SHOCK SHIM STACK TUNING - Yamaha XTZ1200Z Super Tenere 2015 -

Discussion in 'Land of the Rising Sun: ADV Bikes from Japan' started by Xfool, Aug 24, 2017.

  1. Xfool

    Xfool Been here awhile

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    De Carbon gas pressurized shock cavitation limit
    As a sanity check you can figure out the cavitation limit of a basic gas-shock from the gas pressure. With 190 psi on top of the 40 mm shock piston and vacuum on the back side the force on the shock piston is 370 lbf. Push the shock beyond that and the pressure drop across the piston sucks the back side to vacuum and that cavitates the shock.

    The shocks damping force curve shows the Tenere shock hits that 370 lbf damping force limit at shaft velocities around 30 in/sec. More-or-less inline with the chamber pressure plot showing the shock cavitates at that speed.

    De Carbon style gas shocks used in the Tenere, V-Strom, DR650 and KLR all suffer from a compression damping limit set by the shocks gas pressure.

    11-comp-limit.png

    That 370 lbf compression damping limit happens at about the same point where the shim stack hits the backing washer so the whole thing blows up at the same time. The shock was well designed.
    #41
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  2. Xfool

    Xfool Been here awhile

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    Compression stroke cavitation in shocks
    Stiff compression shim stacks generate high pressure drops across the shock pistion. When the shock is pushed to high speed that pressure drop pulls a vacuum on the back side of the piston, cavitates the shock and foams out the oil.

    That happens more often than you might think in a shock. Roehrig made a video demonstrating the basic problem to shock tuners (1:cavitation, 2:Charlotte track).

    12-cavitation.png

    The problem in that video is when the shock flips around into the rebound stroke the piston just wings back through foamed out oil. That generates zero rebound damping force and loss of control of suspension movements.

    The problem with cavitating a shock isn’t loss of compression damping, it’s loss of rebound!
    #42
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  3. Pipe dreams

    Pipe dreams Adventurer

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    That is a remarkable vid!
    I know years ago we spent considerable time trying to radius the piston ports sharp edges with the thinking that would minimize the chance of cavitation.
    Never saw this on any serious race shocks so figured it was a fools errand.
    #43
  4. Xfool

    Xfool Been here awhile

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    Shock cavitation fix
    The oil foaming in that video is caused by the piston pulling a vacuum on the rebound chamber. When the pressure drop across the piston is greater than the gas pressure the piston ends up pulling a vacuum on the back side. And that foams out the oil.

    The fix is simple: Put a compression adjuster on the shock. The compression adjuster sits between the shock and gas reservoir. Oil driven out of the shock on the compression stroke is forced through the compression adjuster. High flow resistance through the compression adjuster back pressures the shock creating the pressure force needed to push fluid through the shock piston into the rebound chamber. That back pressure is typically in the range of several hundred pounds, way more than the gas pressure in the shock.
    13-comp-adj.png

    That $40 yzf600 shock I dragged out of the junkyard has a compression adjuster on it. The reason I bought that shock in the first place was to get my hands on the compression adjuster. The fact that the yzf600 shock piston just happens to fit the Tenere turned out to be a huge bonus in fixing the blown piston band.

    There are two styles of yzf600 compression adjusters:
    • ‘01 and later shocks use an orifice style compression adjuster

    • ‘00 and earlier shocks use a blow-off style compression adjuster
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  5. Motociclo

    Motociclo Without motion, nothing.

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    The comp valve/ base valve restricts oil flow to bladder chamber and creates higher pressure income chamber. This makes it easy for oil to flow through midvalve and fill rebound chamber. Less pressure drop is less chance of cavitation.
    The S10 has plenty of Techy stuff, makes you wonder why it was given such a budget shock?
    Simple fix, nice work.
    #45
  6. Xfool

    Xfool Been here awhile

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    '01 yzf600 compression adjuster
    The ‘01 yzf600 shock has an endlessly rotating six position adjuster selecting orifice sizes from 1 to 2.4mm. The orifice style adjuster generates a boat load of back pressure at high speed (+2,000 psi) but not much at low speed.

    14-yzf600-06.png
    #46
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  7. Xfool

    Xfool Been here awhile

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    '00 yzf600 compression adjuster
    The ‘00 and earlier yzf600 shocks use a spring loaded blow-off style compression adjuster. Cranking the adjuster increases preload on the blow-off spring and delays the point where the valve cracks open.

    15-yzf600-00.png

    The Tenere needs a boat load of low speed damping to control the chassis weight. The low speed back pressure generated by the blow-off style ‘00 and earlier yzf600 compression adjusters looks to be the better choice for the Tenere, imo.
    #47
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  8. Xfool

    Xfool Been here awhile

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    Compression adjuster alternatives
    Both of those yzf600 compression adjusters are pretty crude compared to the shim stack controlled compression adjusters used on dirt bikes. With a shim stack the back pressure from the compression adjuster can be tuned to match the main piston pressure drop and keep the rebound chamber pressurized at a constant value over the stroke velocity range. That is know as “pressure balancing” the shock. The yzf600 compression adjuster just kind-of bangs up to 1,000 psi and sits there. Crude, but effective.

