Flex index - an idea

[BlueB]

It would be great if manufacturers could standardise their stiffness / flex indexes and publish the information with the specs. Or at least if we, the carvers, could adopt something for our use, so when buying a board one would know what to expect...

I've put my thinking cap on and came up with a simple test and formula:

1) Support the board at the lines of contact (limits of the Contact Length "CL"), say on two chairs.

2) Measure the CL in centimetres. Determine the Centre of Contact Length "CLC".

3) Measure the Unweighted Distance "UD" from CLC, on the base/ edge, to the floor in millimetres.

4) Place a standardised weight, 10kg (10kg sounds good - almost every household bucket would have measurement and take 10L of water = 10kg, or just use 10kg fitness weights), on CLC).

5) Measure the Weighted Distance "WD" from CLC (base/edge) to the floor in millimetres.

6) Deduct the WD from UD. The result is the measured flex "MF".

7) Divide CL by MF, and round off to 1 decimal. The result is what we need: the Universal Longitudinal Flex Index "ULFI". Smaller numbers mean soft, bigger stiff.

It is probably fair removing the bindings, as they could stiffen the board...

Example on my Hooger 168:

CL = 143

UD = 443

WD = 419

ULFI = CL / (UD - WD) = CL / MF

ULFI = 143 / (443 - 419) = 143 / 24 = 5.9583

ULFI ~ 6.0

and another on an Alp 69:

CL = 147

UD = 449

WD = 426

ULFI = CL / (UD - WD) = CL / MF

ULFI = 147 / (449 - 426) = 147 / 23 = 6.3913

ULFI ~ 6.4

I might be wrong, but I brought the length and flex ratio in the play, because out of the two boards of the same absolute stiffness, the longer one will flex more due to greater leverage.

Isn't this a perfect summer tread, and also a chance to do something with your boards when not carving? :)

If there's interested people, I could do a photo story of how it's done.

Now further, boards of the same ULFI could still flex differently in real world - when used, or dynamically. By repeating the test with different standardised weights (10kg,- 20kg, 30kg; or 10kg, 15kg, 20kg, 25kg, 30kg ?), one could determine the Dynamic Flex Index "DFI". Simple graph would show weight "W" on horizontal axis, and measured flex "MF" on vertical axis. Resulting Dynamic Flex Curve "DFC" could be a parabola, hyperbole, straight ascending line, or an erratic curve (requires at least 4 weight test to show).

Uniform straight line would mean Uniform "U" dynamic flex / performance.

Parabola would mean Fast "F" action, meaning that board would probably be easy to initiate, but then require efforts for more aggressive bending / inclinations / turns.

Hyperbole would mean Progressive "P" action - hard to initiate, but then requires small tweaks to bring you deeper into the carve. I'm not too sure if this kind of performance would be nice.

Erratic curve you do not want to see - unpredictable performance...

(For this "performance hypothesis", let's suppose we are working with the boards with same SCR.)

Thus your board's Dynamic Flex Index - DFI could be rated as U, F and P.

I didn't try this test on my boards yet, I have to find the suitable weights first...

Now, I'm sure there's quite a few mechanical engineers here to give us some more input on all this - I am just an industrial designer ;)

Also, Bruce, Chris, Sean, could we hear some thoughts from you?

Boris

P.S. Other very important factor is the torsional flex / stiffness, as the torsionally stiff board should carve better than one that is not.

The testing would require a bit more hardware, but it can be done. Maybe I'll write about Universal Torsional Flex Index "UTFI" next?

[mirror70]

There are two bad assumptions with that system:

1) The rider's stance is centered

2) The board has even stiffness along its length

The problem with your dynamic test is that "dynamic" implies motion while the test is steady-state.

A better test, imo, would use a rig something like this:

W is a weight. The rig, when loaded, will bend the board from the front binding out to the nose. The stiffness of the nose can then be rated according to how much weight it takes to get the base of the rig to come a given distance off of the ground. The rig and then be turned around and mounted on the rear inserts and the test and be repeated for the tail. An alternative, using the same rig, is to use a known weight and then measure the angular deflection of the rig. Neither one of these needs to be normalized for board length.

[Chris Houghton]

It's been done and discussed to death over the last few years, search under BOBSI to see it. The flex index cannot be a linear relationship, so BOBSI used the square of the effective edge as a closer estimation. Still meant nothing if the board construction, flex pattern and camber were at all different between the boards being compared. I can tell you about a 10 and a 14 BOBSI that ride almost identically, so I find the scientific evaluation a bit entertaining, but not good for much of anything else.

The only way to know is to ride!

[underdog]

sorry bored watching it rain at work and decided to stir the pot :)

-chris, 228pounds 6ft0inches, libra likes hardpack and golf

[NateW]

I don't care what system gets used as long as all manufacturers pick one and be consistent about it. It just ticks me off that there's no way to even get a clue about how brand X compares to brand Y.

So what if it varies between tip and tail, or varies between the center and the ends... I just want a clue, that's all I ask...

[skywalker]

Hi to all,

as I don't have a lot of time at the moment, only one short point about it:

Now further, boards of the same ULFI could still flex differently in real world - when used, or dynamically. By repeating the test with different standardised weights (10kg,- 20kg, 30kg; or 10kg, 15kg, 20kg, 25kg, 30kg ?), one could determine the Dynamic Flex Index "DFI". Simple graph would show weight "W" on horizontal axis, and measured flex "MF" on vertical axis. Resulting Dynamic Flex Curve "DFC" could be a parabola, hyperbole, straight ascending line, or an erratic curve (requires at least 4 weight test to show).

