What exactly causes shock fade?

[JonathanHanson]

To the engineering experts here - I hope you won't mind a question from a non-expert, although I'm familiar with the general theories of shock absorbers and the different types - monotube, twin-tube, etc., and gas charging.

I'm writing a review of a new set of adjustable, twin-tube shock absorbers I installed on a Toyota Tacoma, which employ a closed-cell foam insert and a hefty volume of oil rather than a nitrogen charge.

In researching the causes of shock fade, I find varying opinions and even contradictory information. So far I've read or heard of a couple of contributing factors:

1. As the oil in a shock heats up under severe use, its viscosity drops, reducing the effectiveness of the valving and making the shock "softer." There is also mention of aeration in twin-tube shocks in which the nitrogen charge is not separated from the oil, as it is in a de Carbon monotube shock.

2. Cavitation, caused by voids forming in the oil on the low-pressure side of the piston. I've seen a couple of videos of this.

However, it remains unclear to me why cavitation causes fade. Does the momentary formation of those voids also reduce the viscosity of the oil? For cavitation to cause fade - which by definition is a progressive condition - I assume it must increase under severe conditions. Why would this be? Does hot oil cavitate more than cool oil?

Finally, no one has explained to my satisfaction how a pressurized shock reduces cavitation (or fade). Also, the shocks I'm testing are under essentially no pressure at rest. The closed-cell foam compresses and expands as the shock extends and contracts, but there is no gas pressure as such. So I wonder about cavitation and fade in this type of shock (which seems to be highly regarded in Australia for use in severe conditions).

Is anyone here willing to opine/pontificate/argue about these issues?

Thanks for any clarification, the more detailed the better - I want to understand this stuff!

Regards,

Jonathan

 

[BrianPetersen]

Cavitation happens when the pressure somewhere in the shock body drops below the vapor pressure of the lowest-boiling-point constituent of the oil in the shock.

At higher temperature, that vapor pressure is higher, so cavitation will happen easier for a given speed of movement of the shock.

Cavitation doesn't necessarily reduce the viscosity ... but if you have (say) 51 psi absolute on one side of the piston and 1 psi absolute on the other side of the piston without cavitation, and now you raise the temperature (vapor pressure) so that the vapor pressure is now (say) 11 psi, you are now going to get 51 psi on one side and 11 psi on the other side with vapor pockets and less oil flow through the orifices (even though the shock piston is moving the same speed), and now when the shock gets to the end of its stroke, instead of having all incompressible oil on the side that just flowed through from the other side, it now has a mixture of oil and vapor pockets, which present little resistance to the piston returning back in the other direction until all the vapor pockets collapse.

If you raise the bulk pressure inside the shock via gas pressurization to (say) 100 psi absolute and you move the piston the same as above, you might get 100 psi absolute on one side of the piston (or more, but let's say the piston is moving towards the side that has the pressure reservoir) and 50 psi absolute on the other side - but this is more than the vapor pressure, so cavitation doesn't happen.

I have no experience with using closed-cell foam instead of a gas reservoir. All of the GOOD shocks (Ohlins, Penske, Elka) in the application that I'm most familiar with (motorcycle roadracing) are gas pressurized. So-called "emulsion" shocks, that are supposedly designed to work with a mixture of oil and gas bubbles, are generally regarded as garbage.

 

[JonathanHanson]

Brian, thanks very much - your description of the pressure differentials illustrates exactly what I wasn't grasping.

Is the theory regarding heated oil simply losing viscosity also a factor in fade, or is it all about cavitation?

My experience with shock absorbers is in the realm of long-distance expeditions, such as in heavily loaded Land Cruisers/Land Rovers. I had a chance to put 3,000 kilometers on a Land Rover equipped with Koni Heavy Track Raid shocks (here) - obviously a respected brand - which were not gas charged (nor did they have any foam insert), but relied on a massive volume of oil. I don't know about their tendency or lack thereof toward cavitation, but I do know that after a pretty brutal 200-mile day in Tanzania, I could comfortably rest my hand on the shock body, and they appeared to have lost no function at all.

The shocks I'm reviewing now are from Boss, and they are similar to the Konis except for the addition of the foam. There are several other brands using this approach, especially in Australia where long cross-country trips are the norm. In researching the field, I noticed that every company there offering both oil/foam-cell shocks and oil/gas shocks, the oil/foam cell shocks are sold as the heavier-duty version, and priced higher. When I asked one rep about this, his response was, "Nitrogen is cheap. You can build a shock with very little oil, add gas to it, and it will work fine. An all-oil shock will stand up better to long-distance abuse."

Any thoughts there? Thank you again for the education!

 

[Fabrico]

You might take a look from the other side, where these concerns have been thoroughly addressed. For heavy use, nothing compares to a remote reservoir. These are used around the world for just what you are doing, Dakar, Baja, and a whole lot more.

http://www.kingshocks.com/

http://www.swayaway.com/

http://www.ridefox.com/

 

[BrianPetersen]

Heating the oil will also make it fade because the viscosity drops; it's not all about cavitation. The better shocks rely on spring-loaded orifices (shim stacks) which at least somewhat counteracts this. It will take quite a bit of working the shock to make for a noticeable temperature change. Motocross may have a greater issue with this. In street/roadrace bikes, the bigger issue is when the shock body or the reservoir have to pass in close proximity to the exhaust system ... some bikes that have fashionably high-mounted mufflers in the tail section route the exhaust through the same vicinity as the shock; there is only so much room to work with. In really high powered examples, the airflow warmed up through the radiator can be significant as it passes back over the engine and then around the shock.

Bilstein also use gas-pressurization.