    An alternative to the compression adjuster is pressurize the shock with 1,000 psi of nitrogen. That pressure would pretty much lock up the shaft seals and destroy the shocks low speed compliance over things like highway dots. The beauty of the compression adjuster is the back pressure is only generated when the shock is pushed to high speed, otherwise the back pressure is zero. A compression adjuster keeps the strain and drag off of the shaft seals.

    Would love to find a junkyard version of a shim stack controlled remote reservoir. Those are available aftermarket for a couple hundred bucks, but nobody is giving those away at junkyards. Maybe you could hacksaw the top off of a dirt bike shock and stick a 4an hose fitting on there to use it as a remote reservoir. Might work and you could get that KLR “Road Warrior” look of functional simplicity with the hacked remote reservoir. Maybe next time -- for now pretty happy with the yzf600 unit.
    #48
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  9. Motociclo

    Motociclo Without motion, nothing.

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    Great work as always.
    With a bit of machining, I would think you could convert to a shim style adjuster while still incorporating the current spring system. All depends on how further you would want to pursue adjuster mods.
    Your quoted figure of 1000psi is what I found also when a made a mistake on port size. Small changes can have dramatic effects.
    #49
  10. Oyabun

    Oyabun 親分

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    Wow... Actually I'm just in the process of trying something similar - but in a different way. bought a few used shocs for a couple bucks to experiment - so waiting at the moment, but what I1m trying to pull off, is to install a HI-LO type comp adjuster from a GSX-R integrated reservoir shock, to an external reservoir. If I'll fail, I'll have at least some experience on how these are working and constructed. In case I manage I'll have a fully adjustable shock for peanuts and still comparable to aftermarket ones. Will keep you guys posted.

    Wish I'd read the idea of hacking an offroad shock apart - most probably would have been easier that way. Might give that a try also.
    #50
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  11. Xfool

    Xfool Been here awhile

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    Oyabun - post up a pick of whatever you come up with.
    Would like to see it.

    For the record a $40 junked yz250 shock and some 4an hose fittings.
    xt-yz250-shock.png
    #51
  12. Oyabun

    Oyabun 親分

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    Just got busted... Living in Europe severely limits my possibilities it seems.
    For some reason ebay global shipping program does not deliver shock absorbers, so my gsxr donor shock were just returned to their origin in the states. Over here used bike parts cost way more even if they are available.
    Gotta rethink this project a bit.
    #52
  13. Xfool

    Xfool Been here awhile

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    Bummer about junked shock availability.

    But frankly, to get the shock to work right the valving needs to be tuned to match the bikes weight and spring rate. And the clickers really need to be set at “10” to get the low speed bleed the wheels need to float over junk.

    Cranking High speed / Low speed adjusters around can make an ill tuned shock work better. But to get it to work right the valving needs to be tuned to match the bikes suspension with all of the adjusters more-or-less centered.

    Since your options for other shocks is limited, why not just re-tune what you’ve got?
    #53
  14. Motociclo

    Motociclo Without motion, nothing.

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    Given most stock pistons don't have addition bleed holes, it doesn't hurt to drill a bleed.
    With ReStackor can experiment with size to get adequate bleed. Probably find about 1.5mm will suit most applications.
    #54
  15. Oyabun

    Oyabun 親分

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    Unfortunately the stock shock on a cb500x is very basic. Means not rebuildable. And I'm about to fiddle a bit further with height and stroke. So I've got a only a very few non-expensive options.
    I have planned to build frankenshock with specific parts selected from different shocks and valved to my application. I will still build one, just that my options are more limited.
    #55
  16. Xfool

    Xfool Been here awhile

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    yzf750 compression adjuster
    Pawing through the junkyards the other thing I ran across is this ‘97 yzf750 compression adjuster. The feature is the compression adjuster has a knob on it so you can adjust it while riding. Inside, the guts of the compression adjuster uses a slightly smaller valve and softer spring – those two offset so the back pressure ends up about the same as the yzf600 unit. I’m going to use the yzf750 compression adjuster so I can play around with the adjuster on long highway stretches – sort of like an ES (LOL).

    16-yzf750-comp-adj.png
    #56
  17. Xfool

    Xfool Been here awhile

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    yzf750 Shock
    The yzf750 shock uses a 46mm piston so the piston in that shock isn’t going to fit the 40mm Tenere shock.
    But the port geometry looks to be a photo scaled version of the Tenere.
    17-yz750-valve.png
    #57
  18. Xfool

    Xfool Been here awhile

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    There is definitely some magic involved in valve port geometry design. Manufactures stick with stuff that works.
    #58
  19. Oyabun

    Oyabun 親分

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    What's the general consensus. Which one os better? Larger diameter valves? What is the benefit of larger valve sizes? Not port sizes, those I have an idea. Is it that they simply able to accommodate larger ports?
    #59
  20. Oyabun

    Oyabun 親分

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    This is the older type gsxr compression adjustment. I assume it is also the preload type one the same as you explained before. However this is how much I could remove it from the piggyback reservoir. Where do I go from here? I assume I have to unscrew the peppersprayer style bottom?

    Attached Files:

    #60