File Name: Twins vs Mono
Shocks control spring motion, that is, they slow down and reduce the magnitude of the spring’s oscillation. The process is known as dampening. In technical terms, a shock controls the frequency and amplitude of the suspension's oscillation. In layman’s terms, a shock controls how fast and how much the suspension compresses and rebounds.
By dampening the movements of the suspension, the tires remain in contact with the track surface. This prevents the tires from bouncing and skipping with every bump and dip on the track. Tires that do not stay in contact with the track can’t provide good traction, steering stability or braking friction. Dampening the movements of the suspension also results in resistance to vehicle bounce, roll, and sway during weight transfer and therefore resistance to brake dive and acceleration squat.
How Shocks Work
A shock absorber is basically a hydraulic piston pump that converts the energy of motion (kinetic energy) into heat energy. One end of the shock, the shock body or “tube”, is the cylinder of the pump, the other end of the shock is the rod and piston. One end of the shock is connected to the chassis and the other end is connected to the suspension. As the wheels move up and down relative to the chassis the piston pumps up and down in the cylinder.
The cylinder (shock body) is a tube filled with hydraulic oil. When the piston pumps up and down through the hydraulic oil, the oil is forced through holes in the piston, called orifices. The flow of the oil through the orifices is further regulated by the deflection of special spring-loaded metering valves, or deflection discs, located on either side of the piston.
Because of the valves and the fact that the piston orifices are so small, only a small amount of fluid, under great pressure, passes through the piston as it pumps up and down. The resulting resistance slows down the piston and creates heat from friction. This in turn slows down spring and suspension movement and converts the spring’s kinetic energy to heat in the oil that is then dissipated as the oil cools.
The amount of dampening a shock absorber provides depends on the number and size of the orifices in the piston as well as the valving. By changing the design of the valves, the pressure at which they open and close can be altered and a shock’s damping characteristics can be tuned as needed. The higher the opening pressure, the firmer the shock, and the lower the opening pressure, the softer the shock.
Obviously, shock absorbers must work in two directions - compression and rebound. The compression stroke occurs as the shock gets shorter and the piston travels into the cylinder. The rebound stroke occurs as the shock lengthens and the piston extends from the cylinder. The compression stroke controls the motion of the vehicle's unsprung weight, while the rebound stroke controls the heavier sprung weight. Accordingly, a typical race car shock will have valving designed to provide more resistance during rebound than compression. This is mainly true in a twin tube shock such as a QA1 or Afco style shock and not so true in a Bilstein or Pro gas shock.
Close-up view of shock piston.
Note: This shock uses small bleed hole to move oil though when the shaft speed is below 3”per second. The number of holes and the size of them determine how much feel the driver has over the car at slow speeds. Not all shocks use hole for bleed, many companies use bleed plate, this allows them to adjust the amount of flow with out changing piston, it also makes for a smoother transition between bleed and stack opening.
Compression Valving discs is on the rod-side of the piston. This is a standard pryimid stack on a flat piston.
Rebound valving discs are on the free-end of the piston.
Rebound valving discs removed from free-end of the piston.
Looking very much like a series of washers (and often referred to as such) they are stamped spring steel deflection discs that react to pressure and velocity to meter, or regulate, the amount of fluid that flows through the piston and thus the amount of resistance or damping the shock provides. A particular number, size, and arrangement of valving discs are often referred to as a "shim stack".
Heat and Shock Fade
Because of the tiny orifices in the piston, the viscosity of the shock oil has a large effect on the resistance the oil presents to the piston’s movement, and hence the dampening the shock provides. Viscosity changes with temperature –hot oil is “thinner” and pours more easily than cold oil (which is why you change your oil when the engine is warm). It stands to reason then, that to provide consistent dampening; we would want the shock oil to maintain a consistent viscosity. However, we just said one of the purposes of the shock is to dissipate the spring’s kinetic energy by converting it to heat that is absorbed by the hydraulic oil, so we can see the potential for trouble here. If the shock is used hard enough, it will eventually heat the oil to a point where its viscosity changes (becomes less). This thin, overheated shock oil now offers less (possibly much less) resistance to the piston’s movement and the shock’s dampening capability is reduced – sometimes drastically. This is called “shock fade.” There are strategies to combat shock fade that we will cover shortly.
Most shocks produced today are either of twin-tube or mono-tube design. Before we look at each individually, let’s look at another important aspect of modern shock design that is often applied to both twin-tube and mono-tube shocks – gas charging.
