or Connect
New Posts  All Forums:Forum Nav:

# There will be math involved...

Approximate width of edge material on my new skis = 2mm / .07874 in.  (I'm measuring across the base of the ski, btw)

Oh no! That fly by night ski tune guy just put a 1.5 degree base bevel on them and now I have to get them stone ground flat. How much material has to come off?

2mm x 1.5 degree (tan) = .05237mm / .00206 inches!

About half the thickness of a piece of cheap 20lb copy paper.

Discuss...

This assumes you want to restore a 0 rad base bevel. Do you?

If you are trying to get to 1 degree then you need to remove 1/3 of that, so 1/6 of a 20lb copy paper (whatever that is).

True, lets just remove enough base material to reshape the edge angle. Don't want to shorten the life of my skis!

To get to a 1 degree bevel:  (gonna need some more decimal points!)

2mm x 1.5 degree (tan) = .0523718mm / .002061811 inches

2mm x 1 degree (tan) = .0349101mm / .001374413 inches (subtract this number from the top number)

.0174617mm / .000687468 inches (or 17.416 microns) This dimension is approximately the same as a very fine human hair - blond.

However that difference could be corrected by leaving the ski alone and moving my knees over by about .1832 inches (3/16") That would put the metal edge back where it should be if I had a 1 degree bevel.

Quote:
Originally Posted by Fuller

True, lets just remove enough base material to reshape the edge angle. Don't want to shorten the life of my skis!

To get to a 1 degree bevel:  (gonna need some more decimal points!)

2mm x 1.5 degree (tan) = .0523718mm / .002061811 inches

2mm x 1 degree (tan) = .0349101mm / .001374413 inches (subtract this number from the top number)

.0174617mm / .000687468 inches (or 17.416 microns) This dimension is approximately the same as a very fine human hair - blond.

However that difference could be corrected by leaving the ski alone and moving my knees over by about .1832 inches (3/16") That would put the metal edge back where it should be if I had a 1 degree bevel.

Unfortunately, it doesn't work like that in the real world!

YOu're skis will feel loose as a goose!

Mathematics have no feelings... and what part of my math lesson is not part of the real world?

Yea, I'm sure the guy will be more careful the next time he throws your skis into the machine.

That's why so many of us do our own tuning. I haven't had a bad tune since doing it myself. Been 15 years now.

Clearly there is no appetite on this forum for a rigorous examination of the facts - especially if they include 10th grade geometry. Isn't anyone even curious as to why a pair of skis can be transformed from "Unskiable" to "Magic" by removing a layer of base material no thicker than a sheet of aluminum foil? I'm not discounting your experience guys, I'm just wondering what is really happening when a ski meets the mountain. If the real world is real, then someone should be able to prove it somehow.

Think of all the other variables that get thrown out of the discussion.

Are there any analog situations that are easier to measure and could inform the discussion?

Has anyone ever attempted to measure different setups empirically?

How about a double blind Ski tune challenge? Three identical pairs of skis tuned differently, three different riders, could they tell which one has the 1.5 degree base?

Does a ski edge really get hardened by grinding it? Is it really that hot? Has anyone ever measured it?

Can you harden an edge by skiing over a rock? If so, how is happening, heat? Impact?

So many statements make me go hmmmm? All these squishy words that get used to describe what's happening, but no data.

And of course - HUMANS! They're mostly unreliable witnesses.

Well, no geometry expert here, but assuming this paper thick angle change at the ski, what does that translate to as you travel up the leg to the hip? Certainly more than a paper thickness. How far do you need to move your center of mass to compensate? That's the biggie.

Personally, I want a CONSISTENT bevel first. After that, I want it around a 1, and I definitely don't want burrs mucking things up, but if it was a 1.1, I'd take that before a .9.
If what you are trying to say is a 1.5 degree angle (or base bevel) over an adjacent length of 2mm is so fractionally small that it couldn'possible be significant, then I agree. The only way to notice it would be to ski on an ice rink.

But what matters is the net angle from both base and side bevels. If this angle is accute, you will notice it over one that is oblique.

The difference between a dull knife and a sharp one is a million kms in terms of ability to cut, but probably only a few microns in reality.

Assuming your upper and lower leg are lined up and leaning into the turn then that .5 degree angle becomes 3/8" (36" away from the ski). Of course a lot more than just the 2mm edge is sitting in the snow.

Quote:
Originally Posted by joe strummer

If what you are trying to say is a 1.5 degree angle (or base bevel) over an adjacent length of 2mm is so fractionally small that it couldn'possible be significant, then I agree. The only way to notice it would be to ski on an ice rink.

But what matters is the net angle from both base and side bevels. If this angle is accute, you will notice it over one that is oblique.

The difference between a dull knife and a sharp one is a million kms in terms of ability to cut, but probably only a few microns in reality.

