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Differences/Benefits of a Monotube & Twintube Shock Absorber, Also Info on Inverted

I think this information is pretty helpful when deciding and purchasing or replacing your suspension systems. As you can see there are different designs when selecting and they offer certain benefits when use for roads or track/circuit. Since there isn't a thread on here dedicated to explaining the differences between the two, I figure I put this together for you guys and I hope this helps in some way on your next purchase :thumbsup:

*I picked up this info from another forum, some say it was written by Tein, but some say otherwise. The info on Inverted, I got that from Tein Website. Anyway I added a few things to both*

Mono-Tube Shock Absorber & Twin-Tube Shock Absorber
Which is really better?​

The Structural Differences Between Mono-Tube & Twin-Tube

These are the mono-tube and twin-tube shock absorbers you often find in magazines etc.
For those of you who have ever thought to yourself, "I think I know the differences in the structure, but what exactly is the difference?" Here is the explanation. (mono - left/twin - right)



The main difference between mono-tube and twin-tube systems, as shown above, is the number of tubes used for the shock absorber.

♦ Mono-tube: the shell case itself works as a cylinder and oil, gas, piston valve, etc are all set inside that single tube.
♦ Twin-tube: there is a separate cylinder set inside the shell case and the piston valve moves up and down within the inner cylinder.

Other differences in construction
Mono-tube uses a free piston that completely separates the oil chamber from the gas chamber. While for twin-tube, nothing separates the oil and gas chambers within the shell case.
The differences between mono-tube and twin-tube shock absorbers, as you can see, are not only the number of tubes but also the inner mechanism as well.

Mono-Tube - Pros & Cons

Let's have a look at the pros and cons of the mono-tube shock absorber.

Pros
♦Stable damping force can be generated continuously, because of the larger oil capacity and improved heat dissipation.
♦The larger sized piston valve creates a wider area to receive pressure, and thus even a subtle damping force can be generated precisely.
♦The structure allows no restrictions in installation angles.
♦Oil releases heat easier as the temperature increases.
♦Aeration doesn't occur because oil and gas are completely separated.

Cons
♦Maintaining sufficient stroke is difficult because the oil and gas chambers are positioned serially.
♦Because an injection of high-pressure gas is required structurally, he ride tends to be stiffer.
♦High-pressure gas can lead to greater stress on seals and hence can cause larger friction.
♦Compared with the upright type, the inverted type tends to have more friction, because it has more moving parts.
♦External damage to the shell case directly affects the inner cylinder.



Twin-Tube - Pros & Cons

Let's have a look at the pros and cons of the twin-tube shock absorber.

Pros
♦It is easier to secure sufficient stroke, because the oil and gas chambers are separated and not positioned serially.
♦The use of base valve enables to keep gas pressure low, allowing for a more comfortable ride.
♦Low gas pressure avoids stress on seals and keeps friction low.
♦Compared to the inverted type, friction can be suppressed.
♦Even if the shell case is slightly damaged, the function of the shock absorber remains unaffected.
♦Superior manufacturing processes allow to keep production costs low.

Cons
♦Oil capacity is not as large as the mono-tube.
♦Size of the piston valve is not as large as the mono-tube.
♦The structure restricts installation angles.
♦The oil and gas chambers are not separated, so there is a possibility of aeration.



Comparing Mono-Tube and Twin-Tube Directly

1. Ride Quality of Street Shock Absorber.

Some manufacturers say the mono-tube is superior, but is that really true? We will take a look here

♦Maintaining sufficient stroke is difficult because the oil and gas chambers are positioned serially.
♦Because an injection of high-pressure gas is required structurally, there is a tendency for a stiffer ride
♦High-pressure gas can lead to greater stress on seals and hence can cause larger friction.
♦Compared with the upright type, the inverted type tends to have more friction, because it has more moving parts
♦External damage to the shell case directly affects the inner cylinder.
In recent years, the ride quality has become most essential for street shock absorbers.

To ensure optimal ride quality with street shock absorbers, it is vital to have the correct settings, but also to secure the sufficient stroke. As oil and gas chambers are divided and positioned serially inside the mono-tube, it is harder to ensure the sufficient stroke compared to the twin-tube of the same length.



A race shock absorber designed for flat circuits does not require as much stroke as a street shock absorber which always has to deal with uneven surfaces, bumpiness and other conditions of ordinary streets. There is a direct link between the insufficient stroke and the deteriorated ride, such as sudden jolts etc, for street shock absorbers.

2. What is high-pressure gas?

In addition to the insufficient stroke problem, the use of high-pressure gas leads to a stiffer ride, increases friction etc. and is the main reason for deterioration in ride quality. Some manufacturers have the base valve positioned above the free piston to lower the gas pressure, but this results in even further sacrifice in stroke and has an adverse effect on ride quality.



Why the Mono-Tube Requires High-Pressure Gas Injection.

