Flex experiment

[Neil Gendzwill]

Just received a new soft boot carver from Coiler and was giving it the old shop flex. It’s stiffer than my Jones Flagship but not that stiff. Got me to thinking, exactly how stiff are my various decks? So I set up a crude test rig using my two vices (comically disparate in size as you will see in the pictures). The support points are 110 cm apart which is about the max the Jones can do. I put 100 lbs of weight over each insert pack and measured the deflection. Results:

2022 Jones Flagship 162W: 55 mm

2024 Coiler Contra 166: 50 mm

2015 Coiler AMT 167: 44 mm

2012 Coiler NFC 180: 29 mm

The Nirvana is definitely the stiffest both by test and by riding but it is not an overly stiff board by our standards. 
I’m guessing this kind of deflection test doesn’t correspond linearly to the formally defined flex properties whatever those are. Not a mechie. Corie?

Pics here: https://imgur.com/a/Ze2cHZ9

[slabber]

What are the stiffness numbers on your 3 Coilers?  

[Neil Gendzwill]

Those stiffness numbers aren't really comparable.

[slabber]

Yeah, they're relative to the design/length of each deck from what I understand.  

[SunSurfer]

The relationship between SCR, flex, and board performance is complex. In addition, wider stances result in less bend for a constant rider mass and G force.

Edited December 30, 2023 by SunSurfer
typo

[dredman]

4 hours ago, SunSurfer said:

In addition, wider stances result is less bend

But with a wider stance you can increase or decrease the amount of bending between the bindings by resisting/torquing legs together or apart.  

Another factor is what parts of the board are stiffer than others? Tip, mid, tail or evenly?

Agreed...very complex...what about how quickly or slowly the board wants to return any bent energy.  (Think rebound and compression on off road shocks). What about how much deflection before the board gets stiff?  Oh so many variables!

 

[BlueB]

Uniform distance between flex test points doesn't cut it either, because on snow, you press all the way to the contact points. 

[Neil Gendzwill]

You could probably test with the support points at either edge of the effective edge and then adjust the result mathematically to compensate for the difference between EEs on the board. Not sure that would be accurate. 

[staples156]

My first thought was also to move the supports to the ends of the effective edge. If you do that, the deflection should scale with the cube of the distance between supports*. So if you rerun your tests with the supports at the ends of the effective edge, then divide the deflection by the distance between supports cubed (make sure to use the same units for all measurements), that will give you a number that should allow you to compare stiffnesses between boards of different effective edge lengths. This will be a very small number, so scientific or engineering notation will be your friend here. As with all mathematical models, it's relationship to reality is questionable, but hopefully the error is the same across all tests so you can at least use it to show the general trend and compare one board's stiffness relative to another one.

 

*This is assuming the board acts as a simply supported beam with two point loads, and ignores quite a few complicating factors, so it probably won't be totally accurate.

[Neil Gendzwill]

So if I am reading you correctly and I measured two equivalently stiff boards of differing lengths, I would expect:

D1 ~= D2 * (L1/L2)^3

where D is deflection and L is the distance between the support points. 

[Jack M]

8 hours ago, BlueB said:

Uniform distance between flex test points doesn't cut it either, because on snow, you press all the way to the contact points. 

True. I wanted my Winterstick 185 to be softer than my 170, so Rob built it identically to the 170 - same materials, same thickness in the middle, just longer.  The length made it softer. If you measured the deflection of the two boards with the same distance between supports, you’d get similar results, but that wouldn’t be the correct representation. 

https://www.omnicalculator.com/construction/beam-deflection

[staples156]

1 hour ago, Neil Gendzwill said:

So if I am reading you correctly and I measured two equivalently stiff boards of differing lengths, I would expect:

D1 ~= D2 * (L1/L2)^3

where D is deflection and L is the distance between the support points. 

Yes, that's correct. That's assuming the loads and stance widths both stay the same. Also assumes that the inserts are centered on the board. My gut says that for most boards with only mild stance offset, it isn't worth correcting for, but you could technically do it (the math is just more annoying).

 

If you're curious about the math, go here and scroll down to the "Simply Supported, Center Load" case and look at the formula for δ_max. It's not technically the right case, but the math is much simpler and therefore easier to understand the general trend, which is mostly what matters here. The E and I variables are the two related to stiffness and are technically what's of interest here. You could solve the equation for an E*I term and compare that between boards, but I wouldn't. Given that this is just an approximation, I think it makes more sense to just use the equation to spot that deflection varies with L^3 and then just use L^3 to normalize the deflection that you measure.

[Jack M]

9 minutes ago, staples156 said:

If you're curious about the math, go here and scroll down to the "Simply Supported, Center Load" case and look at the formula for δ_max. It's not technically the right case

Looks like the (mostly) right case is the next one down, "Simply Supported, 2 Loads at Equal Distances from Supports" - cool!

[Cthulhufish]

Hmm... My inclination is to support the board with a rod under each contact point, stack weights between the bindings, and create weight vs deflection curves.

Not sure if relative deflection or absolute (more camber means a more negative initial value) would be more useful.

Probably only good for comparing similar camber profiles, but most of us like traditional camber anyway.

Regardless, the thing to do is probably to measure a bunch of boards a bunch of ways, ride them all, then assess which methodology produces results that feel right subjectively. E.g. does it matter if you stack weights at the center vs at the binding mount points?