Strange SCX10 type drivetrain phenomenon!
 

I have an SCX10 esque vehicle, think all metal (Aliexpress) Chinesium, which is properly shimmed and buttery smooth. However, today I have received a 55T motor and I've been busy messing.

I noticed, after removing the spur gear and the attached clutch/friction pad, that when I rotated the opposing alloy friction disc it was buttery smooth but in the other direction there was resistance. By resistance I don't mean friction but more like magnetic or gravitational, if you get my meaning.
I was using the pad of my finger on the edge of the friction plate and it was not a problem in the one direction but slipped in the other, regardless of pressure.

I'll say again, everything is shimmed to perfection but what comes to mind is the differentials and the direction the gears rotate.

Is there some sort of mechanical phenomenon that produces this? What is also strange, is that if I lift the vehicle off of its tyres the buttery smooth prefection is the same in both directions of drivetrain rotation.

Any ideas?

Tom.