What CVA means

[Saito]

Ever wondered what CVA means (in terms of CVA shock)? Tower Hobbies states CVA refers to the material they are made of, however I believe they are wrong. I swear I heard/read it stood for "Constant Velocity Adjustable". What has everybody else heard (and boy do I wish they still made 'em in yellow)?

[terry.sc]

CVA stands for Constant Volume Adjustable. The CV part refers to the oil reservoir, the diaphragm in the top keeps the volume of the oil chamber the same as the piston rod goes in and out.

[DeeMiller]

quote:Originally posted by terry.sc

CVA stands for Constant Volume Adjustable. The CV part refers to the oil reservoir, the diaphragm in the top keeps the volume of the oil chamber the same as the piston rod goes in and out.

---

id="quote">id="quote">

The volume of air isn't the same as the volume of the rod.

The air is there to give little back pressure on the first step of travelling.

Works as "air suspension".

The next step fully compresses the air and the oil takes over.

As you know, liquids can't be compressed so there will be more resistance when the travel is bigger.

This is the absorber part.

So it's kinda interactive suspension damper.

Grtz Dee

[terry.sc]

quote:

The volume of air isn't the same as the volume of the rod.

The air is there to give little back pressure on the first step of travelling.

Works as "air suspension".

---

id="quote">id="quote">

The air is there to compress when the piston moves up, to compensate for the damper rod. If the volume of air in the top of the damper, separated by the diaphragm, was not as big as the volume of the section of rod that actually moves in and out of the damper(not the same as total volume of rod) as soon as the air pocket was compressed you will get a hydraulic lock and the damper moves in no more. The oil does not 'take over' the compression because you just cannot compress a fluid. Several dampers are available with a small hole in the top cap to remove the pressurisation effect of the air pocket, although the advantage of the pressurisation is that any air trapped in the oil after filling is compressed so it's effect on the damping is less noticeable.

If it is just there to provide 'back pressure' try filling the damper up to the top without the diaphragm, so no air pocket, and try to compress it. A chamber completely filled with pressurised oil as you described is used as suspension, but you need something big such as a full size aircraft to be able to provide enough pressure to compress them.

Dampers work by controlling the flow of oil through the holes in the piston, by changing the pistons and oil viscosity you adjust the damper.

[robster959]

Can you recommend a damper oil for use in the 959? I was running mine for the first time today and noticed that at high speed over a rough surface, the rear end kept bouncing, while the front was fine.

Rob

[DeeMiller]

The volume is smaller, otherwise the rod would touch the bottom end and you don't want that at all!

To avoid serious damage you can add "bump stops" when you expect heavy abuse (due to surface conditions, like rocks/holes etc.).

Of course, you can also use stiffer springs.

Besides, you want to absorb the shock, that's why the oil is compressed and it will absorb the energy by turning it into heat while, as I said before, liquids can't be compressed.

Says mr. Boyle-Gay-Lussac...(PxV/T = constant).

The spring is also used to return the rod and due to the viscosity of the oil, it travels back slower (re-bound).

So, you can use a varity of stiffer/weaker springs in combination with more/ less viscous oil.

Depends all on what you want to adjust (comfort<=shock absorber/spring=<response)

Grtz Bounching-back-Dee.[]

[DeeMiller]

quote:Originally posted by robster959

Can you recommend a damper oil for use in the 959? I was running mine for the first time today and noticed that at high speed over a rough surface, the rear end kept bouncing, while the front was fine.

Rob

---

id="quote">id="quote">

Use less viscous oil OR weaker springs on the back.

Grtz Dee.

[Odd-N]

Ehh.... I suspect you both mean the same but just say it differently...

First it is not quite correct to say a shock absorber absorbs the shock, it slows the oscillation down after the shock. It is suppose to compress relatively easy and then decompress a bit slower. This applies to real car shocks, on RC cars most shocks have the same rate both ways.

