Ill try to answer some of that.
There isnt any support for materials and weight yet. But that isnt the problem with the piston model.

Regarding joints. Here is a simple rule that might help. If you want a joint to move its got to be in a group with an object the moves.

Here is how I updated the piston model.
-First I disconnected all the joints.
-Then I grouped the wheel and the 2 hinges that will connect to the pistons. This will allow those 2 hinges to move when the wheel spins.

Now using the Joint connection tool.
-Attach each of the pistons to the sliders.
-Attach each piston to its hinge in the wheel group.
-Attach the wheel group to the center hinge. Note: this hinge inst in a group so it wont move. That will keep the wheel from falling.
-Right click the center hinge and select "joint settings". Set accel to 1.0 and damp to 0.2. This will act like a motor.

BUT. If you play the model you will see it still doesnt work. The reason is because when the wheel turns it wants to push the pistons left and right as well as up and down. The slider prevents the pistons from moving left right so it stalls. A real piston has a rod and a head for that reason.

Chris

piston - 2.skp

Yeah they are all there. And then some, it looks like you have SketchyReplay installed too. You can probably remove that. It is way out of date.

Chris

Not quite the same thing. But this guy did a bridge!

@unknownuser said:

Convexhull:
Is this simply a cube (re-assinged to covexhull)and the form adjusted as necessary?
Forgive me if I have missed something obvious.

C

It doesnt have to be a cube. Any shape will work. The easiest way to imagine a convex hulled object is to picture it wrapped in plastic. Here is a more detailed explanation:

(www.rustycode.com)

If you want to see what the physics engine thinks an objects shape is:Right click the object and select Debug->Readback Collision Geometry. Then press play.

Chris

@unknownuser said:

Ok, so I'm playing around with my bridge destroying it, throwing the car around, etc. when I come up with the idea of rendering the bridge, and once I had a nice render of it I couldn't help but make one of it destroyed .

These renders are pretty sweet. I assume it is Kerkthea?

@unknownuser said:

Thanks for showing me what "convexhull" does, with that info I have revised it again.

Very nice! It looks elegant and its really stable. I vote for this one.

@unknownuser said:

Wow... what a balancing act... I like it... To bad it violates rule #1, but otherwise I like it. And I revised mine, it looks a lot better now.

FYI, You can make perfect arches if you use the convex hull shape. Here is an example:

arch1.skp

@unknownuser said:

No, you don't need to be scared, you're supposed to fix that and make it a working bridge. Like this one. And I'll add another rule to the list #7 You may make minor additions to the abutments.

That is amazing! Very nice construction ArchitectBoy.

I was going to do a arch bridge but yours is better. Maybe I'll try a trestle bridge.

@unknownuser said:

I downloaded SketchyPhysics and I don't know how to do it. Will you help me?

You need to explain the problem.