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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    Weight scaled rebound damping
    To match the target rebound curve I added a 20.15 and 24.15 shim pair under the 16.3 clamp. A single 19.2 shim under the clamp would work, but I didn’t have one. With those stack mods the damping force curve is pretty darn close to the target over the entire speed range.

    That weight scaled shim stack is more-or-less equivalent to running the stock shock with the clickers fully closed – except at low speed. At low speed the stack loosens up, in proportion to the spring rate change, to duplicate the stock suspension response. That is important to avoid packing the rebound stroke.

    7-mod-rebound.png

    Good enough for a test ride....
    #21
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  2. Motociclo

    Motociclo Without motion, nothing.

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    Some top info. ReStackor makes it is indeed. Very addictive!
    I am impressed that the Soqi shock can provide that amount of rebound control. Stock Af Twin is poor even with stock spring.
    Thanks for link ratio on S10. My figure was only an estimate.
    #22
  3. Xfool

    Xfool Been here awhile

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    Digressive damping
    MX dirt bikes weigh about 220 lbs. With a 50/50 front/rear weight split and 40 lb wheels the chassis to wheel weight ratio works out to around 1.75:1.

    Adventure bikes weigh over 500 lbs. Doubling the chassis weight requires the shock to produce the proverbial boat load of low speed damping to get control of the chassis. Wheel weights on the other hand are about the same as dirt bikes. That pushes the sprung to un-sprung weight ratio into the 5.3:1 range. About three times higher than a dirt bike.

    Shaft drive on the Tenere actually helps in that regard. Increased wheel weight drops the sprung to un-sprung weight ratio to around 3.6:1. Either way, adventure bikes require a boat load of low speed damping to get control of the chassis and that damping has to drop way-off at high speed to prevent the wheels from being over-damped. That requires the shock to produce a strongly digressive damping force curve.
    #23
  4. Xfool

    Xfool Been here awhile

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    Digressive damping force target
    You can get a handle on how steep that digression needs to be from suspension response zeta values for the chassis and wheel. Setting zeta values to be equal is the simple approach.
    xt2-digression-ratio.png
    The link ratio and spring rate are the same on both sides of that equation so those terms cancel out. That gives a simple relation showing the damping force fall-off needs to be equal to the square root of the wheel to chassis weight ratio.

    Compressing the Tenere suspension to the bump stop and allowing the chassis to spring back gives a maximum rebound shock shaft velocity of about 10 in/sec. When the bike is in the air and the wheels springing out the maximum velocity is 30 in/sec. Those numbers come out of ReStackor suspension response calculations.
    xt3-max-rebound.png
    At those max suspension speeds the ratio of wheel to chassis damping is 0.62. The square root of the wheel to chassis weight ratio on the Tenere is 0.53. That’s within 10% and pretty close considering the wild-ass assumptions going on here. If you went back to the zeta equation and set chassis damping at zeta=0.7 and wheel damping at zeta=0.83 that relation would be exact. So there’s a range of digressive damping and that range depends on how loose you are willing to run chassis damping (zeta<0.7) and how stiff you think you can get away with on wheel damping.

    The above example is for the Tenere shaft drive. A non-shaft drive bike needs an even steeper digressive curve. Design the suspension around carrying luggage and the damping force fall-off gets steeper still.
    #24
  5. Xfool

    Xfool Been here awhile

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    Tapered stack shim stack tuning
    The usual gig for suspension tuning is working on shocks that run simple tapered shim stacks. Conventional wisdom is adding shims to the low speed stack increases low speed damping. MXScandinavia over on ThumperTalk ran a bunch of dyno tests trying out those basic theories and found that DOES NOT WORK. Adding shims to the low speed stack increases the damping force everywhere, not just at low speed. ReStackor shows the same thing.

    Anyway, you could take a simple tapered stack and hack around adding shims to the low speed stack to get low speed damping to line up with the Tenere rebound curve by some area under the curve approach.
    xt4-tapered-stack.png
    That simple tapered stack produces a digressive damping force curve and the fall-off in that curve is about right for the 1.75:1 weight ratio of a dirt bike. But, the higher weight of an adventure bike needs a whole lot more digression in that curve. Comparison to the Tenere damping force curve (orange line) shows how large that difference is.
    #25
  6. Xfool

    Xfool Been here awhile

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    Motociclo: What do you think about moving your Africa Twin rebound stacks over to a ring shim configuration?

    You would have to adjust the ring-shim preload to get low speed damping to lineup with the stacks you have already developed through ride tests. Then back way-off on the stack stiffness to get high speed to drop-off to something more compatible with the 5:1 weight ratio of an adventure bike. The goal here is keep the low speed chassis control you have and back-off on high speed to get improved wheel compliance.