Second you say in the same sentence that the oil is compressed, and that a liquid can not compress..... obviously only the last is correct[]

By the way the Boyle's law of termodynamics PxV = nRT is only useable on gases. But if you lower the pressure on a liquid enough it will turn into a gas.

[DeeMiller]

quote:Originally posted by Odd-N

Ehh.... I suspect you both mean the same but just say it differently...

First it is not quite correct to say a shock absorber absorbs the shock, it slows the oscillation down after the shock. It is suppose to compress relatively easy and then decompress a bit slower. This applies to real car shocks, on RC cars most shocks have the same rate both ways.

Second you say in the same sentence that the oil is compressed, and that a liquid can not compress..... obviously only the last is correct[]

By the way the Boyle's law of termodynamics PxV = nRT is only useable on gases. But if you lower the pressure on a liquid enough it will turn into a gas.

---

id="quote">id="quote">

Yes, we mean the same but not all is/was correct.

Ok, what about this:

Liquids can't be compressed, so the volume before/after stays the same, right?

So: P/T=c this means when you build up pressure (Pa.), the temp (K.) rises accordingly.

Or am I a bit too creative now?[]

Never mind, I was also talking about the air trapped in the shock absorber.

Odd-N, it sounds quite funny when you say:"First it is not quite correct to say a shock absorber absorbs the shock"...

Gtz Dee.

[OCD]

This is so very similar to an argument I've had about nitro motors:

Is the 'compression' a funtion of pressurizing the cylinder gasses, or is it nothing more than resitance from the 'pinch zone' at TDC...

As for the damper bladder question:

I think you both had it, but overlooked the obvious.. There is very little pressurising of this small amount of gas. After all, high pressure would force the air through the seal and into the oil! (think of squeezing a resealable bag: most of the air get forced out of the bag though very small holes- well before any real pressure is made) Most of the 'spring' action created by the air bladder is from the flexing of the rubber itself. Rod goes in, rubber is deformed. Rod goes out, rubber returns to it's natural shape. Think of this as a way of creating a 'negative' pressure on the oil resevior. Otherewise- continually leaking shocks like you remember from the early days of RC. (Think Super Champ!)

[terry.sc]

quote:Originally posted by DeeMiller

Odd-N, it sounds quite funny when you say:"First it is not quite correct to say a shock absorber absorbs the shock"...

---

id="quote">id="quote">

The spring is really the shock absorber. When the wheel hits a bump the movement of the suspension causes the spring to compress and the spring compression absorbs the shock. The oil filled damper is used to stop the spring rebounding back uncontrolled.

Model car dampers just aren't pressurised, although Kyosho used to use 'pressure oil shocks' for marketing years ago. The volume of the air pocket at the top is greater than the volume of the part of the rod that moves in and out to prevent any problems caused by compressing the air pocket. Yes, most of the 'pressure' that pushes the rod back out is caused by the diaphragm returning to its original shape.

Because the diaphragm moves as the piston rod goes in the volume of the actual oil chamber stays the same. As the volume stays the same the oil is not compressed, so the oil is never pressurised. The only pressurisation is where the oil is squeezed through the piston holes.

The original Frog dampers were the first dampers that came with one way valves on the pistons. They compressed quickly with very little damping so the spring dealt with absorbing the impact, then the valve shut so the damper controlled the spring return. I presume performance wise the difference is so small that they never bothered with it in later shocks.

[terry.sc]

quote:Originally posted by robster959

Can you recommend a damper oil for use in the 959? I was running mine for the first time today and noticed that at high speed over a rough surface, the rear end kept bouncing, while the front was fine.

Rob

---

id="quote">id="quote">

Depends. If the oil is quite thick it will prevent the springs from absorbing the shocks, in which case you need thinner oil.

I expect it is more likely that the car is continuing to bounce after the bump, in which case you need thicker oil to increase the damping.

