What the 'Fric' is it?

[DaveW]

Dave, with fluid assumed "incompressible" it can't slow it down, but it does generate a pressure increase before the valve, which translates into the added force at the piston rod. (Obviously the damping might slow the suspension motion - if that's what you meant )

I suspect that most damper manufacturers would not accept the idea that damper fluid is incompressible - probably because it usually carries "dissolved" air (most dampers have an effective series spring with stiffness somewhere between 2 & 6 KN/mm, depending on CSA and architecture). But you are right in the second part - not much fluid would pass if the shims remained closed. To be fair I probably didn't express myself very well.

[gt6racer]

Dave, I added the "incompressible" provision as that is the property which defines it can't speed up/slow down as a gas could by changing density. I agree completely that damper fluids are not incompressible, but to be pedantic it's not the dissolved air that is the issue (dissolved air doesn't affect the bulk modulus) - it's if that dissolved air comes out of solution and forms bubbles. If you evac and fill a closed system with specially prepared evacuated fluid you can get it pretty stiff.

[Pierce89]

Dave, I added the "incompressible" provision as that is the property which defines it can't speed up/slow down as a gas could by changing density. I agree completely that damper fluids are not incompressible, but to be pedantic it's not the dissolved air that is the issue (dissolved air doesn't affect the bulk modulus) - it's if that dissolved air comes out of solution and forms bubbles. If you evac and fill a closed system with specially prepared evacuated fluid you can get it pretty stiff.

I assure you that DaveW needs no education on the basics of suspension. He runs the post rig at a well known racing engineering company. I don't know if he wants the name divulged on the forum so I won't.

[mep]

The assumption that hydraulic fluid is incompressible is wrong when investigating interlinked suspension. It is around hundred times softer than steel. Also the pipes which connect the front to the rear are not of infinite stiffness. These things have to be taken into account when laying out the system. Especially because the stiffness of the link has to be tuned to the suspension stiffness, otherwise the system is either not working at all, or it is causing significant oscillations between the axles. It is quite tricky to do that for a race car because typically the tires are too soft and the springs too hard. Probably the reason why this sort of things are used in road vehicles preferably. The only reason why the front to rear link is used in F1 now is to keep the vehicle in a specific ride height range. The high rake RedBull shows how a F1 car should be run to generate maximum downforce. The other teams just struggle to get the aero working with higher rake.

[gt6racer]

mep, the difference between road cars and F1 is a very good point. Within a range (where I typically operate) compliances can be factored and accounted for in overall system stiffness, but once you get to the point that the unavoidable compliance is already more than you want, then things get difficult. Small diameter lines can reduce fluid volume, and hence compliance, and also improve pipe stiffness, but then line damping and fluid column mass effects impact system performance/function.
When you refer to the FRIC function as maintaining a specific height range, do you mean to say that FRIC impacts average ride height (heave) as well as pitch ?

[ringo]

This thread is getting a bit too wordy.
Can you clarify the term "modal damping", you mentioned that on the previous page. Also "modal stiffness system" as you relate it to your idea of FRIC.
Elaborate on that please, i'm not understanding what you are illustrating.

[henry]

My reading is .....

Good comments, but surely the "damping" of the central module would slow down the transfer of fluid between the two axles.

I think slowing down the movement is what this is for. I looked again at the pictures in the referenced document. I don't speak German so I may be getting the wrong end of the stick. In the example of braking from 100kph the non-DRC car has a pitch oscillation at around 2Hz-ish. The DRC car has a lower initial pitch and oscillates much less. My interpretation is that the pitch is more highly damped on the DRC car because of the fluid movement across the central damper(s).

As an observation, I would have expected the mean pitch angle to be the same and they are not. I'm not sure what conclusion I can draw from this.

Having looked at this device again it wouldn't help the aeroynamicists with gross pitch or roll attitude so it is probably not something F1 would find useful. Even if they did the much higher pitch and roll frequencies would no doubt make it more difficult to implement.

[DaveW]

... I think slowing down the movement is what this is for....

Exactly. But wouldn't that suggest they wanted the steady state characteristics, but without the dynamic consequences. mep suggested why, perhaps.

Much earlier in this thread I discussed the Moulton "Hydragas" suspension, and I commented that "It worked because it didn't" (work as advertised). I suspect that there are many similarities between "Hydragas" & the present system (even, perhaps, to the similarities between the plot that Audi produced and my "guestimated" plot showing the effect of the front/rear link).

[gt6racer]

Ringo,
By "modal" stiffness or damping I refer to a stiffness or damping that is largely effective in a particular defined mode of loading/motion. My primary "modes" would be pitch, heave, roll, single wheel bump and two wheel (same axle) bump.
Consider a stabilizer bar, which is connected to the two wheels of one axle. It's stiff in the role and warp modes, but not in pitch or heave, but does also have a significant impact on single wheel. Now imagine you use the same device to instead connect a front and rear wheel on the same side of the car. Now you have a device that is stiff in pitch and warp modes, but not in roll or heave, with significant impact on single wheel.
Going one step further, if you replace one of the drop links of each fore/aft "stabilizer bar" with a hydraulic cylinder, and connect the two cylinders together such that only opposite direction motion is allowed, you can remove the warp stiffness and half the single wheel impact, such that you have a device that is stiff in pitch, but not in heave, warp or roll, and has about half the single wheel impact of the original pitch device.
Replace the "bars" by hydraulic compliance (to achieve this the system appearance will change significantly - there would be a cylinder at each wheel) and this is what I think the basic FRIC function is. Anyone agree, or do we think it's more complex than that ?

