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The Myth of ski bend - Page 7

post #181 of 205

IT'S ALIVE!!!!!!!

post #182 of 205
Thread Starter 

Someone mentioned that the underfoot of race skis was relatively soft. So...

The sawcut near the boot sole center allowed the skis to decamber enough so the edge underfoot could touch the snow. The ski is still exceptionally stiff - but it now bends a tiny bit right in the middle.

 

Skiing the early morning hardpack was kind of fun. While not the pure race skis that the racers on Red Dog were using, it was better than the previous iteration of this ski. It held an edge reasonably well. I could do my crappy railroad tracks. Icy bumps were skiable (still uncomfortable but icy bumps are always uncomfortable). Actually fun on the firm groomers. Quite different from two days ago. And much better. 

 

Inconclusive whether the ski is bending more or just getting better edge pressure. This whole carving thing is quite complex.

 

Eric

post #183 of 205
So I was going to try and explain to you why rollerblades and skates turn but the explanation turned ou way more complicated on paper than in my mind. So I'm going to try and go with a simpler explanation which is the case of two wheels connected by axle. When one wheel goes faster than the other you create a turn. This is quite obvious when push one wheel and it spins in place. This is similair to how your legs connect the skates/roller blades.

Now we have to consider the influence of Newton's laws specifically his first which states an object in motion stays in motion. As you exert more force onto one leg, or rather to the outside of the turn, you get an opposite force pushing inwards which begins to cause the skate to change direction. As one skate is accelerating, that is changing directions, and the other is not you now have different speeds causing a turn.

This is vastly simplified and doesn't account for things like the fact that the round cross sections of the wheel also cause the skate to turn, or flex in the blade of the skate or the case of a single skate but it gives you an idea.

Also Gs have nothing to do with the number of feet you are standing on. It has to do with acceleration which on earth is defined as 9.8 m/s^2 when you enter a situation in which you excced this acceleration you are experienceing more than one G

For back ground I have a degree in Astronautical engineering with a specialty in orbital mechanics

Edit: I forgot to mention you are probably getting more edge pressure becasue the ski is bending more and if in fact your ski didn't bend before you are now engaging more of the edge.
post #184 of 205
Thread Starter 

@lonewolf210 Your explanation made lots of sense. Especially relating to the first iteration ski mod which felt like the brakes were on. A little differential braking from edge pressure might make my railroad tracks feel pretty good. That might be a better explanation than pure ski bend.

 

Then I thought about rollerblades. I can carve turns both ways on one foot. Certainly no differential braking or bending going on there. My edge pressure theory doesn't work very well either. Just rotational torque.

 

The sawcut allowed the skis to bend. Sort of. They would decamber then hit a wall as the gap created by the thin sawcut closed up. Not bending to the shape of any turn, just flexing enough to get some edge underfoot in contact with the snow. The ski performance was much better - especially on the ice. The difference was more associated with a better camber than more flexibility.

 

So many factors go into a carving turn that isolating one factor (ski bend?) is a silly design parameter. "Bend your skis" is a great coaching tip but great coaching tips do not correlate well to physical ski characteristics.

 

Eric

post #185 of 205
Well on roller blades one footed turns are greatly aided by the shape of the wheels but to be honest that's starting to go beyond my off hand knowledge of physics. I'm sure if you were to do some research you could find an answer but then again maybe not there is still no accepted theory as to why a bicycle balances upright... If you really want a challenge you should try tackling that. There was a theory posed a year or two ago that I believe was fairly good but not sure what became of it.

Now back to skis. While the explanation I gave works fairly well for ice skates and roller baldes but your neglecting the length of skis. INcreasing the length greatly increase the size of the "carved turn" so while you are correct that it is technicaly possible to make a carve without ski bend your talking about turns in the 10's if not 100's of meters. The size of turns being achieved today on skis is just not possible without a parabolic edge and by extension ski bend as the edge would not contact without the bend of the ski.
post #186 of 205
Quote:
Originally Posted by lonewolf210 View Post

Well on roller blades one footed turns are greatly aided by the shape of the wheels but to be honest that's starting to go beyond my off hand knowledge of physics. I'm sure if you were to do some research you could find an answer but then again maybe not there is still no accepted theory as to why a bicycle balances upright... If you really want a challenge you should try tackling that. There was a theory posed a year or two ago that I believe was fairly good but not sure what became of it.

Now back to skis. While the explanation I gave works fairly well for ice skates and roller baldes but your neglecting the length of skis. INcreasing the length greatly increase the size of the "carved turn" so while you are correct that it is technicaly possible to make a carve without ski bend your talking about turns in the 10's if not 100's of meters. The size of turns being achieved today on skis is just not possible without a parabolic edge and by extension ski bend as the edge would not contact without the bend of the ski.

 I'm new to this discussion, but I don't understand your comment about why a bicycle stays upright? Either I am misreading it or am really missing something. The rotating mass of the wheels wants to resist a change in direction keeping it upright. Ever hold a bicycle wheel by the axle and spin it, then try and move it. 

