Shockabsorber 101

[Sydneykid]

For some time lot's of guys have been asking me to explain the differences between a good shock and a bad one, what sophisticated valving really means, why a monotube shock is better than a twin tube etc etc. So this thread is an attempt to answer those questions and any others that might come up in the process.

To kick it off I thought I would start with why a monotube shock is better than a twin tube. Monotube shocks have a larger piston area than a twin tube for the same given outside diameter. This larger piston OD/monotube design means that;

1. There is room for more valves

2. The valves themselves can be bigger

3. More hydraulic fluid is moved for the same stroke, this means superior metering of the fluid transfer though the piston.

4. There is more fluid in the shock, which means higher temperature capacity. The movement of the fluid through the valves generates heat (Newtons law at work) by transforming the kinetic (up and down) energy.

5. The fluid is in direct contact with the shock body meaning rapid transfer of heat to the outside )ambient) atmosphere.

Following is a shock dyno graph (courtesy of Bilstein) showing the effect of heat on dampening of a typical Japanese twin tube shock. As you can see after 2 minutes, yes that's 2 minutes, of running at 550 mm/sec the damping force (the vertical axis) drops off dramatically. For those that haven't seen a shock dyno graph before the vertical axis is the damping force and the horizontal axis is the speed (frequency) of the up and down.

This means is the damping efficiency of shock drops away due to the rapid overheating of the fluid. Because of the twin tube design there is insufficient fluid to absorb the heat and the two tubes slow down the heat transfer to outside atmosphere. What you feel in real life is a gradual decrease in damping effect, the shock slowly ceases to control the spring and the chassis. You get move vertical/roll movement, the shock no longer helps to keep the tyre in contact with the road.

Hope that was in some way informative, the next subject (with pictures) will be a comparison of the design of the shock piston and the valve pack.

Cheers

Edited May 3, 2006 by Sydneykid

[nuffsaid]

Great stuff SK!

Now, how can you tell whether a shock is mono or twin tube?

Would you happen to have a list of commonly used euro and jap performance shocks and whether they are mono or twin tube?

[dangerous_daveo]

This NEEDS to be sticked!

[Duncan]

it has been stickied

[Bliksem]

I hope this isnt a stupid question, but where do gas struts come into the equation?

[Sydneykid]

I hope this isnt a stupid question, but where do gas struts come into the equation?

Gas (nitrogen) under pressure (45 psi or so) is used to reduce the foaming of the hydraulic fluid as it passes through the valves. Air bubbles are bad for damping consistency as they whip through the valves much easier/faster than the fluid and so give zero damping.

Most shocks these days are gas pressure, pretty much only the vintage shocks are not.

cheers

Edited September 3, 2006 by Sydneykid

[Sydneykid]

Great stuff SK!

Now, how can you tell whether a shock is mono or twin tube?

Would you happen to have a list of commonly used euro and jap performance shocks and whether they are mono or twin tube?

It's not easy to tell the difference some times, the best way is to look at the specifications of the shock. If it doesn't say "monotube" then chances are it's a twin tube. The other way is to look at the shock body, if it has a step in the outside diameter (usually towards the bottom) then it's a twin tube.

cheers

PS; the object of this thread is to educate, so the readers know why one shock costs $100 and another costs $500, even though they look basically the same. I am trying not to mention brand names or say "this shock" is always better than "that shock".

Edited September 3, 2006 by Sydneykid

[anthony84]

im geting new shocks 4 my r32 gtst what brands are good just heavy duty shocks not coil overs?

[Sydneykid]

Following are pictures of some twin tube shocks, the first is a standard GTR front shock. Below the spring seat you can see the step up in diameter, showing where the ouside chamber is located for the oil displaced by the shock shaft.

The second picture is of a standard R33GTST front shock. As you can see it has the same step down below the spring seat. Plus it has another step down, just above the mounting eye, after the full travel of the piston. This is used commonly to give the shock more length (raise the ride height) without increasing the shock travel.

The third picture is of a generic Japanese shock commonly used in a coil over kit. As you can see it also has the step down in diameter just above the mounting eye. This indicates that, depite being adjustable, it is still a twin tube shock.

Hopefully this will help you in identifying a twin tube shock from a monotube. Unfortunately some shock manufacturers don't make it this obvious. The next best way is to look at the specifications of the shock. If it doesn't say "monotube" then chances are it's a twin tube.

Cheers

Edited September 3, 2006 by Sydneykid

[Sydneykid]

This is a picture of 2 shock pistons, the one on the left is a generic Japanese twin tube piston found in a 50 mm OD shock (Skyline front). The one on the right is a Bilstein piston from a Skyline front shock, also 50 mm OD.

The are two things to note from the picture;

1. The diameter, is is rather obvious that the twin tube piston is much smaller in diameter than the monotube piston. This means less oil is moved for the same stroke. Plus there is only room for 4 major valve orifices and 6 drilled holes. The importance of these two items was discused in a previous post.

