Why is the roll center of the car suspension designed lower than the car's cg?

Question

Why is the car roll center always designed to be lower than the car's center of gravity

In my humble opinion, despite the fact that the car suspension will be harder to install and that there will be interference between the wheel and the car body, having the car's suspension roll center above its cg is a lot more beneficial as:

1.) the roll of the car will counter the car's lateral weight transfer during cornering, which helps the car to corner better. 2.) the roll motion if the car also helps the car roll into the center of the cornering, which prevents rollover of the car.

Please give reasonable explanation about this, and please avoid giving reasons such as

1.) the perception of the driver will be better while cornering if the roll center is lower than the cg 2.) the suspension is harder to install to the car

Because i find that these reasons are not good enough to out weight the benefits gained written above.

Thank you

Answer 1

Essentially, because it is impossible to move the Center of Roll at rest ("RC" so that it matches the pictures) above the Center of Gravity at rest (CoG) on a car.

Center of Roll is determined primarily by suspension geometry. By tracing a line through the Instant Center of Rotation (IC) on each wheel to the point of contact of the opposite wheel, we can find the RC.

The IC is determined by the upper and lower points of the suspension. It is a little bit more complicated for double wishbone and some other more exotic suspensions, but essentially there is always at least 2 vectors which we can extrapolate out to an IC.

So how could you possibly move the RC above the CoG? You would need MASSIVE wheels to keep the hub (the point the suspension connects to on the wheel) as far off of the ground as possible (think Monster Truck, but bigger). Then you would need to hang as much weight as possible as low as possible to keep the CoG down. At this point, the handling of the vehicle is already ruined.

It isn't that the suspension is harder to install, Its impossible to install. Or at least so impractical that you lose much more handling from wonky car design than you gain.

Answer 2

it's not a factor of impossible it is that you'd have too much jacking effect i.e. old VW that were flipping over so easily, rear RC was very high. Also you have too much lateral variation which cause darting on even the smallest bumps. Final, driver does not have imput of magnitude of cornering force without roll so difficult to drive at the limit.

Answer 3

Because it will reduce mechanical grip

Bit of an old question, but it doesn't appear to have been answered accurately. I'll try to explain in layman's terms, which might mean some of the language might not be 100% accurate as per physics, but it helps to understand it!

It is possible to have a roll centre higher than the CoG by making the upper link in a double wishbone setup horizontal, and the lower wishbone angled downwards towards the outside of the car. The Triumph GT6 with rotoflex suspension has such a setup.

Note the very high roll centre

The reason this isn't done is that any roll centre above ground level reduces mechanical grip. Grip at the tyres is a function of the force (weight) they are subjected to. The more weight you have on a tyre, the better it grips (provided it doesn't exceed its coefficient of friction). You want weight transfer onto your tyres.

With a roll centre at ground level, the effective 'angle' of the force the tyres are subjected to is dead vertical. All of the weight transfer in a corner is acting down on the tyre, which is desirable. If you have a roll centre above ground level, some of that force will be taken up laterally, pushing sideways against the tyre sidewall and along the suspension linkages. It is this action that limits roll. However, this means that the tyre will be loaded sideways which is worse for grip. I believe an extreme version of this happens with the RC above the CoG.

However, there are compromises in design that make raising the RC desirable. One of those is if you have a camber curve that does not rise enough to meet your roll degrees, and a moderate amount of roll centre height helps keep the tyres negative. An ARB would also help this without side-loading the tyres, but comes with its own consequences and packaging requirements.

Another example of acceptable compromise in RC height is in high-downforce racecars like F1. If you look at F1 cars they have sky-high roll centres at the front which should result in awful mechanical grip. However, this will work to keep the car dead level in a corner which will allow uninterrupted aero. Any losses in mechanical grip can be offset by massive increases in downforce.

Answer 4

If you think about a ship, then having the CoG above the CoR will mean the ship will invert easily as it is unstable. Designing the CoG to be below the CoR makes the system more stable. However, there are examples of ships that have taken on large amounts of water causing the CoG and CoR to change position leading to them capsizing.