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  1. #1
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    High speed and low speed compression adjustments.. what do they affect?

    High speed and low speed compression adjustments?

    Ok so im a bit confused about what these do...

    Scenario 1)

    Refers to the speed of the shaft.. so even if the shaft is at the start of its stroke, if the movement is fast the high speed compression is affected. Vice versa even if the shock is almost fully compressed if the movement is slow the low speed compression will be controlling it.

    Scenario 2

    It actually affects where the shaft is in its movement. i.e. if the shaft is at the start of its stroke it is the low speed compression that is affected and if it as at the end it is the high speed compression.

    It makes sense that it is the first but I have heard contradictory information which implies it is the latter.

    Also do big drops create high speed shaft movement.. or would they be considered relatively slow to say hitting a rock garden at 20mph

    Does it vary from manufacturer to manufacturer/ I’m running a totem up front with mission control and ccdb at the back so any info specific to that would be much appreciated! However a general discussion / info about this would also be welcome.

  2. #2
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    Quote Originally Posted by Karve
    High speed and low speed compression adjustments?

    Ok so im a bit confused about what these do...

    Scenario 1)

    Refers to the speed of the shaft.. so even if the shaft is at the start of its stroke, if the movement is fast the high speed compression is affected. Vice versa even if the shock is almost fully compressed if the movement is slow the low speed compression will be controlling it.

    Scenario 2

    It actually affects where the shaft is in its movement. i.e. if the shaft is at the start of its stroke it is the low speed compression that is affected and if it as at the end it is the high speed compression.

    It makes sense that it is the first but I have heard contradictory information which implies it is the latter.

    Also do big drops create high speed shaft movement.. or would they be considered relatively slow to say hitting a rock garden at 20mph

    Does it vary from manufacturer to manufacturer/ I’m running a totem up front with mission control and ccdb at the back so any info specific to that would be much appreciated! However a general discussion / info about this would also be welcome.
    Pretty nice set-up, but the amount of adjustability in both products can lead to lots of trial and error as well as something that can be set-up very odd. (I have a Lyric coil and CCDB on my little bike).

    scenario one correctly describes speed sensitive damping. Scenario two describes position sensitive damping.

    The confusion is partially due to mfg using incorrect terminology, and partly due to shocks like the 5th, or manitou SPV, or even the DHX that use both speed sensitive and position sensitive damping...so although these shocks have both, the knobs and literature will usually only refer to one type or the other.

    The vast majority of adjusters on forks and shocks effect the low speed of compression (they open and close a free-bleed), where as the high speed compression curve is determined by the internal shim stack

    Where high and low speed damping change (the threshold) is different on differnt products, and can be adjusted on some products. The mission control damper allows seperate adjustment of low speed compression as well as the threshold (or blowoff)....the amount of force/speed required to 'become' a high speed damping event.

    The ccdb is a bit different in internal construction using popet valves rather than shims (yes I know there are shims, but they are not used the same way in the CCDB as other shocks). Again the shock offers the ability to adjust low speed compression, and the threshold is sort of tied to the high speed adjuster.

    Generally a large drop (especailly to a transition) is predominantly a low speed damping event. You might open the high speed compression circuit right at touch down, but the shock movement rapidly slows and is generally controlled by the low speed circuit.


    Rebound is a bit more confusing, as you cannot seperate shock displacement (position) from speed when it comes to rebound damping. This is because the rebound force comes from the spring which increases its force linearly with displacememt. So you cannot have a small displacement, yet high speed rebound event...there is just not enough energy stored in a spring at small displacements to move the shaft 'that' fast.
    In a compression event, the force comes from rider speed combined with impact....you can have a high speed event by hitting a small rock while riding fast (shock only moves a fraction of an inch...but at high speed)...or you could hit a 1 foot tall wall at 40 mph....again a high speed event, but this time the suspension displacement will be much greater...you could even hit a pebble after landing a large drop (suspension almost fully compressed) and have a secondary high speed event due to the pebble.....

  3. #3
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    Low speed compression damping affects very light compression inputs from rider weight shift's that cause wallowing in bumps and pedal bob and fork braking dive, also very small bump compliance and traction feel is affected.

    High speed compression damping affects heavier weighted compressions like landing jumps and sudden compression g-outs like dropping into the bottom of deep washouts.

