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Shock valving vs. re-charging

EQuin

Well-Known Member
Hi Folks,

I'm considering some 2.0 coil-overs for my daily driven, disco-ish Tacoma Prerunner that sees some occasional off-road use. Before I settle on a particular set, I was wondering what the difference is between shock "valving" and the pressurized gas charge within the shock. If I understand correctly, the valving of the shock is set by some washers inside the shock that control the flow of oil inside. Is that right? What is meant by "re-valving", and how does re-valving change or affect the shock's function?

Secondly, what is the purpose of the pressurized gas inside the shock? And how does increasing or decreasing the pressure affect the shock's dampening function? For instance, if the shock came pressurized at 150 psi, how does adding an extra 50 psi (for a total of 200 psi) affect the shock?

Thanks for any educational info. Take care,


Ed Q.
"If you're going through hell, keep going." Sir Winston Churchill
 

John Bitting

Administrator
Experts let loose on this question. Since no name brand are mentioned it is acceptable, I say a lot of people can learn a lot from this Thread including myself... Kreg, Greg, anyone.. Thanks in advance.
 

Mike_HKmtrsprts

Well-Known Member
I know that re-valving a shock means changing washers to thicker or thinner to make it harder or easier for the piston to move through the oil, but I would like to know some anwers to your other questions. I always thought that the nitrogen was there to keep the oil from turning into a milk shake inside the shock. Im by no means an expert so some one let us know....

You must be Fast cuz I was HAULIN ASS when I passed you...
 

EQuin

Well-Known Member
Yeah, I was careful not to name any shock brands in the hopes of eliciting a brand-free, "shock science for dummies" response from our reknowned experts, me being a reknowned dummy - lol.

Thanks for the support and for bumping it up for me! And GOD BLESS AMERICA!!


Ed Q.
"If you're going through hell, keep going." Sir Winston Churchill
 

EQuin

Well-Known Member
Thanks for the info, Mike. At least I sort of understand the washer part. So changing to a thicker washer (which I take is also known as "re-valving") makes it harder for the piston to move through the oil? Or is it the other way around? And by making it harder for the piston to move through the oil, the dampening effect of the shock is increased, albeit at the risk of sacrificing a more comfortable ride?

Hopefully the other experts will chime in and explain more fully the purpose of the nitrogen gas, and how changing the pressure affects the dampening of the shock.

Thanks again and take care, and GOD BLESS AMERICA!!!


Ed Q.
"If you're going through hell, keep going." Sir Winston Churchill
 

BIG_FAT_LOSER

Well-Known Member
correct me if I am wrong:

nitrogen keeps pressure on the oil to prevent it from cavitating (bubbles, like a hand moving rapidly under water) a fast moving valve stack moving through oil creates a vacuum on the back side. The pressure keeps air bubbles from forming in this vacuum.

a shim stack or valve disk is like a flap on a big rig exhaust. Thin discs or washers cover the oil passages and the thinner or thicker the washes are, the more or less they flex to allow the oil through the passage and the slower or faster the valve moves through the oil on compression and rebound

reservoirs absorb hard fast inpacts (curb at 45 mph) that the vavle does not have time to respond to. the reservoir has a piston separating oil and gas. upon a fast inpact the valve cannot respond and the oil compresses into the reservoir and moves the piston and compresses the gas behind it. when the valve responds the gas pressure behind the piston move the oil (very little) back into the shock

shocks with out reservoirs use the gas as a sort of bump stop. as the valve cannot respond the gas compresses and allows the shock to move without locking

the amount of gas pressure basically makes the shock stiffer or softer under hard fast impacts.

bypass shocks usually have pistons without valve stacks. as the piston moves up oil is diverted through a hole in the side of the shock (not through the piston so it has nowhere to go) through an externally adjustable valve and back to the top of the shock. there are several passages at different points along the shock body as the piston moves upward. and each external valve can be adjusted (like a faucet) to allow more or less oil through it. there is also a passage on the rebound as well. these valves are one way flow.

Did I get an A? or an F?

<font color=red>PAT KAPKO</font color=red>
"DISCO FEVER"
 

TimHayosh

Well-Known Member
Here's my $0.02...

The washers are the valves. They are stacked on both sides of the piston (imagine a pyramid-shaped stack of washers that go from large to small diameter). Re-valving means you are changing the characteristics of the way the fluid is allowed to pass through piston. When the piston is in motion, the fluid goes through the ports in the piston and deflects the valve stack. The valves do not cover the ports completely. Therefore, at slow piston speeds, the valve aren't doing anything other than covering a portion of the port. At higher piston velocities, the fluid rushing through the port will deflect the valve stack. It is this deflection that you can alter by changing valves. Bigger (dia.), thicker valves will cover more port area and take more fluid pressure to deflect, i.e. a stiffer shock. Smaller (dia.), thinner valves (washers) will result in a "softer" shock. Keep in mind that there are valve stacks on both sides of the piston. This is how you adjust for compression AND rebound. Some shock mfgr's will have adjustable ports as well. Kinda like a jet in a carburetor. Others are just holes. There are also bleeder screws (in the piston) that can also adjust the low speed characteristics of the shock (when the valves aren't being deflected).

