Inside the struts & shocks ...
I was taking a hacksaw to an old pair of struts as the first step to a pair of strut extenders, and I decided that I would open up the strut whilst I was about it.
My apologies in advance for any blurry images, the pictures were taken in the late afternoon with a cell phone.
This is a Mitsubishi OE front strut, MR319792, made by KYB, and should be equivalent to the KYB 334442 aftermarket offering - from a technical standpoint it is a twin tube, gas filled strut, with dual stage valving, with a 22mm diameter hard chromed piston rod, a 32mm diameter piston, and a double lip seal.
Since I figured Claude would probably ask, I measured the inner diameter of the outer tube - it's 44 mm.
Here's the assembled strut ..
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And the outer body with the actual damper assembly removed - the space between the inner and outer sections is a reservoir for oil and where the nitrogen is stored.
A close-up of the bottom end of the damper, showing the foot valve.
A close-up of the foot valve as seen from the bottom or outer end.
A second view of the foot valve - from the inside.
The piston...
The assembled piston is shown here - note the spring - this is a much heavier spring than the one in the foot valve.
The disassembled piston and shim stacks - upper & lower.
Close up of the piston.
As with the foot valve, the upper shim stack closes the larger holes, and the lower shim stack closes the smaller ones.
There is however, one big difference, on the downward (compression) stroke, the piston is moving through the oil, so oil flows through the larger holes, controlled by the upper shim stack - this is the reverse of the foot valve, which is stationary - on the up (rebound) stroke, the lower shim stack comes into play as the oil flows through the smaller holes.
Back to those springs now
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The piston spring is on the left, the foot valve spring on the right - notice how much heavier the piston spring is?
The heavier piston spring will keep the piston valving closed when the piston movements are small (low speed piston movements are typical on smooth roads), forcing the oil through the foot valve, with it's lighter spring, but as the piston movement increases (rougher roads), the pressure of the oil will overcome the pressure of the large piston spring and flow through the valving on the piston.
I would describe these struts as being typical "street use" products - shocks designed for off road use will have more stages of valving and many more discs in the shim stacks - as an example, the Monroe Adventure series that I have on the rear of my iO claim "three stage" damping, and OME, which I have on the Grand Vitara simply claim "multi-stage" damping without going in to more details.
Rear Shocks...
Mitsubishi MR319750 - made by KYB - it's a "twin tube" gas filled shock with a 12.5mm diameter piston rod and a 25mm diameter piston.
This first picture shows two different shock absorbers side by side - the top, black one, is the one we're going to be dissecting today - the lower, yellow one is a Monroe Adventure Series unit that I will be taking apart in the not too distant future.
In this shot the outer tube has been cut & removed and the inner tube is visible - the space in between these two tubes is a reservoir for oil and the nitrogen gas.
Up next is the actual damper assembly - the inner tube minus the outer tube and the stone shield - ignore the two items below the damper for now, I will get to those in a minute.
This is a close up of the bottom of the inner tube showing the foot valve.
Continuing with the internals...
This is a close up of the foot valve, as seen from the bottom or outside of the tube - notice the four tiny slots around the inner circles - those are actually holes and what you're seeing through them is the back side of a valve shim.
This is the upper side of the foot valve
And here's a "low angle" shot of the valve with the spring removed - the spring would normally be just under the top button of the foot valve - holding the shim against the valve body - look closely at the shim - there are curved cutaway sections underneath the button - on the compression stroke oil flows through those cutouts, on the rebound stroke oil flows through the rectangular slots and lifts the shim against the spring.
More internals ...
This is the piston rod outside of its tube
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And the disassembled piston & valve stack - the piston is the black disk in the centre
From the left - the piston, a spacer, two valve shims, a spring seat, the spring and the nut that holds everything onto the piston rod - this is a part of the rebound valve stack.
From the right this time, the piston followed by two valve shims, a washer, a spring, a spacer, and what appears to be a hard rubber rebound stop - these are a part of the compression valve stack.
