Fully independant suspension or floating axle discussion

[gomulletgo]

This is a question I have been meaning to ask for sometime, but seeing as everyone is going on about the rollovers at sheffield it makes it alot more relevant.

I want to know if the type of suspension a vehicle runs has an effect on how "roll happy" it is.

Lets say we have two vehicles, both with identical weights, identical center of gravity (COG) etc. the only thing being different is that one vehicle is fully independant suspension the other has floating or live axles...........if these two vehicles were to be put on a sidling which would be the first to tip over and why? or would they tip at the same time?

Something about the theory of live axles and how they articulate "pushing the other wheel on to the ground" made me think that it may be a disadvantage over fully independant suspension on sidlings.

Any thoughts and explanations would be great

[albundy]

sounds like we need mythbusters.
Al

[SupraLux]

I'd say it would have everything to do with the way it was set up (softnes, shocks, height for COG etc...) and nothing to do with the type of suspension.

On a side-slope a low COG, wide track and stiffer suspension would be an advantage - this can be done with any suspension setup. Live axle or IFS.

Rollovers are caused mainly by bad luck, COG too high for the slope, and of course bad judgement (driver error).

IFS may seem better for a side-slope simply because as a rule solid axle trucks are easier to lift, so get lifted higher, so end up with a less stable stance. The IFS truck will be lower, less modified and more stable as a result (generally speaking).

Steve

[4WDbits]

If both vehicles have rock hard suspension that doesn't move as the sidling increases then they will both tip at the same time.
What will make one roll over first is the movement of weight to the downward side of the vehicle as the sidling angle increases. As has been pointed out this would depend on how soft/hard the suspension is. Assuming both are the same then it comes to roll centres which affect the amount of weight transfered to the downward side as the sidling angle increases. As the weight shifts it will also affect the forces on the suspension, for example a solid axle vehicle with the springs and shocks near the centre of the vehicle will rollover before one with the springs and shocks out near the wheel.
I'm sure someone who knows more will correct me.

[KiwiBacon]

What you're talking about is the "roll centres" of the suspension and yes they have a big effect.

The roll centre describes what arc the wheels move in relation to the body. Generally solid axle suspension has a higher roll centre than independent. This means that on a sideslope and without the aid of a swaybar, a solid axle suspension will lean over less than an independent suspension with the same effective spring rates.


If your roll centre and centre of gravity were at the same height, then your vehicle wouldn't lean on it's suspension when on a sideslope. But there's a big downside.
The downside is, you don't feel the transition until it's too late and the top wheels start to lift. Once that happens you're on the way over.

In a cornering situation, lower rollcentres cause the vehicle to lean more, but also pull itself lower. With the addition of a swaybar, the body roll is controlled but the vehicle still pulls itself lower, becoming more stable.
In the same situation a vehicle with a very high roll-centre will suffer jacking, it will rise up and become less stable, leading to the rollover mentioned earlier.

It's quite difficult to explain, but basically what makes for a more stable vehicle onroad can make for a less stable one offroad.