  Thermal friction offers a promising solution to the Hubble and the
large-scale structure (LSS) tensions. This additional friction acts on a scalar
field in the early universe and extracts its energy density into dark
radiation, the cumulative effect being similar to that of an early dark energy
(EDE) scenario. The dark radiation automatically redshifts at the minimal
necessary rate to improve the Hubble tension. On the other hand, the addition
of extra radiation to the Universe can improve the LSS tension. We explore this
model in light of cosmic microwave background (CMB), baryon acoustic
oscillation and supernova data, including the SH0ES $H_0$ measurement and the
Dark Energy Survey Y1 data release in our analysis. Our results indicate a
preference for the regime where the scalar field converts to dark radiation at
very high redshifts, asymptoting effectively to an extra self-interacting
radiation species rather than an EDE-like injection. In this limit, thermal
friction can ease both the Hubble and the LSS tensions, but not resolve them.
We find the source of this preference to be the incompatibility of the CMB data
with the linear density perturbations of the dark radiation when injected at
redshifts close to matter-radiation equality.