The primary function of gas charging is to minimize aeration and foaming of the hydraulic fluid by reducing the chance of cavitation. When the shock cycles, the motion of the piston through the oil creates an area of high pressure ahead of the piston and an area of low pressure behind it. If the pressure of a fluid is reduced below its vapor pressure, the fluid will spontaneously change state from a liquid to a gas. This means that tiny air bubbles can form in the low-pressure area directly behind the piston. The process is called cavitation. Left unchecked, the tiny air bubbles that form will mix with the oil - a process called aeration. The resultant mixture of oil and air inhibits the functioning of the shock as the piston and valving are designed to produce the required damping by operating in a column of incompressible oil. Once the oil is mixed with air, or aerated, it is no longer incompressible. The faster the piston pumps up and down, the more rapidly cavitation aerates the oil on both sides of the piston, and eventually the oil will be churned into foam. The resulting foam offers little resistance and causes extreme fade of the shock’s dampening ability.
The addition of pressurized nitrogen gas to the shock helps to prevent the low-pressure zone behind the moving piston from falling below the vapor pressure of the oil, reducing the chance of cavitation, aeration, foaming, and eventual fade. In the case of cavitation a gas charged shock will force the air from the oil removing the cavitation almost instantly, where a twin tube shock will need to rest in order to recover from this state.
Shock fade is a condition that occurs in the shock when the oil either becomes to hot or it caviates due to loss of control in the pressure tube. This is a condition that usually will cause total failure of the shocks performance. This condition can be over come with the use of high performance oils or the increase in gas pressure. When using a twin tube this option of more gas is not available and you must rely on better oil. Most shock companies use what they feel is the best oil for their product, and some times that is determined by cost and not by performance. This is why there are companies out there that sell after market oils for shock. One thing to remember is that when changing the oil velocity you can change the out come of the shock. You should be safe in using 5wt or 10wt oil, but would be best if you consulted your shock guy first before making this change.
Depending on the pressure used and the diameter of the rod, gas charging can also cause the shock to provide a small increase in the spring rate of the suspension (in fact, if the vehicle were light enough, the rod diameter large enough, and the pressure high enough, the pressurized nitrogen alone could act as the spring – hence air shocks). This mild boost in spring rate is caused by the pressurized gas acting on the rod through which there are, obviously, no orifices; as shown at left. Therefore, the larger the rod is in diameter, the larger the area for the pressure to act against. This gas pressure acting on the rod through the center part of the piston is also the reason why an uninstalled, unloaded, gas-charged shock absorber will extend on its own.
The best way to handle rod force is to reduce the surface area of the shaft or reduce the amount of gas in which you put into the shock. Most shock companies use too much gas pressure in their shocks, they over compensate for extreme conditions which has an ill effect on a race car. Gas pressure is used to hold the oil column from moving as the piston travels though it, when this does not happen you will have a low pressure area behind the piston causing caviatation. This is also true in a twin tube shock, but a twin tube shock controls it in a different way. We will cover that more in the twin tube section.
The amount of gas pressure is determined by the amount of compression that a shock has, the higher the compression the higher the gas. Likewise the higher the gas the more rod force in which you have. If you are running shocks that you can adjust the gas force in this is a formula used to determine that pressure.
(Compression Force @ Max velocity / Piston Area)
This will take some information such as compression force, which is what the shock is rated at full velocity. A shock is rated on a dyno and determines it’s dampening value which is the shocks compression/rebound force. That force is divided by the piston area and gives you the amount of gas pressure you need in the shock. In most cases the lower amount of gas is what you will want with out causing caviatation in the shock. One thing to remember is that cavitation will cause heat in the shock and that will cause the shock to fade. Always watch your shock for heat build up, it is common to see heat in the shock such as 130 to 150 degrees, but unusual amounts of heat are a sure sign of caviatation in the shock. Use your head on this, if you feel that your car went away from you, and you see a high amount of heat in your shocks than it is a good sign that you do not have enough gas in them to control the internal pressures.
