This is a clear example of mono-tube shock bodies with and without a base valve in them - behind them is a Pro inner tube with base valve
This is a selection of base valves of different types - they all serve the same purpose.
Here is a comparison of a standard mono-tube shock with a base valve mono-tube shock - you can see the bright silver piston in the shock on the right. Above this piston is the gas chamber and below it is the main valving chamber which is full of oil. The shock on the left basically has the same set-up but with a base valve that can be seen just below the gold piston.
Afco 16 Series twin tube, Pro twin tube, Penske Mono tube - 7500 series. You can see the plastic gas bags in the twin tubes.
In this photo you can see inside the inner tubes of the twin tubes where the base valves are.
Here is a great shot of a Pro twin tube - you can see the inner and outer tubes and the space where the gas bag is contained as well as the main piston assembly
    I hope you find this page helpful in understanding some of the basics of shock terminology and design. Below you will find a list of "Buzz Words" related to shocks along with explanaitions of them for the average racer to understand. If you don't see what you are looking for explained here please e-mail me with your question or subject and I'll be happy to add it to the page.
Shock Technology 101.....
Base Valves?

       A base valve is basically a pressure valve built into the head of the main body of a shock. All twin tube shocks regardless of brand have base valves in them. If you think back about the paragraph where I talked about the twin tube shock – the inner tube of the shock has a base valve built into the end of it which restricts the oil from escaping the inner tube when the shock is moving in the compression stroke. It creates hydraulic back pressure forces the oil through the valving in the shock. The base valve is designed to displace a small amount of oil into the space between the inner and outer tubes where the gas bag is as the shaft enters the shock and then allow the oil to return as the shaft exits the shock on the rebound stroke. 
     A gas shock can operate with or without a base valve – the pressure in a gas shock is what takes the place of a base valve. In a mono tube shock the gas pressure has to be high enough to hold the oil into the main body of the shock as the shaft forces the piston through the oil on the compression stroke. If the pressure in the gas chamber isn’t high enough to force the oil through the valving in the shock as it moves it will actually compress the gas too easily and allow the entire column of oil above the piston to slide up creating a vacuum effect below the piston which will instantly turn the oil to foam – this is known as cavitation. Since gas pressure is widely regarded as a negative in some conditions you can run much lower gas pressure by having base valves in your gas shocks. The base valve can be tuned to do the majority of the work forcing the oil through the valving that would normally be done by the gas pressure. This allows you to run much less gas pressure as an assist to the base valve to control the oil flow in the shock instead of the higher pressure required when the gas pressure was doing all the work alone.

Gas vs. Oil Shocks?

       Many of my customers refer to their shocks as gas or oil shocks which is fine but to my surprise some of them actually think that their gas shocks are full of gas – not oil. “Isn’t that why they are called gas shocks and oil shocks?” All shocks have oil in them and all shocks have gas in them – with each providing the necessary elements to make a shock work correctly. For some reason only the shocks that you can change or adjust the pressure externally are referred to as gas shocks. Even your most basic twin tube shocks have a bag in them that is full of gas – but since you don’t pressurize them externally they are referred to as oil shocks by many people.
       The oil inside a shock - while providing some natural lubricating properties – acts as the tool by which the shock dampens the movement of the suspension. Dampening happens by the building and releasing of hydraulic pressure when the oil is forced through the valving as shaft moves in and out of the shock. The valving inside the shock restricts oil flow in each direction as the shaft is moving in and out of the shock creating resistance to movement. The more restrictive the flow through the valving – the stiffer the dampening it will produce and vice-versa the less restrictive the flow – the softer the valving will be.
       The gas inside a shock acts as an air space that compresses to allow for the oil to expand or displace when the shaft is pushed into the shock. If you had a shock with no air space in it - you would not be able to push the shaft into the shock because oil alone will not compress. To visualize this you can take a simple cup of water filled to within 1” of the top then stick your finger down in the water and you will see the water level rising as your finger goes farther into it. The same thing happens inside a shock – when the shaft goes in the oil has to make room for the volume of the shaft as it enters so the oil “rises” or displaces and compresses the gas. As the shaft goes back out the gas expands and forces the oil back to fill the void from the exiting shaft.
       These are the basics of what happen inside any shock and there is much more that goes into the design and manipulation of the valving forces for different applications.

