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Mechanism - Cam Connections bug?

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Mechanism - Cam Connections bug?

Using Wildfire 2.0, I am trying to run a dynamic analysis on a simple bicycle model rolling down a sloped surface. The bicycle assembly simple has a chassis plus two wheels each pin jointed to allow rotation. This model is placed into an assembly by a planar connection. Also a sloped 'track' part is placed into the assembly below the bicycle, and cam connections created between the bicycle wheel surfaces and the track surface, to give the effect of friction acting between the wheel/ground. Gravity and mass are defined. When I run a dynamic analysis, the model falls under gravity until it hits the track (as it should), but then it stops dead there, held due to the friction coeff set in the cam connection - the wheels won't rotate (so it doesn't roll down the hill) and I can't understand why. It seems to be ignoring the rotation of the pin joint in the wheels. (when the coeff of friction is reduced, the bicycle wheels slide down the hill - however the wheels are NOT rotating) Also the bicycle itself remains in whatever angle it is originally placed at, when one wheel hits the ground, by laws of physics the model should then rotate under gravity so that the other wheel then subsequently hits the ground. However it does not do this. This seems to be ignoring the rotational degree of freedom within the planar connection definition. I have spent hours trying to correct this. Got nowhere. I am suspicious that it may be a bug in the software... Has anyone else had this problem? Does anyone know if I am doing something wrong? I would really appreciate some help with this! My dissertation relies on it! Thanks, Jon.
3 REPLIES 3
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RE: Mechanism - Cam Connections bug?

Sounds like (without seeing it) the fact that you have the cam connection applied to the sloped surface only (and not the horizontal surface) is causing you trouble. When it is reaching the end it is stopping. You could try enabling liftoff in the cam connection.
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RE: Mechanism - Cam Connections bug?

Thanks for the reply. The problem is, the bicycle never gets anywhere near to the end of the cam surface. Lift-off is enabled, and friction is defined between the wheel surface and the ground - this friction should surely encourage the wheels to spin as it hits the ground. Here is an image of the model in the snapshot that is set as the initial condition before the dynamic analysis is run (i've clicked on gravity to show its direction on the image): Note the yellow representations of the connections - clearly the wheels should have rotation. And here is where it lies after running a dynamic analysis (gravity & friction are enabled, duration 4 secs): As you can see, it has dropped onto the track, but stopped on one wheel. No rotation of either the bicyle chassis or the rear wheel has taken place - it seems to be ignoring these degrees of freedom that are set in their connection definitions. Any chance I could email a zip of the files to you? Thanks for the help, Jon.
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RE: Mechanism - Cam Connections bug?

Jon, Sounds to me like you are way ahead of me already (I don't use mechanisms a ton). I would try posting this problem on both mcadcentral.com and the pro-user and sim newsgroups at ptcuser.org. You will likely get a better response than I can give. Regards
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