maybe acknex uses that minimum time per frame
to handle unwanted imprecisions.
A too big framerate could cause problems,
for example when using the frametime as factor in a calculation.

If the frametime gets too small, the factor might turn to
be zero -> crashing a function that depends ons positive numbers.

If you run an old 3D Dos game, it might crash, because they
calculate too many frames, wich messes up the calculations.

Anyway. 512 should be big enough...