Problem in modeling a robot

[skye53969]

Hi,

This is my first attempt to model a differential drive robot. I try to follow the BubbleRob tutorial, and other existing models, but the caster wheel of my robot was dismantled after it ran for a while. What am I doing wrong? This is my model:
https://www.dropbox.com/s/cdlvu1dnzv8zqui/econ.ttt?dl=0

[coppelia]

Hello,

make sure to carefully read this page, in particular design considerations 7 & 8.

Cheers