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Sidecut profile & long axis flex profile - interaction and performance


SunSurfer
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I have definitely had boards where the flex was not a good match for the sidecut.  It was like the sidecut wanted to make a shorter turn and the flex wanted to make a longer turn.  The board seemed to fight itself.

I find the Kessler 168 to be very easy to ride at all edge angles.  Could just be a better board for eastern North America.  I'll never forget demo'ing a Madd 180 in Aspen at the SES in 2005.  I loved it and bought it.  When I got it back to Maine it was a completely different board.  I still loved it, but it suddenly rode a lot longer.

Great discussion!

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No one seems to have talked much about what seems to me to be the wild card in the pack, which is torsional rigidity and especially the way in which torsional rigidity progresses along the board. I'd be interested to hear your thoughts. It's true that TR is closely related  to thickness and longitudinal flex for a given board width but it does vary from that relationship, especially when the layup features multiple axes. The magic baked into some Coilers I've ridden (a 2002 180 Racecarve was my favorite for many years) seemed to me to owe more to the gentle progression of TR, which allowed the board to feather into a carve really smoothly. The same applies to my current Proteus but the boards Sean was building back in 2002 seemed much harsher to me, and I think it was more related to this feature than anything else.

Long ago I had a homemade jig (big clamp, long weighted lever arm, twang it and measure frequency) I used to use to measure the TR of my race skis and it was often the decider between the race-day skis and the trainers and the hard-snow pair (very stiff) and the rut-runners  which I preferred softer.

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8 hours ago, johnasmo said:

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.

Yes, it's challenging to explain all the ways pressure can be distributed/controlled on a snowboard.

It's also challenging to explain the intricacies of pressure, edging and tracking that lead to a clean carve (where the tail/back of the board follows in the thin path set by the nose/front), and also relating that to how much friction there is because skidding/slipping can be caused by poor pressure distribution/control (e.g. too much pressure or not enough pressure) regardless of the amount of board tilt (edge angle).

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7 hours ago, Jack M said:

 When I got it back to Maine it was a completely different board.  I still loved it, but it suddenly rode a lot longer.

Great discussion!

Your statement makes me go back to when i first started making plates and i came to the realization that certain plates could make a short board ride as though it was longer and a long board ride as though it were shorter. Down a rabbit hole indeed ! New riders might want to ignore this thread for a while.

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4 hours ago, lowrider said:

Your statement makes me go back to when i first started making plates and i came to the realization that certain plates could make a short board ride as though it was longer and a long board ride as though it were shorter. Down a rabbit hole indeed ! New riders might want to ignore this thread for a while.

When you talk about typical short board performance are you thinking of the relative lesser stability at high speed, or that shorter boards generally turn more tightly. The tighter turning is probably due more to the sidecut tending to be shorter too. Not a lot of 160cm long boards out there with 20+m sidecuts! 

Thankyou to everyone who has posted so far. I am having a great time pondering all the various factors that are being brought to this discussion.

 

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SunSurfer  I was recalling the thought that a plate on a board moderates the flex . Sometimes it makes the middle of the board less flat than it would be without the plate and sometimes it allows the board to flex more than it would if ridden without a plate. Sometime the impression is that it's due to peddling perhaps other factors are at play. Pay me and I'll buy a thousand sensors buy some more boards and test out the theory and while I'm doing that you and your wife can come and lamb out the ewes and bale hay .Oh wait you can't were on a 6 week lock down police now have the right to stop and ask where you live and where you are going . The message is stay home but people don't know what that means so here we are exactly where no one wants to be , Having fun waiting for your report of the upcoming season.                                                                                                                                   Update April 18 th police decided they wouldn't stop people so gov't yanked the order. New reality you can't cure stupid or quarantine it either .

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55 minutes ago, lowrider said:

Pay me and I'll buy a thousand sensors buy some more boards and test out the theory and while I'm doing that you and your wife can come and lamb out the ewes and bale hay

Careful brother @lowrider, I suspect the @SunSurfer would be a dab hand at lambing, with the added bonus of applying Propofol as required. While he may pay up front for the pleasure, I suspect the account may swing in his favor once his professional skills are tallied.

