Implement a tire friction model through plugin?

[Sciancisco]

Intro

Hi!

I am a graduate at Université de Sherbrooke, I'm investigating the use MuJoCo for my research on car control.

My setup

Latest stable release from PyPI, Ubuntu 22.04, eventually 26.04.

My question

Since the most important part of a car model is the tire model, I want to know if I could write a plugin with the passive capability to implement various friction models of tires (e.g. Pacejka's Magic Formula).

From what I could understand from the documentation, the friction cone is only used to scale the static friction coefficient and switch to the kinetic friction coefficient once static friction is saturated. I also grasp that the collision detection / contact handling / friction computation is quite core the physics engine of MuJoCo.

I guess I'm wondering if a plugin could only use the contact forces and slip/slip speeds and substitute the friction machinery altogether.

I found this issue from a year ago, unanswered sadly [https://github.com//discussions/2838].

If impossible, what parameters could be used to model "simply" a tire.

Minimal model and/or code that explain my question

For now I have a simple bicycle model to prototype.

scene.xml

<!-- Copyright 2021 DeepMind Technologies Limited

     Licensed under the Apache License, Version 2.0 (the "License");
     you may not use this file except in compliance with the License.
     You may obtain a copy of the License at

         http://www.apache.org/licenses/LICENSE-2.0

     Unless required by applicable law or agreed to in writing, software
     distributed under the License is distributed on an "AS IS" BASIS,
     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     See the License for the specific language governing permissions and
     limitations under the License.
-->

<mujoco>
  <compiler meshdir="asset" texturedir="asset"/>

  <statistic meansize=".05"/>

  <visual>
    <rgba haze="0.15 0.25 0.35 1"/>
    <quality shadowsize="4096"/>
    <map stiffness="700" shadowscale="0.5" fogstart="1" fogend="15" zfar="40" haze="1"/>
  </visual>

  <asset>
    <texture type="skybox" builtin="gradient" rgb1="0.3 0.5 0.7" rgb2="0 0 0" width="512" height="512"/>
    <texture name="texplane" type="2d" builtin="checker" rgb1=".2 .3 .4" rgb2=".1 0.15 0.2"
      width="2048" height="2048" mark="cross" markrgb=".8 .8 .8"/>

    <material name="matplane" reflectance="0.3" texture="texplane" texrepeat="1 1" texuniform="true"/>
  </asset>

  <worldbody>
    <light diffuse=".4 .4 .4" specular="0.1 0.1 0.1" pos="0 0 2.0" dir="0 0 -1" castshadow="false"/>
    <light directional="true" diffuse=".8 .8 .8" specular="0.2 0.2 0.2" pos="0 0 4" dir="0 0 -1"/>

    <geom name="ground" type="plane" size="0 0 1" pos="0 0 0" quat="1 0 0 0" material="matplane" condim="1"/>
  </worldbody>
</mujoco>

bicycle.xml

<mujoco>
    <include file="scene.xml"/>

    <compiler angle="degree" coordinate="local" inertiafromgeom="true" settotalmass="100"/>
    <default>
        <joint armature="0" damping="0"/>
    </default>
    <asset>
      <texture builtin="flat" height="1278" mark="cross" markrgb="1 1 1" name="texgeom" random="0.01" rgb1="0.8 0.6 0.4" rgb2="0.8 0.6 0.4" type="cube" width="127"/>
      <texture builtin="checker" height="1" width="1" rgb1="0 0 0" rgb2="0.8 0.8 0.8" name="wheel"/>
      <material name="wheel" texture="wheel"/>
      <material name="geom" texture="texgeom" texuniform="true"/>
    </asset>
    <worldbody>
        <body name="body" pos="0 0 5">
            <camera name="follow" mode="targetbody" pos="-4 0 1" xyaxes="0 -1 0 0 0 1"/>
            <camera name="follow lat" mode="targetbody" pos="0 -3 .6" xyaxes="1 0 0 0 0 1"/>
            <camera name="hotwheel" mode="trackcom" pos="-0.152 -6.978 6.034" xyaxes="1.000 -0.017 -0.000 0.010 0.607 0.795"/>
            <geom fromto="-0.5 0 0 0.5 0 0" type="capsule" size="0.05"/>
            <joint axis="1 0 0" limited="false" name="rootx" pos="0 0 0" type="slide"/>
            <joint axis="0 1 0" limited="false" name="rooty" pos="0 0 0" type="slide"/>
            <joint axis="0 0 1" limited="false" name="rootz" pos="0 0 0" type="slide"/>
            <joint axis="0 0 1" limited="false" name="rootyaw" pos="0 0 0" type="hinge"/>
            <!-- <joint axis="0 1 0" limited="false" name="rootpitch" pos="0 0 0" type="hinge"/> -->
            <body name="front-tire" pos="0.5 0 0">
                <!-- <geom type="cylinder" size="0.2 0.05" axisangle="1 0 0 90" material="wheel"/> -->
                <geom type="ellipsoid" size="0.2 0.2 0.05" axisangle="1 0 0 90" material="wheel" condim="3"/>
                <!-- <joint axis="0 0 1" limited="false" name="direction" pos="0 0 0" range="-30 30" stiffness="400" damping="100" type="hinge"/> -->
                <joint axis="0 0 1" limited="true" name="direction" pos="0 0 0" range="-35 35" stiffness="100" damping="100" type="hinge"/>
                <joint axis="0 1 0" limited="false" name="fr-rot" pos="0 0 0" type="hinge"/>
            </body>
            <body name="rear-tire" pos="-0.5 0 0">
                <!-- <geom type="cylinder" size="0.2 0.05" axisangle="1 0 0 90" material="wheel"/> -->
                <geom type="ellipsoid" size="0.2 0.2 0.05" axisangle="1 0 0 90" material="wheel" condim="3"/>
                <joint axis="0 1 0" limited="false" name="rr-rot" pos="0 0 0" type="hinge"/>
            </body>
        </body>
    </worldbody>
    <actuator>
        <motor joint="direction" name="servo" gear="-120"/>
        <motor joint="fr-rot" name="prop" gear="120"/>
        <motor joint="rr-rot" name="trac" gear="120"/>
    </actuator>
</mujoco>

Confirmations

• I searched the latest documentation thoroughly before posting.
• I searched previous Issues and Discussions, I am certain this has not been raised before.

[Miko997]

A passive-force plugin can add your tire forces, but it won't automatically replace the contact solver.

The timing matters: passive forces are evaluated before the current constraint forces are solved. You can't read the current normal contact force there as an already-computed input. Your load estimate or coupling scheme needs to account for that.

There's also a detail in your XML: the ground uses condim="1", but the tires use condim="3". At equal priority, MuJoCo uses the larger value, so those contacts still have tangential friction.

If your custom model supplies the tire's tangential force, make sure the wheel-ground contact isn't also supplying it. I'd first validate that separation and the normal-load model in a small wheel test.

Relevant docs: [pipeline order](https://mujoco.readthedocs.io/en/stable/computation/index.html#stages) and [contact parameter mixing](https://mujoco.readthedocs.io/en/stable/modeling.html#contact-parameters).