[OCD]

quote:Originally posted by terry.sc

quote:Originally posted by DeeMiller

Odd-N, it sounds quite funny when you say:"First it is not quite correct to say a shock absorber absorbs the shock"...

---

id="quote">id="quote">

The spring is really the shock absorber. When the wheel hits a bump the movement of the suspension causes the spring to compress and the spring compression absorbs the shock. The oil filled damper is used to stop the spring rebounding back uncontrolled.

Model car dampers just aren't pressurised, although Kyosho used to use 'pressure oil shocks' for marketing years ago. The volume of the air pocket at the top is greater than the volume of the part of the rod that moves in and out to prevent any problems caused by compressing the air pocket. Yes, most of the 'pressure' that pushes the rod back out is caused by the diaphragm returning to its original shape.

Because the diaphragm moves as the piston rod goes in the volume of the actual oil chamber stays the same. As the volume stays the same the oil is not compressed, so the oil is never pressurised. The only pressurisation is where the oil is squeezed through the piston holes.

The original Frog dampers were the first dampers that came with one way valves on the pistons. They compressed quickly with very little damping so the spring dealt with absorbing the impact, then the valve shut so the damper controlled the spring return. I presume performance wise the difference is so small that they never bothered with it in later shocks.

---

id="quote">id="quote">

All good points, Terry.

As you said, no RC shocks are really under static pressure. However, there is a pressure dynamic that is created when the piston travels through the fluid:

High pressure in front of the piston, low pressure behind. The damping action is a function of the equalization of these two pressures. The size of the hole(s) in the piston determines the rate at which this differential becomes 'equal' (in this case, no pressure or vacuum[] )

One-way valves like those in the Frog have sown up in many products over the years. I remember in the '90s some manufacturer had released some damper pistons using the same idea, only theirs used tapered holes rather than a slider. (as in the Frog) Neat concept, but usless if you want dampening on both compression and rebound. (who wouldn't?)

[DeeMiller]

quote:Originally posted by terry.sc

The spring is really the shock absorber. When the wheel hits a bump the movement of the suspension causes the spring to compress and the spring compression absorbs the shock. The oil filled damper is used to stop the spring rebounding back uncontrolled.

Model car dampers just aren't pressurised, although Kyosho used to use 'pressure oil shocks' for marketing years ago. The volume of the air pocket at the top is greater than the volume of the part of the rod that moves in and out to prevent any problems caused by compressing the air pocket. Yes, most of the 'pressure' that pushes the rod back out is caused by the diaphragm returning to its original shape.

---

id="quote">id="quote">

So when you pressurise the air in the absorber high enough we could remove the spring, right?

There you go!

quote:Originally posted by OCD

Originally posted by terry.sc

Neat concept, but usless if you want dampening on both compression and rebound. (who wouldn't?)

---

id="quote">id="quote">

Actually, you can.

This is used in RL-(racing)cars.

Put in a curved piston with holes in it which has more resistance on it's way back, called "fast bump/slow rebound damper"...

By adding different shapes of pistons or holes you can manage the response of the damper.

[DJTheo]

OCD and Terrys posts are quite correct, here just some additions :

Shocks

are in their generic definition absorbed by the dampers where the

kinetic energy of the suspension is transformed into heat on the oil. A

spring cannot absorb a shock as it just temporarily coverts kinetic

energy into potential energy but sooner or later transforms it back

into kinetic energy.

Actually everything (even solids) are

compressible, but their compressibity (fractional change in volume per

unit increase in pressure) is very low so at pressures we usually use

it can be neglected. On design of most hydraulic devices with pressures

over 100 bar like hydraulic drives or transmissions or a common rail

diesel injections those effect aren't neglectible at all anymore and

have to be considered.

The equations writen above are only valid

for gases and the temperature in them is the state of the gas which has

nothing to do with heat from friction as on dampers, a famous German

comedian Dieter Nuhr has a nice saying which is oftzen used in German

forums but I will be a good boy and won't write it

Cheers