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Sidecut profile & long axis flex profile - interaction and performance


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@johnasmo
I was reading your just posted comments (Whitefish videos) about the complex curves used for the sidecut shape of the Contras you were testing at Whitefish at the end of the 20/21 season.

I can see that at board inclination angles below 45 degrees sidecut is probably the major factor affecting turn shape, but it would seem to me that the flex profile curve would transition to become the major determinant above 45 degrees.

This interaction and transition is something I've not seen addressed here before.

Could you comment on the interaction of flex & SCR generally, but also for the Contra prototypes you had designed and were testing?

Thanks in anticipation!

Edited by SunSurfer
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Yes, exactly.  They explain how they don't want the ski to twist enough to change the direction of carving, but they acknowledge too much TR is a problem for how the ski tracks.  If you don't allow so

I think I'll quote myself. If you've got a lot of time on your hands due to government lockdowns maybe you could suspend a board at the ends of it's effective length using rotating sleeves to r

I've thought about TR a lot.  I find the torsional rigidity in the nose to be a big factor in how harsh a ride feels.  The nose is the front suspension of the board and it can be tuned for the mix of

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Timely topic,

Gloria rode a friends board for a few days. It was a 160 Stubbet build with a soft flex.  She really liked it. Liked the short length and it was 19cm wide about 1cm narrower. SCR was probably a bit bigger than her 172 nirvana, but she found it easier to ride. Shes not getting the board really high on the edge so I’m also interested in SRC vs flex and flex pattern. How soft of flex can you go?

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Posted (edited)

We can precisely describe/draw the sidecut shape. Our ability to describe flex is very limited. "Soft", "firm", various numbers with one or more decimal places. To me this is part of the problem we have in describing and understanding the interplay between sidecut and flex. Another part is how reproducible a flex curve is i.e. can a builder make a number of boards with an identical flex profile, or how much variation should we expect given that most are wood cored, and wood is not a consistent material.

Edited by SunSurfer
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11 minutes ago, SunSurfer said:

i.e. can a builder make a number of boards with an identical flex profile, or how much variation should we expect given that most are wood cored, and wood is not a consistent material.

I just assumed the wood gives an approximation to flex and is then supplemented by more precisely measurable inlays, such as carbon. Am I on the wrong track here? 

Edited by daveo
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You guys are starting to make my brain ache. It's not so much the flex in general but more specifically where on the board the flex is occurring  as the rider transitions through each turn. Too many variables for me to worry about. Sunsurfer call Bruce and have a chat forecast is for rain tomorrow so he probably won't be on the links. 

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@digger jryou can always have fun on a soft board, one too stiff will be terrifying.  Soft flexing boards can be easily bent/flexed into much smaller turn size, or you can resist the flex and lengthen the turn size.  Say Hi! To Gloria! 

Here we go!  Down the Rabbit Hole! 

So many factors....and how do they change performance? I am still trying to get my head wrapped around how all of these variables interact.  How do you quantify these things to test hypotheses?  Because we do not have the luxury of building and testing boards that only have one change in design parameter to help make these determinations it leads to lots of speculation  and even more questions.

@johnasmo and I’s chair rides have become discussion sessions on board design.  Some of the factors we are discussing and how they relate to performance are:

How does the flex differ in the 3 segments of the board (tip, between bindings, and tail)? 

How far does it flex or range of motion before it gets stiff? 

What is the rate of flex? (compression/rebound, think car/,motorcycle suspension) 

Camber, reverse camber, decambered tip and tails?

Taper?

How different building materials interact to give desired flex, range of motion, rate of dampening/rebound, and torsional flex patterns?

How are all of these factors interacting with the sidecut or sidecut with other factors?

Our latest question is about torsional flex.  We were twisting boards and noticed different flex patterns.  Some had more twist at the tips and tails, some more between the bindings.

We both seem to be aiming for a board that has the widest range of conditions that are fun to ride, are “Ride all day” boards with centered body positions, and have great suspension (without a plate) to make them smooth on the not so great groomer days.  

 

 

 

 

 

 

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Posted (edited)

@dredman
You're primarily dealing with BV for the Contra, although I know you've ridden and are riding other makers boards (edited) so have a broad board experience. You've mentioned a range of different aspects of flex and that's going to help me start to understand this.

