Hey all,
Missing motion blur on cached geometry is one of the most-asked questions in Houdini's history — and when it's not missing, it often overshoots into long streaks at every impact. Here's the complete method, plus a free kit if you want the code ready to paste.
Why per-frame caches lose blur
Per-frame caches (.bgeo.sc, per-frame Alembic, VDBs) throw away the between-frame info. To create blur the renderer needs either a velocity attribute v to extrapolate along, or two or more time samples to interpolate between. Changing topology (particle birth/death, fractures, fluid meshes) makes interpolation impossible — those cases must use velocity blur.
The decision table

Particles / changing point count -> velocity blur (v)
Deforming mesh, constant topology -> time samples / Cache LOP
Volumes (pyro, smoke) -> velocity blur from vel
Camera / transform animation in LOPs -> Cache LOP + matching camera shutter

The fix — if v got stripped, rebuild it before the cache (Trail SOP -> Compute Velocity), or by hand in a Solver SOP:
vector prevP = point(1, "P", @ptnum);
v@v = (@P - prevP) / f@TimeInc;
Then enable velocity blur on the renderer. In Karma/Solaris it's read from v automatically on most geometry; animated transforms need a Cache LOP so the USD stage carries multiple time samples.
When blur overshoots at impacts, clamp velocity in the render branch only — never touch the raw cache:
float maxspeed = ch('max_speed'); // ~2x your hero speed
float spd = length(v@v);
if (spd > maxspeed) v@v *= maxspeed / spd;
Works on Houdini 19.5 / 20.0 / 20.5 / 21.0. In H21's Karma (Hydra 2) these rules are unchanged.

That's the full method. I packaged it into a free kit that adds the clean copy-paste VEX + Python files, a third fix (neighbour-smoothing for impact spikes), a one-click batch-render tool, and a printable reference card:

Free on Gumroad (secure checkout):

https://worldvfx.gumroad.com/l/motion-blur-fix-kit [worldvfx.gumroad.com]

It's a slice from a larger Houdini Automation Cookbook I'm building. Feedback welcome.