 

Sorry if I am misreading.

post #187 of 205
Quote:
Originally Posted by ericz721 View Post
 

 I'm new to this discussion, but I don't understand your comment about why a bicycle stays upright? Either I am misreading it or am really missing something. The rotating mass of the wheels wants to resist a change in direction keeping it upright. Ever hold a bicycle wheel by the axle and spin it, then try and move it. 

 

Sorry if I am misreading.

 

i don't really want to get back into this discussion, but i just wanted to point out that the gyroscopic thing is NOT what makes bikes stay upright.

 

edit: which is what lonewolf was saying (didn't read that before). the gyroscope thing has been disproved...and no one's really sure what the right answer is.

post #188 of 205
Quote:
Originally Posted by treeski7 View Post
 

 

i don't really want to get back into this discussion, but i just wanted to point out that the gyroscopic thing is NOT what makes bikes stay upright.

 

edit: which is what lonewolf was saying (didn't read that before). the gyroscope thing has been disproved...and no one's really sure what the right answer is.

ok so I thought I was going to get my mind blown but here but I guess I didn't in my very quick research. The gyroscope was not disproved per say, more that it is not the only factor that keeps a bike upright. You can keep a bike upright with the gyroscope force cancelled meaning more forces are at work. But it is still a very real force that is very easily proved, like I mentioned holding a wheel.

 

It is really felt in dirtbiking. A larger bore or increased engine rpm adding stability or taking away agility. Or the change from riding a two stroke four stroke, with the added mass of spinning cams...

post #189 of 205

With a fresh perspective here I think a ski has to bend to carve. Isn't the whole definition of a curve is it is constantly changing direction(or slope), getting in to the whole definition of a derivative. I mean would you need an infinitely small contact point with the ski edge if it was at least not bending a little. 

 

Could a simple experiment be to take a piece of pencil lead and hold it long ways so it is all touching the paper and then try and draw an arc? I imagine one could not be drawn.

 

Maybe I need to go back and read 7 pages haha...

post #190 of 205
Quote:
Originally Posted by ericz721 View Post
 

ok so I thought I was going to get my mind blown but here but I guess I didn't in my very quick research. The gyroscope was not disproved per say, more that it is not the only factor that keeps a bike upright. You can keep a bike upright with the gyroscope force cancelled meaning more forces are at work. But it is still a very real force that is very easily proved, like I mentioned holding a wheel.

 

It is really felt in dirtbiking. A larger bore or increased engine rpm adding stability or taking away agility. Or the change from riding a two stroke four stroke, with the added mass of spinning cams...

 

ok, poor/hasty choice of words on my part. it exists, obviously...i just meant it's not necessary (as shown by the counterbalancing).

 

and as for the carving...many similar attempts to use logic have already gone nowhere, so don't get your hopes up.

post #191 of 205
Quote:
Originally Posted by treeski7 View Post
 
Quote:
Originally Posted by ericz721 View Post
 

 I'm new to this discussion, but I don't understand your comment about why a bicycle stays upright? Either I am misreading it or am really missing something. The rotating mass of the wheels wants to resist a change in direction keeping it upright. Ever hold a bicycle wheel by the axle and spin it, then try and move it. 

 

Sorry if I am misreading.

 

i don't really want to get back into this discussion, but i just wanted to point out that the gyroscopic thing is NOT what makes bikes stay upright.

 

edit: which is what lonewolf was saying (didn't read that before). the gyroscope thing has been disproved...and no one's really sure what the right answer is.


OK then, please explain why a toy motorcycle that cannot turn will stay up upright as long as the wheels are going around, but will topple over when it stops.

post #192 of 205
Quote:
Originally Posted by Ghost View Post
 


OK then, please explain why a toy motorcycle that cannot turn will stay up upright as long as the wheels are going around, but will topple over when it stops.

 

i don't know, and neither does anyone else as far as i can tell...it's apparently a combination of things, but it definitely isn't just that particular thing.

 

check this out. http://www.popularmechanics.com/outdoors/sports/physics/physics-of-a-riderless-bike

 

didn't mean to derail this thread (if it was even on the rails before).

post #193 of 205
Well that opened a can of worms. I have some of the links saved on my computer at home. I'll post them when I get home but at least it's a topic we don't have an answer for...

Also just for transparency the explanation I posted about skates could be wrong that's just my understanding. That being said the shape of the wheels has way more impact
post #194 of 205
Quote:
Originally Posted by treeski7 View Post
 

 

ok, poor/hasty choice of words on my part. it exists, obviously...i just meant it's not necessary (as shown by the counterbalancing).

 

and as for the carving...many similar attempts to use logic have already gone nowhere, so don't get your hopes up.

I'm on the same page as you, and apparently most people tree...and 

 

I didn't mean to derail this thread either. I learned a little something at least.  

post #195 of 205
Thread Starter 

I demoed some Bent Chettlers. The turn initiation felt exactly like the first day on the modified stiff skis. The Bent Chettlers were not super stiff nor did they make easy railroad tracks. I was able to make them work but the turn initiation took some extra work. Surprising that such different skis shared the turn initiation quirk. Skiing is a very complex activity.