    Low speed affects high speed and vice-versa.

    Sudden obstacle hits, such as hitting larger rocks at faster riding speed, blow off the damping thresholds of both circuits until the spring resists travel speed enough for damping resistance to have some affect again.

    Most shocks and forks have one compression adjuster for low speed or preloaded platform threshold compression adjustment with high speed set internally.

  4. #4
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    Quote Originally Posted by derby
    Low speed compression damping affects very light compression inputs from rider weight shift's that cause wallowing in bumps and pedal bob and fork braking dive, also very small bump compliance and traction feel is affected.

    High speed compression damping affects heavier weighted compressions like landing jumps and sudden compression g-outs like dropping into the bottom of deep washouts.
    .
    No, not quite. "High speed" and "Low speed" correspond to the shock;s shaft speed, that is what those names traditionally refer to, that threshold between high speed and low speed is not as low as you're making it out to be. The "weight" has nothing to do with it, the shaft speed has everything to do with it, and landing jumps and similer moves are traditionally low-speed events (shaft speed). The first guy explained it much better. Obviously the further you drop, the faster you get going, and the faster the shaft will move when you touch down, so it's not to say that there's no high-speed event, but Derby is confusing force with impulse, and the impulse (force and time) determines the shaft speed and the high vs low speed situation.
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  5. #5
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    Quote Originally Posted by Karve
    High speed and low speed compression adjustments?

    Ok so im a bit confused about what these do...

    Scenario 1)

    Refers to the speed of the shaft.. so even if the shaft is at the start of its stroke, if the movement is fast the high speed compression is affected. Vice versa even if the shock is almost fully compressed if the movement is slow the low speed compression will be controlling it.

    Scenario 2

    It actually affects where the shaft is in its movement. i.e. if the shaft is at the start of its stroke it is the low speed compression that is affected and if it as at the end it is the high speed compression.
    Scenario 1 is the normal. When shock manufacturers build a platform into the damper it can have some elements that are position sensitive.

  6. #6
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    Quote Originally Posted by Jayem
    No, not quite. "High speed" and "Low speed" correspond to the shock;s shaft speed, that is what those names traditionally refer to, that threshold between high speed and low speed is not as low as you're making it out to be. The "weight" has nothing to do with it, the shaft speed has everything to do with it, and landing jumps and similer moves are traditionally low-speed events (shaft speed). The first guy explained it much better. Obviously the further you drop, the faster you get going, and the faster the shaft will move when you touch down, so it's not to say that there's no high-speed event, but Derby is confusing force with impulse, and the impulse (force and time) determines the shaft speed and the high vs low speed situation.
    I’m not clear on your confusion or misunderstanding of my post. I was of course referring to shock shaft speed damping. Shock shaft speed not isolated from the whole bike, shaft speed IS activated by weighted leverage of the suspension between the ground and unsprung weight (which is about 90% rider weight).

    I elaborated more on the very good first reply, and gave more examples where low speed and high speed compression damping is very differently activated while riding. This based on expert rider and shock designer information I’ve gathered. I don’t have a separately adjustable low and high shaft speed shock to test the opinions of these authorities, so my opinion is only reasonably educated.

    If high speed compression external tuning is available it would be tuned by testing heavy weighted ride situations, such as g-outs and landing jumps. Low speed compression damping would be externally tuned separately by testing low rider weight-shifting inputs, such as wallow, pedal bob, very small bump feel, handing traction grip balance front to rear, and brake dive. Adjusting either circuit affects the other. Very sharp compressions, such as hitting a sharp rock at high rider speed blows open both damping circuits, which will reactivate when the shaft speed slows enough (by spring compression resistance) within range of the damping effect.

    Yes low-speed damping can encroach significantly into the high shaft speed range of damping when adjusted to an extreme firmness, but then is no longer really low-speed damping. And any amount of low speed damping does affect high speed damping as shaft speed accelerates and decelerates in compression. And vise-versa, high speed damping affects low speed - and this is where PUSH tuning improves a single external adjustable shock, buy tuning the high speed for a particular rider weight and suspension design, and lowering the low speed thresholds for smoother while stable reactivity to lightly weighted inputs I mentioned before (or raising the low speed resistance if more platform effect is desired).

