Vehicle tracks animation

I’m looking at animating the tracks on this vehicle.
I’ve had a look through the Bongo forum and found 1 message years ago which sort of stopped mid discussion and didn’t seem to get resolved.
The rotation of driving front axle and the rollers I can setup for animation.
I’m looking for idea’s on animation setup for the tracks.
There are 75 individual pieces per track each side, they follow a path around axle and rollers.
Cheers Cian


As far as I know the structure of caterpillars is similar to that of a (bicycle) chain. Obviously, the idea of using Inverse Kinematics comes in mind first.
Caterpillar_IK_20.3dm (98.3 KB) shows such an attempt. The chain has only been build halfway because the required IK-structure involves a hierarchy of nested constraints, i.e. the result of a constraint based upon another constraint which is based upon another etc… Increasing the number of chain-links results in an exponential increase of the time needed by the IK solver. Solving Caterpillar_IK_20.3dm (with only 20 chain links) takes nearly 3 minutes - depending on the speed of your processor.
You’ll probably notice that the IK-step-counter freezes although solving continues. On my machine the preview is also somewhat jerky.

30 chain links take nearly 20 minutes to solve.

Caterpillar_IK_40.3dm (160.6 KB) All 40 chain-links: nearly 1 hour solving. The preview is a disaster. The caterpillar in your model even counts 75 parts so… the IK-approach is hardly practical.

To prevent IK solving to start automatically I save the files with IK disabled. Make sure IK is enabled before starting IK solving with the BongoPreCalculateIk command.

However… when a slight deviation in the connection of the chain-links is allowable an alternative can be found in the use of Simple Connections rather than IK.
Caterpillar_SC.3dm (316.2 KB)
An array of auxiliary points marking a hinge of each chain-link is positioned on the track curve by adapting the parameters of a ‘To Path’ constraint (0.000 … 1.000, 0.025 … 1.025, etc, till 0.975 … 1.975). The chain-links themself are attached to the auxiliary points by using an ‘Object To Pivot’ constraint and oriented by a ‘Look At’ towards the previous auxiliary point.
An ‘Object To Pivot’ Simple Constraint by its nature works as a universal hinge.
Since the distribution of the auxiliary points is linear along the curve there is a slight shift of the chain-links where the track bends. The length of the chain-link (center to center) is longer than the chord of the bow.
But I guess the anomaly will probably remain unnoticed by the viewer of the film… unless you are planning a close-up of the caterpillar.

Modeling all of this takes quite some time and patience but Simple Constraints are instantly solved!


Hi Luc,
Thanks for the reply, I’ll give option Simple Connections a try and see how I go.

Cheers, Cian

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Just let me know If any more issues arise… and otherwise please show us some footage.