 

@lonewolf210 Railroad tracks have radii in the tens of meter range - within your unbent ski range. Yet bending is the only factor? Or even the major one?

 

"Bend the ski to make it carve" is an excellent visualization tool and a great coaching point. It will help a skier carve (and an instructor get to the next level).

 

But carving is far more complex than that. Figuring out more of the factors that make a skier carve may lead to more versatile skis and improved coaching techniques. I certainly have learned a lot about ski design as it relates to carving from my experiments. Plus it's been fun.

 

Eric

post #196 of 205
Figuring out what makes a ski carve isn't hard. If you over think it then its you, not the ski, that makes it hard.
post #197 of 205
Quote:
Originally Posted by eleeski View Post

Skiing is a very complex activity.

Or...

Skiing to maximize racing results when one is not racing is a very complex activity.
post #198 of 205
Quote:
Originally Posted by clink83 View Post

Figuring out what makes a ski carve isn't hard. If you over think it then its you, not the ski, that makes it hard.

 

Well said!  Now understanding what makes some shaped skis NOT carve, that is deserving of a more complex discussion.

post #199 of 205
Quote:
Originally Posted by SandwichTech View Post
Originally Posted by clink83 View Post
Figuring out what makes a ski carve isn't hard. If you over think it then its you, not the ski, that makes it hard.

Well said!  Now understanding what makes some shaped skis NOT carve, that is deserving of a more complex discussion.

 

Lack of torsional firmness.  Tip and tail twist away from the snow, not gripping.  Ski may bend some, skier may be balanced appropriately above, but it won't track if the tip is not up on edge when the middle of the ski is edged.

post #200 of 205
Thread Starter 
Quote:
Originally Posted by clink83 View Post

Figuring out what makes a ski carve isn't hard. If you over think it then its you, not the ski, that makes it hard.

I can't disagree more. Some skis carve for me, some don't. I am unable to find a single trait that makes a ski carve well. It is a complex balance of many factors. 

 

I'm not sure any of the skis I ride twist much torsionally. Most manufacturers are doing a pretty good job of building torsional rigidity nowadays.

 

The only sure thing to make a ski carve badly is a dull edge. But a sharp edge doesn't help some skis much. 

 

Oh, having a bendy ski is the least correlated to a good carver and maybe correlated to a poor carver. 

 

Eric

post #201 of 205

I can make just about any ski carve, but some will carve with more g force than others, and some or more tolerant of abrupt loading than others, and some (cough) early rise (cough) require more abrupt movement.

post #202 of 205

Ghost, have you ever tried to carve with a limp noodle of a twin tip park ski?  

post #203 of 205

Lower the speed?

The older Head SS Magnum was extremely soft in 170. I had two versions. Carved pretty well but not necessarily in all conditions.

However that had a lot of sidecut.

post #204 of 205
Quote:
Originally Posted by LiquidFeet View Post
 

 

Lack of torsional firmness.  Tip and tail twist away from the snow, not gripping.  Ski may bend some, skier may be balanced appropriately above, but it won't track if the tip is not up on edge when the middle of the ski is edged.

 

@LiquidFeet, I just wrote a blog article (w/ a torsion testing vid) on this exact same effect!

 

Quote:

Originally Posted by eleeski View Post
 

I'm not sure any of the skis I ride twist much torsionally. Most manufacturers are doing a pretty good job of building torsional rigidity nowadays.

 

 

 

Oh, having a bendy ski is the least correlated to a good carver and maybe correlated to a poor carver. 

 

 

I think the effect you are noticing is a soft flexing ski that is also torsionally soft (common combo).  The poor edge hold is almost certainly due to the compliant torsional flex, not the longitudinal - if either.  Also worth noting: there is a significant difference in torsional stiffness between skis.

post #205 of 205
Thread Starter 

@SandwichTech Cool video with good information! I wonder how skis that are crappy edgers fare in that test. That seemed like a LOT of weight to get the skis twisting - more than a skier could normally load. Also, how does ski width affect the test? Your testing apparatus looks like it would be sensitive to ski width in a contrary way. A wide ski would need to be stiffer in real life as there is a greater lever arm on the edge to twist. But your bar gets supported by the longer lever of the wide ski. Still, maybe your test reflects real life which is why some wide skis can carve well.

 

My Race Tiger SL skis are the best hard snow carvers in my quiver (except for certain versions of the test skis). Their performance did deteriorate over time (too many moguls mashed) but what appears to have changed is the camber not the flex. Adding a strip of carbon unitdirectional in the middle of the ski to restore the camber helped the performance. This did add a bit of stiffness to the ski but did very little for torsional stiffness.

 

Torsional stiffness appears to be just one factor in edge performance. Certainly a useful one. This begs the question, what is the optimal torsional flex pattern? Perhaps a very soft tip and tail after the rockered area with strong torsional rigidity in the traditionally cambered area? Really, there are a lot of design considerations.

 

Sandwich Tech skis do look intriguing!

 

Eric

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