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Why Isaac Sim Doesn’t Always Match the Textbook
Collisions, friction, and contact materials are central to realism in robotics simulations, and Isaac Sim currently doesn't support soft contact compliant or frictional materials, or full contact geometry. Find out how Champion can help fill the gap.
why-isaac-sim-doesnt-always-match-the-textbook-the-gap-in-contact-impact-models
Last updated:
September 3, 2025
Isaac Sim gives robotics teams a powerful, GPU‑accelerated sandbox. But if you’ve ever noticed robots jittering, boxes sinking a few millimeters, or wheels spinning forever, that’s not your CAD or URDF misbehaving—it’s the physics engine making trade‑offs.
Here are the biggest gaps between ideal contact physics and what Isaac Sim actually does under the hood:
1. Friction cones → Friction pyramids 🟦⬛
- In theory: friction resists motion equally in every direction.
- In Isaac: PhysX uses a pyramid approximation (a couple of fixed axes). It’s faster, but motion along diagonals can behave differently.
2. Exact inequalities → Iterative solvers 🔁
- In theory: contacts are solved so there’s no penetration and perfect stick/slip transitions.
- In Isaac: an iterative Gauss‑Seidel solver only approximates the solution. You may see tiny penetrations or jitter if iteration counts are low.
3. Instant impulses → Soft contacts 💥🪵
- In theory: collisions are instant (impulses) or continuous (springs/dampers).
- In Isaac: by default collisions are impulses. You can enable compliant mode with
compliant_contact_stiffness, but restitution (bounce) gets ignored when it’s on.
4. Tangential & torsional restitution ↔️⟳
- In theory: some models allow bounciness in tangential or rotational directions.
- In Isaac: not supported. Tangential impacts are always inelastic. Torsional friction exists (via patch radius), but it isn’t exposed in the default schema.
5. Curved surfaces → Point contacts ⚪🔺
- In theory: smooth curves touch along areas or lines.
- In Isaac: everything is reduced to a handful of point contacts. Curves are approximated with triangles or convex hulls, which can cause jitter unless you use primitives or SDF colliders.
What this means for you 🚀
- Slipping or jittering? Increase solver iterations or switch to the TGS solver.
- Collisions too bouncy or stiff? Tune
restitution,contactOffset, or try compliant contacts. - Wheels spin forever? Add torsional friction or script rolling resistance.
- CAD geometry sticking weirdly? Simplify meshes or switch to primitives/SDFs.
⚡ Bottom line: Isaac Sim trades some physical purity for real-time performance. Most of the time that’s a smart call, but knowing where the engine cuts corners helps you tune your simulations and spot engine limits—not physics limits.
👉 At Champion3D.io, we’re building CAD-to-physics enrichment pipelines to bridge this gap—so your assets are truly simulation-ready from day one.
