# solref\solimp Parameter Cheat Sheet

This cheat sheet provides recommended **solref** and **solimp** parameters for common materials in MuJoCo. These values serve as **practical starting points**, not absolute rules—MuJoCo’s contact model is approximate, and optimal settings depend on:

* mesh quality
* object mass & inertia
* simulation timestep (`timestep`)
* scene stability

Fine-tuning may still be required for precise results.

***

### 🔧 Understanding Parameters

#### **Solref**

Controls **contact stiffness** and **damping**, defined as:

```
(solref_time_constant, solref_damping_ratio)
```

* Lower time constant → stiffer contact
* Damping ratio controls bounciness

***

#### **Solimp**

Controls **contact transition region**, defined as:

```
(dmin, dmax, width, mid, power)
```

For most rigid objects, the **default solimp is sufficient**.\
You only need to change solimp when simulating **very soft** or **special contact behaviors**.

***

## 📋 Recommended Material Parameters

<table><thead><tr><th width="121.5">Material Type</th><th width="147">Typical Applications</th><th width="136.5">Solref </th><th width="173.5">Solimp</th><th>Physical Property Description</th></tr></thead><tbody><tr><td>Rigid Metal / Concrete</td><td>Robot Bases, Connecting Rods, Ground Surfaces</td><td>0.002 1</td><td>0.9 0.95 0.001 0.5 2</td><td>Extremely hard. Almost no deformation upon impact and no rebound. Suitable for high-precision mechanical structures.</td></tr><tr><td>Hard Plastic / Hardwood</td><td>Building Blocks, Desktops, Housings</td><td>0.006 1</td><td>0.9 0.95 0.001 0.5 2</td><td>Hard yet stable. Slightly "softer" than metal, which can enhance simulation stability and reduce jitter.</td></tr><tr><td>Hard Rubber / Tires</td><td>Robot Foot Pads, Wheels</td><td>0.015 1</td><td>0.9 0.95 0.001 0.5 2</td><td>Slightly tough. Provides grip and allows minimal deformation upon contact.</td></tr><tr><td>Soft Silicone / Finger Tips</td><td>Soft Fingertips of Dexterous Hands, Sealing Rings</td><td>0.025 1</td><td>0.95 0.99 0.001 0.5 2</td><td>Soft. Allows visible compressive deformation at contact points, increasing contact area and facilitating grasping.</td></tr><tr><td>High-Elasticity Ball</td><td>Table Tennis Balls, Marbles</td><td>0.02 0.1</td><td>0.9 0.95 0.001 0.5 2</td><td>High resilience. Extremely low damping (0.1), retains most energy and rebounds after impact.</td></tr><tr><td>Sponge / Shock-Absorbing Foam</td><td>Cushion Pads, Anti-Collision Strips</td><td>0.04 4</td><td>0.8 0.99 0.001 0.5 2</td><td>Energy-absorbing. Over-damped (>1), similar to hitting dough—energy is quickly dissipated with no rebound.</td></tr><tr><td>Biological Soft Tissue</td><td>Human Skin, Muscles</td><td>0.03 1</td><td>0.9 0.95 0.001 0.5 2</td><td>Flexible and tough. Similar to soft silicone, but exhibits compliance over longer time scales.</td></tr></tbody></table>


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