Hi Chao,
Sorry, currently there is no option to make cartoon tube helices straight.  There is a request already in our ticket database, but I do not know if the feature will be added or not.
<https://www.rbvi.ucsf.edu/trac/ChimeraX/ticket/1242>

If you don't mind just having a separate cylinder display for each helix that is not connected into the cartoon, then try Axes/Planes/Centroids tool and/or "define axis" command.  The tool is very new (not in ChimeraX version 1.4) so you would need to download a new daily build to use it.
<https://rbvi.ucsf.edu/chimerax/docs/user/tools/axesplanes.html>
<https://rbvi.ucsf.edu/chimerax/docs/user/commands/define.html>

I hope this helps,
Elaine
-----
Elaine C. Meng, Ph.D.                       
UCSF Chimera(X) team
Department of Pharmaceutical Chemistry
University of California, San Francisco

On Aug 26, 2022, at 7:47 PM, 王超(Chao Wang) via ChimeraX-users <chimerax-users@cgl.ucsf.edu> wrote:

Dear All,

After displaying helix in cylinder mode, I found that it is curved by default. I would like to ask whether it can be changed to a straight shape with two ends as fulcrums or other straight way?

<2D08A285@47B9C47A.AA850963>

Thanks,

Chao.

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