For twin-tube, piston valve and base valve share the role of generating compression damping force, and hence there is no need to inject high-pressure gas. However, the mono-tube creates compression/rebound damping force with just the piston valve. The mono-tube strokes as the free piston is forced down by the displacement of contracting piston rod. If there isn't enough pressure on the other side of the free piston while this happens, the free piston would move quite easily without generating the adequate compression damping force and thus the piston rod would fully contract. To prevent this, there needs to be enough high-pressure gas injected into the gas chamber to control the free piston movement. When the gas chamber is filled with high-pressure gas, there will be an equal amount of pressure exerted inside the shock absorber, as the effects of Pascal's Law. In such state, there will be a high pressure applied to the oil seal, which will lead to increase in restraining force on the seal and friction. Pascal's Law http://en.wikipedia.org/wiki/Pascal's_law



When there is low gas pressure.


When there is appropriate gas pressure


3. Which is better suited for the street?

You never know what is lying on the surface of ordinary roads.
If something strikes the shock absorber and damages the shell case, the mono-tube, which is constructed of only one cylinder, will not be able to stroke any longer.

Considering this, is the mono-tube design really suitable for the street?
The answer, unfortunately, is "NO". Now, take a look back at the pros of the twin-tube.

♦It is easier to secure sufficient stroke, because the oil and gas chambers are separated and not positioned serially.
♦The use of base valve enables to keep gas pressure low, allowing for a more comfortable ride.
♦Low gas pressure avoids stress on seals and keeps friction low.
♦Compared to the inverted type, friction can be suppressed.
♦Even if the shell case is slightly damaged, the function of the shock absorber remains unaffected.

The twin-tube design can solve all the street shock absorber problems. Furthermore, it has lower production costs due to superior manufacturing processes, compared to the mono-tube design. Extremely important factors for street shock absorbers are high quality and reasonable price. The twin-tube design is rather better suited for street shock absorbers.



4. Mono-Tube for the Circuit

Ok, now we compare them for the circuit. The mono-tube's cons seem to stand out for street use, but its full potential is exhibited on the circuits. Take a look back at the pros of the mono-tube.

♦Stable damping force can be generated continuously, because of the larger oil capacity and improved heat dissipation.
♦The larger sized piston valve creates a wider area to receive pressure, and thus even a subtle damping force can be generated precisely.
♦The structure allows no restrictions in installation angles.
♦Oil releases heat easier as the temperature increases.
♦Aeration doesn't occur because oil and gas are completely separated.

Racing on a circuit is extremely harsh for the shock absorber. When racing continuously for long hours, not only the shock absorber itself generates heat but also the brake components nearby can heat up to 1,000 degrees C. The shock absorber is fully affected by such heat. For this reason, it is vital for circuit shock absorbers to perform well consistently even when used under tough conditions for long hours. So the mono-tube's pros are truly suited for hard-driving on circuits. The stroke problem is not really a concern at the circuit because the flat surfaces and stiffer springs don't require as much stroke as on the street. These are the reasons why the mono-tube design is better suited for the circuit.



The Twin-Tube Design, on the other hand...

♦Oil capacity is not as large as the mono-tube.
♦Size of the piston valve is not as large as the mono-tube.
♦The shell case cannot be inverted for strut type suspension.
♦The oil and gas chambers are not separated, so there is a possibility of aeration.

Because of these cons, unfortunately, we cannot say that it is considered suitable for the tough circuit conditions. But this is only based on direct comparisons with the mono-tube. In fact, the twin-tube design system has landed the class victory in the 2005 Nürburgring 24Hour Race. There is not much of a difference between mono-tube and twin-tube in terms of durability etc. Only a certain people, such as professional drivers, feel a difference in performance under very limited conditions.

As described earlier, both the mono-tube and twin-tube designs have their pros/cons and both have respective realms of expertise. It is not a matter of which design is better than the other, but providing the most suitable design for the purpose is.

Inverted Mono-Tube Shock Absorber
What really are inverted shocks?​
What is the point of inverting the system?​

What is Inverted Type?

Put simply, the standard upright type has a shock absorber placed upward, while the inverted has it downward.



Strut Suspension Requires Strength

MacPherson strut is a type of suspension system most commonly used for the front suspension of various vehicles. With this system, the shock absorber itself also works as a part of suspension arm; hence maintaining sufficient strength is absolutely imperative. If it does not have enough strength, it not only has negative effect on car's controllability and could also causes breakage of suspension leading to a major accident.



How to Secure Sufficient Strength?

Twin-tube system, widely adopted in OE suspensions, uses thicker piston rod to ensure strength in strut types. Because of its structure, gas pressure can be kept low even with thicker piston rod used and thus the adverse effects on ride comfort can be minimized. But the story is quite different for mono-tube system.
For its structural reasons, piston rod can not easily be made thicker.

General Mono-Tube Structure



In most commonly-used mono-tube structure, both compression & rebound damping force is generated only at the piston valve. Because of this, compression damping force can not be generated properly if the gas pressure is not sufficient.

