Spiral Stair problem

I'm trying to make a pair of curved stairs, that will sweep up to a landing, with a stairway each side.

I'm making them from cylinders, 10x9.5x6. Path cut B to 0.98. Hollowed 35. And then I twist them. And then it gets strange.

One stair which is twisted to -180 works fine. The other, twisted to +180, looks fine, but has a "ghost prim" that prevents anyone walking up or down the stairs.

I've made several of these trying to figure out whats happening, my partner's made them, and its always the same. If you take a good stair, twisted to -180, and change the twist to +180 the ghost disappears. Change the twist back, and the ghost reappears.

Anyone got any ideas, both as to what is happening and how to get round it?

the colission box of a prim barely matches if a prim is twisted.
that is so for all prim types.

Yeah I figured it was something to do with the collision box, but what I don't understand is that if twisted one way it's fine, collision box seems to match the shape of the prim, but twist the other way and the collision box seems to be the same as for the original uncut/untwisted cylinder.

If there's no way round it,I'll have to rethink the design completely.

How about make the offending rail or rails phantom, then put a series of shorter, non-bent rails that are invisible along side or through the phantom ones so people won't fall off the stairs?

Hi Wulfric.
Try using a tube, these setings will give you a starting point:
X=10
y=10
z=10

Path begin 0.0 end1.0
Hollow 85 skew 0.8
Twist 0 0
Holesize 1.0 0.4
shear 0. 0
Profile cut 0.05 0.25
Taper 0. 0
Radius 0 revoulutions 1

Repeat this changing the profile cut to 0.5 - 0.8 then again with 0.05 - 0.25

The result should be a three prim spiral stair with handrails.

If you get stuck IM me and I'll send a LM where you can see one.

Thanks Roland, I'll give that a try.

Still wondering why a twist one way works, and the other way causes problems.

The twisted cylinder sems to be ok if it is only half cut before twisting but as you reduce the thickness but cutting more of the cylinder the problems get worse not only with the invisible barrier but also with avies feet sinking in.
btw, using the tube method you can make the stair and sides with a single prim, but the texturing is much better with three prims.

There have been bugs with regards to that. When Havok 4 comes along to your sim, that SHOULD fix it. I managed to reproduce your problem in a Havok 1 region (after figuring out through trial-and-error that you meant twist 180 & 0, rather than something like -180 to 180) and I found that taking the same prim to a Havok 4 region fixed the problem.

If you find any such error still occurring in a region running Havok 4, it would be great to make a JIRA report of it.