mep wrote:Careful Ringo,
Typically and unlike most of the other users here I inform myself before I comment on something. The values are from the following references:
http://books.google.ch/books/about/Hydr ... edir_esc=y
http://books.google.ch/books/about/Ange ... edir_esc=y
It is pretty straightforward to compare the stiffness of different materials isolated of any geometry. In this case is the compression coefficient of hydraulic oil stated to be between 1 to 3.5 x 10³ MPa. Water by the way has around 2 x 10³ MPa. Everybody should know that Steel has 210 x 10³ MPa. Hence it is fair to say that Steel is around 100 times stiffer than hydraulic fluid. Going further you also need to take the containment of the fluid into account. If you reasonably think of typical flexible hydraulic pipe you would not expect them to provide a high stiffness. Their stiffness is stated to be around 0,3 x 10³ MPa.
Now making things more complicated you also need to take the mounting conditions into account. For stiffness this is the motion ratio squared!
You cannot do that. You cannot apply those numbers in such a manner. Can you put solid steel in a brake line? lol
I am not arguing to stifness of the material.
I am saying to you that you can use the stiffest material known to man, and create a very "soft" spring with it. Agree?
You cannot create a sane spring with hydraulic fluid. It doesn't have any shape, and it cannot carry any tension. Your better of creating a spring out of a cinder block. You simply do not have the choice to make a "soft" hydraulic spring. Don't make the mistake of looking at this on a level of just a number value.
That number has nothing to do with spring design. Example can you make a torsion spring with hydraulic fluid? How do you apply it's compressibility to that? In fact lets do the opposite and create a tension spring. how do you do that?
Containment of fluid is a none issue. The brake pipes aren't going to be moving in any meaningful direction and theres no external forces acting on them besides the fluid. the oil will more readily go through the orifices than expand an anullus of metal. And even if it does, the time frames we are dealing with, and also the lack of control is crazy. If you do an atcual calculation on the fluid pressure and pipe diameter, you will see that the strain in the metal is minimal, and so will the force be. Simply useless for F1.
We have motion ratios from the wheel to the hydraulic cylinder but also from the piston area to the pipe diameter. So the relative small force acting on the wheel causes some surprising high forces in the hydraulic system. For static conditions this can be fine, but we are dealing dynamics when the car runs with 300km/h over bumps. The system adds at least one more spring mass system connecting the unsprung masses of front and rear axle. This is not ideal for the dynamic response.
This is over reaching. There is no piston area to pipe diameter motion ratio. You have the fluid pressure acting on the pipe wall. the wall will expand with pressure, but if you are familar with material properties, we are talking micro strains; very small movements. And these movements will be slow, maybe so slow that once the fluid passes through the orifices for damper purposes, that microscopic expansion hasn't even completed. It's too far fetched to depend on that as a "spring".
Even with this, you basically end up agreeing that hydraulic fluid is transmitting forces since It is not behaving like a spring, the pipe wall is.
And you are probably better off using rubber lines if you want some pipe movement, but i know you wouldn't want to go there.
Hydraulic brakes are perfect to experience the compression of fluids. The pedal would be super hard once the pads touch the disc if there would not be any compliance in the system. The elasticity of the system actually makes it possible that the brake force can be modulated so easily. For brakes the elasticity helps. To link a very stiff suspension you need to adjust stiffness to the requirements. I am not saying the system must be of infinite stiffness.
You are pushing it man. You should watch a gear pump pump oil or water into vessel. If that think keeps pumping it will explode that vessel. Hydraulic fluid will destroy any container before you get any useful compression out of it. Same for the brake pipes. The fluid is assumed in compressible. It simply will not compress before the vessel containing it does.
I'n not talking about some 12 inch thick cast iron vessel obviously.
But i think it's best we drop that rumor floating around the blogs about purely hydraulic springs. Whoever wrote that obviously was throwing it on the wall nonchalantly to see if it stuck. I like those articles still, they keep the discussions on F1 technical alive.
I have one last question:
If the wheel travels 2 inches, What kind of displacement will a 10mm hydraulic pipe wall see? What will be the change in diameter if the pipe doesn't go beyond it's ultimate strain?
