In the first part of this analysis of Porsche 919 front suspension, we analyzed how a suspension system like the one described in a Porsche patent and aiming to decouple roll and heave control could actually work.
As I mentioned, one area of concerns could be to achieve a roll damper motion for each degree of roll big enough to ensure an effective damping and avoid the undesired influence of damper internal friction (although latest technology dampers, using much lower internal gas pressure, probably reduced already internal friction).
Let’s now take a look at what the actual 919 front suspension (could) look like. As for the patent scheme, we rebuilt the suspension layout in CAD in order to show how its kinematic could work. Now, please note the model obviously doesn’t have the right dimensions, but should only help to explain Porsche’s front suspension working principle, above all in term of springs and dampers activation. Also, it doesn’t represent a working design, above all in terms of components stiffness and strength: it should only serve as a visualization aid to explain how the mechanism works (a picture is worth a thousand words, after all). Finally, as I said, what we are going to show here is only our taking about how Porsche’s scheme could look like; Porsche is most probably not keen to reveal any of their secret to us, so we cannot be totally sure that what we sketched is absolutely right.
Again, a number and a colour are assigned to each component. Parts n.1 (dark grey) are lower and upper wishbones, Part n.2 (pale brown) is the tie rod, Parts n.3 (orange) the two pushrods and Parts. n.4 (grey) are the two rockers. The pushrods move the rockers, which rotates around a pivot axis represented by the blue bracket attached to the chassis. When moving, the rockers activate a mechanism that works on a very similar principle to the one described by the patent. More specifically, one rocker is connected to a rod (Part n.5, pink) and the other one to the heave spring-damper unit (Part n.8, red) and to a second link (Part n.7, purple). Let’s now take a closer look at the central components of this layout.
The main element, here, is the Central Rocker (Part n.6, yellow), which pivots around an axis fixed to the chassis (protrusion in its lower part) activating, most probably, a coaxial torsion spring that works as an anti-roll device. Its upper extremity is directly connected to the roll damper (Part n.9, light blue) that, as we will see, only moves in roll.
As in the patent’s scheme, the central rocker is not activated directly by the rockers and the lateral links. The latter are actually connected to a central lever, functionally very similar to the Watt linkage we have seen in the patent’s scheme. It can be better seen in the following picture where, for clarity, we removed the yellow central rocker to make the inner components visible:
As we can see, both the pink (part n.5) and purple (part n.7) links activate a central lever (part n.10, dark green) which is free to pivot around an axis fixed to the central rocker. This lever is also connected, on its upper part, to a short rod (part n.11, dark purple) which then activates the other side of the heave spring-damper unit (part 5, in red).
Please note that the design of the short rod (part n.11) and of the central lever (part n.10) are here just a sketch to show how each component is activated and represents in no way a real working design. We know they would need to build in a different way, above all in their common interface, to ensure proper stiffness and strength. A similar scope could be achieved, for example, using a ball joint between part n.5 and part n.10 and an axial bearing between part n.10 and part n.11. But such a detailed study is out of the scope of this article. Each and every 3D model shown in our pictures only wants to be an aid to explain how the kinematics works.
The reader can probably see how similar this scheme is to the one shown in the patent, although it was only a simplified layout to describe the working principles of Porsche’s concept.
Of course, the control arms protrusions of the patent are here gone and have been replaced by a double wishbone setup with pushrods and rockers. Still, the key elements of this layout are the central rocker, taking the place of the sway vertical element of the patent, the central lever, that replaces the watt linkage, the heave spring-damper unit and the roll spring and damper.
We can now take a look at how this layout behaves when the suspension moves. We will analyze two situations: a pure heave motion, with a jounce (body moving vertically, downward in our case) of 25 mm and a pure roll motion of an angle of 1 degree. For clarity, the central rocker has been made partially transparent in following pictures, to let the reader see what happens to the central lever in both situations.
Also, the heave spring has been removed, since its deflection would anyway be equal to the one of the heave damper, being the two coaxial. Finally, the length of both the roll damper and the heave damper will be shown, to analyze how much they deflect.
Let’s first start with the design condition:
As we may see, in this condition the heave spring-damper unit is 260 mm long, while the roll damper is about 300 mm long.
If we now consider a heave motion, we would a see a displacement in the heave unit while the roll damper maintains approximately the same length.
As shown in our picture, the roll damper nearly doesn’t move, staying at a length of about 300 mm. The heave damper displaces, going to a final length of about 224 mm. The system we built up in CAD is not perfect. The roll damper has here a deflection of 0.023 mm, which confirms it is possible to achieve a nearly perfect decoupling.
As we can see in the picture, in a pure heave movement, the central rocker doesn’t practically move. The central lever, instead, rotates about its pivot on the central rocker and, through the short rod, compresses the heave damper which is, on the other side, also pushed by the rocker.
Since the central rocker doesn’t move, this is also the case for the roll damper, on top of it.
Key to making all of this happen is the motion ratio of the central lever and of the corner rocker. Referring to the next picture, this means that the ratio between the distance of point A to point B and the distance of point B to point C must be the (close to) the same of the ratio between the distance of point D to point F and the distance of point E to point F.
Considering a roll motion, on the other hand, we would see the components moving as shown in the following picture:
Here, the heave damper nearly doesn’t move (we talk here of a 0.17 mm displacement over a 1-degree roll suspension movement, which is unrealistically big for such a car) while the roll damper is now activated and displaces pretty significantly (about 21 mm). How much the roll damper moves directly depends on how far its attachment point on the central rocker seats from the pivot.
As the reader can see, now the central lever doesn’t move at all. As we said, our CAD model doesn’t produce an ideal separation but the error we have is in the region of parts manufacturing tolerances and can be neglected. Also, we can assume that Porsche put much more effort than we did in optimizing their kinematics to extract the maximum out of it.
This simple study shows what should have been the working principle of Porsche front suspension (rear suspension was using a similar principle, but achieved it with a different layout, as explained in my Blog) and why it is so interesting. As we mentioned, Porsche engineers developed an extremely elegant and neat system to completely decouple roll and heave motion, ensuring not only tuning advantages but also a very compact package and the use of lower numbers of components.