# Model constraint: Simulating rolling casters

**Brian Mac G**Apr 17, 2013 8:39 PM

Hi all and thanks in advance for helping out. It's so great to have a forum like this, a trouble shared is a trouble halved and all that. Anyways, I have a kart (http://imgur.com/KhEdGVf) that rolls on four casters and I want to investigate how the legs of this kart will splay outward when it carries a load. The casters are connected to metal plates welded onto the bottom of the kart's frame; I call these "caster pads".

What I'd like to do is fix one caster pad and allow the other caster pads to deform outward. Ideally this would be done as follows:

- Constrain all three translations of a single point at the centre of one caster pad, while allowing rotations.

- Constrain a point on each of the remaining caster pads to translation along a horizontal plane, again allowing rotations.

I can acheive a something close to this by fixing one caster pad *surface *( ie. all translations and rotations) and allowing the other caster pad *surfaces* to roller/slide horizontally. However, I don't this it's realistic to constrain the caster pad surfaces against rotation from horizontal. A workaround I've used is to shrink the size of the constrained surfaces so that they approximate points, but this obviously causes a high stress spike around those constraints.

So, which this in mind I have a few questions:

1: I don't seem to be able to constrain a point on the solid mesh while restraining *translations *only; the constraint symbol that results when constraining a point indicates rotations are prevented for that point. Now, a point can't really rotate, can it? Is fixing translation of a point while allowing rotations of the geometry around that point possible?

2: The advanced restraints options only allow me to constrain a point on a plane and have it move in either a positive *or *negative direction in x or z, not both. Any idea why this is? I ask because I'm not certain which direction my caster plates might play out along although I can guess.

3: Adding the constraints the way I've described using the advanced restraints menu (and ignoring the issues raised by 1 and 2) just causes my solver to fail and make me sad. Any general thoughts or comments that may help? Perhaps having all the vertical load supported by points only isn't workable as the weight is supported by zero area?

Thanks again everyone!