Magic Shock Oil?

       Shock oil is a subject that gets a lot of attention from novices to experts. I’ve seen it time and time again where a shock technician will tell you they have this new “magic shock oil” that will make you faster. This is one of the biggest misconceptions of all. Oil is in the shock to allow the valving to perform properly but if a shock isn’t built or valved properly – there is no oil that will make it good – period. During my time on the Cup series we have tested everything in shocks from water to WD40 and the truth is that there is no magic in shock oil. Synthetics are overall better oils but the important part about shock oil is using the correct viscosity. If you have a shock with a thicker viscosity of oil in it – the dampening of shock will change a lot more from cold to hot than it will if your shock has a thinner viscosity oil in it. In years past Pro shocks used a thicker viscosity oil in their shocks and as they ran they would lose about 20% of their dampening force as the oil heated up and thinned out. Shock oil will only thin to a certain point as it gets hot so the way that you minimize the change with temperature is to start with an oil that has a relatively thin viscosity when it is cold – this narrows the change from cold to hot making it more consistent. It is similar to the change if you start you engine and the oil is cold – the oil pressure is very high and as the engine and oil warm up the pressure drops considerably – though this isn’t the same as a shock it is a good example of how oil heating up and thinning out changes pressures related to them. Most valvings used on dirt are pretty soft in general so they don’t work the oil very hard but when you are dealing with very stiff valving like we use on the front of asphalt late models will you see very tiny differences in oils of the same viscosity. 
       The other side of this is that thicker oil are actually better dampening fluids but it is important when using thicker fluid to make sure your valving has the desired forces when at operating temperatures. We won several Cup races on 30 wt. shock oil. We used this for impound races where we would qualify and then the cars were impounded until the start of the race. The thicker oil would increase the rebound forces on the front shocks for qualifying when they were cool – the oil would stay relatively thick during the 2 laps but when they were ran for several laps and got good and hot at the start of the race they oil would thin down to where it needed to be for the correct rebound forces. 

Rod Pressure?

  Rod pressure is a force that is created by the internal pressure of a gas shock. It is an added force that resists the shaft being compressed into the shock. To compress the shock you must first overcome the rod pressure and seal drag. Depending on gas pressure and shock configuration it can be a significant amount. As the shaft compresses into the shock and the oil displaces as talked about in previous paragraphs – the gas will compress and the pressure goes up accordingly thus increasing the rod pressure as the shock compresses. You can watch this happen by putting a pressure gauge on your shock and push the shaft in and watch the pressure go up and down with the shaft movement. The other factor that affects rod pressure is called “seal drag” – it is the amount of drag that the seals, wipers and internal piston band have on the shaft as it moves in and out. Anything that touches the shaft as it move will create some amount of drag. Too much seal drag is a bad thing so minimal seal drag is optimum. I see shocks all the time with enough seal drag that the shaft doesn’t naturally extend from the internal gas pressure because the amount of seal drag on the shaft is greater than the than the amount of rod pressure pushing it out.  
  There is a simple formula for figuring out how much rod pressure a shock will have for different settings. You take the area of the diameter of the shaft in your shock and multiply it by the amount of gas pressure you have in the shock. This gives you the actual rod pressure of the shock before seal drag.
  For example – I’ll figure the rod pressure of a shock with a 5/8” shaft at 50 psi. To figure the area of the shaft – PIxR2 or 3.1416 x (.312 x .312) = .306 sq. inches. So the area of the shaft is .306 of a square inch so we now multiply .306 x 50 = 15.5 pounds of rod pressure before seal drag. You can see now that a smaller shaft will have less rod pressure than a larger shaft does at the same gas pressure setting because the area of the shaft is smaller.
  During my years on the Cup series - at times we were running front shocks with 1500psi in them – these were 5/8” shaft shocks and would produce about 450 pounds of rod pressure per shock. We did this so that the shock would do much of the work of holding up the nose of the car in the garage. We would unwind the front spring and the shocks would hold the nose up until we were on the track – the shocks were built in such a way that they would trap the gas pressure internally once they had been compressed on the track and the rod pressure would go away so the nose of the car would stay much lower around the track – this was before bump stops and we were coil binding springs to get what we needed for height and doing this allowed us to run softer springs that we ever could before. Once the car came off the track the rod pressure would slowly come back and raise the nose of the car for post-race inspections. Once NASCAR found out what we were doing they made everyone take the shocks off the car until after inspections heights were done both before and after the race taking away the advantage of the high rod pressure deal.