The knife analogy is a good one, I certainly can tell the difference in the kitchen when the knives are done right. The edge of a ski is the focal point for all the other forces at work. But a knife is literally the "thin end of the wedge" that becomes progressively thinner, sharper and more fragile. A 1/3 tune is a net 89 degree edge. 89 degrees vs 90 degrees is the difference? Something else is going on here. I'm guessing the sharpness and quality of the edge is much more of a factor than the angle (at 89 vs 90)

The difference is not that different from how a ski performs when you don't scrape quite enough of the wax off.  Fortunately the wax problem usually corrects itself quickly.

I want these guys tuning my skis!

Quote:
Originally Posted by Fuller

Clearly there is no appetite on this forum for a rigorous examination of the facts - especially if they include 10th grade geometry. Isn't anyone even curious as to why a pair of skis can be transformed from "Unskiable" to "Magic" by removing a layer of base material no thicker than a sheet of aluminum foil? I'm not discounting your experience guys, I'm just wondering what is really happening when a ski meets the mountain. If the real world is real, then someone should be able to prove it somehow.

Think of all the other variables that get thrown out of the discussion.

Are there any analog situations that are easier to measure and could inform the discussion?

Has anyone ever attempted to measure different setups empirically?

How about a double blind Ski tune challenge? Three identical pairs of skis tuned differently, three different riders, could they tell which one has the 1.5 degree base?

Does a ski edge really get hardened by grinding it? Is it really that hot? Has anyone ever measured it?

Can you harden an edge by skiing over a rock? If so, how is happening, heat? Impact?

So many statements make me go hmmmm? All these squishy words that get used to describe what's happening, but no data.

And of course - HUMANS! They're mostly unreliable witnesses.

NO one said they could tell a 1.5 from a 1. Or a 2 from a 1.5. That is not the point and really serves no purpose.

What I am sure I can tell you with a high degree of certainty is which ski has more or less base bevel comparatively.

Don't need no stinkin' data........I've tuned it and skied it!  'Nuff Said!

Have you ever tuned a ski yourself???

If not I understand your bewilderment!   If you have tuned, then I can't understand your bewilderment.

Or maybe your ski skills are such that none of this matters anyway???

I love the guys who have never tuned a skis and can't ski worth a sh_t, who get on here questioning what experienced skiers know to be true FROM EXPERIENCE!! Now I don't know you so I don't know if this applies to you

Next you are going to tell me that strings, string gauge, the string pattern, string tension, racquet weight and balance, and construction material of my tennis racquet all make no difference cause I can't prove it to you with an equation.

And by the way, I thinkyour calculations could be inaccurate. A 1 degree base edge bevel creates a 1MM gap 60MM across the ski when a truebar is matched to the edge angle.

So the difference between a .5 base edge bevel and a 1 degree is .5MM 60mm Across the ski!

You are calculating how much P-tex is removed to flatten the base of course depending on existing base discrepancies not to bevel the base edge!

Edited by Atomicman - 2/4/15 at 8:09am
Honestly just go get a ski tuned to 1.5 and ski it a while. Then go ski something that was tuned by somebody that knows what they are doing that has a real .5 edge.

If you can't feel the difference, I don't know what to tell you.
Quote:
I'm not discounting your experience guys, I'm just wondering what is really happening when a ski meets the mountain. If the real world is real, then someone should be able to prove it somehow.

Allow me to quote myself here to unruffle your feathers before I gore your sacred cow...

Atomicman, yes, "a 1 degree base edge bevel creates a 1MM gap 60MM across the ski when a truebar is matched to the edge angle." (The actual number is 1.0473mm). What I am talking about is the gap only across the 2mm metal edge. Now you're down to :

2mm x 1.5 degree (tan) = .0523718mm / .002061811 inches

2mm x 1 degree (tan) = .0349101mm / .001374413 inches

I have a brand new pair of EXP 88's that have way more variation in flatness across the entire base than that. They are slightly concave with a gap underneath the truebar. With all the slop in measuring and the extreme subjectivity when assessing the results it seems ridiculous to worry that my actual metal corner is 17 microns too high in the perfectly groomed snow.

The factory base bevel was right on 1degree and I've increased the side bevel to 3 degrees in deference to the Grand Poohbahs of Epic School of Tuning. So despite my obvious trolling I'm taking your advice and thoroughly enjoying my new skis and my new tuning kit. It is a pleasant way to occupy one's time with a beer close by.

So don't take any of this personally but if all these micro adjustments make a physically measurable difference, someone should be able to measure them. But they haven't.

Someone should be able to demonstrate statistically (the double blind challenge) that making one adjustment at a time to a pair of skis will elicit the same opinions from a group of experienced skiers. No one's done that either.

So now were left with a bunch of guys who all think they tune better than a 250k machine (me included) and a couple of six packs of opinions. Maybe I'll feel different about it after I get back from Montana. I'll have plenty of time to ski and tune. I might even buy a crappy pair of "subject" skis to experiment with using my own unreliable senses instead of "conventional wisdom".