Stay safe, some day this war will end.

Edited by Lurch
Not sure on his baling, loose string is a curse. 
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Posted (edited)
3 hours ago, Lurch said:

@SunSurfermore fuel to stoke your cerebral fire; no talk yet on sidecut depth - think it was @crackaddicts 25m moon rocket that raised the topic of "sidecut depth being better predictor of turn size than sidecut radius" 💣

I can't agree with the text in italics. Turn geometry doesn't work like that. 
So to reduce that to the absurd, that would mean a 6 metre long board and a 1.5 metre long board, both with a 3cm deep sidecut would turn roughly the same.

To carve, there must be a groove (trench for Mario) that the board initiates, and then compresses both walls as the board tracks the groove, and the curve of the groove and the resistance of the snow to compression changes the board's (and rider's) direction. 

The radius of that curved groove at any point in the resulting turn will depend on the combined curves of board sidecut and board flex. The higher the lateral wall of that groove the more the board flex curve is contributing to the turn radius. 

A typical days carving for me will wear away the wax sheen on the 2-3cm just in from the steel edge. On my 174 Nirvana T4 Energy the wax wear starts where the board edge curves to the nose and runs to the tail. The rest of my wax job is almost untouched. That wear pattern is a reflection of the friction on the board base from the pressure against the snow in the lateral wall of carve groove. That same pressure creates the flex curve along the length of the board. 

If it was just the steel edge sidecut shape that determined the turn shape then there would be a much more even wear pattern on my wax. On an inclined plane slope of perfectly smooth ice, with a perfectly engaged edge, then the maths of Nate's old calculator of how sidecut curve radius changed with edge angle would be accurate. EXCEPT that even on ice a carved edge creates a groove, and that groove has measurable depth in millimetres, and that's enough to throw off Nate's calculator significantly. 

The bending force applied along the length of the board, and that pushes against the compressed snow that keeps us carving and turning, will depend upon the rider's mass, where along the board that mass is applied, their speed and the radius of the turn. Bending force increases with greater rider mass, the closer to the centre of the board that mass is applied, the greater the speed, and the smaller the radius of the turn.

The where the mass is applied effect is not immediately intuitive. This is an important part of the effect of isolation plates that attach to the board at just two points and allow the board to curve freely under load (e.g. Bomber Boiler Plate). Under the effect of gravity, with a board suspended at tip and tail, if all of a rider's mass (e.g. 80kg) is applied at the centre of board then the maximum bend for that mass is produced. If you divide that mass in two, and apply it above the suspension points at tip and tail no bending occurs. If you mount a BBP plate on the board and put the mass in the centre of the plate the board bends less than if the mass was in the centre of the board. Distribution of mass along the plate makes no significant difference to the resulting curve in the board. The wider the plate axles are apart, the less the bend for any given mass. If you have an isolation plate you can demonstrate this for yourself. 

But the point I want to make is this. The lateral wall of the groove is what keeps us from falling in the depths of our turns. It's what we balance on at every point of the turn, and what forces (as in changes our momentum) us to turn. And that means that groove geometry and board flex are key parameters in the production of any carved turn.

 

Edited by SunSurfer
add a reason for rejecting the assertion.
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Posted (edited)

Dangerous time walking the dog. I get thinking.

So, given the effect of weight distribution along the board on flex, perhaps rider BMI is a better indicator of how a board will flex under load.

A shorter rider of nominal BMI will have less mass, but likely a shorter stance, so their mass is more concentrated towards the middle of the board.

A taller rider of same BMI will have more mass, but likely a longer stance, so their mass is spread over a longer mid-section of board.

This mass distribution relationship might enable a board designed to perform satisfactorily for a rider of given mass and physique, to perform satisfactorily for a wider range of physiques than we might have originally thought.