But, and it's a BIG but, what repeatable and reproducible methods are there for measuring these different aspects of flex? Unless we can measure it and determine just how the flex curve is shaped at various degrees of loading, and as you point out how it behaves dynamically, trying to understand what's happening as a result of changes in design and construction becomes a fuzzy gestalt of what "feels good".

Edited by SunSurfer
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1 hour ago, SunSurfer said:

@dredman trying to understand what's happening as a result of changes in design and construction becomes a fuzzy gestalt of what "feels good".

(SunSurfer) If you are looking for a specific feature your present boards don't have to justify to your wife why you need a new board just tell her everyone of your imaginary internet snowboard friends say you need it to keep a smile on your face !

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howdy sunsurfer

thank you for starting another thread and not filling the whitefish videos with comments that is not about whitefish videos.

thanks, west carven

 

I think rider weight and flex should be close. a soft board can't be pushed too hard and a stiff board needs to be pushed hard, both will work but not ideal. I also think effective edge and scr should be close and work together.

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I think that at heart there are two items on the table here. 

First is, with certainty, how do we identify and quantify a system performance measurement (or figure of merit or key performance indicator, depending on what your day job is) for the snowboard.  In very generalized terms:  [system performance measurements] = f(length, min_width, sidecut, rider_weight, snow_type, ...).  Given a "simplified" analytical model where the snowboard is quantified as a set of functional properties (side-cut, width, stiffness, camber, damping, etc), you can pick a working point and perform a sensitivity analysis on the different inputs to identify which parameters will have the greatest effect on improving the system performance.  I think a lot of the theory needed to model this is available, where the whole thing is underpinned by a model of how the snow reacts to the board (here discrete particle modelling looks promising).  At this point we want to generate a generally "optimal" description of side-cut, camber, stiffness distribution, damping, etc.

The second problem is how do we fabricate a board that displays those attributes?  Here comes FEM/FEA, manufacturing expertise etc.  And maybe the optimum theoretical board is not practical to manufacturer, then you need to refine and adapt your simplified analytical model to reject those designs and try again.

Human intuition works well sometimes, but when the problem definition has remained vague for so long and the best solution is elusive or "an art", it is worth looking into systematizing the problem and approaching it with analytical tools, maybe?

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If the same materials are used tip to tail, then board thickness is a very good proxy for relative stiffness. Look up "bending moment of inertia" if you want to get all mathy. The fun starts with varying materials, and of course the carbon & fiberglass laid on top/bottom of the outer titanal layers don't have to be uniform. Plus board width changes as you go along the length. 

Shifting the thicker part of the core fore-aft changes our perception of the board being stiffer in the nose vs. the tail. Pull out some calipers and plot your board thicknesses along their length for some end of season fun. Raise the thickness to a power of 3 on that graph for a better approximation of relative stiffness. 

The Coiler Nirvana boards shifted the core profile noticeably between models, with the Energy variants having a thicker core under the rear foot than front for more pop at the end of a turn. Same for the Donek Proteus models. Coiler Nirvana Balance is closer to the same thickness under both feet. 

I was lucky enough to see Donek's program for spec'ing board shapes. It calculates shape based on all of the above. Set rider weight, board width, model, sidecut parameters, select a nose/tail profile, and length, and it generated a code that gave the core width and thickness at every spot along the length. Ready for the CNC! Extremely cool! Of course that's completely proprietary and Donek's intellectual property so he won't be sharing it, but it sounds like John is generating similar. 

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the minutia involved to model flex variabilities caused by human body positioning and snow consistency make my brain lockup and reboot :freak3:

imma just let the art happen and enjoy the ride:1luvu:🏂

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I am not any kind of engineer, but have wild ideas about putting a bunch of sensors on a board with a connection to a data-logging device. Sensors would just give relative position. The data could then be processed to give a sort of movie of board flex as it's being ridden, available to analyze any which way. Add to it some accelerometers and you could get some idea of pressure distribution, and GPS data to get an estimate of path of center of mass. Then you'd have a data set you could maybe do a lot with in terms of board response.

I even looked at suppliers for sensors that might be suitable, but it's still at pipe dream stage for me. I wonder if ski manufacturers already do such things, or have blow past armchair stuff like that long ago...

