DyMESH Version 3

DyMESH Version 3 (Engineering Dynamics Company, 2026 HVE Forum) adds a collision model for the vehicle wheels — DyMESH Wheels — so that impact forces and moments developed at the tires/wheels are computed and fed into the vehicle dynamics. Version 3 also finalizes the user-defined third-order force-deflection stiffness described in the Collision Model chapter.

Problem statement

• DyMESH Version 2 calculated 3-D collision forces and moments only for the sprung mass (the vehicle body).
• However, significant collision forces and moments can occur during impact at the wheel(s) — e.g. an angled wheel-to-wheel or wheel-to-body strike.
• Ignoring these forces affects the collision dynamics (post-impact heading, spin, and trajectory).

Solution criteria

The wheel collision model had to:

• Calculate forces and moments from the collision at each individual wheel.
• Integrate the results directly into the equations of motion.
• Be robust, accounting for:
• Wheel shape — a round cylinder.
• Wheel location / displacement (the wheel can be moved by the impact).
• Wheel steering and camber change.
• Wheel spin (tangential friction).

Solution

The wheel model reuses the DyMESH machinery by treating each wheel as its own DyMESH body:

• Treat each wheel as an independent DyMESH object.
• Create the wheel mesh based on tire size. In the code (InitializeDyMeshWheels / calcWheelMesh in Dymesh.cpp) a cylindrical wheel mesh is built from the tire radius and width, with NUM_DYMESH_WHEEL_INC (80) angular increments. Wheel material stiffnesses are set to their own constants — TRUCK_WHEEL_A_STIFF / TRUCK_WHEEL_B_STIFF (500 / 100) or PASS_CAR_WHEEL_A_STIFF / PASS_CAR_WHEEL_B_STIFF (250 / 50) — and a saturation deflection tied to the wheel half-width is assigned per vertex (deflMax).
• Place transforms in front of and following the DyMESH calculation to carry the wheel's location, steer, camber, and spin into and out of the collision computation.
• The user can select individual wheels for analysis (for computational efficiency — only the wheels expected to be involved need be simulated).

Flow charts

Version 3 adds a wheel loop nested within the existing sprung-mass collision loop.

Sprung Mass loop — for each pair of vehicles:

1. Collision Test → if no collision, skip.
2. Swap Vehicles (assign Master / Slave).
3. Get Vehicle Matrices.
4. DyMESH (Master, Slave) — the body-to-body collision.
5. DyMESH Wheels (Master, Slave) — the wheel collisions.
6. Smooth Collision Pulse.
7. Separation Test.

Figure: Sprung Mass flow chart.

Wheel loop — for each wheel:

1. Collision? → if no, skip.
2. Get Wheel Matrices.
3. Update Wheel Mesh (apply location/steer/camber/spin transform).
4. DyMESH (Master, Slave) — run the contact algorithm on the wheel mesh.
5. Add Wheel Force to Sprung Mass.
6. Update Wheel Mesh Damage.
7. Calculate Wheel Displacement.

Figure: Wheels flow chart.

(updated: In the current code these steps are implemented in Physics/Source/Simon/PHYMODEL.CPP as DyMeshWheels(), UpdateWheelMesh(), AddWheelForceToSprungMass(), UpdateWheelMeshDamage(), and DyMeshWheelDispl().)

DyMESH Wheels user interface

The wheel model is driven by three areas of the interface: the DyMESH Options, the per-wheel Set-up, and the Output vs. Time results.

DyMESH Options dialog

The DyMESH Options dialog (Options menu) as shown in the deck contains:

• Use DyMESH (master enable).
• Include Environment — also run DyMESH contact against the environment.
• Force To x-y Plane — constrain the collision deformation/force to the horizontal plane.
• Search — Automatic (use the calculated box size) or Set Box Size (use a user-entered box size, e.g. 20.00 in).
• Force Smoothing — Version 1 or Version 2.
• Timesteps — Use DyMESH Time Intervals, with Vehicle Trajectory (e.g. 0.0010 s) and Output Time Interval (e.g. 0.0200 s).

Figure: "DyMESH Options" dialog.