So, the higher the gas pressure, the better?

No. It’s not that simple. If the gas pressure is too high, the repulsive force and friction increase, thus causing the ride to worsen.

As you can see, mono-tube structure can only work well at finely-balanced gas pressure.

What Happens if Ticker Piston Rod is Used in Upright Mono-Tube Damper...?



Compared to the thinner piston rod, the volume of piston rod slides into the oil chamber is larger and hence the gas chamber is compressed more and the repulsive force increases.

Also, the amount of oil would be smaller as the piston rod volume increases, thus one of the benefits of mono-tube structure “larger oil capacity” can not be fully utilized.

Further more, the amount of oil flows through the piston valve is less with the thicker piston rod used, for the same stroke distance, thus the damping force can not be generated stably. To overcome, the gas pressure should be increased, but that would lead to worsening of the ride quality.

This poses a dilemma;

If possible, the piston rod should not be much thicker, in order to prevent a decline in performance. The piston rod needs to be thicker to some extent, to secure a sufficient strength. Weighing the strength against the performance, there need to be some compromise on the damper design. So, with the standard upright mono-tube system, widening the piston rod only makes the cons worse. It would not be an overstatement to say that there is no advantage in using mono-tube system this way.

(This does not apply to some standard upright mono-tube system with wider piston valves, shorter strokes and/or reservoir tanks, because of sufficient room for installation, such as for racing-use shocks. Abovementioned disadvantage can be eliminated in such cases.)

Just for the reference, most of overseas super low-price mono-tube height adjustable dampers widely available on the market nowadays use standard upright system for struts. One of the investigated samples uses dia.20 piston rod in upright mono-tube system for front struts on 86/BRZ. This is thinner than the original shocks, which use dia.22 piston rod. If the high-strength material is used, the use of the thinner rod would probably not be any problem. If the same material is used, it is obvious that this one is of lesser intensity than OE components.

With these facts, you can catch a glimpse of the reasons why mono-tube strut cannot use the thicker piston rod.



Balancing Strength and Performance?

How can both the strength and the performance of mono-tube struts be satisfied?
The answer is to use a separate pipe for reinforcement, just as with the inverted systems.



As shown in the picture, a large pipe is used as a sliding cylinder and adds strength against lateral force. This allows securing the sufficient strength without the use of thicker piston rod and without sacrificing the performance and the pros like large oil capacity.

Then, why do those low-price mono-tube struts not use the inverted system, but use the standard system with lots of compromises? The main reason is that the standard upright type uses fewer parts and thus the cost is lower.

The standard upright system is cheaper to make than the inverted system which uses more parts and requires complicated assembly process of high accuracy.



In the past, mono-tube strut generally meant inverted system, but this is no longer true as more and more low-price standard upright mono-tube shocks came into the market.

To get the most out of mono-tube systems for strut suspensions, TEIN strongly recommends choosing “inverted” types which provide both the sufficient strength and the high performance.

With the performance of shock absorbers being the first priority, TEIN uses “inverted” systems for all mono-tube strut suspensions.

We are sure that all TEIN users can fully enjoy the performance of mono-tube dampers to the full extent, as the skilled TEIN staffs at the factory in Yokohama are putting all the high-quality and high-prcision parts together one by one in a very careful manner.

I'll keep this thread open for simple discussions and farther questions on these two systems. Please no useless chatting or random postings, we have other threads dedicated to them.

Also use these links for other information on suspension components and reviews

Basic Suspensions http://forums.clubrsx.com/showthread.php?t=570483

Sway Bar Review http://forums.clubrsx.com/showthread.php?t=178149

Strut/Tie Bar Reviews http://forums.clubrsx.com/showthread.php?t=178151

Spring/Coilover & Shocks Reviews http://forums.clubrsx.com/showthread.php?t=168184
 

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Yes but He mentioned only the inverted information was taken from Tein. I'm stating all the information came from Tein.
*I picked up this info from another forum, some say it was written by Tein, but some say otherwise. The info on Inverted, I got that from Tein Website. Anyway I added a few things to both*
Really?

I'm just nitpicking, here.
 

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Discussion Starter #12
it might've all been written by tein, I didn't know for sure at the time of thread creation. I found the info on another forum I was browsing, the inverted stuff I got directly from tein website like I said.

tein written or not, I thought it was still useful to have it on here for those that don't know.
 

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Gonna go ahead and bump this from the dead because last summer, I found out strength really does matter in a McPherson strut car. After 3 seasons of autocross with a K swap and Hoosiers on my EM2, my front right Koni finally gave up the ghost.

I hit a bump mid sweeper, and after pulling off track I discovered that I had sheared the Koni shaft at the upper spring perch :eek: The leading theory is that my camber bearings had loosened up over the years and allowed some extra lateral force to get put into the shaft. Yikes.





 
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