@tm1v2 Best I could describe is that if I'm going between 50 to 80 mph on the highway, I experience 2 different behaviors with encountering a bump. Either the car goes over the bump and I barely feel it, which is great, or, it if it's a larger bump, then I get the feeling of the tired get knocked into the air. It's significantly worse in the rear. My wife and cargo will get airborne like you'd experience in the back of a school bus!
It's hard for me to predict which bumps will cause this to happen. Which sucks because this is really reducing my confidence while driving in the highway.
@nadkoolio I'm now thinking it's both the spring rate and compression damping, but more the damping.
For a comparison point, I haven't felt that on my Model 3. I have 11kg rear springs, vs 12kg for your R&T, but my M3P should be about 500 lbs lighter than a dual motor Model Y. Assuming 250 lbs lighter in the rear (could be wrong), and assuming motion ratio is very similar between 3 and Y, my ratio of rear spring rate to rear vehicle weight should be very similar.
So what's actually softer on my car? I think it's the damping / valving.
Now to be clear if it's a big bump you can certainly feel it in my car, I don't have the soft GT kit, but I've
never felt like my wheels left the pavement over any bump. In fact quite the opposite, I feel like the suspension on my car does a supreme job of keeping the wheels planted and the car's weight settled. Any time I've tried to describe how this suspension feels, "planted" and "settled" are pretty much the first words that come to mind.
Only going fast over a crest have I felt it go light and struggle to maintain contact with the road. But doing jumps in the family car is bad mmmkay?
I still really want to see pictures of these bumps though, or a detailed written description. Many of the roads where I am are in pretty poor condition, but it's possible yours are even worse and that's the real difference. Can you pull up pictures of some bumps where the rear got light in Street View (in Google Maps)?
I think you're right that the combination of the strength of the compression damping and the stiffness of the spring isn't allowing the spring to compress enough to absorb the impact.
I'm considering switching to a softer spring (I might just through the OEM spring on there and see how it feels), but my fear is also what you suspect that the main culprit is an excessive amount of compression damping.
Swapping in the OEM rear springs would be a nice experiment. If you do that let us know how it feels! I'm really curious now if a softer spring alone would solve this issue.
Btw it sounds like some sandbags in the trunk might be the easiest solution.