    Regarding position sensitive compression damping, I’m not familiar with details of a couple designs that have been used for mountain bike shocks. Progressive shocks designed an air pressure activated platform (adjustable lockout damper) and bottom out resistance, so deeper into travel increased compressed air pressure would activate to open the lockout valve and additionally resist bottom out by adding air spring resistance in the compression oil reserve chamber to the main spring. DHX “Boost Pressure” apparently worked similarly to activate valve resistance for increased high speed damping increasingly in deep travel. I don’t have detailed knowledge of these systems.

  7. #7
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    DO A SEARCH!!!!!!!!!!

  8. #8
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    i did.. if you know of a definitive thread which covers off all the questions asked then let me know...... waiting.... still waiting....

    I think you can see from the discussion that a lot of people are not clear on what high and low compression are affected by and this is further clouded by the fact that manufactures don't add clarity.

    I believe that RS with their new vivid shock referred to the high speed compression adjustments affecting the end of the stroke rather than speed etc

    http://www.sram.com/en/rockshox/rear...on/vivid/#tab1
    Damping Adjust External ending stroke rebound, beginning stroke rebound, and compression - ???????

    So its not a question which has a definitive answer on this forum.. Each shock manufacture seem to consider it relevant to adjust a differnt range its thus worthy of discussion.
    Last edited by Karve; 07-15-2008 at 09:55 AM.

  9. #9
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    Keep this going. I'm a holder of a liberal arts degree who is struggling to understand all this stuff. Keep it coming until I can wrap my head around it all.
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  10. #10
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    Quote Originally Posted by Karve
    i did.. if you know of a definitive thread which covers off all the questions asked then let me know...... waiting.... still waiting....

    I think you can see from the discussion that a lot of people are not clear on what high and low compression are affected by and this is further clouded by the fact that manufactures don't add clarity.

    I believe that RS with their new vivid shock referred to the high speed compression adjustments affecting the end of the stroke rather than speed etc

    http://www.sram.com/en/rockshox/rear...on/vivid/#tab1
    Damping Adjust External ending stroke rebound, beginning stroke rebound, and compression - ???????

    So its not a question which has a definitive answer on this forum.. Each shock manufacture seem to consider it relevant to adjust a differnt range its thus worthy of discussion.

    Go back and read my explination of how position and speed are linked when it comes to rebound........ RS is going back and forth (I am guessing) because it is easier for people to understand what 'end of stroke' is versus high speed...it is more intuitive if you are not familiar with the terminology.....and in the case of rebound, the terms are interchangable (not so with compression). Many people would assume (incorrectly) that 'high speed' ment high rate of riding speed....rather than high rate of shaft speed...something most people are just not familiar with or know how to measure. Saying end of stroke is technically correct (when talking about rebound) and easier for the user to understand.

    Jayem's post is pretty spot on. There are some things Derby wrote that are not correct, like g-outs being high speed events..they are NOT...neither is landing large drops to transition. It is a bit hard to understand some of this....but it really comes down to shock shaft speed...not rider speed, not size of impact (well generally a larger impact is a slower speed event..counter intuitive sort of). Jayem touched on impulse....and that is key. A large drop to transition has a large net force, yet that force is acting over long period of time. Because of the time involved, the shaft speeds are low. A smaller net force impact, like hitting a breaking bump at speed, can cause a high speed event because the impulse is for a much shorter time.

    Classic example of high speed is breaking bumps.....low amplitude, high frequency
    classic example of low speed is a g-out ...........large amplitude and low frequency.


    The bike biz is NOT the place to be looking for propper explinations for this kind of stuff. Everything available in the bike biz is ripped off from another application where it was actually designed...and then re-named in an attempt to patent, or copywrite, or just sound cool.
    The peter verdone design web site has some good explinations of dampers(motorcycle) as well as internals and how they work. Penske used to have some good tech articles around about their auto race dampers....I have seen some info from Ohlins as well....you could also find a copy of Tony Foale's book on motorcycle chassis and design. It is very applicable to MTB design and might help with understanding the forces involved in 2 wheel vehicles.