Gas pressure in a shock will increase as the shaft is pushed in, this will increase the amount of rod force that you have in your shock. This amount is small but it is worth knowing so you can understand how that would affect your car. There are mathematical formulas on the internet that can help you to see how the pressure increase as the compressed volume decreases; this is called Command Gas Law. Most factory built shock used in racing have a set amount of gas in them. Bilstein for example uses 185 psi of gas in their shocks which creates about 40 lbs of rod force in their shock. BSB uses 100 psi in a lot of their shocks which produces about 20 lbs of rod force. As you can see the force is not just mathematical as it goes down. You can reduce your gas pressure in other ways such as a bigger gas chambers or the use of a base valve, both commonly used in higher dollar shocks. We have found out that 20 lbs of rod force is about the max you want to use and still have a good feel on the race track. One other things to remember is that the lower you go on gas pressure the more at risk you are for leaking. Gas pressure helps hold the seal to the shaft and to the body of the shock as you reduce this pressure you will increase the chance for leakage.
One last thing and that is, rod force will hold up your car because it acts as a spring, but if you keep it low enough you should be alright when scaling your car and setting ride heights. I would suggest that if you are using a high pressure Left rear shock that you remove it from your car when scaling or setting ride heights because it will change the out come.
Despite the effective spring rate of the pressurized shock, in a normal hydraulic shock, pressure should probably not be used to compensate for incorrect spring rates or worn / broken springs. However, it can help reduce body roll, sway, brake dive, and acceleration squat compared to a vehicle with identical springs but non-gas-charged shocks.
Shock top-out occurs when droop travel (at the wheel) exceeds the shock’s available droop travel and the shock is forced to be the physical limit to droop. At top-out, internally, the shock piston bottoms against the bearing cap which will likely cause damage.
Shock bottoming occurs when bump travel at the wheel exceeds the shock’s available bump travel and the shock acts as the bumpstop. This should be avoided to prevent harsh ride and shock damage. The small rubber snubbers that come installed on the shaft of many coilover shocks are intended to prevent shock body to lower spring-seat contact but are not sufficient to be used as a true bumpstop.
The twin tube design is as applied, it has two tubes. The inner tube known as the working or pressure tube and an outer tube known as the reserve tube. The pressure tube holds the oil through which the piston moves. However, because the oil is incompressible, it must have somewhere to go during shock compression as the piston rod displaces a certain volume of fluid. The reserve tube provides a place for this hydraulic fluid that is displaced as the rod travels into the pressure tube. The reserve tube also creates a space for the fluid as its volume expands due to heat during use. The reserve tube may also contain a low-pressure charge of nitrogen or what is known as a gas bag. The pressure of the nitrogen in the reserve tube normally varies from 100 to 150 psi. And the pressure in a gas bag varies from 5 to 15 psi. The gas bag is by far the more popular of the two designs because it allows you to run the shock in any direction. A gas charge twin tube shock must be run right side up or it will not function correctly. Twin-tube shocks have a valve located at the bottom of the pressure tube called the base valve. The base valve is the compression valve - it controls fluid movement during the compression stroke of the shock by metering the flow between the pressure tube and the reserve tube.
Because they use a “tube within a tube”, twin-tube shocks are compact in length, making them easy to fit or package, particularly on cars where space is extremely limited. Their design also lends itself to relatively cheap mass production while retaining effective performance without requiring the strict tolerances (and associated manufacturing costs) of a mono-tube design.
Twin-tube shocks also have some limitations. They tend to trap heat within the pressure tube as it is insulated from cooling air by the outer reserve tube, making them prone to heat buildup and fade under hard use. They also have a tendency to cavitate the oil due to lack of control of the gas bag. These shocks are fine for very slow speed movement but have a hard time performing in more harsh conditions. Because the base valve is inside the pressure tube they are also not designed for the user to be able to alter or customize the Valving. Over the past few year companies such as QA1 improved on base valve designs to allow revalving for the end user or custom shops.
Mono-tube shocks have only one tube, the pressure tube. They are longer than twin-tube shocks because the single pressure tube must have sufficient length to store the hydraulic oil that is displaced as the rod travels into the pressure tube as well as provide space for the fluid as its volume expands due to heat during use.
If a mono-tube shock is gas charged, as most are, it must also have a place to store the high-pressure nitrogen charge. There are two methods of accomplishing this. In some shocks, the nitrogen is stored with the oil in the pressure tube. The gas and the oil are, in effect, mixed together in an emulsion. Not surprisingly, this style of gas-charged mono-tube shock is known as an emulsion shock.
A better way to contain the nitrogen charge is to separate it from the oil with a floating piston, also called a dividing piston. The floating piston moves up and down as the piston moves up and down in the cylinder, keeping the oil and nitrogen from mixing. However, the shock must now be much longer because the tube must be of sufficient length to accommodate the full stroke of the piston, all the hydraulic oil, the heat expansion of the oil, the gas charge, and the floating piston. If the shock is a long-travel shock, (meaning the piston stroke alone is 12 inches or more), the total tube length required can become prohibitive. In this case, an external reservoir is used to house the nitrogen charge, the floating piston, and some of the hydraulic oil. Mono-tube gas charges normally range from about 50psi to 200 psi. depending on their use and how much compression valving they have.
