Stiffness Index?

[trikerdad]

After riding the Prior 185 WCR that I picked up last month, I'd like to know where it is on the 'stiffness index'. How is stiffness measured? I know that the Prior's a lot stiffer than the Donek 182 Axis at 7.8, but that's the only thing I have with a known stiffness to compare it to. It would sure be nice to have some way of getting a rough estimate of where a board falls on the stiffness index other than just flexing it by hand. Sean?

[Guest tigger]

have a look at

http://www.extremecarving.com/forum/viewtopic.php?t=1104

[trikerdad]

Thanks, exactly what I was looking for.

[trikerdad]

I'm having problems with the formula: (98.1/x)x(effective edge squared)=BOBSI. I end up with huge numbers nowhere close to what I would expect. I assume that the / means divided by x or the deflection of the board with the 10 kilograms of weight on it. What am I doing wrong. I have an effective edge of 159cm and a deflection of 18mm. It's a Donek Axis and is supposed to have a stiffness of 7.8. I'm just trying to see if I can duplicate the stiffness number to see if I'm doing it right.

[Donek]

I get asked a lot of questions about this index... It's kind of broken.

It is just intended as a guide to indicate relative stiffness within our line. It is based on theoretical numbers for the overall stiffness of a board. The problem is, I have a value off somewhere in my calculations that throws the numbers off slightly as the width of the boards change. If this error were corrected, I could provide an accurate BOBSI value. I simply have not had the time to track down this value. I hope to be able to at some point in the next year or so.

[Chris Houghton]

Looks like an issue with the units of measure in your formula. 159cm eff edge and 18mm deflection with 10Kg is a stiffness of 13.8 according to my calculations - pretty stiff. If you multiply your result by 0.0001, that should fix the problem.

Most of the PRODUCTION boards I've measured (about 40) from Oxygen, Prior, Donek, Hot, Burton etc. are between 10 and 12 on this index. GS/SG race boards from Fawcett, JJA, Ian Hadgkiss and so on are more in the 14 or 15 range, so definitely stiffer than "production".

All this is great if you're comparing the same construction, because torsional stiffness does not enter into this calculation. My new Prior with the Quadrax glass measures 12 but rides like 15 when on edge. How the @#$%# am I going to change the formula to account for that! Softer flex with killer edge hold.

[Neil Gendzwill]

I think that 13.8 BOBSI calculation must be off as the Axis 182 is only a 7.8 by Donek's scale, and some of the Donek customs push into the 14s. Still I get the impression that the Axis is generally stiffer than it's competitors. Correct or not?

[Mike T]

Originally posted by Neil Gendzwill

Still I get the impression that the Axis is generally stiffer than it's competitors. Correct or not?

Haven't ridden the Prior, but I have both an Axis 172 and a Coiler AM 182 Custom. They feel about the same stiffness wise. Based on the Donek stioffness index, the Donek 182 would be substantially stiffer than my Coiler.

Mike T