    Beyond that, is just an education/understanding of fluid dynamics as well as understanding the formal definitions of damping (over, under, critical)...system harmonics, etc.....as well as the classic physics deffs like force, energy, work, etc...
    Last edited by davep; 07-15-2008 at 11:26 PM.

  11. #11
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    whatever...all this is far too confusing for me.

    my advice is to buy the shock/fork that looks the coolest on your bike

  12. #12
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    Not to Hijack but... Say I wanted to get a little more "pop" off lips of jumps rather than have my suspension eat a lot of that force. Would I want to up the HSC or LSC on my fork? Thanks in advance.
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  13. #13
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    Quote Originally Posted by njhcx4xlife
    Not to Hijack but... Say I wanted to get a little more "pop" off lips of jumps rather than have my suspension eat a lot of that force. Would I want to up the HSC or LSC on my fork? Thanks in advance.
    I found that backing off the rebound damping (making it faster) gave a little more pop. A little more LSC damping might help, too by keeping the bike riding taller while pumping the face of a jump. Just don't get so crazy that you mess up the ride.
    You better just go ahead and drop that seatpost down to the reflector... the trail gets pretty rough down there.

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    davep - spot on. what you've just explained is exactly the feelings I've noticed while riding - I can do a decent job of explaining them, but not well enough - people tend to just look at me like I'm crazy.

    I'm fairly certain I understand the rebound thing as well, but since you know your business I'm fairly certain that if you tell me I'm wrong, I'm actually wrong.

    Rebound force is a function of spring compression, and so the deeper you are into your travel the more rebound force there is. The shock (or fork, or whatever) will rebound faster if its at bottom out then it will if its 1/4th of the way into the travel. Thus, when you get a basic rebound adjuster, its just a rebound damping adjuster that hits somewhere in the middle, so that on bottom outs etc it doesn't spring back too fast, but that its fast enough to track changes in the terrain. The idea of separate rebound adjusters is so that when you hit hard you can control the rate the shaft speed returns and make it taper off - the harder the shaft pushes, the more damping, the less the spring pushes, the less damping there is.

    This is what makes the vivid and CCDB so attractive is that you can get something that doesn't buck you on rebound from a bottom out but still tracks well. That seems pretty simple so far, but heres another question: what's the benefit of having two tubes? CC/Ohlins make a big deal of that, I understand that they're entirely correct, I just don't know why

  15. #15
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    Quote Originally Posted by Uncle Six Pack
    I found that backing off the rebound damping (making it faster) gave a little more pop. A little more LSC damping might help, too by keeping the bike riding taller while pumping the face of a jump. Just don't get so crazy that you mess up the ride.

    Cool, thanks. My rebound is good but I guess I phrased it wrong. I needed it to not eat so much of the force while pumping the face of the jump.. I knew more compression would help but I'm new to having the option of HSC and LSC coming off a zoke.
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    I'm no suspension expert, but I will share my findings.

    Fork is a Talas 180 RC2 on an Ibis Mojo HD.

    High speed compression adjustment kicks in when you're in a situation that will bottom out the fork (or shock).

    You can pump up the air chamber until it doesn't bottom anymore, or you can let the HSC (high speed compression) help out to prevent or soften the bottom out.

    Why wouldn't you just pump it up or get a stiffer spring to not bottom out?

    Because you might be running so high in the travel that your tire is coming off the ground in bumpy sections. When your tire isn't on the ground in bumpy sections, you have no traction or braking.

    -=Invicta Rocks.

  17. #17
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    Quote Originally Posted by invictarocks View Post
    I'm no suspension expert, but I will share my findings.

    Fork is a Talas 180 RC2 on an Ibis Mojo HD.

    High speed compression adjustment kicks in when you're in a situation that will bottom out the fork (or shock).

    You can pump up the air chamber until it doesn't bottom anymore, or you can let the HSC (high speed compression) help out to prevent or soften the bottom out.

    Why wouldn't you just pump it up or get a stiffer spring to not bottom out?

    Because you might be running so high in the travel that your tire is coming off the ground in bumpy sections. When your tire isn't on the ground in bumpy sections, you have no traction or braking.

    -=Invicta Rocks.
    Not quite.

    Insufficient low speed compression damping could bottom the fork off a drop, jump or big g-out. This is actually one of the more common "bottoming" events with a fork.