There's little question that using a larger volume of oil will slow down the effects of temperature changes but I don't see how that has anything to do with using closed-cell foam as opposed to a floating-piston gas-charged remote reservoir - and surely the floating piston and its associated seals etc is more expensive.

My initial impression is that you are being sold a bill of goods. I could be wrong.

 

[Tmoose]

I wonder if the outdoor temp sensor on an indoor/outdoor thermometer could be used to monitor shock body temperature during your evaluation.

In the mid 1970s Skyway was a manufacturer of quality innovative offroad motorcycle products. Now they just make composite wheels for wheelchairs, BMX bikes and industrial applications.
One of their mid 70s products was a pair of small plastic bags filled with freon or a similar gas. the advertised purpose was to boost the performance of twin tube shocks being called into abusive service with the new-fangled "long travel" modified rear suspension.
Air mixing with oil to form a "low viscosity" emulsion given the chance whether squirting thru holes or riding in a cocktail shaker sounds fairly believable.
A few paragraphs in this 1976 dirt bike magazine pages describe their performance.

http://davestestsandarticles.weebly.com/uploads/4/8/4/5/4845046/dirtb_nov76_mr250.pdf

The ones I had have been sitting for over 3 decades unused in the original package. A few years back I happened to check on them and the gas was gone, or at least the bags were no longer plump.

As you noted some modern shocks brag of their freon bag technology. I wonder if something similar resides inside any of the nearly modern low pressure gas shox.

In the mid 1970s Girling, a preemininent suspension supplier made their gas shox supposely to operate correctly with an air oil emulsion, thus bypassing the necessity of techno-efforts to prevent the twain from meeting. The inverted mounting with a "gas recuperation space" and lack of separating piston in the cutaway shown in the adonb page 1 here.

http://i.ebayimg.com/00/s/MzAwWDQwMA==/$(KGrHqR,!roE-Yzw6fBRBPwwKN(CTg~~60_12.JPG

 

[JonathanHanson]

Thank you once again Brian and Fabrico. Brian, thanks for the clarification regarding overheating and fade. I agree with you regarding cost and the de Carbon monotube design vs. the foam-cell insert. Might be a different story in a twin-tube shock I suppose. In any case I have a vastly greater understanding of the physics. I must dig deeper into the marketing to find out why several of these makers are positioning the foam-cell shocks higher than their own gas-charged models.

Just for information, here is the text from one Australian manufacturer (who sell both styles) on their top-end shocks:

The use of Micro-cellular foam inserts has produced a new breed of shocks, which are more resistant to fade. After heavy work, all shock absorbers, whether they are hydraulic, gas or foam cell shocks, show signs of fade or lack of damping force. However, Foam Cell shock absorbers are far more resistant to fade than other types of shocks. Tests have shown that foam cell shocks maintain almost optimum efficiency in conditions that can cause gas and especially hydraulic shocks to lose much of their dampening force.

To understand how foam cell shock absorbers could produce such superior performance, we will look at the common causes of fade and how foam cell inserts affect shock absorber operation.

AERATION is the mixing of the oil with gas or air in the oil reservoir, i.e. the outside cylinder of the shock. This causes the oil to "FROTH", reducing its viscosity so that it passes more freely through the valving. This causes the shock to lose some of its damping force and the air bubbles passing through the valving causes the shock to operate unevenly and noisily.

Also, when aerated, the oil is compressible. Instead of the oil being forced through the valving and into the outer cylinder, the action of the shock absorber compresses the oil within the pressure cylinder. When extremely aerated, a shock provides a cushioning effect rather than a controlled resistance to movement. This is loosely called "FADE".

CAVITATION occurs when the oil inside the pressure chamber starts to boil. This is due more to LOW pressure than to high temperature, although both contribute to cavitation.

The pressure drop is caused when the shock extends Rapidly, but the flow of oil into the pressure chamber is insufficient to fill the space left by the retreating piston and rod. A vacuum is created in the pressure cylinder and the oil begins to boil. Under low pressure, a liquid can in fact boil without heating, and this is what occurs when a vacuum is created inside a shock absorber- instantaneous boiling due to a sudden drop in pressure. The boiling of the oil produces air bubbles within the shock, which produces the same effects as aeration of the oil.

Gas pressurization of shock absorbers can significantly improve their performance, however gas shocks are also subject to aeration & cavitation. Under unrelenting and extremely rough conditions a gas shock provides only marginally resistance to fade than a standard hydraulic unit. The FOAM CELL shock absorber, on the other hand, performs with an almost negligible reduction in dampening force. To do this, it must virtually eliminate aeration & cavitation. How does it do this?. Quite simply. The foam cell insert occupies the space in the oil reservoir which in the case of hydraulic or gas shock absorbers would contain air or gas. While oil can mix with air and even pressurized nitrogen, it cannot mix with the foam. The foam used in shocks has tiny closed cells, which contain bubbles of gas. The oil inside the shock cannot mix with the gas bubbles inside the foam because they are enclosed inside the foam.

When the shock is compressed, the foam works not like a sponge absorbing the oil, but rather like a spring or air bag. At the end of the compression stroke, the compressed gas bubbles inside the foam push the oil back into the pressure cylinder.

No oil / air or oil / gas mixture has occurred, so there is no risk of aeration. The pressure extended by the foam pressing the oil back into the pressure cylinder prevents cavitation occurring. Hence, no aeration and minimal cavitation for unrivalled resistance to fade.

It mostly seems as thought they're describing things properly, although they seem to be ignoring monotube shocks with a piston to separate the oil and gas, and describing only twin-tube designs.

Thanks again for all this help!

 

[JonathanHanson]

Tmoose, out posts must have passed in the ether. Thanks for the additional input!