Microtype: tracking for bold fonts

Question

There are some new fonts (e.g. Fira, now IBM Plex) and even packages for pdftex. The fonts include extrabold and even black subfonts. I'd like to use them, but it seems necessary to add more space / kerning between the bold letters.

According to this answer I tried in vain to change the tracking amount. See MWE:

\documentclass{article}

\usepackage[sfdefault]{FiraSans}
\usepackage[tracking]{microtype}
\usepackage{blindtext}
  \DeclareMicrotypeSet*[tracking]{bold}
   { encoding = *, series = bf* }
\SetTracking
  [ unit = 1pc ]
   { encoding = *, series = bf* }
   { 550 }


\begin{document}

\textbf{Es irrt der Mensch.}

\section{First section}

Some more text.



\end{document}

If I swap Fira again lmodern, it works as expected! It doesn't work with the new package plex-sans instead of FiraSans either. »Doesn't work« means, no tracking, independently of the value of \SetTracking.

Here as screenshot with lmodern, so how it is supposed to look:

And here the »doesn't work« with Fira-Sans, as in my MWE:

Answer 1

The bf* in the expression series = bf* in your set declarations translates to \bfdefault, which in LaTeX is defined as bx. However, the FiraSans package does not provide a bx series; instead, it declares the four bold series sb (semibold), b (bold), eb (extrabold), and h (heavy).

Therefore, to adjust the tracking of all bold fonts, change your microtype declarations to:

\DeclareMicrotypeSet*[tracking]{bold}
   { encoding = *, series = {b,sb,eb,h} }
\SetTracking
   [ unit = 1em ]
   { encoding = *, series = {b,sb,eb,h} }
   { 450 }

(I'd also suggest to specify a unit of 1em so that the tracking is adjusted relative to the design size of the fonts.)