What the 'Fric' is it?

[Tim.Wright]

Interesting stuff, but i'm yet to see how a passive hydrodynamic suspension has any advantage over a more mechanically efficient conventional system.

The advantages of a hydraulic system over a mechanical system is in the packaging. Where would you package a front/rear connection mechanically? Also, every time you add a mechanical link, you add friction and possibly freeplay

[Dragonfly]

Ride height control now is interesting as fluid will need to be introduced. The good thing is that introducing more oil does not affect the spring forces.

As I mentioned, if there is a hydro accumulator, additional fluid can be introduced from there once the fuel is filled in the tank. The amount needed is very small as the volumes are small. Don't know how it is in the real cars but a thin hydro cylinder may be built in the pull rod. The "Activa" version of Citroen has such, mounted as anti roll bar links - one front, one rear, diagonally.
During a race you start with full fuel load. As the car gets lighter you can simply bleed some liquid to compensate the height.

[Dragonfly]

The advantages of a hydraulic system over a mechanical system is in the packaging. Where would you package a front/rear connection mechanically? Also, every time you add a mechanical link, you add friction and possibly freeplay

That's what I was trying to say previously but not so short and clear.

[Tim.Wright]

Ride height control now is interesting as fluid will need to be introduced. The good thing is that introducing more oil does not affect the spring forces.

As I mentioned, if there is a hydro accumulator, additional fluid can be introduced from there once the fuel is filled in the tank. The amount needed is very small as the volumes are small. Don't know how it is in the real cars but a thin hydro cylinder may be built in the pull rod. The "Activa" version of Citroen has such, mounted as anti roll bar links - one front, one rear, diagonally.
During a race you start with full fuel load. As the car gets lighter you can simply bleed some liquid to compensate the height.

Just my opinion, but I don't think these systems as are complicated as people are making out. I don't think there are any hidden actuators which act based on the fuel load or any other tricks. Its just a normal, hydraulic interconnected system which has been done many times before.

I think the system is merely aiming to obtain different stiffness' of the suspension in each mode (heave, roll, pitch warp) as compared to a conventional suspension.

I.e. maybe they want a specific pitch stiffness which is not possible to reach using the heave springs which are sized control the ride height down the straight. Therefore perhaps they are putting in an "anti pitch spring" using a hydraulic interconnection.

[autogyro]

I think that Bose would claim rather more... see this.

The Bose system has coil springs.
They also developed counter sound frequency technology to prevent road, wind and powertrain noise as you well know.

How to maintain a constant ride height using hydropneumatic suspension should be fairly obvious.
On topic again, perhaps you could explain how. We can worry about reliability later.

Topic is fine.

Just place an hydropneumatic displacer unit under the fuel load connected to the suspension system.

Simple enough, I am sure the math will keep you busy.

[DaveW]

But I own and drive an aging Citroen "Xantia". I took it because I was curious about the suspension since I was a boy. Initially it seemed complicated, but now I know it and it is a relatively simple and reliable thing.

Snap! I did something similar, but before the Activa made its appearance. The car rig tested very well (it achieved a comfort rating well above its "class"), but had an unusually high front strut friction (around 250N, I recall) caused, no doubt, by the seals required to contain the fluid. I somewhat reluctantly parted company with it because my back was incompatible with the seats supplied, & I was too lazy to replace them.

It is easy to demonstrate the high friction. With the engine running and car stationary, simply lift up the front of the car by hand, and watch while it sticks there for a time (up to 20mm high, I recall), before finally the ride height control kicks in & compensates. Ultimately, I concluded that I could detect the ride height chasing around friction in normal use, but it really wasn't an issue.

Interestingly, the press at the time were not very positive about the Activa development. But what do they know....

Anyway, thanks for the excellent reference you supplied.

The advantages of a hydraulic system over a mechanical system is in the packaging. Where would you package a front/rear connection mechanically? Also, every time you add a mechanical link, you add friction and possibly freeplay

You are correct, but don't forget seal friction, fluid flow forces, fluid (& hose) compliance and temperature sensitivity of an hydraulic linkage.

[ringo]

If we keep this in the context of the current Formula1 cars, i don't think it will be packaged better than a torsion bar suspension.
I believe it has more friction as well.

As for the anti roll bars, the article with the Dakar truck does not have one because of the cross linked piston and rods, but the citroen example had a mechanical roll bar, because the hydro system did not activate the air springs in roll.

The thing with the fuel tank providing more fluid for the system, is that the pressure of the suspension will be acted on the tank. There would have to be a check valve for all 4 corners for the tank support if this oil is to support the car. It cannot be freely acting back and forth on the tank.

The advantages are still not clear in terms of what it offers in a passive state. I still see it as the same of a mechanical suspension, only with bigger packaging. Remember i'm referring to an Modern F1 suspension.
So there must be more to the system than what we have here.

[ringo]

The advantages of a hydraulic system over a mechanical system is in the packaging. Where would you package a front/rear connection mechanically? Also, every time you add a mechanical link, you add friction and possibly freeplay

I agree that a front to rear connection will be packaged better hydraulically.
I was disputing a conventional suspension with no front to rear connection versus the front to rear hydraulic suspension.
The conventional one surely is packaged smaller.

Since we haven't established what the FRIC is doing as yet, this is why i'm somewhat looking at it as if why would you move to FRIC from conventional suspension.

