Here's a thought on the unsprung weight thing;
Consider a tipper lorry running on leaf springs. It's got 20 tons of gravel (or feathers) on board and it goes over a bump. This bump is a very short bump and the lorry is travelling at say 40 mph. The back axle is a stonking great big thing wot weighs a ton. The axle bumps up and down over the bump. The lorry doesn't bump much cos it's so stonkingly heavy it can't move that quickly up and down in response to a short shove from the stonking springs. The unspung weight here is as people have implied, the one ton axle cos it's the road side of the leaf springs.
Now, the driver goes and tips out his gravel/feathers. He drives back down the same road and hits the same bump at the same speed. This time the axle follows the bump as before, at least tries to. The lorry gets a short shove from the stonking springs. But this time its so light, that the springs don't flex much and the whole lorry is pushed up and bounces off the road. Bounce, bounce bounce it goes. (And this is the really good bit.) Because the whole lorry is now unsprung, cos it hasn't got enough inertia to flex the springs.
Don't I go on 🤔
Susser
1/2 🙆🏻d lorry driver and 1/3 🙆🏻d physicist.
PS
Watch the wheels of one of them next time you're out.
It doesn't work that way with air springs cos ,,,,,,,,,I'm going for a drink.