Suspension Discussion Thread: Hydraulic linked suspensions

[Raptor22]

This thread is to further useful engineering and scientific discussion on hydraulic linked suspensions utilising or not utilising fluid inerters and or inertia valves.
Some premises for the thread:
Platform valved dampers are in widespread use.


To kick off the discussion:
If a current formula ! car were to employ a hydraulic linked suspension to not only counter roll but also pitch, what would be the advantages of such a system?

[Jersey Tom]

...better attitude control?

[gato azul]

maybe it helps to explain first, what you (we?) think, is the "problem" with pitch,
why would they (F1 teams) want to avoid it?

in a second step, we could define what causes the "pitch angle [pitch attitude]" of our car, and which parts of the car have an influence on the [final] pitch attitude/angle of our car.

answering these questions, may help to understand the ideas behind any RRH system

[hardingfv32]

And the disadvantage? There are always compromises or this would have been in use from decades ago.

Less compliance?

Brian

[Raptor22]

Good Point Gato.

Pitch would be the tendency of the front of the car to dive under braking. This presents a multitude of problems.
1) it changes the aerodynamic balance of the car. With the nose of the car closer to the ground the splitter will tend to scrape the ground. The resulting change in flow under the car leads to sudden changes in grip that can make the car unpredictable initially on the brakes and at turn in.

2) changes in pitch also result in a sudden change in the shape of the tyres contact patch. This creates disadvantageous wear patterns relative to a car which does not exihibit dive tendencies

3) Couple 1 & 2 and you have a car that would exhibit characteristics ranging from over to understeer turn in, high front wear rates. May need to run disadvantageous camber angles resulting in tyre drag, high wear and low grip.

4) All the above leads to high work rate for the driver.

But a wing in ground effect is also affected by roll and to a lesser extent , yaw. The latter is not a major concern for this discussion since roll would result in the one wing tip sinking deeper into ground effect while the other moves away from it.


What causes pitch sensitivity?
The most common cause is a poor trade off in wing profile. Stability vs down force.
Some profiles are more sensitive to the ground effect they run in but produce lots of downforce while other wings are less sensitive to height but produce less downforce.
Front wings work on a ground effect principal. The closer to the ground they get, the more down force they produce. If the wing is lowered too quickly the down force in front increases too rapidly, changing the aerodynamic balance too quickly for the driver to catch it.
In extreme cases the wing can slip in and out of ground effect within a few mm setting up an oscillation which the suspension system has to damp out. (porpoising)

So damping and spring rate of the front suspension is vitally important in maintaining the performance of the front wing through a wider performance range, especially under braking and even through a corner such as Eu Rouge at Spa-Francochamps where the transition from steep downhill to steep incline is quite rapid. The front suspension has to cope with massive loads as the car is forced own into the ground.

The traditional way of coping with the requirement for constant ride height of the front wing is to run a stiff front spring set up. But having a "stiff front end" brings with it associated problems of maintaining a constant contact patch shape and the car had to be kept off the kerbs as much as possible.

Compromises had to be found in damping and spring stiffness. Then around 2001 platform dampers started to appear in F1. These dampers have a check valve in their circuit that allows the damper to have two or three different rates depending on the force the piston sees. Essentially either an air spring or coil spring shut off valve on different circuits that responded to different loads. This allowed the suspension to sag slowly under braking (heavily damped slow speed circuit) but still respond to curb hits and under loads that resulted in high piston speed. This gave cars like the Ferrari F2002 a huge advantage since the suspension could be set up to ride the ripples and small bumps whilst still being able to cope with high suspension loads separately. But dive was still there.

Other tactics to ensure the front responded more "fluidly" included the mass damper as raced by Renault in 2005/6 and then more lately the fluid inerter and hydraulically linked suspension on the rear ( to reduce the cars roll thus keeping the wings downforce distribution more constant.

All of these mechanisms have the result of aiding the front suspension to maintain a constant flow around the front wing. Rear suspension has slightly different requirements but benefits of these devices can also be extended to diffuser performance and improved roll control to maintain tyre contact patch shape and improving grip and or tyre wear

[gato azul]

Good post Raptor22,

Now, this is "your" thread, so you will layout the rules, but if I can make a suggestion, I would recommend to look at each possible scenario, one at a time.
Maybe starting with "pitch" [control], as this seems to be the main idea behind the different RRH systems proposed. This includes the now banned Lotus-Renault
system, as well as the "rumored" system of Mercedes [and perhaps others].
Later you can expand this line of thinking onto roll, and other conditions.
Otherwise, I feel we will have a load of "cross talk", and it will be difficult to follow the debate [for some].