[GSpeedR]

I agree with Tim Wright. Everyone seems to be jumping straight to 'closed loop' suspension and 'active ride height control'. I am of the opinion that a passive interconnected suspension can have enormous benefit on its own. Ignoring for a moment how the actual system is implemented, Interconnected suspension allows independent control of rigid-body suspension modes, which is more difficult to achieve with traditional suspension components (corner springs, third springs, anti-rollbars). Stiff heave, pitch, roll and a much softer warp mode can greatly improve tire load variation without sacrificing body control. There is also more flexibility in setting modal damping ratios.

Issues such as 'porpoising' (unstable aero-body coupling) and tire resonances could be addressed more effectively than a conventional vehicle. There have been a lot of (relatively unnoticed!) publications from people like Ortiz, Zapletal, and Smith that discuss the theory and implementation. Smith - Interconnected Suspension and Zapletal - Balanced Suspension. Ortiz wrote articles in Racecar Engineering on the subject back in 1999.

[Tim.Wright]

I'm still not getting what you're talking about with the fuel tank. Anyway such a system would likely be illegal since it is affected by forces coming from somewhere other than the wheels. I think we can safely forget that completely.

The advantages are still not clear in terms of what it offers in a passive state

The advantages are likely what I said in my last post. I.e. to have a better control of one or more of the suspension modes. I'm guessing its the pitch mode because of the front/rear connection.

To control pitch in a conventional suspension, you have the heave springs at the front and the rear. Now these springs don't only work in pitch but also in heave movement which occurs when downforce acts on the car. So the heave springs have 2 jobs:
1. Control the ride height and body attitude as downforce increases
2. Provide a portion of the pitch stiffness under braking and accel

Its very likely you can't have the optimum of both of these at the same time. For example, if you setup the springs to give a good ride height control, maybe your pitch stiffness is too low...

By connecting the front to the rear, you can add a spring/damper which only works in the pitch movement and does not affect the vertical dynamics. Therefore you can have the pitch stiffness AND the vertical stiffness that you want.

So basically, the key points are:
1. This is something you can't do in a suspension without a front/rear connection
2. The only front/rear connection viable for an F1 car is hydraulic
3. Such a system involves no external tricks (i.e. using the fuel level) of power sources
4. As I said before, its been done many times before and there is quite a lot of literature around on it

[ringo]

Ok we are now defining what advantages there are.

I see the pitch control as the only advantage currently, also twist. Ie from looking at the system in the articles.

Left to right is basically the same as a conventional spring, damper and roll bar setup.

Front to back and twist, hydraulically linked is probably where i agree there is an advantage. Otherwise for a passive system i see no difference with conventional.

edit: Also note some of these systems are quite cumbersome, some also contain conventional springs and much more.
One article even noted packaging as a consideration.

[Tim.Wright]

As you said, there is also the possibility to reduce the warp stiffness while keeping the other modes stiff which is an advantage from a pure mechanical grip point of view. In fact, this is the primary reason these interlinked suspension were invented.

[DaveW]

I'm still not getting what you're talking about with the fuel tank. Anyway such a system would likely be illegal since it is affected by forces coming from somewhere other than the wheels. I think we can safely forget that completely.

I suspect that autogyro is thinking about an independent suspension, connecting the fuel tank directly to the uprights. I guess you would also require Lotus 88 style "twin chassis" system to handle the aero.

The Bose system has coil springs.
They also developed counter sound frequency technology to prevent road, wind and powertrain noise as you well know.

So did the Lotus Active Suspension system....
I thought that "counter sound frequency technology" was developed by Lotus, in conjunction with ISVR of Southampton University. Perhaps I was wrong.

By connecting the front to the rear, you can add a spring/damper which only works in the pitch movement and does not affect the vertical dynamics. Therefore you can have the pitch stiffness AND the vertical stiffness that you want.

I like the reasoning, but I think the solution is rather more complicated than you describe. Recall my post about the "hydragas" system, which (arguably) worked because it didn't work. A straight connection between the axles was intended to minimise pitch response. If it had achieved that, then the system would have had no pitch control at all. In reality the system was saved by the "damping" caused by pipe flow forces and fluid compliance. Added to that, I think that the axle-based stiffness ratio (required in F1 to manage the aerodynamics) would have a large effect on the design of the system.

[DaveW]

Ignoring for a moment how the actual system is implemented, Interconnected suspension allows independent control of rigid-body suspension modes, which is more difficult to achieve with traditional suspension components (corner springs, third springs, anti-rollbars).

Thanks for your helpful post, GSpeedR. You might be interested to know that some F1 suspensions for the last few years have achieved a roll/warp stiffness ratios of around 5. Perhaps an interconnected suspension allows similar ratios to be achieved with fewer contraints.

[elclingo]

Ok we are now defining what advantages there are.

I see the pitch control as the only advantage currently, also twist. Ie from looking at the system in the articles.

Left to right is basically the same as a conventional spring, damper and roll bar setup.

Front to back and twist, hydraulically linked is probably where i agree there is an advantage. Otherwise for a passive system i see no difference with conventional.

edit: Also note some of these systems are quite cumbersome, some also contain conventional springs and much more.
One article even noted packaging as a consideration.

I totally agree with you.
Additionally, as I was reading about the subject I found this article where they mention that if vertical frequency is set sufficiently high, then anti-roll bars are no needed to control the roll of body.

http://www.autospeed.com/cms/A_112686/article.html