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2 hours ago, teach said:

I wonder if ski manufacturers already do such things, or have blow past armchair stuff like that long ago...

Hard because every rider interacts differently with each board and environment.

I think that's why some riders feel more comfortable on Oxess vs Kessler vs SG. Not necessarily because one manufacturer is better, sometimes the signature geometry/flex of a specific brand of board just interacts more favourably with the rider's biomechanics/style.

It is often why one teammate is often better than the other in F1. Think 2005/2006 Renault. No one can drive with Alonso's aggressive early understeer style, but it worked for him and they built the car around it. They couldn't change the fundamental design of the car to suit 2 totally different driving styles.

I think this is similar for snowboard manufacturers. They each have their own standard geometry/flex and can only change a limited number of factors before the performance of the board starts to deteriorate or a total redesign is in order. I think there is a lot less flexibility in terms of design changes in the upper echelon of the WC scene where boards are a lot more finely tuned and the purpose is more specific than recreational carving.

Perhaps doing some sort of analysis with sensors and perhaps video recognition would help to further customise and fine tune a board for a rider whom the board already works exceptionally well for.

If there were any money in the sport, I'm sure it would already be explored. I did something similar for our aus swim team a few years back. Using video recognition and a bunch of neutral networks to help the Olympic athletes with stroke efficiency. But that was purely for biomechanics without any interaction with a snowboard and it was quite expensive. Didn't require sensors, though. 

In reflection, the relevance of my post is questionable. 

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Always nice to think about how a snowboard behaves:

On hard snow (no significant trench), turn radius is mostly determined by the sidecut and edge angle

On soft snow, turn radius /shape is mostly determined by board flex and pressure exerted by rider.

With stiff laminates like used in alpine boards, the core only contributes 15-30% of the stiffness. The stiffness is quadratic with the thickness (only the core contribution is 3rd power with the thickness)

You can quite easily build a board within 10% accurate of your design stiffness

 

A bit on camber: The stiffness multiplied by the deflection of the board distribute pressure towards tip and tail. Deflection consist of camber (first the board needs to be bent straight) and deflection in the turn. If you turn tighter (increase edge angle), the board deflects more so the pressure on tip and tail increases. But you also exert more pressure on the board (because of the tighter turn). So with a soft board, most of the extra pressure will end up under your feet (because the board is not stiff enough to get it to the tip and tail). WIth a very stiff board, most of the extra pressure will go to the tip and tail (because a lot of force is needed to increase the deflection).

A board with a lot of camber will need little stiffness to distribute pressure to the tip and tail. On the other hand, because of the low stiffness the pressure on tip and tail with not increase a lot when you increase edge angle.

A board with little camber needs more stiffness to distribute pressure. However, the pressure on tip and tail will increase a lot more when the edge angle (and thus board deflection) increases.

I did the math in the graph below for boards with different amounts of camber. Shown is the stiffness needed to distribute a given % of the total pressure to the tip and tail. Camber is given as a percentage of sidecut depth. E.g. if you take a board with a 100% camber and a stiffness of '7', at 45 degree edge angle you get a certain pressure distribution. If you carve a shallower turn, the stiffness is lower than would be needed to get the same relative distribution, so more pressure ends up underfoot. The same at higher edge angles, because of the lower stiffness it cannot get all pressure to the tip and tail, and you could overpower the board.

Now if you'd  go for a 33% camber board and you make it with a  stiffness of 11 to distribute the pressure at 45degree edge angle. However this stiffness would be too much at 70 degrees, and bring a lot of pressure to the tip and tail. So the board would have a narrow operating window, being soft at low angles, much to stiff at high angles.

The bottom section of each curve would be where you get a proper operating window. So a 100% camber board with ~7 stiffness would be nicely responsive from 30 to 60 degree edge angle, a 50% camber board with 9 stiffness would have it operating window at higher edge angles.

Hope this makes sense

 

camber.png.66b606684ccb4ffe2d1de7190bb925b5.png

Edited by TimW
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The stiffness number is just a number for comparison. Stiffness needed also depend on board length, sidecut radius, weight, design, torsional stiffness, etc. 

