# 2 pivot points on one object?

So I’m stuck trying to make something work, not sure the best way to go about it. I want to create the sketch below, where the purple is the “track” and the white is a rigid bar with wheels at each end which rides in the track. I’ve constrained the lower wheel to the blue line, but i cant figure out how to constrain the upper wheel to the same line and have it work the way I would expect. Is there a way to have multiple pivot points? The bar is a child to the lower wheel so it rotates with it but i cant figure out how to get the upper wheel to track AND keep that end of the bar in the right place. Any ideas?

Much appreciated!

I don’t know how to do this in Bongo, but it’s relatively easy in Grasshopper. The vertical pivot is constrained to the vertical line and the horizontal pivot is constrained to the intersection of the horizontal axis and a circle centered on the vertical pivot with radius the length of the bar. I suppose you can add these kinds of constraints in Bongo as well…

Short video…

–Mitch

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Mitch, thanks for taking the time to respond and to make that video! I assumed grasshopper would be another way to go but this is only part of a larger mechanism. There are additional components attached to the moving piece that change positions as well depending on the angle of the bar, and more from there. I could do it all in grasshopper but I was hoping bongo would eliminate a lot of work since that patch would get fairly large quickly and also it would be nice to be able to animate automatically.

I assume it can be done with Bongo but i’m missing something. Everything else seems to be working. If I can’t get it working ill probably switch to GH, in which case I may have a more questions!

Thanks again!

You should create a structure like this:

The lower wheel can be made moving horizontal by means of keyframes (Position transform). When you want it to move by means of a Simple constraint ‘To Path’ (using the blue line as a path) you’ll have to add an extra proxy (in the center of the wheel) as the ‘head’ of the chain, because a Simple constrained object can’t be part of an IK chain.
The upper wheel must be constrained as ‘Part of an IK chain’ to the blue line.
The lower wheel must be a Hinge, hence allowing the whole to rotate.
Succes.

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a demo.
rollbar.3dm (32.7 KB)

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Luc, thanks for your response. I think i figured it out based on that! Lots of other stuff to work out now, but this issue is resolved. I wasn’t doing the IK chain right - and I had the upper wheel as a joint & constraint. Now that I have a better idea of how it needs to be set up the rest of it should go much smoother. Thanks!!

My pleasure. Any more issues: keep on posting.
Maybe you can have a look at a little PDF tutorial I once made:
[http://bongo.rhino3d.com/forum/topics/the-basic-why-s-of-IK][1]
[1]: http://bongo.rhino3d.com/forum/topics/the-basic-why-s-of-ik.

I must rectify this; an Simple constrained object can sure be part of an IK chain but it cannot be a joint (Hinge, …) simultaneously.

And how about 3 pivot points on one object?
I’m trying to animate a object like this without results.

I don’t understand how to allow every bar to rotate in 3 points: at the ends and in the middle.

Any ideas?

Please have a look at this old post with instructions:

It should give you an idea on how to get it animated.