What is a 3” Number? Zero Number?

       The 3” number seems to be the most popular way many racers describe a shock valving they are looking for. The term 3” number relates to an amount of force that a valving creates at 3” per second shaft speed. The 3” per second shaft speed does not mean the shock is moving 3 inches. It means that the shaft of the shock is moving in or out at a speed where if it traveled for a full second it would travel 3 inches. During operation on the car a shock is constantly going in and out in mostly small movements. If a shock were to move .300 of an inch in 1/10th of a second it would have moved at 3” per second speed even though it had only moved .300 of an inch. The drawback of focusing so much on a simple 3” number is that there are many factors that go into creating a correct valving and the 3” number is simply 1 part of the puzzle. I can make you 2 shocks with the same 3” numbers that will be completely different shocks but it’s the shock builder’s job to make sure the rest of the valving is what it should be for the particular application. 
       The Zero number is another big buzz word – a “Zero” number refers to an amount of force that a shock has stored in it during the “millionth” of a second that the shock actually stops moving at the end of the rebound stroke as it begins its compression stroke as well as when the shock stops at the end of the compression stroke as it begins the rebound stroke – this being the point on a shock graph at the line marked “0” velocity. The zero number is a by-product of the valving in the shock and shouldn’t be focused on too closely when tuning a shock. 
       The biggest problem with comparing numbers is that you need to make sure that the shocks you are comparing were tested in the same manner on the dyno. The “Zero” number will change with different temperatures and even more at different test speeds. If you have access to a dyno – test a shock at different speeds and look at the differences in the numbers – especially things like the Zero number. A shock doesn’t just produce a specific set of numbers on a dyno – the numbers that they produce are relative to the way they were tested. If you test them different shocks different ways then try to compare the numbers you will be chasing your tail so to speak.

Twin Tube vs. Mono-Tube?

  Twin-Tube and Mono-Tube are the 2 main designs of nearly every shock on the planet. The terms “twin tube” and “mono tube” refer to the construction of the shock. A mono tube shock has a single tube construction for the main body of the shock with the gas chamber built into the head of the shock. Mono tube shocks are referred to as gas shock since they are nearly all built with the ability to add or change gas pressure externally. Twin tube shocks have 2 tubes that make up the main body of the shock. The inner tube where all the valving happens is surrounded by a larger outer tube. The thin space between the inner and outer tubes is where the gas is contained inside a sealed plastic bag. I’m sure most of you have heard of a shock having a “bad bag” in it. This refers to when the gas bag inside the shock has ruptured and allowed the gas inside the bag to escape into the shock and find its way into the inner tube causing issues with the performance. Many twin tube shocks you see will have small dents in them from use which doesn’t affect their performance as long as the dents aren’t deep enough to reach the inner tube. A mono tube shock however is much more sensitive as the single tube body must remain free from any dents which could damage the performance. 
  The fact that a shock is a mono tube or twin tube doesn’t necessarily mean that one is better than the other but just that they are constructed differently.

L-R FK,Pro,Penske,Afco,Super Shox,VRP,Pro Gas
"The only "Pot of Gold" at the end of this rainbow is the one it cost you to buy them. It's less about what color or brand they are and more about having them right no matter what brand you choose."
"The Shock Rainbow"
Base Valve Examples
Top Row L-R
Penske, Penske, Ohlins
​Lower Row
Afco, Pro
Shock Body Tubes 
Top - Pro Twin Tube Inner with Base Valve
Lower - Penske with and without Base Valve
Cutaway Shocks - Twin Tube 
Afco, Pro
Mono Tube 
Penske 7500
Cutaway Shocks - Mono Tube
 Penske 7500 without Base Valve
7300 with Base Valve