My understanding of the gas pressure (nitrogen) is that it helps to keep the fluid from cavitating when the shaft speed is high. In otherwords, when the shaft is moved quickly, a vaccuum (or just low pressure) is created on the back side. By having the fluid under pressure, it can more readily fill the void. I don't think gas pressure has a huge impact on the dampening charaterisics of the shock. You might be able to do some very fine tuning, but I don't think it will affect the shock in a big way. I think SAW and Fox recommend about 200 psi. I run mine at 210: big deal.

As an addendum, bypass tubes/valves only serve to change the valving characteristics when the piston is at a certain part of the shock travel. They allow fluid to port AROUND the piston valve stacks and go back to the back side of the piston. Once the piston moves past the last bypass port, all fluid must pass through the piston valve stack. Bypass tubes are directional. Therefore you have compression tubes/valves and rebound valves/tubes.

The only thing reservoirs do is add fluid QUANTITY. This allows the shock to disipate more heat. They have NO EFFECT on the dampening characteristics of the shock. On a street driven (or occasionally off-road driven) vehicle, reservoirs don't do anything for you. You will never generate enough heat to gain any benefit from a reservoir under those conditions.

So that's all I know about shocks. Flame away!

For more info, call a pro:
Lee at RaceShock Co. : 602-493-3700
Mike Arthur at Kartek: 909-737-RACE
 

frankh

Well-Known Member
You can not fill a non-reservoir shock all the way up with oil because you need some place for the oil displaced by the shaft to go. You need the fill that gaps with nitrogen or the oil will arrant (spelling ?). A reservoir shock pushes the extra oil into the reserve and the nitrogen pressure pushes it back. I think a by-pass shock still has valuing otherwise it would hydraulic when it passed the last tube. But we will all find out when someone that is knowable checks in.
 

BIG_FAT_LOSER

Well-Known Member
Reservoirs do allow for some heat disipation through radiating it, there is not enough oil flow between shock and reservoir to allow much cooling benefit. if there is no gas in a shock to absorb the fast hard hits it will lock hydroiclly, like putting your finger over the end of a syringe. you can move the syringe a bit because of the airspace. if it were filled completeley you could not.

<font color=red>PAT KAPKO</font color=red>
"DISCO FEVER"
 

vwguy

Well-Known Member
everything here is right
as pat said the resivoir kind of acts as a bump stop when the fluid can not flow through the piston fast enough the piston in the resivoir compress's the nitrogen and then bounces back
i believe frank is right if there was no valving in the piston of a bypass shock it would lock up having no where for the fluid to go if it passed the last hole

how ironic is it that most people slow down for speed bumps yet almost all of us here im sure pin it
 

FABRICATOR

Well-Known Member
Pat deserves the "A" as he is the one who pointed out the important fact that the piston in the reservoir separates the oil from the pressurized gas. When set up right there is no nitrogen, air, or any other gas in contact with the oil. This way the oil can expand as it heats up, contract as it cools, but it cannot foam up. This separation, by whatever means, is critical to any off-road racing shock absorber.

Adding pressure does have some effect on damping, as does changing the amount of oil. (as in more or less room for gas, ie; progression). It also can add some preload (as in springing). It can be used for very fine tuning but really should not be used for this purpose to any great degree. Having more pressure than what is necessary to prevent foaming just makes it harder on the seals.
 

rdc

- users no longer part of the rdc family -
yah fabricator is right when i re-charged my shock, i boosted it from 200 to 220 it helped out alot, it rides alot better for me now
Dan

DRIVE IT LIKE YOU STOLE IT!
 

EQuin

Well-Known Member
You are all awesome! Thanks a bunch to all of you for taking the time to explain and help me learn and understand all of this. Man, I love this forum!

Take care,


Ed Q.
"If you're going through hell, keep going." Sir Winston Churchill
 

TimHayosh

Well-Known Member
Pat said: "Reservoirs do allow for some heat disipation through radiating it, there is not enough oil flow between shock and reservoir to allow much cooling benefit."
What then is the purpose of the reservoir? Yes heat is disipated via radiation, but it is also CONDUCTED through the fluid. The fluid does not have to go into the reservoir for the reservoir to aid in cooling. Thus, it takes more (kinetic) energy to heat 10 gallons than it does to heat 5 gallons. The reservoir is all about heat. If reservoirs had some kind of "bump absorbing" benefit (that non-reservoired shocks don't have), wouldn't ALL cars have them? The same airspace exists above the fluid in a non-reservoir shock as exists in the end of a reservoir. A solid piston separates the fluid from the air in both. No valving here. (An emulsion shock does NOT have this piston and the gas is mixed in with the fluid. Not common in off-raod racing.)

How ironic that as I was writing this, Lee from RaceShock called! I asked about the "bump stop" uses of the reservoir. He said you could manipulate the bump characteristics of a shock absorber by changing the installed position of the dividing piston. Remember that as the piston/shaft move into the tube, the dividing piston is also moving in the same direction. So, if the dividing piston is installed with say 3" remaining before it hits the end of its travel, and you put 200 psi of gas in, that 200 psi is raised way up in just a few inches of travel (as it is compressed into a very small space). However, if you installed the dividing piston with say 8" remaining of reservoir travel, the piston/shaft could go farther before the same gas pressure (bump) is achieved. This, however, applies 100% to a non-reservoir shock too! Again, emulsion shocks are not a part of this discussion.

