Krio

iTrader: (159)

Some designs are intentionally looking to achieve longer arms and that is why you see the hinge pin inside or outside the king pin. Other times, there are design constraints or other suspension choices that have been predetermined that make the hinge pin location change.

Using the AE B6 and B64 for an example:

AE uses the same arm length (hinge pin to hinge pin) on the B6 as the B64. The B6 C-hub is just like you mentioned on other 2wd vehicles with the hinge pin further inside than the king pin. The B64 hinge pin is pretty much in line with the king pins. For AE, the main reason for this difference is compensation for the wider differentials on the B64. The inner hinge pins are further apart on the 4wd, so the outer hinge pins are that much further out. They have an arm length they chose to stick to, so they design the rest of the suspension around that.

More to the point of what you pointed out on average, there are two main things you need to keep in mind when it comes to the roll center:
1. Where it is when the car is sitting at ride height
2. How quickly it moves up and down due to the suspension compressing/rebounding

Where the roll center is at ride height determines how quickly the car initially reacts to lateral forces due to steering input. How quickly the roll center moves up or down is one factor that determines how quickly the car settles as you are taking the corner. Other things like roll bars and springs can change this, but that's the gist.

Lets assume that we have cars that are identical, but one has long arms and one has short. They also have the same spring and damping rate at the wheel. Also, at ride height, the arms are parallel with the ground. This means they both have the same roll center when the car is at ride height and will react identically the instant you turn the steering wheel. Once the suspension compresses at all, they are now going to react in different ways.*

With everything else equal, a longer arm will make the roll center move less as the suspension compresses. This means the car will settle into the corner more quickly because the chassis will roll less. A shorter arm does the opposite. The shorter the arm is, the more the chassis wants to keep rolling as the suspension compresses. In the example outlined with two cars where everything is equal except the lower arm length, the one with shorter arms will roll more and take longer for the chassis to stop rolling as you enter a corner. Whether this is beneficial or not totally depends on a multitude of factors, such as the traction level, class of vehicle, how rough the track is (thus changing the oil and springs that are used), and even the drivers skill level.

2wd offroad vehicles usually benefit from being a tad slower reacting. Quick changes on the loads the tires experience is a good way to break the rear end loose and cause a spinout. On the flip side, a car that reacts too slowly can also become unpredictable when you need to make quick directional changes such as through a chicane or if you need to dodge a car that is crashed on the track. It's a never ending tug of war to find a balance, but on average the roll center and arms on a 2wd offroad vehicle are lower and shorter than an somewhat equivalent 4wd vehicle.

For 4wd offroad vehicles, the additional stability, grip, and speed they typically experience means you want to take advantage of a suspension that reacts more quickly. Longer arms do this, but you can also make the suspension react more quickly with stiffer springs and swaybars. The stiffer they are, the more quickly equilibrium is reached between the forces trying to roll the car (roll center acting on the center of gravity and centripetal force loading the outer tires) and the forces that are resisting that roll (swaybars and springs).

When you get to on-road, you are looking at even faster left/right changes in direction, so you want an even quicker reacting car. This leads to suspension arms that put the hinge pin out past the king pin to make it as long as possible without resorting to unique inner arm mounting locations. They also want the center of gravity to be closer to the roll center than on offroad vehicles.

*disclaimer: all examples above assume that the static roll center at ride height is below the center of gravity of the vehicle. If you move the roll center above the center of gravity by raising the inner hinge pin really high, the roll center now resists roll instead of aiding it. This is a bad idea 90% of the time, so it is ignored.