at the moment when the paper goes free. Your weight minus
this figure is the stiffnes of your pair of skis.
In my experience this is a pretty precise method - as good as 1 kg, no
worse than the madshus flex tester i saw in the shop which had 2 kg
marks on it's scale. I used this method to choose my classic skis in
another shop.
You can also test skate skis which are stiffer than your weight - you
shall need a heavy backpack or another person who is heavier than you.
If bindings are installed, you shall need to carve dips in the wooden
bricks to accomodate the bindigs. If you want to test the skis
individually you shall need a level surface instead of the lower ski.
Hope that helps. Enjoy :)
Sorry if something is wrong with my English.
Vladimir
dardruba
Its a neat description.
Helgor Teupern
>
> lgs...@yahoo.com писал(а):
>
>> Does anyone have some plans or ideas to make a simple flex tester for
>> nordic skis. I would prefer to make one which measures skis
>> individually not in pairs. I've given up trying to get my local shop
>> Reliable Racing/Inside Edge to flex test for me. They always have to
>> "locate and set up the tester" which never happens. I will not buy skis
>> anymore unless they are properly flexed...been burned to many times.
>
> Hi from Russia.
>
> Bathroom scale and two pieces of wood can substitute a flex tester in
> some ways. Wooden bricks should be wide enough to stand on them and
> narrow enough to put the pressure on a specific area - about 10-20 cm.
> Tie the skis at their ends, place one ski on one piece of wood, put
> another wooden brick on the upper ski, put a piece of paper or a gauge
> between the skis. Stand on the upper brick with one leg, and on the
> bathroom scale with another leg, so that the piece of paper sticks
> between the skis. Try to pull the paper, slowly transfering your weight
> from the skis to the scale, looking at the numbers on it. Try to note
> the weight at the moment when the paper goes free. Your weight minus
> this figure is the stiffnes of your pair of skis.
I do it almost the same way. But using a c-clamp. Put the scale on the
table. Put both skis as described above on the scale and squeeze the
table, the scale, your skis and the wooden pieces with the
c-clamp together. You may now observe calmly the weight force applied and
measure with gauges the chamber at different forces. Don't forget to
substract the weight of the clamp, and the wooden pieces. Works.
> In my experience this is a pretty precise method - as good as 1 kg, no
> worse than the madshus flex tester i saw in the shop which had 2 kg
> marks on it's scale. I used this method to choose my classic skis in
> another shop.
>
> You can also test skate skis which are stiffer than your weight - you
> shall need a heavy backpack or another person who is heavier than you.
> If bindings are installed, you shall need to carve dips in the wooden
> bricks to accomodate the bindigs. If you want to test the skis
> individually you shall need a level surface instead of the lower ski.
No problem with a strong c-clamp.
> Hope that helps. Enjoy :)
Nothing replaces a shop where they know what they are doing. But often ...
Cheers
Helgor