    Based on your comments I’ve got a feeling Honda may have set rebound to get wheel damping in the ball park and that left you with sloppy chassis control. To get chassis control back you had to double rebound damping. Thats good for chassis control but forces the wheels into an over-damped condition.

    If you can get a digressive stack to work the fall-off at high speed should give better wheel compliance over washboard and a little better traction on rough hill climbs. Those differences are kind-of subtle and no doubt more than one professional tuner would tell you: "Look, just deflate your tires already”

    I wouldn’t argue with them if they would follow me around and re-inflate my tires every time I got back on tarmac.


    …...... just thinking out loud here, a ring-shim based stack might be a useful alternative if you could get it to work. That weight ratio thing gives you a target to shot for.
    #26
  7. Motociclo

    Motociclo Without motion, nothing.

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    I have considered using preloaded stacks.
    Did spend a little time playing with stacks on restackor. I was happy with linear damping in the end. Rebound feels quite good.
    I feel digressive valving may be more benefit to comp valving.
    Don't know why digressive is not used more often. Only reason i can come to is plush feeling linear or progressive damping offers.
    Digressive will give the firm but plush feeling that riders like.
    I have seem digressive set ups used more for road racing applications.

    Certainly would look into again. Digressive damping does have its benefits.



    KTM 1190 adv uses a digressive rebound stack. It has a spring rate and link ratio very similar to the tenere.

    The unspung/sprung weight ratio for af twin is around 4:1.

    You have posted some great info here. Nice work.
    #27
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  8. Xfool

    Xfool Been here awhile

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    ktm 1190, great example. Looked around but could not find a thread listing the shim stacks.

    Some say if ring shims were important dirt bikes would use them. The thing is dirt bikes just happen to be light enough the natural digression of shim stack controlled valves is enough to get the low speed damping needed for the light chassis weight of a dirt bike.

    Move into a heavier bike like a KLR or 701 and digressive ring-shim stacks start to make more sense. But you can’t just toss a ring shim in there to see what it “feels” like. You have to back way-off on shim stack stiffness and then preload the stack to get the low speed damping back. That makes for a wimpy looking stack. The Tenere shim stacks pretty much look that way compared to a dirt bike shock. Wimpy.
    #28
  9. Motociclo

    Motociclo Without motion, nothing.

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    Preloaded stacks do look wimpy, but do have alot of power.
    I think may not be used in dirt bike world is partly because they are tricky to tune, and mostly because current stack styles performed well.
    Super moto bikes would benefit from digressive valving. Likely it does get used for this. These bikes have interesting suspension needs.

    http://www.ktmshop.se

    This Web sites has manuals for Ktm's. In Swedish. On left choose type, eg "Adventure", then year, model then at bottom of pages, documents.
    From here you will find the wp manuals for Ktm model you picked. I have down loaded a number of these for my nerdy suspension interest. They have all the data you would need.
    Hope this helps.
    #29
  10. Xfool

    Xfool Been here awhile

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    Tenere Stock Compression Damping
    The compression shim stack on the Tenere shock also uses a ring shim. Stack preload cranks up compression damping to about 100 lbf before the shim stack cracks open around 3 in/sec – The so called “nose” of the damping force curve.

    8-stock-comp.png

    The compression shim stack does not have much space between the face shims and backing washer. At 35 in/sec the 32.25 shim backing the ring shim hits the clamp washer preventing further deflection of the stack. That causes the damping force to kick up at high speed around 35 in/sec and adds some bottoming resistance to the Tenere compression stroke.
    #30
  11. Pipe dreams

    Pipe dreams Adventurer

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    Xfool this is a fascinating thread.
    I have an AF twin and am preparing to re-valve.
    I have a couple of compressed vertebrae that are not happy with the Socal freeway expansion joints.
    Off road I can deal with a less than optimum setup but getting to the trails is taking the fun out of riding.

    This digressive shim stack looks promising.
    A freeway expansion joint is not unlike a washboard surface as far as amplitude just frequency is less.
    Please excuse my ignorance when you refer to a ring shim is that a shim with a separate slightly thicker ring around the outside?
    I had seen those occasionally back in the day (my shock re-valving experience ran from 1976 through 1988!)
    Really impressed with the knowledge you and Motocicio share with the forum.
    Also the resources available like restacker and the collective knowledge of the interweb make what used to be considered a black art into a fine science.
    #31
  12. Motociclo

    Motociclo Without motion, nothing.

