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phys_lengthconstraint is a point entity available in all Source games. It is a constraint that restricts the maximum (and optionally minimum) distance two objects are allowed to be from each other. It is most often used to simulate rope, though it does not support the "stiffening" of rope attachment points that can be observed in reality; an object can rotate a full 360° in fact, clipping right through any rope that may be attached.

Note.png Note: Constraint is calculated not from the origin/mass centre of the attached entities, but from the entity's own origin and the location of its Hammer helper (attachpoint). Be sure to place them appropriately!

The constraint can be made rigid with its second spawnflag. It then behaves much like a phys_ballsocket, except that the objects can freely rotate around their own centres.

Example map: sourcesdk_content\hl2\mapsrc\sdk_phys_lengthconstraint.vmf

Tip.png Tip: To suspend a standard wooden crate and allow the player to break its constraint by pulling, use a force limit of 1350.


Additional Length (addlength) <float>
Elasticity of the constraint. It is allowed to stretch this many units over or under its maximum and minimum length. The constraint will try to spring back to its preferred length.
Minimum Length (minlength) <float>
Minimum distance between origin and attachpoint. Irrelevant if the constraint is rigid. Default is 0.
Attached object 2 point (attachpoint) <vector>
The second end of the constraint (the other is the origin). The second attached entity will be rigidly constrained to this location, and the first to the origin.


Entity 1 (attach1) <targetname>
Entity 2 (attach2) <targetname>
The entities to constrain. Leave one or the other field blank to constrain to the world.
Note.png Note: Only one entity will be constrained, even if several share the given targetname.
Constraint System Manager (constraintsystem) <targetname>
A phys_constraintsystem that this constraint should be a part of. This avoids the "jiggling" caused by constraints applied to the same set of entities fighting with each other.
Force Limit to Break (forcelimit) <float>
Impact force required to break the constraint, in pounds. 0 means infinite. A way of calculating this is to set it to the weight of an object that would break the constraint if it were resting on its objects.
Torque Limit to Break (torquelimit) <float>
Torque required to break the constraint, in pounds*inches. 0 means infinite. A way of calculating this is to multiply any reference mass by the resting distance (from the center of mass of the object) needed to break the constraint.
Play Sound on Break (breaksound) <string>
A sound played when the constraint is broken.
Follow teleport distance (teleportfollowdistance) <float>
If one constrained object teleports more than this many units away, the other will teleport with it.


Name (targetname) <string>
The targetname that other entities refer to this entity by.
Entity Scripts (vscripts) <scriptlist> (in all games since <Left 4 Dead 2>)
Space delimited list of VScript files (without file extension) that are executed after all entities have spawned. The scripts are all executed in the same script scope, later ones overwriting any identical variables and functions.
Script think function (thinkfunction) <string> (in all games since <Left 4 Dead 2>)
Name of a function in this entity's script which will be called automatically every 100 milliseconds (ten times a second) for the duration of the script. It can be used to create timers or to simulate autonomous behavior. The return value (if present) will set the time until the next call.
Note.png Note: Try to avoid expensive operations in this function, as it may cause performance problems.


  • 1 : No Collision until break
  • 2 : Keep Rigid
  • 1: No Collision until break
  • 4: Start inactive
  • 8: Change mass to keep stable attachment to world
  • 16: Do not connect entities until turned on



Force the constraint to break.
Enable the constraint; do this if the constrained objects don't exist when it spawns. Note that broken constraints cannot be turned back on as they have been deleted.
Disable the constraint.


Removes this entity from the world.
Removes this entity and its children from the world.
Note.png Note: Entities already remove orphaned children upon being removed, but this input removes all children on the same frame, being marginally faster than Kill.
AddOutput <string>
Adds a keyvalue/output to this entity. It can be potentially very dangerous, use with care.
KV Format: <key> <value>
I/O Format: <output name> <targetname>:<inputname>:<parameter>:<delay>:<max times to fire, -1 means infinite>
FireUser1 to FireUser4
Fire the OnUser outputs; see User Inputs and Outputs.
Use  !FGD
Same as a player invoking +use; may not do anything depending on the entity. Can also be invoked by firing an output that does not specify an input.
RunScriptFile <script> (in all games since <Left 4 Dead 2>)
Execute a VScript file from disk, without file extension. The script contents are merged with the script scope of the receiving entity.
RunScriptCode <string> (in all games since <Left 4 Dead 2>)
Execute a string of VScript source code in the scope of the entity receiving the input. String quotation may be needed when fired via console.
Bug.png Bug: In <Left 4 Dead 2>, the code is executed in the script scope of the entity that fires the output, not the one receiving the input.
Warning.png Warning: Never try to pass string parameters to a script function with this input. It will corrupt the VMF structure because of the nested quotation marks, which then must be removed manually with a text editor.
CallScriptFunction <string> (in all games since <Left 4 Dead 2>) !FGD
Execute a VScript function in the scope of the receiving entity.
SetLocalOrigin <coordinates> (in all games since <Alien Swarm>) !FGD
Send this entity to a spot in the map. If the entity is parented to something, it will be offset from the parent by this amount.
SetLocalAngles <angles> (in all games since <Alien Swarm>) !FGD
Set this entity's angles.



Fired when the constraint breaks.


OnUser1 to OnUser4
These outputs each fire in response to the firing of the like-numbered FireUser1 to FireUser4 Input; see User Inputs and Outputs.
OnKilled  (only in Left 4 Dead <Left 4 Dead 2>)
This output fires when the entity is killed and removed from the game.