Quote:
Originally Posted by Fuller

The factory base bevel was right on 1degree and I've increased the side bevel to 3 degrees in deference to the Grand Poohbahs of Epic School of Tuning.

I'm wondering why you bothered?

If your skis came 1/2, the difference between a 2 and a 3 is just a few microns.
Quote:
Originally Posted by sofort99

I'm wondering why you bothered?

If your skis came 1/2, the difference between a 2 and a 3 is just a few microns.

Exactly!

34.9 microns to be precise.

It was by going through the motions that I realized how little was required to change the angle. Then I did the math. Then I started thinking...

But you can't fight the Grand Poobah so I ended up at 1/3

Quote:
Originally Posted by Fuller

Allow me to quote myself here to unruffle your feathers before I gore your sacred cow...

Atomicman, yes, "a 1 degree base edge bevel creates a 1MM gap 60MM across the ski when a truebar is matched to the edge angle." (The actual number is 1.0473mm). What I am talking about is the gap only across the 2mm metal edge. Now you're down to :

2mm x 1.5 degree (tan) = .0523718mm / .002061811 inches

2mm x 1 degree (tan) = .0349101mm / .001374413 inches

I have a brand new pair of EXP 88's that have way more variation in flatness across the entire base than that. They are slightly concave with a gap underneath the truebar. With all the slop in measuring and the extreme subjectivity when assessing the results it seems ridiculous to worry that my actual metal corner is 17 microns too high in the perfectly groomed snow.

The factory base bevel was right on 1degree and I've increased the side bevel to 3 degrees in deference to the Grand Poohbahs of Epic School of Tuning. So despite my obvious trolling I'm taking your advice and thoroughly enjoying my new skis and my new tuning kit. It is a pleasant way to occupy one's time with a beer close by.

So don't take any of this personally but if all these micro adjustments make a physically measurable difference, someone should be able to measure them. But they haven't.

Someone should be able to demonstrate statistically (the double blind challenge) that making one adjustment at a time to a pair of skis will elicit the same opinions from a group of experienced skiers. No one's done that either.

So now were left with a bunch of guys who all think they tune better than a 250k machine (me included) and a couple of six packs of opinions. Maybe I'll feel different about it after I get back from Montana. I'll have plenty of time to ski and tune. I might even buy a crappy pair of "subject" skis to experiment with using my own unreliable senses instead of "conventional wisdom".

I LIKE IT!

But, for what it's worth, every person I've ever changed the 1/2 tune on their skis to a 1/3, noticed a significant difference with regard to how their skis turned, for the better, and so far, they all have liked it a lot better.  So, even though the "numbers" may show it as being insignificant to you, I'd be very surprised if you felt the same way after skiing both tunes back to back.

Quote:
Originally Posted by MoJo23

I got his point loud and clear - someone should do a double blind test.

I think it would be kinda fun to be part of something like that.

The other point of his numerical breakdown is to cause us to ask more questions - like for example how does the obvious more-than-microns mismatch in longitudinal curvature between a side bevel guide's baseplate and a cambered ski affect the applied tool angle - and is that comparable in scale to the results we're supposedly trying to achieve.

It would be absolutely hilarious if we were creating 34 micron precision with  only, say, 90 micron accuracy.

I LIKE THAT!

(credits to Atomicman)

Apparently sometimes hard to reconcile the experience with the numbers.

(But interesting to try - at least when there isn't a lot of snow).

This is so awesome!  How about .5 degrees change of base bevel and the relationship of that to the angle of the lower leg!

Quote:
Originally Posted by jdleuck

I LIKE THAT!

(credits to Atomicman)

Apparently sometimes hard to reconcile the experience with the numbers.

(But interesting to try - at least when there isn't a lot of snow).

I liked it, but still Don't need no stinkin' math or theories or double blind tests or worship to pagan symbols to know what the seemingly minor difference in bevel angles does to ski performance in the real world!

I'VE SKIED THEM!

Quote:
Originally Posted by Jacques

This is so awesome!  How about .5 degrees change of base bevel and the relationship of that to the angle of the lower leg!

Covered that Jacques:

Quote:
However that difference could be corrected by leaving the ski alone and moving my knees over by about .1832 inches (3/16") That would put the metal edge back where it should be if I had a 1 degree bevel.

See numbers are fun!

Quote:
Originally Posted by Fuller

Quote:
Originally Posted by Jacques

This is so awesome!  How about .5 degrees change of base bevel and the relationship of that to the angle of the lower leg!

Covered that Jacques:

Quote:
However that difference could be corrected by leaving the ski alone and moving my knees over by about .1832 inches (3/16") That would put the metal edge back where it should be if I had a 1 degree bevel.

See numbers are fun!

Awesome!  So a long bevel on a .75MM edge vs. a NOT long bevel on a 1.90MM base edge?

New Posts  All Forums:Forum Nav:
Return Home
Back to Forum: Tuning, Maintenance and Repairs