Mercedes: Innovative Linked Rear Suspension

As we get towards the end of the season we often see teams start to get relaxed with the usual secrecy in the pitlane. This weekend in Korea was no different with several technical details being bared to the cameras for the first time.
In particular was the first picture I’ve seen of the Mercedes rear suspension, http://www.f1talks.pl/?p=11598&pid=6274  (Credit to F1Talks.pl and SuttonImages.com for the picture)
The surprise is that Mercedes appear to have adopted a hydraulic solution for managing rear roll andor heave stiffness. Nothing is new in F1, this solution closely matches the aims of the 1995 Tyrrell Hydrolink system, which I hope to cover in detail in a future blog post. Indeed this is not even new in current F1, as several other teams already run similar and perhaps even more developed systems. But this is the first evidence I’ve had of teams interconnecting the suspension with hydraulics.
I spoke to renowned race car designer and suspension expert Andy Thorby about the use of just such a system, “I think most or all the teams are using linked hydraulic actuators on the corners.” adding “they allow you to tune the attitude change of the car under aero load, independently of corner spring rates” by altering both heave and roll stiffness.

Background
Mercedes were one of the many teams to switch to pull rod suspension for 2011, to gain the aero benefits and a lower CofG. With space at a premium at the back of an F1 car, compromises in packaging the various suspension elements need to be made. At its launch it was clear the Mercedes pull rod arrangement placed the rocker quite rearward, in comparison to other pull rod arrangements which place the rocker towards the front of the gear case. The conventional forward rocker placement, puts the heave spring and antiroll in the space at the front of the gear case, packaged around the clutch.
In Mercedes case the rocker is packaged the other side of the gear cluster, just under the gearboxes cross shaft. This leaves little room for the antiroll bar and heave spring. It does however place the rocker and torsion bars very low for the benefit of packaging, aero and CofG. Albeit these are small benefits, perhaps Mercedes choice of a short wheel base did not leave space for the suspension to be packaged around the clutch, as the gearbox length is a critical factor in wheelbase length.
So left with the lack of space to place either a mechanical heave element or a antiroll bar, it appears that Mercedes have opted to create a passive hydraulic system. This is not to be confused with any form of active suspension or the cars high pressure hydraulic systems, this system will be entirely self contained to remain within the rules on suspension design. As the system reacts only to suspension loads, it is clearly legal and there is no question of interpretation in its acceptance by the FIA.

What Mercedes have in place of a conventional anti roll bar and heave spring are hydraulics units (yellow), which probably also act as the dampers. These are connected via fluid lines (blue) to the central valve block and reservoir (red). Springing for the rear wheels is managed by the torsion bars. One end of which is conventionally located within the rocker pivot, the torsion bar then leading forward and connecting to the front of the gear case.
There is still some hardware at the top of the gearbox, which looks like it might be the mounting for an anti roll bar (ARB). But in this set up, its hard to see how the suspension rocker will act on the ARB. So Its not clear if the car started the season with a mechanical system, or whether it was designed purely with this solution in mind.
The teams early season struggle with rear tyre wear, may or may not be attributable to this system. My feeling is that other rear suspension and car layout factors have influenced the tyre problem, to a greater degree than this hydraulic solution. Although in a car that had a difficult pre-season and fundamental design problems. Getting the hydraulic suspension to work as well, may have been just another drain on resources for a team trying to recover its pace.

How it works
A cars individual wheel dampers displace hydraulic fluid as the suspension moves, creating higher pressure in one end of the damper and lower pressure in the other. To act as a damper, valves in the damper control the rate in which the fluid moves between the two chambers to create the damping effect.

In the passive hydraulic unit, the fluid is displaced not from one chamber to another, but via pipes through a valve block and into the opposite hydraulic unit. How the upper and lower chambers are interconnected left to right make the system react differently to inputs from the suspension. These being a resistance to roll or heave.

In a simplified view we can see the system working in two modes, with the fluid lines in ‘Parallel‘, where one units upper chamber connected to the opposite units upper chamber. Or, in ‘Crossover‘, where the upper chamber in one unit is connected to the lower chamber in the opposite unit.
In each mode we can see the effect of the car in roll (tilting from cornering loads) or heave (going down from aero or braking loads).