(updated: The current CDyMeshOptionsDlg differs from the deck screenshot. The General area now also carries a DyMESH Version No selector — radio buttons Version 3 and Version 4 — a Tow Vehicle / Trailer Contact checkbox (DyMeshGeometryConnection), and separate DyMESH start time and Environment start time fields. The Force Smoothing radio labels have been renamed to "Version 1 and 2" and "Version 3 and Later". The stand-alone Timesteps group shown in the deck is commented out in the current dialog code. An Advanced tab exposes the low-level search controls — Stop After one/two/all vertices, inside-vehicle test (master normal vs. CG), inside-polygon test (sub-areas vs. cross product), pushback method, "don't deform a single vertex", and "don't restore a large distance".)

Set-up (Wheels)

Each wheel is configured on the Damage tab of the Wheel Data dialog: Axle No., Side (Left / Right), Wheel is Damaged, Auto Start, Start Time, Duration (e.g. 0.1 s), and Peak Lock-up Torque. A Use DyMESH section supplies the wheel force-deflection and moment-deflection parameters:

Parameter	Example	Meaning
Max No-Damage Force	3000 lb	Force threshold below which the wheel is not permanently displaced
Displacement Rate	300 lb/in	Force per unit wheel displacement (stiffness)
Max Force	30000 lb	Saturation force
Max No-Damage Moment	3000 ft-lb	Moment threshold below which no permanent reorientation
Moment Displacement Rate	300 ft-lb/deg	Moment per unit angular change
Max Moment	30000 ft-lb	Saturation moment

Figure: "Wheel Data" dialog, Damage tab, with the Use DyMESH force/moment fields highlighted.

See also the wheel-data / wheel-displacement inputs documented in ../../05-tires-wheels/WheelsDlg1.md.

Output vs. time

The Key Results output reports, per wheel, the collision impulses and moments in addition to the usual kinematics and kinetics — e.g. for "Axle 1, Right": Fx Imp, Fy Imp, Fz Imp (lb) and Mx Imp, My Imp, Mz Imp (ft-lb), alongside the whole-vehicle (Kinetics) impulses. This lets the user see how much of the collision load each wheel carried.

Figure: "Key Results" output table highlighting the per-wheel impulse rows.

Wheel displacement capping (updated)

(updated: The current code adds a refinement not present in the 2026 decks. In PHYMODEL.CPP, UpdateWheelMeshDamage() now tracks the largest actual movement of any wheel-mesh vertex during contact, MaxContactDispl (the magnitude of the damaged-minus-undamaged vertex displacement). DyMeshWheelDispl() then caps the computed permanent wheel displacement by that physical contact motion: the force-derived displacement Displ = (Ftotal - FConst)/FLinear is limited by Displ = min(Displ, MaxContactDispl), and the per-time-step change in the x/y wheel displacement is likewise scaled so it cannot exceed MaxContactDispl. This prevents the wheel from being "bent" further than the collision physically moved it. See commits 12131c2 / 16a1825, "Cap DyMesh wheel displacement by contact motion.")

Validation

Validation is described as under way, starting from the initial conceptual design (see the Dial Engineering white-paper presentation at the 2012 HVE Forum), covering vehicle-to-vehicle collisions and other configurations.

Examples

The deck shows the wheel model applied to: an intersection collision, a truck tractor, a trailer under-ride, and (memorably) monster trucks. The main technical deck additionally shows angled collisions — a Ford pickup vs. Explorer on a graded road, and the RICSAC 1 (20 mph) and RICSAC 2 (32 mph) staged tests — as sample DyMESH validations.

Figure: Angled collision, Ford pickup vs. Explorer.

Figure: RICSAC 1 (20 mph) angled collision.

Figure: RICSAC 2 (32 mph) angled collision.

Summary

• DyMESH Wheel Impact is robust.
• Clean integration into the existing vehicle dynamics model (SIMON).
• More validation is needed.
• Initially released as a Beta option, now included in the commercial release.

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Source: DyMESH Version 3 (2026 HVE Forum) — organized and verified against DYMESH.H / Dymesh.cpp, 2026-07-05.

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