-------------

A rider can only cope with a certain level of rate of change of direction before they get get thrown off. Our brain and muscles need to be able to keep up with the changes. Sudden changes in direction are harder to cope with.
If the rate of direction change produced by the edge engagement curve of the sidecut at lower angles and the flex curve at higher angles are very different then that might make for an uncomfortable ride, especially for riders rapidly moving between low and high edge angles. Kind of expanding on what @Jack M was describing in the the first lines of his post above.
An easy riding board would seem to need reasonably matched sidecut and flex curves, and in use by the intended rider not produce direction changes of greater rapidity (really tight radius curves i.e. too soft) than the rider's body could keep up with. A board that is hard to initate a turn on unless the rider is going faster than speeds they're comfortable with is too stiff.
An easy riding board is like porridge for Goldilocks, it's "just right!"

Edited by SunSurfer
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6 hours ago, SunSurfer said:

Dangerous time walking the dog. I get thinking.

 

Spreading a barn full on sheep manure last week i had a glitch with the machine. Reflecting on what a mental health expert was advising on dealing with Covid " remove negative thoughts  and redirect to pleasant thoughts" I told my wife i had a few thoughts I'd like to share with the expert ! SunSurfer you sure the dog isn't talking back ?                                                                                                                         Is it time to order a new board with a different sidewall profile on each side for comparison. Better be carful in your discussions with Bruce about that idea I think it might take an order of flowers for Bruce's wife before he would be  softened up enough to consider it !

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11 hours ago, SunSurfer said:

On an inclined plane slope of perfectly smooth ice, with a perfectly engaged edge, then the maths of Nate's old calculator of how sidecut curve radius changed with edge angle would be accurate. EXCEPT that even on ice a carved edge creates a groove, and that groove has measurable depth in millimetres, and that's enough to throw off Nate's calculator significantly. 

The calculator is correct for ideal carve, where all parts of the edge follow the same track and are into the snow at the same depth. Now, it is possible to push the mid parts of the edge further out from the centre of the arc, than nose and tail, while none is actually skidding. The resulting groove is wider than a "pencil line", but still uniform - carved. 

It's easier to achieve the above on a longer board, as there's more leverage to work with. Longer board has more sidecut depth than a shorter one of the same radius. I guess, that's where the observation of the cut depth comes from. 

While I enjoy the power and rebound of the "overflexed" turns, the "ideal carve" feels the nicest and "flowiest". To me, at least... 

Edited by BlueB
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On 4/16/2021 at 8:28 AM, Jonny said:

No one seems to have talked much about what seems to me to be the wild card in the pack, which is torsional rigidity and especially the way in which torsional rigidity progresses along the board. I'd be interested to hear your thoughts.

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 performance and comfort that you are after.

Back in 2015, I had a build done that had a big carbon butterfly under the base going from ahead of the front inserts to behind the rear.  What was known as the Torsion Plus option from Bruce.   In almost all other respects, it was to be like a previous build (a Nirvana precursor from 2011).  I thought at the time that my mediocre skill level required every degree of edge angle the board could muster for better edge hold in icy conditions.  What I found out was how high TR translated to being a harsh suspension.  It cut slush like a knife, but was noticeably less comfortable otherwise, transmitting every irregularity in the snow to your feet, reducing confidence in hold rather than increasing it. 

It made the board feel wider because of it, even though it was same dimensions are predecessor.  I feel the same harshness and width illusion whenever I demo a T4 metal board, including Bruce's T4 boards and my K168.  They are super controlled, but the thicker titanal also increases TR and my legs feel it and my brain can't ignore it.

Realizing what had happened, my next build in 2016, which I called the the Triple-X, suspended each section of the board (nose, mid, tail) independently.  It used mini butterflies under each binding on the base, and then three independent carbon X's on the top.  It proved quite smooth yet performant.  But the X's were a hassle to layup, so it remains a one of a kind.

I was happy with that board until jumping down the Contra rabbit hole in the 2018/2019 season.  Today my builds keep the mini-butterflies underfoot and then just uni-directional fibers everywhere else.  The full sheets of T3 give all the TR I want.  Bruce has tried notching the titanal in the nose to relieve some TR in the tip, but I find that T3 doesn't need to be notched, at least not in the width boards I ride (<19cm).

