Why do vehicles handle better with heavy loads?

Question

Inspired by comments here: I'm not the only one who has experienced an improvement in ride and road-feel with a heavily-laden vehicle. I'm recalling a long drive I made a decade ago in my otherwise crappy 1995 Mitsubishi Galant with nearly half a ton of equipment. That load turned it into a better car: Its ride went from flighty and harsh to soft and planted. It was not only more connected to the road, but its response to imperfections were more dampened – characteristics I have only seen together in much more expensive cars.

All I did was add weight to transform it from the car I had to drive into a car I wanted to drive. The question is: Why? And more particularly: Can that effect be reproduced without actually adding half a ton of unneeded weight?

We know that much weight made several changes to the vehicle:

1. Shifted the center of mass to the rear, and lowered it a bit.
2. Compressed the suspension.
3. Which would have also increased negative camber.

More recently, I have taken to aligning all my cars to the limits of their negative camber specs, so I can appreciate what that does, but it doesn't nearly account for everything I experienced saddling that old sedan with weight.

I've spent plenty of time in near 50/50 RWD cars, so I know what neutral weight distribution feels like.

So I'm very convinced that whatever the weight did to the suspension had a great deal to do with the car becoming "smooth and connected." And in retrospect it seems like that would make sense: After all, with static suspension components manufacturers have to design in enough room for the vehicle to take a load up to its gross vehicle weight, without riding on the stops or pushing the wheels out of line. With a single passenger a standard sedan is half a ton shy of its design weight. Is it plausible that an unladen standard suspension allows more noise and is less connected? Or, put another way, if you knew you were never going to put more than a 200-pound passenger in a sedan, could you take the "excess" out of the suspension and expect a better ride?

Answer 1

The smoothness of the ride is determined by the ability of the suspension to respond to changes in the road surface without affecting the body of the car. The ratio of sprung to un-sprung weight will greatly affect this. By increasing sprung weight with more luggage, your car body will have greater inertia and therefore will allow the wheels to move up and down on the road without moving the body, thus making the drive more comfortable.

Answer 2

Presumably the bulk of the equipment was in the boot / low down. If you'd strapped it to the roof you might not have found the same improvement in handing came about.

What you had effectively done was lower the centre of gravity of the vehicle. This is the same reason that ground-up designed fire engine such as the Dennis Rapier sling their water tanks low between the axles. Traditional truck conversion based fire engines become an absolute handful when they're water tanks are full, the Rapiers handling improves.

There will be various other reasons for the improvement too such as the height of the vehicle will have been reduced, the pre-load on the springs and dampers will have been increased and possibly if the gear was in the boot, the weight distribution of the vehicle will have been centralised as you've effectively counteracted the weight of the engine and gearbox up front. However, the biggest single reason would be the lowering of the centre of gravity of the vehicle.