2. The valve orifice sophistication, once again it is obvious that the Bilstein piston has 50% more outer valve orifices (6 compared to 4). In addition the 6 simple inner holes in the twin tube piston do not offer the variability in damping that the triangular shaped inner valve orifices do on the mono tube piston. As the valves open a larger portion of the triangular orifices are exposed, giving a great deal of tuneable variation in the damping. With the simple holes, the valve is either open or closed, there is therefore no potential for progression in the damping.

This is why the sophisticated valve orifice design can give good control without excessive harshness. Simplistic valving, as a result, can never meet those twin objectives.

In the next post I will discuss the valves themselves and how they contribute even more to the handling/ride comfort equation.

cheers

Edited September 3, 2006 by Sydneykid

[T04GTR]

excelent. i wait paitantly

[simpletool]

"In addition the 6 simple inner holes in the twin tube piston do not offer the variability in damping that the triangular shaped inner valve orifices do on the twin tube piston" just one typing error SK,

that should read mono tube piston at the end yeah.......just thought it'd save confusion if I pointed it out.

Wow, I was impressed by the size difference of those valves. Got to say the japanese shocks on mine are harsher than need be on fast movements. (ie. Road works)

[dangerous_daveo]

Well I got my car back yesterday after it had been sydneykidised. And yeah. DO NOT WASTE YOUR MONEY ON CHEAP JAPANESE (or expencive for that matter) SHOCKS. DO IT RIGHT ONCE.

Its like night and day. Its almost like they say on all those car shows about supercars 'awesome in the turns yet still driveable!'. Just pay the money, even if you dont get it thru SK (as much as you would have to be stupid not to).

[GTSTGIRL]

Hey. My R33 has HKS hyper D's and they need rekitting. Is is worth it to rekit them or just better off buying a new set?

[Sydneykid]

Hey. My R33 has HKS hyper D's and they need rekitting. Is is worth it to rekit them or just better off buying a new set?

This is not really the thread to be discussing rebuild options, I will send you a PM tonight with my thoughts.

cheers

[AlexCim]

Cant wait to read more about the shocks and shock valving SK

[lukits01]

hey SK,

I know most of the time, you are anti-Tein due to their twin tube design and somewat hard spring rate, was wondering if you had any experiences with CUSCO coilovers? Ive always liked the brand and their line of zero coilovers, just wondering if you have any thoughts on it.

And whats your thoughts on helper springs? the pros and cons?

cheers

[Sydneykid]

hey SK,

I know most of the time, you are anti-Tein due to their twin tube design and somewat hard spring rate, was wondering if you had any experiences with CUSCO coilovers? Ive always liked the brand and their line of zero coilovers, just wondering if you have any thoughts on it.

And whats your thoughts on helper springs? the pros and cons?

cheers

Helper/tender springs help keep the main spring trapped at full droop. They do that quite effectively. The only con is that you have to spend money to buy them and the intermediate seat that they require.

This shock is for a MacPherson strut based car (S13/14/15, Cefiro or equivalent) hence why it has the large diameter shaft. The extra rigidity is necessary because the strut has to not only act as a damper, it also has to stop lateral movement of the suspension. Since R32/33/34 and Stagea have double wishbone suspension, they upper wishbone does that job. All the shock has to do is dampen the spring and chassis movements. Hence I wouldn't use that design of shock in a Skyline.

As you can see this design of shock has the more normal shaft diameter and hence is most likely for a double wishbone car. However it also has the larger diameter lower body indicating it is most likely a twin tube design.

cheers

Edited September 3, 2006 by Sydneykid

[lukits01]

What about the pros and cons of inverted monotube?

most coilovers that boast monotube design are inverted monotube, does this actually count for something?

[Sydneykid]

What about the pros and cons of inverted monotube?

most coilovers that boast monotube design are inverted monotube, does this actually count for something?

The only place you would use an inverted monotube is on a strut based car, to take advantage of the extra shaft strength to maintain the wheel alignment angles. For example Bilstein for Cefiros and S13/14/15 utilise the inverted montube design. Using an inverted monotube shock on a double wishbone car (like a Skyline R32/33/34 and Stagea) is simply not necessary. That's why the Bilsteins for those models are the normal/straight monotube design.

Summary follows;

Pros

Holds alignment in strut based cars, the shaft doesn't bend under cornering, acclerating and braking loads (as occurs in strut based suspension, not double wishbone)

Lower friction when subject to side loads (as occurs in strut based suspension, not double wishbone)

It's a monotube, so all the usual advantages over a twin tube (larger piston area, more fluid, room for more sophisticated valves etc)

Cons

Not necessary for double wishbone cars

Complex design, more parts to fail

Costs more to make

Heavier than a straight monotube

Difficult to service (particulalrly nitrogen regas)

Larger seal area, so more prone to leaks

Fitting travel limiters is more difficult

Replacing and tuning the internal bump stops is also more difficult

Slightly reduced piston area to outside diameter compared to a straight monotube design

Hope that answered your question.

cheers

Edited September 3, 2006 by Sydneykid