    The other big way a fork can bottom is due to a high speed impact, like say a sharp 4" fork you hit going mach 5 with your 5" travel fork. Supposedly it has enough travel to deal with this, but you can get the fork to hydro-lock if it can't pass enough oil quick enough or slam into the opposite end (bottom) if it doesn't have enough high speed damping. Most fork manufacturers like to be a little conservative here, so it's more common to get the hydrolock and spiking, but bottoming is still possible.

    Of course you could have a combination of the two at once, in which case you have to address the proper parameter.

    That aside, often the "high and low" adjusters on suspension forks and shocks don't *really* adjust the circuits independently like they claim, even when they do, the function may be counter-intuitive.
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  18. #18
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    Most dampers with HS and LS adjusters are indeed two circuits, namely a freebleed, and a shimstack. The LS adjuster opens the freebleed, while the HS adjuster usually increases preload against some kind of shim stack, or a spring-loaded valve.

    I made a simple graph in paint trying to explain its usage:
    High speed and low speed compression adjustments.. what do they affect?-graph.png

    Here the green curves show the different ways the first LS circuit absorbs a hit, while orange shows the opening of the HS circuit. As you can see, the freebleed has a exponential curve, and would cause spiking at higher speeds if no HS circuit was present. The HS circuit has a linear starting curve, because it slowly forces a spring to open up bit by bit, until it can't open up any further, causing the final ramp-up. The strength of this spring/shimstack is the defining characteristic of the Slope at which the orange curve is set.

    The area where the two curves meet ("the knee") can be used as a useful tuning element. By setting the LS so that the knee coincides with brake-dive speed, or the speed at which you hit most drops, you can make sure that you are not blowing through your travel. While at the same time, the secondary curve can still absorb fast rocks and roots.

  19. #19
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    OK, so now diving into this arena and found this thread, started in '08 and then resurrected in '13, so thought I'd bump it up again since it has in some good info. Also to ask Derby if he stands by his posts here that are now about 6 years old and he's been doing this as a profession for a while?
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  20. #20
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    Davep and jayem are always reliable sources for suspension information, and two-one and UncleSixer are right on target.
    Last edited by scottzg; 07-26-2014 at 09:26 AM.
    affect befect cefect defect effect fect

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    Apologies for the necro-posting. This thread is extremely illuminating; the awesome distinction made above, between what type of situation creates high speed compression vs high-speed rebound was awesomely helpful. I only wish suspension manufacturers would be clearer about this. The other thing that manufacturers muddy the waters with is when they use vernacular like "increase your LSR" when in reality they mean "increase your LSR DAMPING" (the exact opposite of increasing LSR!).

    Honestly, this would all be so much clearer if they (a) simply spoke of tightening and loosening the movement in either direction and (b) re-worded HSR as "deep-travel rebound" and LSR as "low-travel rebound" while keeping "high-speed compression" and "low speed compression" or perhaps calling them "fast-impact compression" vs "slow-impact compression".

    Another fault with manufacturer tuning guidelines is that they never talk about things like potholes, ruts, or steps... cases where your wheel suddenly drops below you, instead of hitting and clearing an obstacle. These are just as important to the ride as clearing roots, washboards, and rocks. Going out on a limb, I assume what happens when you hit a small dropout like a pothole (or hoof-hole), is that your low speed rebound damping will slow your wheel from dropping into it and hugging the trail... the effect being that you lose traction while the wheel is not tight against the trail. Yes? No?

    My takeaway is that I generally want low speed rebound to be as fast I can handle*— as long as it's not bucking me too badly on the rock gardens. While I want high speed rebound to be as slow as possible (given high speed rebound events are relatively uncommon and I'd rather have the bike recover relatively slowly from a deep travel event)... though I'm not sure how such a rebound would affect the steep climb back out from a deepish g-out.

    Quote Originally Posted by two-one View Post
    Click image for larger version. 

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    This diagram confused me. Why are there three lines on the left and two on the right? What's the difference between yellow, light green, and dark green? And what is "force" referring to here and what it it's relationship to velocity? The force the shock is going to rebound with (stored energy)? The force required to achieve a certain velocity of travel? The force applied to the shock regardless of velocity of travel? Why would this force become proportionately smaller as velocity increases (according to the lines drawn, the further right you go, the less the force increases)... or vice-versa, why would higher force be correlated with lower velocities?