So, I take it from your post, that one aim is, to control the height of the front wing, in relation to the ground, under different conditions, e.g. low speed vs. high speed, and braking vs. acceleration &/or driving a constant speed.
This is a good start - IMO.

Now I´m thinking about the following point:

2) changes in pitch also result in a sudden change in the shape of the tyres contact patch. This creates disadvantageous wear patterns relative to a car which does not exihibit dive tendencies

Is the change in pitch really the cause for the change in tyre CP size/shape?
Or is there another reason for this?
Following this line of thought, can a suspension (in this case I mean the combination of springs/dampers, not geometry as in camber change) affect the "shape/size" of the CP?
And if yes - how does this work?

[Jersey Tom]

Ehh I'm a bit wary of endorsing the chain of thought of footprint shape being tied to pitch and having to make up for things with camber and getting wear, blah blah. That's diving deep into hand waving... though I agree with the general premise of wanting to control pitch as ONE aspect of body attitude.

Ultimately attitude control (pitch, heave, roll, yaw) is a big deal. Engineers do setup development to try to decouple effects and control each aspect individually to some degree. Adding ARB's to a car architecture helps decouple roll and heave. Varying amounts of damper or inertial elements can change frequency response in any mode. Etc.

Adding a hydraulic system on top of everything can really do some impressive stuff. Just about any FSAE / F-Student graduate is familiar with this and the Kinetic (brand name) suspension used on UWA's cars for many years (maybe still?). I've seen it myself where a person can lift a single corner of the car quite easily but still have immense roll stiffness. The decoupling is impressive.

There's no magic to this stuff. It's just adding extra knobs to play with when you tune a dynamic system. Why isn't it used more broadly? Not allowed in many series. I'm pretty sure even the old Williams active suspensions used hydraulic accumulators, etc. Used to have a good technical schematic of the system around here, not sure where I've put it.

[Raptor22]

Jersey Tom Said :"Ehh I'm a bit wary of endorsing the chain of thought of footprint shape being tied to pitch and having to make up for things with camber and getting wear, blah blah. That's diving deep into hand waving... though I agree with the general premise of wanting to control pitch as ONE aspect of body attitude."

Gato Said:"Is the change in pitch really the cause for the change in tyre CP size/shape?
Or is there another reason for this?
Following this line of thought, can a suspension (in this case I mean the combination of springs/dampers, not geometry as in camber change) affect the "shape/size" of the CP?
And if yes - how does this work?"

Yes good questions. I've been thinking about this for a long time without much practical experiment to back it up so its just a theory at the moment. To set up a practical test I need a large sheet of glass and some cameraa's to take pictures of the contact patch of a tyre as a wheel is braked. Of course with a passenger car the pitch is relatively severe in relation to an F1 car so there a significant camber change through the travel of the suspension. The F1 cars tyre profile lends itself to creating more contact patch changes then just pitch on its own. Roll and pitch come into play as do the lateral and longitudinal loads on the tyre.
Tyre pressure, load, and tyre shape and design, in conjunction with suspension geometry all play into this. The most important property is something that has not been raised yet and that's the kinematic centre of the vehicle and effects on suspension.

so over he net few days or weeks I'm going to write up some long posts and get these concepts on the table. Its going to require a lot of typing since theres a lot to unpack.

[Jersey Tom]

or weeks I'm going to write up some long posts and get these concepts on the table. Its going to require a lot of typing since theres a lot to unpack.

Brevity is a virtue.

[xpensive]

The suggested AMG merury-hydraulic leveling system is getting some interesting numbers here;

http://abulafiaf1.wordpress.com/

[gato azul]

so over he net few days or weeks I'm going to write up some long posts and get these concepts on the table. Its going to require a lot of typing since theres a lot to unpack.

I didn't thought, that the answer to my question, would require all this, at least not as an "first order" approximation - but please by all means go ahead Raptor22.

Maybe we wait and see how the blog, mentioned by Xpensive, develops, looks like someone is investigation a similar topic.

[Raptor22]

Oh crack!! SO Mercedes are writing their own Bl**dy blog about it :/
spoil sports.

Well the answer to your question requires the problem to be looked at from a multitude of facets. I was planning to offer as complete an answer as possible. The time is taken in drawings and write up when its not my full time job.

I'll work on it anyway but from reading the part I of the blog their approach is similar to what I am doing.

cover the vehicle dynamics
examince the influence this has on the aerodynamics and tyre contact patch
look at possible solutions to the specific problems at hand
and then look into how a Mercury Suspension would work to offer a solution to those.
Looks like the horses mouth is stepping in ....