What you want to show is linearity or progressivness of flex. Your suggested test has nothing to do with dynamics. Taking away the load suddenly and measuring the board's movements would be a dynamic way of testing. This would lead to another important characteristic of snowboards which is dampening / hysteresis.

I would like to have a system consisting of two charts. One would be the arc a board shows along it's axis when loaded with a given force. Maybe a differential diagram would show very precise the stiffer and softer areas of the board. Second chart ould have to show torsional stiffness, of course also as an arc along the board axis.

My 2 cts

Tom

still believing that estimating a board's characteristics based on numbers / diagrams must be possible. I would be in the wrong profession if I would not ;)

[BlueB]

Thanks for responses, everyone!

Ok, I see, I was reinventing the wheel a bit... But I think it is worth it. As Skywalker said, there must be a way to ESTIMATE a boards characteristics by numbers. Again, I tried (just like authors of BOBSI) to come up with a simple test that everyone could perform at home without complicated rigs, and a formula just to give an overall idea of the board's stiffness. Not to predict the ride completely (where would be the fun then - we could all just crunch the numbers instead of riding?), just to give an ESTIMATE. Instead of saying to a buyer "look, it is quite stiff when I flex it with my hand..." or "I am 80kg (~176lb for pound lovers ;) ) and it rides fine...", one would say BOBSI 9.2 or ULFI 6.4. It should have more meaning?

I have read all the articles I could find about BOBSI (probably there's more on old forum?). Great effort - why no one refers to these numbers any more? Where I can find the data base of measured boards?

Interesting, my ULFI uses similar techniques and maths as BOBSI, so we must be on the right track, no?

Now, few questions about BOBSI.

For those not remembering, formula is (98.1 / deflection) x (effective edge squared). 10kg was used as the test weight.

1) Observations

1a) For the formula to work with the figures given in old examples, deflection has to be taken in millimetres and effective edge in metres.

1b) Effective edge is actually the Contact length, surely? Effective edge is measured along the curve of the edge and it is quite difficult to do for amateurs. Contact length is a straight line. 2 boards of same Contact Length can have different Effective Edge, due to different sidecuts.

2) Could someone (an engineer?) explain the logic behind formula, for public? My knowledge of physics is limited to what I remember from school, so excuse my assumptions.

3) Where the figure 98.1 comes from? My thinking is that the Acceleration of 1G (~9.81 m/s) was multiplied by Mass of 10kg used, resulting in Force of 98.1 Newton? Obviously, it is not a number added in to bring the results into 1 to 10 scale, as many boards go over it, up to 15 - 16.

3a) What the 98.1 (lets call it F) does to the formula? It should be there only if it "levels the field", meaning that the BOBSI number would remain the same irrespectively of the weight used, so long as you adjusted the F accordingly (i.e. for 20kg F would be 196.2)

3b) I tried the test with 10kg and 20kg, and calculated the BOBSI. The results differed about 0.5 - quite a lot for an accidental mistake. It needs much more testing, but maybe, just MAYBE, the 98.1 should not be there at all?

4) Why squaring the Contact Length?

These questions are not to discredit the BOBSI by any means, I just want to understand it fully.

As Nate W said, I do not care what system is used, so long as manufacturers could adopt one and we (the riders) understand how it works. That's why I was hoping to hear from boutique manufacturers at least.

As long as the differences in tail/nose stiffness goes - who cares? In order to carve a perfect arc, you have to pressure the entire edge uniformly, so you would adjust your stance and technique accordingly to the nose/tail differences. Soft nose would help in moguls and maybe in pow, soft tail maybe on ice... For freecarving I would choose uniform flex anyhow. It is the overall stiffness that gives you the first idea. However it is nice knowing the rest too...

Torsional flex is another issue and deserves separate test. The required rig might be complicated for home use, but certainly achievable by manufacturers.

Regarding my proposed "dynamic flex index" test, yes I agree it is done in static state, but it gives you an idea of what would happen when you ride the board, and you pressure it more, and you pressure it more, and you... Trying to insolate/simulate the readings during the different phases of a turn. Therefore I call it Dynamic. And I proposed only 3 ratings, to keep it simple. It is probably less important than torsional flex, anyhow.

Measuring boards movements after sudden release would give a good idea on dampening - very important from what I read. Unfortunately it is completely beyond the means of "home user", and probably beyond the means of small manufacturers.

Again, this is all for fun, and idea exchange, not to start a war. If a benefit for riders would arise from it... well - super!

Boris

[mirror70]

Originally posted by NateW

So what if it varies between tip and tail, or varies between the center and the ends... I just want a clue, that's all I ask...

Without the whole picture, you're better off with none at all.

[CarvCanada]

It would be great to have an entire snowboard industry standard... but yeah what happens when the board is stiff in the tail and nose, but not in the waist?

My 186 "feels" softer than my 171, but I'm sure it takes more force to decamber. A number could give you an idea of how stiff a board is, but the board's shape, structure and "raison d'etre" would give you a better idea.

I'm doing a project for college linking the physics of technique of carving to the board's structure/shape... it fit perfectly into the class material :) I have to start thinking like this now . Oh well it's raining outside and 3 degrees!