Tube helices have wrong curve

[goddard@…]

The following bug report has been submitted:
Platform:        Darwin-19.3.0-x86_64-i386-64bit
ChimeraX Version: 0.93 (2020-04-02)
Description
Tube helices (preset cylinders) sometimes are depicted curved when in fact the helix is very straight.  An example is 1mkj residues 175-190.

Log:
UCSF ChimeraX version: 0.93 (2020-04-02)  
© 2016-2020 Regents of the University of California. All rights reserved.  
How to cite UCSF ChimeraX  

> open 1mkj format mmCIF fromDatabase pdb

1mkj title:  
Human Kinesin Motor Domain With Docked Neck Linker [more info...]  
  
Chain information for 1mkj #1  
---  
Chain | Description  
A | kinesin heavy chain  
  
Non-standard residues in 1mkj #1  
---  
ADP — adenosine-5'-diphosphate  
MG — magnesium ion  
SO4 — sulfate ion  
  
  

> preset cartoons/nucleotides cylinders/stubs

Changed 0 atom styles  
Preset expands to these ChimeraX commands:

    
    
    show nucleic
    hide protein|solvent|H
    surf hide
    style (protein|nucleic|solvent) & @@draw_mode=0 stick
    cartoon
    cartoon style modeh def arrows t arrowshelix f arrowscale 2 wid 2 thick 0.4 sides 12 div 20
    cartoon style ~(nucleic|strand) x round
    cartoon style (nucleic|strand) x rect
    cartoon style protein modeh tube rad 2 sides 24 thick 0.6
    cartoon style nucleic x round width 1.6 thick 1.6
    nucleotides stubs

  

> preset cartoons/nucleotides ribbons/slabs

Changed 0 atom styles  
Preset expands to these ChimeraX commands:

    
    
    show nucleic
    hide protein|solvent|H
    surf hide
    style (protein|nucleic|solvent) & @@draw_mode=0 stick
    cartoon
    cartoon style modeh def arrows t arrowshelix f arrowscale 2 wid 2 thick 0.4 sides 12 div 20
    cartoon style ~(nucleic|strand) x round
    cartoon style (nucleic|strand) x rect
    nucleotides tube/slab shape box

  

> preset cartoons/nucleotides cylinders/stubs

Changed 0 atom styles  
Preset expands to these ChimeraX commands:

    
    
    show nucleic
    hide protein|solvent|H
    surf hide
    style (protein|nucleic|solvent) & @@draw_mode=0 stick
    cartoon
    cartoon style modeh def arrows t arrowshelix f arrowscale 2 wid 2 thick 0.4 sides 12 div 20
    cartoon style ~(nucleic|strand) x round
    cartoon style (nucleic|strand) x rect
    cartoon style protein modeh tube rad 2 sides 24 thick 0.6
    cartoon style nucleic x round width 1.6 thick 1.6
    nucleotides stubs

  

> preset cartoons/nucleotides ribbons/slabs

Changed 0 atom styles  
Preset expands to these ChimeraX commands:

    
    
    show nucleic
    hide protein|solvent|H
    surf hide
    style (protein|nucleic|solvent) & @@draw_mode=0 stick
    cartoon
    cartoon style modeh def arrows t arrowshelix f arrowscale 2 wid 2 thick 0.4 sides 12 div 20
    cartoon style ~(nucleic|strand) x round
    cartoon style (nucleic|strand) x rect
    nucleotides tube/slab shape box

  

> select /A:181

8 atoms, 7 bonds, 1 model selected  

> select up

123 atoms, 123 bonds, 1 model selected  

> preset cartoons/nucleotides cylinders/stubs

Changed 0 atom styles  
Preset expands to these ChimeraX commands:

    
    
    show nucleic
    hide protein|solvent|H
    surf hide
    style (protein|nucleic|solvent) & @@draw_mode=0 stick
    cartoon
    cartoon style modeh def arrows t arrowshelix f arrowscale 2 wid 2 thick 0.4 sides 12 div 20
    cartoon style ~(nucleic|strand) x round
    cartoon style (nucleic|strand) x rect
    cartoon style protein modeh tube rad 2 sides 24 thick 0.6
    cartoon style nucleic x round width 1.6 thick 1.6
    nucleotides stubs

  

> hide

> hide ~sel ribbons

> preset cartoons/nucleotides ribbons/slabs

Changed 0 atom styles  
Preset expands to these ChimeraX commands:

    
    
    show nucleic
    hide protein|solvent|H
    surf hide
    style (protein|nucleic|solvent) & @@draw_mode=0 stick
    cartoon
    cartoon style modeh def arrows t arrowshelix f arrowscale 2 wid 2 thick 0.4 sides 12 div 20
    cartoon style ~(nucleic|strand) x round
    cartoon style (nucleic|strand) x rect
    nucleotides tube/slab shape box

  

> hide ~sel ribbons




OpenGL version: 4.1 ATI-3.5.5
OpenGL renderer: AMD Radeon Pro Vega 20 OpenGL Engine
OpenGL vendor: ATI Technologies Inc.

[Tom Goddard]

I've attached images of 1mkj alpha helix residues 175-190 shown as ribbon, tube, and tube with C-alpha positions. The C-alpha position are what is used to fit the tube. All 3 images are in the same orientation.