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    Compared the tenere comp valving to my current Af Twin valving. Overall quite close in total force.
    Ran a a digressive stack through for Af twin to compare, it smooths out the bumps and hollows that the S10 seems to have.
    Had a curve that mimicked S10 up to about 100mm/sec then a nice consistent linear almost, damping force up to 1000mm/sec.
    Did a comparison again with rebound, similar results, but naturally a lot higher damping force.
    I might just be stripping shock again to give these stacks try.
    Forks a proving to be a bit trickier. But persistence will pay off.
    #32
  13. Xfool

    Xfool Been here awhile

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    Suspension tuning "rules of thumb"
    A “rule of thumb” in suspension tuning (if there are any) is the ratio of rebound to compression damping should be around 2 to 2.5 for a shock. The stock Tenere runs a damping ratio of 3.5, basically the shock has no compression damping.

    9-damping-ratio.png

    To get a damping ratio in the range of 2:1 the Tenere shock needs to produce a whole lot more compression damping. Something like 80% more, that's my target for revalving this thing.
    #33
  14. Xfool

    Xfool Been here awhile

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    Ring Shims
    A ring shim is like you say, a thin centering shim with a thicker ring-shim around the outside. The thicker outside ring forces a slight pre-load bend in all of the shims above it and that increases the force needed to crack the shim stack open.
    xt5-ring-shim.png

    Tuning a shock for washboard or freeway expansion joints are different things. Ring shims help on washboard but they are going to hurt on expansion joints.

    The problem with washboard is closely spaced repeated bumps. Between bumps the suspension does not have enough time to fully extend before hitting the next bump. When that happens the suspension packs down in the travel and continues to pack until the spring force in rebound becomes high enough to return the suspension to some partially extended position. High spring force at that partially extended position makes the bike ride like a jack hammer over repeated bumps. That is know as “packing”.

    The fix for packing on washboard is to make low speed compression damping slightly stiffer than rebound. When the suspension hits a bump compression damping pops the chassis up slightly with each impact. That forces the suspension to hydraulically jack above the normal ride height and ride “high in the stroke”. Lower spring force at that elevated position gives the suspension a little more compliance to float the wheels over bumps.

    You can see that thinking in the stock Tenere damping ratio curve. I replotted the curve on a log scale to get some low speed detail. On the stock setup low speed compression damping is stiffer than rebound out to shaft speeds around 5 in/sec. Stiffer compression damping through that range forces the suspension to hydraulically jack over small bumps and ride “high in the stroke” over repeated bumps. The usual rule of thumb is the damping ratio should be somewhere around 0.8 throught that ultra-low speed range and then switch over to something around 2:1 at high speed above 15 to 20 in/sec.

    For tuning ring-shims you have some options. Increasing ring-shim preload moves that curve to the right by opening the compression stack at higher speeds. Increasing the compression shim stack stiffness drops the rebound/compression damping ratio at high speed and that is what I want to do for the Tenere.
    xt6-stock-damping-ratio.png

    Hydraulically jacking the suspension works over repeated bumps and freeway ripples to get some more compliance out of the suspension. But increased compression damping is the wrong approach for the single impact of expansion joints.

    My experience with back problems is riding over east-coast frost heaves. Any one of those bumps individually is no big deal. But after two days and several thousand hits those frost heaves become unbearable. Increased compression damping would make that worse.

    My advice for expansion joints is open up the clickers. More bleed will soften the impact assuming the suspension is not bottoming, and it may well be. I’ve bottomed my stock Tenere on more than one So. Cal freeway and I’m not blaming Yamaha for that. It’s just a pathetic statement of the state of So. Cal highways. You can’t even “make it” through your morning commute on an adventure bike without bottoming for crying out load.

    If it’s bottoming you need a stiffer spring and I agree with Motociclo’s earlier comment. A stiff spring with soft compression is going to give a plusher more compliant ride compared to a soft spring with stiff compression damping when both are setup to handle the same size bump.

    The other thing that might help is find a more compliant set of tires. My first set on the Tenere were a pair of Heidenau K60’s. Those tires have an extremely stiff sidewall and ride like it. I’m now on a set of Shinko 804/805 big blocks and the softer side walls on those tires handle square impacts a whole lot better. A little surprised by that. Another option might be the radial Pirelli Scorpion MT90’s.

    I’m working on tuning the Tenere. Motociclo is working the AT, he may have some better advice.
    #34
  15. Motociclo

    Motociclo Without motion, nothing.

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    My Af Twin has pretty much a 2:1 rebound to comp ratio. My stacks are on the firmer end of things. I have maybe a touch to much rebound, but it still feels good.
    My thoughts on wash boards and and expansion joints differ.
    I think on wash board you need a bit more bleed, so suspension can react. On expansion joints, given the sudden shaft acceleration would need less.
    On wash board stuff particularly, rebound needs to be spot on. If there is too much and it packs down, suspension will feel harsh and bike will "skate" over bumps.
    Same thing happens with too stiff comp damping, bike will "skate" or deflect off bumps.
    Expansion joints, too much bleed will let it blow through stroke. Compound that with a spring that is to soft, bottoming will likely happen.

