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True bar test easily detects 0.0005 inch concavity

post #1 of 5
Thread Starter 

So, when you see light sneaking through between your true bar and the ski base, how much of a gap is really there?  1/1000 of an inch?  More?  Less?

 

Pondering the best use of a boring moment during a rainy day, I thought I'd do a little experiment and find out.  (Ok, so I was *really* bored, right? redface.gif )

 

Here's what it looked like when I put the true bar up against the ski.  As you can see, the base is concave.  (The ski was actually base-up, it's the camera that was upside down.)

 

DSCF5376.JPG

 

So I got out a machinist's dial indicator to measure the gap with.  (I couldn't find any feeler gauges, which would have been simpler....but, hey, I was bored, right?, so I wanted to make this as complicated as possible smile.gif )

 

I put the base of the dial indicator on a precision parallel bar (the twin of what I use as a true bar) laid across the base, and as a calibration, checked to make sure another parallel bar a few inches away did indeed read flat.  As the three pictures below indicate, it did, since the needle stayed put as the indicator was moved from place to place.

 

DSCF5367.JPG

 

DSCF5368.JPG

 

DSCF5369.JPG

 

Now I repeated the process with the indicator measuring the ski base itself.

 

DSCF5372.JPG

 

DSCF5373.JPG

 

DSCF5374.JPG

 

DSCF5375.JPG

 

Aha, it moved a bit in the middle two positions.  The maximum deviation is one graduation on the dial, which is 0.0005 or half of 1/1000 of an inch!

 

Wow, so looking for light under the true bar is a pretty sensitive test.  And considering the amount of light that comes through in the first picture, it would probably be possible to detect quite a bit less concavity than I had here.

 

Now the next question is, how small a concavity can a skiier feel?  Unfortunately I can't answer that one because I don't have a flat and non-flat version of the same pair of skis to try it (and even if I did, I'm probably not a good enough skiier to feel the difference).

 

Not sure how practical all this is (if at all) but at least I did manage to amuse myself for an hour or so.  Sure beats cleaning up the mess I made last time I scraped my skis! eek.gif

 

I have an idea for a future project....people go through a lot of trouble to get their bases flat.  But it's normally done at room temperature.  And then you ski on cold snow.  I wonder if the shape of the base changes slightly with temperature due to expansion/contraction.  Maybe some day I'll find a flat spot on my ski and then take it outside for a few hours and measure it again to see if it changed.  If so, it might be optimal to grind bases some shape other than flat, so that they become flat in the cold....or refrigerate the stone grinder room!!!  See...there are ways to pass the time when you can't go skiing for some reason or other! biggrin.gif

post #2 of 5
Quote:
Originally Posted by renenkel View Post


I have an idea for a future project....people go through a lot of trouble to get their bases flat.  But it's normally done at room temperature.  And then you ski on cold snow.  I wonder if the shape of the base changes slightly with temperature due to expansion/contraction.  Maybe some day I'll find a flat spot on my ski and then take it outside for a few hours and measure it again to see if it changed.  If so, it might be optimal to grind bases some shape other than flat, so that they become flat in the cold....or refrigerate the stone grinder room!!!  See...there are ways to pass the time when you can't go skiing for some reason or other! biggrin.gif

 

 

 

IMO this is  smarter thinking than it appears, but I think the relevant issue might be going the other way - how much is the base raised relative to the edges with elevated temperatures  (such as during a hot scrape).

 

It would be pretty embarrassing to find that room-temp concavities like the one above can be created by perfectly-flat scrapers during a hot scrape, no?

 


 

 

post #3 of 5
Thread Starter 
Quote:
Originally Posted by cantunamunch View Post

IMO this is  smarter thinking than it appears, but I think the relevant issue might be going the other way - how much is the base raised relative to the edges with elevated temperatures  (such as during a hot scrape).

 

It would be pretty embarrassing to find that room-temp concavities like the one above can be created by perfectly-flat scrapers during a hot scrape, no?

 

That's a very interesting question, and it shouldn't be too hard to answer using a similar methodology to what I did above.  I'll keep it in mind.  Thanks for the idea.
 

 

post #4 of 5

Not sure the amount that you would notice, but I would say that if it is slightly base low, I might not notice, but base high skis are so damn squirrelly that I think I would notice right away. 

post #5 of 5

As long as the base is falt about 10-15MM towards the ski center from each edge, a skier will notice nothing. Center cnacavity is meaningless. If on the other hand the concavity extends from edge to edge,  the ski will act as though it is railed.

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