Most mono-tube shock do not have a base valve. Instead, all of the valving during both compression and extension takes place at the piston. Since the piston and rod are easily removed from the shock, the mono-tube design lends itself to independent tuning of the compression and rebound damping by providing for easy valving changes by the end-user. As a result, mono-tube shock users can individually customize their valving for improved ride and performance. In addition, non-emulsion mono-tube shocks can be mounted in any orientation, including upside down. Because a mono-tube shock doesn’t require a “tube-within-a-tube”, for a given outside diameter, a mono-tube shock will have a larger bore, and thus be able to use a larger piston than a twin-tube design. This can be beneficial to the design as a bigger piston controls more weight with less internal pressure than a twin tube shock. Also the bigger piston design allows for more oil flow and for better low speed control due to better bleed circuits, this will also reduce the heat build up in the oil. A Mono-tube design will also allow the heat in the oil to transfer directly to the outer surface of the shock body, which is in direct contact with cooling airflow, where it can dissipate more efficiently. This reduces heat-induced fade, allowing the shock to maintain full damping characteristics as temperatures rise with hard use.
Finally, mono-tube shocks must be designed and built to exacting tolerances in order to function properly. This results in a high-quality product.
In the mono-tube shock, note the larger diameter piston, and the floating piston. (blue)
More control Dents in body
Bigger piston Rod force
In the twin-tube shock, note the "tube-within-a-tube" construction and the base valve. (red)
No rod force Will fade
Can dent Heat build up
Gas bag design
The larger a shock is in diameter, the larger its bore can be. The bore is the diameter of the piston and the inside diameter of the pressure tube. The larger the piston’s diameter, the larger its surface area. Since pressure is force divided by area, it stands to reason that the larger the area, the smaller the pressure generated by a given force. Inside a shock, the lower the pressure, the lower the temperature. In addition, a larger diameter shock can contain more oil to absorb and dissipate the heat generated, resulting in reduced internal operating temperatures for a given force. The result is, the larger the shock diameter, the cooler it will run and therefore the harder the shock can be worked before fading.
As a result in what we have talked about the best determination of what shock is best for you is really left up to you to decide. In my opinion I feel that a mono tube gas shock with lower gas pressure and linear valving is the best choice for dirt track racing in an open wheel modified. I came to that conclusion because of what I feel are major reason to use that shock.
Over heated or worn out shock oil can smell as bad as over heated rear end oil. Yet on the shock dyno the performance has not changed that much from when the oil was new. Just like engine oil, the problem is loss of lubricity and wear on the shock body and seals. Most shock manufacturers are very proud of their parts so it’s considerably easier on the racing budget to do routine oil changes than replace parts.
Establishing a maintenance schedule involves changing oil at frequent intervals to begin with until one can determine the number of races the oil can stand before needing to be changed. One might think of simply checking the oil periodically but by the time the shock is open, the oil might as well be changed. And while open, it is also easy to check the shims to be sure none have begun to crack.
Some shock companies suggest changing the oil once per season. However, especially on some race cars where the exhaust passes near a shock or under rough track conditions the oil may need to be changed as frequently as twice a year.
It is very important to inspect your shocks on weekly bases for damage. Look closely at the bodies for small dents cause from rocks and debris that can hang up your shaft trivial. The smallest dent can cause a mono tube shock to fail. This dent can come from a control arm that likes to rub or from rocks or mud from other cars. It is important that you get these dents fix before they cause damage to the piston or the divider piston.
On a twin tube shock, tire rub can cause the gas bag to over heat and break. This can cause a shock to fail and over work the oil, and cause for poor handling of the race car. Also inspect the shaft for damage, like dings and dents that can cause damage to the seals which can cause the shocks to leak. Remember that it only takes a small amount of damage to make a shock to fail.
BSB MFG rebuilds and services all Afco, Bilstein, Integra and QA1 shocks. Complete Dyno service available as well as custom Valving and repairs.
Shock Revalve $10 (plus labor)
Shock Body Repair $10
Shaft Replacement $25 (plus labor)
Shock Dyno Test $5
REBUILDING FOR THESE FINE COMPANIES