Physx generally sacrifices real-world accuracy for in-game stability. So I’m unsure exactly how Physx is handling everything on their end, but you can see a suspension system successfully working using ‘Add Force At Location’ in Mhousse1247 ‘Go Kart Physics Template’; which you can download here.
I would suggest digging into that template. The formula for his suspension system does vary quite a bit from yours.
Cheers,
TJ