The flex of the rear axle is a design target in order to "soften" the ride...
I had to go pickup my mudda-in-law a couple months back. About a 420 mile round trip. I did it in the mighty Blueberry. I saved MEEEEELLIONS in fuel expense.
On the way back, there's a railroad track that seemed to bottom out the rear suspension. It felt like the rear tires hopped off the ground after bottoming out. It was rough ... and the mother-in-law had just had shoulder surgery so she didn't like that. But aside from that 1 event in roughly 16k miles, I prefer the softer suspension. When I decide to race the Blueberry in IMSA at Road Atlanta against the experimentals (or whatever they're called), I'll consider a sway bar.
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View my fuel log 2020 Mirage ES 1.2 manual: 42.4 mpg (US) ... 18.0 km/L ... 5.5 L/100 km ... 51.0 mpg (Imp)