The torque controller software, motor, shaft, and your wheel becomes a spring-mass system, that has a natural resonance frequency. But there is no spring and mass - there is torque controller and rotating mass. There is always a tiny bit of noise in the current measurements from the motor, and the encoder is also read at 22bit resolution, so any tiny changes can cause a feedback loop. If any of that excites the system at the natural resonance frequency, buzzing will begin. By installing the shaft, you are changing the natural frequency to something else than the system is tuned for.
Simucube 2’s torque controller is quite tighly tuned for optimal FFB experience, so any new joints in the shafts that have any play in them, can cause the system to hit the natural resonance frequency.
Regarding long shafts, the motor bearings are not designed for extensions, and due to this we recommend an external support bearing for any longer extensions than 100 mm to not cause damage to the motor due to non-axial loads. Warranty does not cover these damages.
Mika of course has the technical answer, but one question after installing the extension did you recalibrate the system…
The reason is as Mila said it controller is very sensitive meaning that with your old calibration you could be slightly off of what it needs and therefore causing it to vibrate where recalibrating may alter the settings a bit allowing it to work properly.
There is no recalibration in the device that would affect the buzzing. There is encoder recalibration that can be used if there is any suspicion that encoder is slipping on the motor shaft, but that is not the case here.
About the weird effects - which sim, which car, which track, which setup?
Actually Motor/Encoder/Centerpoint recalibration was what I was talking about. While @Mika mentions that there is nothing to do with buzzing… I have actually had that issue WHEN you have a setting and then when you hold the wheel naturally and it places it in one of those odd feedback loops… This is why I mentioned that is that a slight change in the angle of the wheel after reconnection of the wheel and extension could do this. so it is worth a try.
Springyness is probably due to filter settings that are now needing to be different due to changes in the static weight of the wheel/Extension/Shaft. this would be something that is probably going to take some balancing of damping and inertia.
The encoder recalibration didn’t fix the issue unfortunately.
The springy feeling I get happens whenever there are any FFB movements. No matter the game, track or car - however I primarily run iRacing.
When the car runs over rumble strips on an exit of a corner, I would feel a light spring feeling. It would become more noticeable on bigger FFB effects e.g. crashing into a wall, hopping over a curb and etc.
Post your settings for the Game being used and the True Drive… as Mika is right in many cases a springy feeling can be due to clipping OR it can be due to misbalanced filter settings allowing the wheel to accelerate very fast to a new imputes.