We have released RumiCar Simulator v7.6.0. The release notes below are also available on GitHub.
v7.6.0
Added a surface type to courses (precision v2 engine only; nothing changes if you leave the default). Writing surface:”loose” in a course definition gives you a loose surface like gravel, dirt or snow. On a loose surface the tire sinks into the top layer and builds a bank of material ahead of itself and pushes it aside (digging). That force is set not by friction but by how much material was displaced, so it grows the more you slide and lateral force stops falling off. This is why the optimal slip angle is 5–10° on tarmac but 20–30° on dirt; in the simulator the peak likewise moves from a slip angle of 8.0° to 22.8°, and the lateral-force-to-peak ratio at large slip goes from 0.89 (tarmac) to 1.19 (loose). Until now a surface was described by grip (peak multiplier) and muDecay (post-peak asymptote), and both can only make a curve that falls as you slide (muDecay is clamped to 0.95 in the implementation, so writing 1.0 still does not give you a surface that never falls off). Its effect on drifting turned out not to be straightforward. Making the low-μ benchmark loose raised the number of drift runs that get round without breaking from 38 to 202 (about ×5.3), and one hairpin that no drift line could clear on tarmac became clearable (this effect stays at ×5.3–5.4 when the sweep is made coarser or finer). But neither of the two expectations we started from was confirmed: ① “the drift-versus-grip lap-time ratio drops on loose” flips sign when the search grid changes, and ② “more ways of getting round with deep slip (sideslip angle above 20°) become available” does not hold in aggregate (28→18 runs). Some cells gain and some lose, and we have not identified the mechanism (the minimum turning radius was not the dividing line). Nor does digging make a surface slippery — adding it to the same grip makes the surface uniformly grippier, so real dirt being “slippery” comes from its low grip. To get dirt-like behaviour, lower grip as well as setting surface:”loose”. No physical invariant is broken: the term changes only the magnitude and never the direction, so the steady-state tire force stays dissipative and the friction circle |F| ≤ μ_eff·Fz is still guaranteed structurally. A standing gate (wf_av1_loose.mjs, 39 assertions) and a real-browser gate (25 checks) verify this by machine, and with the default (no surface field) the frozen benchmarks f0–f3 and the official race verification hash do not change by a single bit. Note that the HUD “friction-circle usage” shows the normalized slip as a percentage, so on a loose surface the maximum-grip point reads 300% (a display-layer matter; the physics is unchanged). We also filled in the missing description of muDecay in the 📐 course data specification, and fixed the section ordering in the physics notes (13.14 had ended up after the index).
Verifying this release yourself
cd Simulator
node wf_run_all.mjs # the full assertion-gate suiteThe suite runs from a fresh clone with no dependencies beyond Node.js.
The real-browser checks under Simulator/browser/ additionally need npm install and a display.