The cog took a little 'persuasion', we welded it on the gearbox shaft in the end.

We thought about using a sprung tensioner, but it's just something else to break. After seeing chains snap in other robots we figured the impact force must be pretty high so let's make this as simple as possible.
Plus given we are over volting the motor by more than double (and it's driving a 2kg drum, not a plastic fa any more!) and have taped up the vents on it it seemed more safe to not let it run for more than a few seconds. Plus it's from a written off Astra and I found a bolt IN it the other day......