Parallel

Heave

When the car is in heave, both upper chambers create high pressure. This creates resistance between the two systems wanting to displace their fluid. This has the effect of increasing the cars heave stiffness.

Roll


When the car is rolling, the upper chamber on one side and the lower chamber on the other side create high pressure. As these chambers are now connected to the lower pressure chambers on their opposite side, the fluid displaced with little resistance. This presents no increase in the cars roll stiffness.

Crossover

Heave


When the car is in heave, both upper chambers create high pressure. As these chambers are now cross connected to the lower pressure chambers on their opposite side, the fluid is displaced with little resistance. This presents no increase in the cars heave stiffness.

Roll


When the car is rolling, the upper chamber on one side and the lower chamber on the other side create high pressure. As these chambers are cross connected to the high pressure chambers on their opposite side. This creates resistance between the two systems wanting to displace their fluid. This has the effect of increasing in the cars roll stiffness.

If a team simply want a hydraulic system to create one suspension effect, then they can rig up a basic system based on one of these patterns. However, with a valve system connecting in the centre of the pipes, then a single pair of hydraulic units and would be able to control both heave and roll stiffness. Such a system would not need external pressurisation or any control software to operate the valve block.

Development issues
However these systems are still present handicaps to development. Friction in the valve seals and the valve block, will create heat and variances in the systems response. This heat will be an enemy of the system, as it effect on the volume of fluid in the system, thus the stiffness the system provides to the suspension will alter. As a result the system will need to be a ‘constant volume’ system. Where the volume of fluid is managed depending on its rate of thermal expansion. This is probably part of the function of the small reservoir mounted to the valve block.
Equally important is the ‘installation stiffness’ of the system, that is the flexibility of any components, especially the flexible fluid lines, as this will alter the systems response.
But these and other issues related to hydraulic systems is already well understood by the teams with similar hydraulics being used both for the braking system and the high pressure electro-hydraulic control systems.

One area which presents trouble to the teams is the modelling of these systems. The design and simulation of the hydraulic element is not necessarily covered by conventional suspension and ride simulation software. I asked , Peter Harman, Technical Director of Deltatheta Ltd (http://www.deltatheta.com) about these issues. “I have advised teams on how best to simulate them“ adding “it sounds like it is a common development“. The problem is the hydraulic elements don’t fit in with conventional suspension design software. As Peter explains “Traditionally car companies have used MSC Adams for suspension modelling, and this has been adopted for ride simulation by most F1 teams, however Adams is really just a mechanical tool and doesn’t do hydraulics well“. Thus teams need to alter their approach, needing specialist add-ons and code to augment the already well established suspension development solutions.
Of course the systems will also be physically rig tested in back to back comparisons with their mechanical counterparts on the teams multi-post rigs.

Overcoming these issues with good approach to the detail design work, a hydraulic system should be able to get very close to the response of a Mechanical system. However the potential of the Hydraulic solution does offer some other benefits over purely mechanical systems.

Other possibilities
Once you have the ability to independently tailor the damping and stiffness of the heave and roll functions. The next obvious step is to control the pitch of the car. Pitch is when the car brakes or accelerates, one end of the car moves down and the other moves up. Braking creates a forward pitch, with reduced front ride height and greater rear ride height. Acceleration is the opposite situation.
As we’ve seen for the past few years controlling pitch is critical to maintaining a low front wing ride height, with out sacrificing splitter wear or excessive rear ride height (thus rear downforce).
Linking the hydraulic unitsvalve blocks between both front and rear axles, will allow the same resistance to pitch, as it does to heave on just one axle. This will increase the front heave stiffness, reducing forward pitch and preventing the splitter grounding excessively. This effect under braking could be further augmented with either gravitationally load sensitive valves, altering the displacement of fluid front to rear. Or similarly, a valve directly controlled by brake pressure. The former G-load system already in legal use on the individual wheel dampers and the latter solution a common fitment to motorbikes in the eighties, often termed Anti-Dive.

Summary
With Rake being ever important to the cars aero set up, such linked systems are increasingly being investigated by the teams. Indeed one team has run such a solution since mid 2009 and at least two other teams (one at each end of the grid) ran them last year.

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