Here's a closeup of the piston - outer holes are compression valving, inner holes are rebound valving.
Third up - the Monroe Adventure Series D7002
I have to admit - having read all Monroe's Adventure Series advertising blurb - I was quite disappointed to see the insides of this one ...
The Monroe is the lower, yellow shock - same length open and closed, very slightly larger on the diameter of the outer tube.
A close up so you can see the number stamped into the body.
And now the innards - as you can see this is a twin tube shock - 12.5mm diameter piston rod, 25mm diameter piston - same as the OEM shock.
A close up of the lower end of the tube showing the foot valve.
What's inside.
The foot valve from below
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And from above
And the disassembled foot valve - as with the other rear shock, if you look closely you can just make out the cut outs in the shim under the button - on the compression stroke oil flows through those cut outs into the reservoir tube, on the rebound stroke, it lifts the shim against the spring.
The piston rod & piston.
It looks like I forgot to take a shot of the assembled piston rod & piston - I'll reassemble it and get one tomorrow.
What we have here are the disassembled piston and valve shim stack - the piston is fifth from left.
From the left - the piston, six shims, a washer and the nut that holds everything in place - this would be the rebound shim stack.
And the upper or compression shim stack - from the right - the piston, a single shim, a star shaped spring, a recessed washer and the rubber rebound stop.
The Monroe Adventure Series hype ...
Monroe had a big advertising splash when they launched the Adventure Series shocks, specially designed for 4WDs that spent the majority of their life on asphalt - they were supposed to be high pressure monotube shocks with larger, stronger piston rods, larger pistons, floating piston to separate the nitrogen from the oil, thicker tube walls to resist damage better, and special three stage valving to cope with motorway use, dirt roads, and off road tracks.
Opening one up was a real disappointment, especially after opening the OEM KYB - same twin tube design, same piston rod diameter, same piston diameter - the only 4x4 specific advantage I could see was that the larger reservoir tube allowed for a greater oil capacity which should help delay the onset of shock fade.
Comparing the valving shim stacks, and bearing in mind that I'm no shock engineer, I would have to assume the OEM KYB to be the better choice - the Monroe appears crude in comparison.
Now - what was not mentioned before is that this shock is perhaps three years old - there are visible scuff marks on the lower tube where the stone shield has been rubbing on it - a problem which I have experienced with all four of the Monroe shocks that I have purchased for this vehicle, and the reason the shock is available for dissection is that the top mount eye broke where it was welded to the piston rod.
Bearing in mind that this is not the bottom rung, OE replacement, but supposedly a premium "upsell" 4X4 shock, what sort of quality should one expect from Monroe?
There isn't a whole lot of choice for shocks/struts for these vehicles, but I don't see another set of Monroes in my future - it will have to KYB or if I scrape together enough - maybe Konis.
Monroe
Interesting to hear your findings about OEM KYB vs "4x4" Monroe!
Heard read quite a few mixed reviews for monroe shock absorbers. Super happy and on my second set of KYB shocks but my IO has 250,000km and spends majority of it's time off road on 3 wheels lifting and dropping the front end from side to side. I'd imagine to be needing a third set closer to 280-300 which, if I still have the car, might mean an investment in coil overs and bigger tyres while we are at it. I wouldn't expect any front shock to cop the abuse I've thrown at the KYB's and not prematurely fail/have a half life.
Syndicate
More pics
This is the foot valve disassembled to show the "shim" stack - a series of very thin flexible stainless steel disks along with a spring and a "bolt & nut" to hold them in place.
A close up of the foot valve minus the "shim stack".
Note carefully the different holes in this valve - the four large rectangular holes are covered by one shim (shown to the left of the valve in the picture above), and the six smaller holes by the smaller circular shims (show to the right).
When the piston is moving down (compression stroke) - the oil is forced through the smaller holes so there is more resistance to flow, when the piston moves up (rebound stroke) the oil flows through the larger holes - from what I have been told, it's normal to have the damping in a 70/30 ratio - 70% compression, 30% rebound.