The profile with which the core thickness in the nose increases affects the distribution of torsional stiffness as that affects the cross section of this titanal beam, but that profile is being designed around the longitudinal flex for now.  The effect on TR distribution comes along for the ride as the T3 is uniform throughout, but I've been really pleased with the shitty snow I can still enjoy on these.  My philosophy is to design boards that don't need plates to be comfortable.  Just say no to plates.  Since I'm not a racer, I *will* sacrifice performance for comfort.

Edited by johnasmo
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Posted (edited)

@johnasmo

Is this the kind of thing you're talking about with TR playing a role in the ride feeling harsh, that too much TR can be a problem. These guys seem to be saying the same thing with skis. Not sure if the English translation captions turn on automatically, otherwise your Swedish better be good. (At least I'm assuming it's Swedish, now I relisten sounds like might be German)

 

 

Edited by SunSurfer
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On 4/16/2021 at 8:18 PM, Lurch said:

@SunSurfermore fuel to stoke your cerebral fire; no talk yet on sidecut depth - think it was @crackaddicts 25m moon rocket that raised the topic of "sidecut depth being better predictor of turn size than sidecut radius" 💣

Yeah, no.  Sidecut depth is a geometric result of the radius.  The sidecut depth can't be a better predictor of turn size than radius since it's the same predictor.  It describes the same thing about the geometry of the sidecut.

But is it a good predictor of turn side?  Sidecut acts as a flex inducer, so it can't be viewed in isolation from stiffness when it comes to predicting turn size.  At low tilt (hence low flex), it's the majority predictor; as tilt increases, stiffness has an increasing say in the turn size.

James is a rock star strong rider.  He's going to be high on edge and pushing the board to its flex limits.  Depth to him probably acts more like a backstop to flex than a flex inducer.  So he can look a depth and know something about how hard he can turn the board without being told what radius it is.

With the variable sidecuts I design, I boil the radius down to a, "radial depth equivalent."  The sidecut may be a long-tight-med-tight-long progression of some kind, but then how to describe it in terms people can extrapolate from prior experience?  By describing it relative to a radial sidecut.  But rather than average the radii in the progression, I beleive (like James) that the best way is to describe it in terms of the resulting sidecut depth.  So when you get a Contra 11.5m sidecut, the radius changes every inch along the effective edge, but the depth ends up matching that of a radial 11.5m.

 

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19 minutes ago, SunSurfer said:

@johnasmo

Is this the kind of thing you're talking about with TR playing a role in the ride feeling harsh, that too much TR can be a problem. These guys seem to be saying the same thing with skis. Not sure if the English translation captions turn on automatically, otherwise your Swedish better be good.

 

 

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 some twist at the tip, all the forces must be dealt with longitudinally, and that adds more bounce to the nose.  So TR is a variable to be used in tuning the front suspension.  Too little is bad, but so is too much.

I feel the right reason to use a plate should not be because the board is too harsh to ride otherwise.  It should be to isolate the board from your bad behavior/inputs so it can win a race, not to isolate you from the board's bad behavior.  If a board behaves badly, change it.

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Yep - exactly what I'm talking about. It is German, although the tech speaks with what sounds like a Tirol accent and the two Swedes speak with a Swedish accent while the coach sounds German to me...

It's really interesting what they say about body geometry and body position and while they don't talk about strength I think that's important too. Lindsey Vonn could just dominate SG and DH courses, partly because she was on men's skis with a much stiffer TR. Ledecka won on Shiffrin's skis which are apparently very soft in that respect, and Ledecka really flowed down that Olympic course.

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4 hours ago, johnasmo said:

Just say no to plates.  Since I'm not a racer, I *will* sacrifice performance for comfort.

Interesting. I use a plate for the comfort factor. Perhaps a Contra will change my mind. August will reveal all the secrets to me.

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@daveoNot necessarily all the secrets. Some of us are still trying to identify which of the various design features of the AllFlex plate design(s) are responsible for it becoming the GS racers current favourite.

Isolation / separation of board from rider

Long distance between board attachment points

Limiting the amount of board flex before the plate must also bend in order for further board flex to occur.

Splitting front and rear of plate, with spring controlled resistance, potentially allowing the racer to manage mid section torsion of the board.

Variation in plate flex characteristics due to different rib patterns on the underside.

Greatest popularity with GS racers on their boards.

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