[GSpeedR]

Ringo,
By "modal" stiffness or damping I refer to a stiffness or damping that is largely effective in a particular defined mode of loading/motion. My primary "modes" would be pitch, heave, roll, single wheel bump and two wheel (same axle) bump.

Typically, rigid-body modes are chosen to be independent (Roll, Warp, Pitch, Heave). 'Two wheel bump' is a linear combination of pitch and heave and 'single wheel bump' is a linear combination of roll/warp/heave/pitch. However, you can choose whichever modes you prefer, just thought I'd clarify for those unfamiliar.

[gt6racer]

I'd agree. I use the term two wheel and single wheel bump, because I perform some rate analysis with body fixed and it's just easier to think that way.

[DaveW]

The assumption that hydraulic fluid is incompressible is wrong when investigating interlinked suspension. It is around hundred times softer than steel. Also the pipes which connect the front to the rear are not of infinite stiffness. These things have to be taken into account when laying out the system. Especially because the stiffness of the link has to be tuned to the suspension stiffness, otherwise the system is either not working at all, or it is causing significant oscillations between the axles. It is quite tricky to do that ....

Absolutely agree. Playing with my simplistic model of a suspension roughly modeled on the illustration posted by matt21, but with links opposed, has shown that.

In effect the central element (labelled, I think, DRC-Ventil - apologies, my German is non-extent) replaces the reservoir of a conventional damper, and is required to store & replenish the fluid displaced or demanded by the shaft area as each the damper moves. Any restriction in the flow between the dampers & the central element can cause dramatic changes in both the effective damping and stiffness of the suspension. Compliance will also have a large effect, particularly on the dynamic response of the suspension.

I discovered that it is relatively easy to change the "static" relationship between ride heights with aero off & on (although the number of variables to be explored is daunting), and it would be easy to accommodate a reduction front spring stiffness (my personal hobby horse).

However, making the system work dynamically is a very difficult problem to solve, and would probably require more degrees of freedom than my current model. It is just too easy to lose control of the contact patch loads.

What I have been doing is very much based on guesswork, but I could imagine that one team (Mercedes?) has poor control of contact patch loads whilst another (Lotus?) has actually improved contact patch load control. The first would be expected to qualify well, but lose out during a race. The second might be expected have issues qualifying, but race well.

[Smokes]

Dave how would one set the rake? would done physically with spacers or hydraulically by varying the Front and rear pressures if possible?

[DaveW]

Dave how would one set the rake? would done physically with spacers or hydraulically by varying the Front and rear pressures if possible?

I started with an old F3000 model, using a rig-based setup from a vehicle that was reasonably successful (but not totally representative of a modern F1 vehicle). I borrowed down forces figures, and scaled these up to represent high speeds. The model did not include bump rubbers, & I kept it that way to make life simple. I then chose a likely CSA ratio (the pressures were equal, but the CSA ratio changed the front/rear loads). I then added the link, which changed static ride heights with weight on wheels. I then chose to change spring rates to recover the ride heights (dropping the front springs and increasing the rears). I found that with no link the ride height difference changed by around 7 mm "air off" to "air on", and with the link in place the difference was -1.6 mm (in my case). Adding the link and changing springs improved minimum load parameters (my measure of mechanical "grip"), but this was quickly reversed when "damping" was added between the two axles.

So, the answer to your question was a combination of link CSA and spring rates. In reality, I am sure that "soft limits" (bump rubbers) would be required to keep things under control.

[ringo]

The assumption that hydraulic fluid is incompressible is wrong when investigating interlinked suspension. It is around hundred times softer than steel. Also the pipes which connect the front to the rear are not of infinite stiffness. .

Haha, ok, so what is softness exactly? I know air is softer than steel, but an air spring can be stiffer than a metal spring.
Careful how you put forward your ideas. As it looks like your being disingenuous.
Even if you compare a metal spring to a solid block of the same metal. Both can have the same toughness and yet different spring rates. I can design a spring of any material to give the softness i want. So "softness" has nothing to do with material.
A column of hydraulic oil is not 100 times more deform-able than a metal spring.

Let's just put it this way. Hydraulic fluid is by no means a spring, especially for use on a car weighing 640kg. It's merely a mode of transporting forces, and multiplying them. It's not intended to store energy like a spring would.

And with accepting the fact that hydraulics makes it convenient to transfer forces, FRIC may be hydraulic just for the sake of creating a front to rear sway bar, where it would be physically limiting to create a linkage based sway bar front to back.