How to set up vehicle collision in Sollumz

A vehicle has no separate collision file. Its collision is part of the fragment, next to the visible model:

mycar                  Armature, Sollumz type: Fragment (bones = the car's parts)
  mycar.mesh           Empty, Sollumz type: Drawable
    chassis            Mesh, Drawable Model (skinned to the bone "chassis")
    bodyshell          Mesh, Drawable Model
    door_dside_f       Mesh, Drawable Model
    wheel_lf           Mesh, Drawable Model, "Is Wheel Mesh" on
    ...
  mycar.col            Empty, Sollumz type: Bound Composite
    chassis.col        Bound Geometry, follows the bone "chassis"
    wheel_lf.col       Bound Disc, follows the bone "wheel_lf"
    door_dside_f.col   Bound Geometry, follows the bone "door_dside_f"
    ...

The Bound Composite is a container. The shapes under it do the work, and each one belongs to a bone, so a door's collision swings with the door and a wheel's collision sits where the wheel is. To see them while you work, use Sollumz Tools > General > View > Show Collisions.

Add shapes in Sollumz Tools > Collisions > Create Bounds with Create Bound and the type menu next to it. Stock vehicles use one bound per functional part, never one merged shape for the whole car.

Part	Sollumz type	Shape
Body (chassis)	Bound Geometry	A simple closed hull around the body.
Each wheel	Bound Disc	A disc the size of the tyre.
Doors, bonnet, boot, bumpers	Bound Geometry	A hull around each part.
Windows	Bound Geometry	A thin shape over the pane, with a glass collision material.
Simple blocks	Bound Box, Bound Sphere, Bound Capsule, Bound Cylinder	Sized with the values in the bound's Shape panel.

The fastest correct start is to copy the bounds of the stock vehicle you imported as reference. A copy keeps the type, the material and the mass. Then replace the mesh of each hull with one that fits your part. The hull mesh is written relative to its bone, so build it with the part at the bone's position.

Two steps, and a bound that misses either one does nothing.

1. Switch on physics for the bone. Select the bone, open Properties > Bone > Sollumz and tick Fragment Physics. The panel then shows the settings of that part: strength, damage health, and the glass options for windows.
2. Make the bound follow the bone. The bound object needs a Copy Transforms constraint whose target is the armature and whose bone is the part's bone. Sollumz Tools > Drawables > Bone Tools > Add Bone Constraint adds it with the right settings; you then choose the bone in the constraint. Older Sollumz versions used a Child Of constraint for this, which stopped working after Blender 4.2.

When a bone has physics on but nothing follows it, the export warns: "Bone has physics enabled, but no associated collision! A collision must be linked to the bone for physics to work." The opposite case matters too: a bone left with physics on after you deleted its bound should be switched off.

The constraint also places the bound. Leave the bound at its own origin and let the constraint carry it to the bone, so it follows when you move the bone later.

Bounds take collision materials only. They decide the sound, the sparks and the bullet behaviour of the surface. Pick one in Sollumz Tools > Collisions > Create Collision Material and press Create Collision Material. For a car body the usual one is Car Metal; there are also Car Plastic and the Car Glass materials for the windows.

Never leave a normal render material on a bound. On one of our cars a visible material on a wheel bound was enough to make the export fail.

Each bound has its own mass, shown on the bound object, and the masses add up to the weight of the model. Sollumz Tools > Fragments > Set Mass writes one value to all selected objects. Copying the bounds of a similar stock vehicle gives sensible numbers from the start.

Mass also sets the centre of gravity. When we added helper bounds at 200 kg each to one car, they moved its centre of mass and changed how it drove. Extra bounds that are only there to collide should weigh about 1 kg. The mass the driving physics uses is still fMass in handling.meta.

Each bound has a Composite Flags panel with two rows of switches: what the shape is, and what it collides with. On the stock vehicles we have opened, these are empty on the vehicle's bounds, and that is normal. Vehicle bounds do not need the map flags.

Map objects and props are different. There, a bound with no flags exports without any warning and collides with nothing in game. For those, apply a flag preset to every bound; Sollumz ships one named General (Default). If you move between props and vehicles, do not carry the rule from one to the other.

Symptom	Cause	Fix
Falls through the road as soon as it spawns	The fragment has no Bound Composite, or it was not exported	Add the bounds as children of the armature and export with the armature selected.
Sits on its belly, wheels in the ground	No wheel bounds, or they do not follow the wheel bones	One Bound Disc per wheel bone, sized to the tyre.
Export warns about a bone without collision	Fragment Physics is on but no bound follows the bone	Add the constraint, or switch physics off for that bone.
Collision is offset from the model	The bound was moved by hand and by the constraint	Reset the bound's own position and let the constraint place it.
Players walk through a part	That part has no bound of its own	Add a bound on that part's bone.
Car drives nose-down or tips over	Heavy extra bounds moved the centre of mass	Lower their mass.

If the model is fine and only the ride height is wrong, that is suspension, not collision: see vehicle troubleshooting.

Export once with CW XML ticked in Target Formats and open the .yft.xml in a text editor. Under the physics section, the list of groups should name every bone you gave physics to, and the list of children should have one entry per bound. If the groups are missing, no bone had Fragment Physics on.

The rest of the model workflow is in how to make a FiveM vehicle in Blender with Sollumz.