    Too soft a spring with stiff comp damping will be harsh. My train of thought is to get as stiff a spring you can that allows you to get rider sag.
    Rebound will need a rework, most stock rebounds have very limited ability to cope with stock springs let alone stiffer.
    As far as comp damping goes, only want enough. If you have proper spring for your weight and it suspension is bottoming easily, you would certainly need a bit stiffer comp damping. If you haven't got to much brake dive or squat, then in a general sense, comp damping won't be too far off ideal.
    The idea with damping is to build pressure quick and bleed off excess pressure. This will give the famous firm but plush ride. Bit like an oil pressure relief in an engine. Side stepping, that is how high speed adjusters in shocks work.
    Preloaded stacks are good for this.
    The preloaded stacks give what is called a higher transmisability at low shaft speed and low transmisability at higher shaft speed.
    An example with this set up would be, hit a speed bump slow and bike tends to ride over bump, higher transmisability.
    Hit the same bump a bit quicker and suspension absorbs bump with less feed back to rider, Lower transmisability.
    This type of thing is not limited to this type of stack, just a little more pronounced than a linear damping set up.
    Tyres definitely play a big part, especially tyre pressure.
    #35
  16. Xfool

    Xfool Been here awhile

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    Tenere Cavitation Limit
    The stock Tenere shock runs a rebound/compression damping ratio around 3.5:1, the usual target is 2:1. Getting to that target requires a compression damping increase of around 80%. That raises the question: Why is compression damping so low on the Tenere?

    I think I got an answer to that running ReStackor. On the compression stroke the pressure drop across the shock piston drives pressures in the rebound chamber to vacuum at shaft velocities above 30 in/sec.
    10-stock-press-balance.png

    Pulling vacuum on the rebound chamber cavitates the shock and foams out the oil. The shaft velocity where that happens is set by the shock gas pressure and piston pressure drop. Stiffer compression damping would force that vacuum condition to occur at an even lower shaft velocity.

    You can get a measure of the gas pressure in the shock by pushing the shock on a bathroom scale and measuring the force it takes to hold the shock an inch or so into the travel. I got 45 lbs and on the 14mm Tenere shock shaft that works out to a gas pressure around 190 psi. That pressure shows up at zero velocity in the above plot.

    The 80% compression damping increase needed to get to a 2:1 damping ratio is huge and I am a little concerned that may upset the touring capability of the Tenere. But I’ve setup my previous dirt bikes with damping ratios of 2:1 and none of those were overly stiff on compression. I expect the Tenere to turn out the same.

    With the above calculations it looks like the reason for low compression damping was Yamaha couldn’t get anything more out of the shock without cavitating.
    #36
  17. Motociclo

    Motociclo Without motion, nothing.

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    80% is a lot, but firmer suspension can feel more plush also.
    If shock is blowing through stroke, it will feel harsh and if it is cavitating, you lose damping and it will be harsh.

    30 in/sec is 756mm/sec, given link ratio, gives just over 2 m/sec at the wheel. Which is about what you would see at the most, unless doing serious off road.
    Did a rough calculation, and I got similar numbers for you comp valving force. Closer to 90%, but that was using assumed unsprung/sprung weight.


    2:1 rebound/comp ratio is only general rule, 3:1 is closer for some bikes..
    #37
  18. Motociclo

    Motociclo Without motion, nothing.

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    Was doing some stuff with my Af Twin in revalving the comp adjuster to help firm up high speed stuff a little. During this, I found a mistake I made with one of numbers in the midvalve, ( for Af twin), to cut it short, your cavitation issue could be relieved a bit by enlarging your comp ports a little. Provided there is room. Also firming up base valve, (comp adjuster),will help also.
    I haven't run numbers for your valve or stack, but I do think it will push cavitation out past where your require shaft speed range.
    Worth a shot
    #38
  19. Pipe dreams

    Pipe dreams Adventurer

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    Thanks for the info guys.
    Running the mitos E-07 dakar tires, so compliance is a bit.. well non compliant.
    Now running heavier springs with small valving change in the rear.
    opened up clickers and ride is better.
    Will move this over to AF twin suspension thread as not to muddy the waters here any further.
    Will continue to Lurk and learn.
    Thanks!
    #39
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  20. Xfool

    Xfool Been here awhile

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    If you get a chance to try a more compliant tire like a regular E-07 or something else would be good to hear how it turns out.
    #40