Edited by TimW
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Wow my head is spinning on this topic, all I know is board stiffness, scr and board tiltness doesn't change the size of the trees at the edges of the runs and if I'm not 100 percent on my game even the best board builder can't make those trees any softer. I don't know what magic Bruce puts in the Contra but I've ridden such a varied range of snow conditions this year and made the biggest turns to the tightest turns that I'm still trying to wipe the grin off my face. Thank you again Bruce

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With the number of years on the clock and a few people that have died hitting trees i have made myself a promise not to ride on the edges but stick to the groom in the middle . I know I'm missing out on some good terrain but I'm willing to trade it for  some extra years . 

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Wow, I see I'm not the only armchair snowboard designer on the forum.  Down the rabbit whole is right.

On 4/14/2021 at 4:22 PM, SunSurfer said:

I can see that at board inclination angles below 45 degrees sidecut is probably the major factor affecting turn shape, but it would seem to me that the flex profile curve would transition to become the major determinant above 45 degrees.

This interaction and transition is something I've not seen addressed here before.

Could you comment on the interaction of flex & SCR generally, but also for the Contra prototypes you had designed and were testing?

I agree that a board's flex is the main thing in the end. 

As I see it, sidecut shape, along with snow compaction or lack thereof, plays a role in influencing the shape it is being flexed into.   The board's resistance to being flexed into this shape affects the pressure distribution along the base.  How well the pressure distribution matches available friction along the edge affects edge hold.  Available friction along the edge is a function of downforce and tilt. 

The transition you mention above, the transition from flat to high on edge, affects downforce distribution.  When running flat, the downforce producing friction is where the rider's weight is pressing down.  The rest of the board can flex away.  As it gets tilted on edge, the downforce gets more distributed because the board doesn't flex sideways.  This shifting downforce is one the things I'm trying to account for, by getting the demand for friction to shift in a similar way along with it.

My hypothesis is that you can use variations in sidecut radius to affect the distribution of load along the base.  That load represents the demand for friction that needs to be satisfied at that location.  How can sidecut affect load distribution?  Sidecut and flex affect the shape that being flexed statically into the snow would like to form.  Differences between that shape and the shape of the carve itself, the trench in the snow actually being followed by the board in motion, will cause some parts of the base to bear more load than others.  So it's a design parameter you can play with to distribute load.

The K168 makes a good case study for this transition from flat to high on edge.  Some of its turns feel perfect.  But it's a traditional high-camber, high-differential VSR.  What I think makes it "demanding" to ride is that it demands to be kept high on edge -- to quickly transition from high to high.  I think that's because having the tightest radii located at the ends of the tip and tail, that's where the demand for friction gets concentrated, but is only satisfied with downforce when the board is high on edge.  At lower tilt, even with the high camber, there's a misalignment of the downforce with the need for traction and you get nose bounce.  One of the reasons I think plates are so often used to help tame this shape even for recreational riding could be that it puts the downforce from the rider's weight onto the board further out than your feet.  That may be helping get the downforce applied out closer to where it's demanded.

With the Contra shapes, we're trying to make the load distribution (need for friction/traction) look like a bell curve that flattens out as you tilt higher to match up with the downforce distribution making the same transition.  You shouldn't need a plate, at least not to help keep the nose loaded with downforce.

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3 hours ago, johnasmo said:

How well the pressure distribution matches available friction along the edge affects edge hold.

You should consider replacing "friction" with traction.  If you lose traction you lose grip (edge hold), however you could still have a lot of friction along the edge but not have good edge hold (e.g. skidding/sliding).

3 hours ago, johnasmo said:

Available friction along the edge is a function of downforce and tilt.

Downforce (aerodynamic grip) is a downwards force created by the aerodynamic features of a vehicle. The correct term for snowboarding (and skiing) is pressure.

Edited by noschoolrider
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Ok, those terms may be better.

But by downforce I meant only the vertical component of the pressure.

By friction I meant the force that once exceeded by the horizontal component of the pressure leads to slipping.  So static friction or traction.

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Yes you got me confused for a second too, as I think of friction as the resistance in my direction travel. Then I realized you must be a side slipper..😜

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4 hours ago, TimW said:

Yes you got me confused for a second too, as I think of friction as the resistance in my direction travel. Then I realized you must be a side slipper..😜

Ouch !!

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