Trying to add....not confuse!
 

Dylan

Well-Known Member
It’s impressive how many of the people here know a lot about shocks!!
Heres some more to through into the mix

The function of the gas chamber in the reservoir is to accommodate the volume change that occurs when the shaft enters the shock body and when the oil expands. The reason that the gas(nitrogen) has to be under pressure is to force the fluid through the various orifices the shock If there is not enough pressure then cavitation occurs. Cavitation is when the absolute pressure exerted on the fluid is lower than the critical vapor pressure for that fluid at a given temperature and the fluid begins to boil or vacuum pockets form
Cavitation happens under two circumstances. What I call gross cavitation and localized dynamic cavitation. Gross cavitation occurs when the force on the shock exceeds the force exerted by the reservoir pressure and the whole column of oil above the piston is shoved into the reservoir and a vacuum is pulled on the bottom side of the piston. Localized dynamic cavitation ocures when oil is flowing at very high speeds and is required to change direction through passages or around corner in the geometry of the piston or valve. This causes low pressure zones, if the decrease in pressure is below the critical pressure or zero than vapor or vacuum bubbles form. This can be very damaging the shock. As the little pockets collapse they cause a hammering action on the surface of the piston or valve. You can see this on a shock piston that has cavitated. Usually around the hole that the compression valves cover or on the face of the valve plate. It appears as abrasion or pits in the surface, it often can eat through the hard anodize on the piston.
In both cases it affects the performance of the shock detrimentally and in both cases more reservoir pressure can alleviate the problem.
I would not recommend using pressure as an adjustment for shock performance. If reducing the pressure alleviates the harshness it is an indicator that you need to ad more high speed flow ie: thinner shims, more stand of to the rate plate, dual stage, ect. As far a letting the reservoir take the big hits, yes this is probably happening to some extent but should be minimized, note that a bypass has the res of the bottom therefore big hits must go through the valving. There is also a high speed throttling effect by the hose restriction casing more than res pressure in the cylinder at high speed, this can also cause harshness.
As far as spring rate from the pressure, with most shocks the res volume is so large that there is usually less than 10 lb/in although it can account for significant preload force for light cars.
Be careful setting the floating piston to close to the shrader end. I’ve seen floating pistons crushed to half there height by hitting the end of the cylinder and hydrolicing
 

toddz

Well-Known Member
The preload generated by the nitrogen pressure can be easily calculated by calculating the surface area of the shock shaft and multiplying by the nitrogen pressure. I'm too lazy to do the math right now, but as I recall, I found that about 180-200 psi of pressure added about 60 lbs. of preload to the 2.5" Kings I was working with awhile back. I talked to all of the aftermarket off-road shock manufacturers a few years back trying to understand the effect of the nitrogen pressure on spring rate, if any. I never felt like I got a good answer from any of them. I found the answer is Carroll Smith's(racecar engineering god) book, Drive to Win.

Todd Z.
 

BIG_FAT_LOSER

Well-Known Member
In hopes to further confuse/educate/lie........Why nitrogen as a shock gas some might ask. Nitrogen is less likely (or not at all) to expand as heat increases.

Another question. Does anyone use different weight oils for fine tuning?

Most of what I have learned comes from Motorcycle shocks and forks. They should be very similar.

<font color=red>PAT KAPKO</font color=red>
"DISCO FEVER"
 

mustafa

Well-Known Member
Nitrogen has a very low coefficient of thermal expansion. That's why it's used to pressurize aircraft and indy car tires.

I'm shaking the tree boss!
 

FABRICATOR

Well-Known Member
Dylan is right in that the main reason for the reservoir is to accommodate the change in oil volume due to the shaft going in and out of the shock body. This is an absolute necessity because the oil is in a sealed space, but in this case the "space" can get larger or smaller.

Bilstein and others use a separated system without a "separate" reservoir (DeCarbon). They use an extra long, single wall body with a floating piston in an area beyond where the shaft mounted piston travels. Very effective and durable. The single wall also aids in cooling. This type of shock has practical limits on travel and especially shaft size. That is why the very good original Bilsteins did not grow with the sport.

The main reason for using nitrogen gas is that it is inert and as Mustafa pointed out, has a low coefficient of thermal expansion. This means it does not react with most other elements and does not radically expand with more heat. It also is fairly cheap. Carbon dioxide is one of the least expensive of all gases (except natural gas or air!) and is generally considered inert. However it does funny things at about 700 PSI (depending on temperature) as it will liquify. It also greatly expands with added temperature. Argon is one of the most inert gases of all however it is fairly expensive. It is also very dense, has a high coefficient of friction, and might add a few degrees of heat. Air is probably the next best thing to nitrogen but it does expand more and can oxidize things.

Anyhow...that's why the nitrogen.
 
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