Looking back to the linked tutorial on cloth parameters, it says:

Quote - Shear resistance has a range of 1 to 1000; it defines how much the cloth tends to retain its shape as it folds. This differs from fold resistance; the shear resistance of a fabric determines how much or little it deforms when actually folding.

This means absolutely nothing to me as far as making predictions and understanding what it does.

I think or thought I know what shear could mean. Consider a 4-point square. Grab two opposite corners and squeeze them closer together, while holding the four side lengths constant. You get a diamond. It is still coplanar, so this is movement within the original plane. That's what I thought shear was. And I thought it did not apply to triangles, since there is no coplanar motion possible with the side lengths held constant in a triangle.

Stretch, I assume (wrongly?), is a variation in the length of edges.

Folding is a changing angle between two adjacent polygons. Maybe.

The collapse of a quad - is that fold or shear or a combination or neither? The crystal ridge suggests it isn't either one, since I was unable to affect crystal ridge with any parameter, other than creating a cloth that stretches however much it needs to in order to avoid a fold. Any quads with a diagonal fold produces crystal ridge.

Now I have another thing happening I'm curious about - the mysterious crossing of a fold through itself. I see it all the time, even with hex tris. Without making any conclusions, I just want to share that I made it stop by altering shear resistance to a low value!!! What?!?! Impossible!

So - this tells me we need to understand shear or we're not going to get anywhere. I reject anything I thought I knew about shearing motion. Starting from first principles, how do we figure this out?

 

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