    Basically... i have no idea what this chart is communicating or plotting. I don't understand what the force axis means, nor how the various lines are supposed to illustrate a fixed and specific relationship to the velocity axis.

  22. #22
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    the shock does not rebound: that's the spring. regardless of the spring material (coil, leaf, air, elastomer) the spring is supporting the bike (and rider), and it oscillates. the purpose of dampers is to restrict the oscillation of the spring, which gives you bump compliance, traction, bottom out resistance, and so on.

    the chart shows compression force. if rebound were included on this chart it might make sense to show it as negative plots below the horizontal (velocity) axis, because it applies force in the opposite direction from compression.

    the lines on the left indicate adjustment range for low speed compression. let's say that green is full firm and yellow is full soft. same for the right side - the lower plot is minimum HSC and the upper is maximum.

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    The chart shows the relation between the amount of damping force at different stanchion velocities... so body-movement and subtle pumping is lowspeed, while rocks&roots are highspeed.

    Most modern dampers consist of 2 channels... one is open, and the size of the port is regulated by a threaded needle attached to the LSC knob. When the LSC is turned clockwise, the needle will slowly fill the port. This port is responsible for the lowspeed exponential shape to the left of the graph. The yellow/green lines indicate a few different settings for the LSC adjuster.

    Because a port-orifice damper (like the LSC port) has an quadratic relationship between speed/velocity, very hard impacts will always feel harsh... so for that reason there is a second channel, which consists of large ports with shimstacks behind them, clamped shut with a spring. So when an impact is hard enough, the clamping force of the spring is countered, and the shimstack will start to flow oil.
    The HSC adjuster is connected to this spring, and preloads it more when it is threaded clockwise... so the blowoff force has to become even higher.

    So in the graph, the green line has the least LSC damping, and the orange line has the least amount of HSC damping.

    So in theory... when you can full close the LSC adjuster, and set an amount of HSC, you basically have a platform damper.

    The slope of the orange line (indicated by the horizontal+vertical lines) indicates the shim stack stiffness. to change this slope you will actually have to open your damper, and rearrange/add/remove shims.

  24. #24
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    Quote Originally Posted by ColinL View Post
    let's say that green is full firm and yellow is full soft
    The other way around Colin

  25. #25
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    Quote Originally Posted by two-one View Post
    The other way around Colin
    DOH!

    Can I claim colorblindness?

  26. #26
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    Quote Originally Posted by two-one View Post
    The chart shows the relation between the amount of damping force at different stanchion velocities
    So... the "damping force" is a force opposite to (but obviously always less than) the force of the stanchion compressing. Is it essentially a "friction force?"

    Therefore, what this graph shows is that the faster a stanchion moves, the more the opposing damping force resists this movement... and that this relationship is "exponential" such that as the stanchion speed increases the opposing force increases disproportionately more. Then, at a given threshold speed, the opposing force falls off, becoming almost linear relative to further increases in stanchion speed for a while.

    Did I get that right?

    I'd understand the graph better with a single line plotted. The "adjuster" arrows make it clear they influence where the threshold occurs and the resulting plot line.

    Am I correct to infer there is no scenario where you'd ever see the force/velocity relationships graphed in the upper left quadrant? I.e., the plot-line tail spiking almost straight up that follows AFTER the adjuster threshold?

    Finally, why should the stanchion force be plotted as velocity — as opposed to force? Not being that great at physics, I'm wondering if that is purposeful? Is the opposing damping force actually a factor of stanchion speed or is it a factor of stanchion force? Aren't they just both forces pushing against each other?

    Could we take that further and simply display both axes as acceleration axes?

    I tried my hand at a graph I would grasp easily... but I'm having trouble believing that changes to LSC and HSC (independently or together) would affect the curve the way the graph implies. So I think I got it wrong

    Sorry if I'm coming across as OCD or obstinate; I'm honestly trying to intuitively grasp what this curve communicates and how it's influenced.High speed and low speed compression adjustments.. what do they affect?-damping-circuit-curves-01.png

  27. #27
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    Quote Originally Posted by calzones View Post
    So... the "damping force" is a force opposite to (but obviously always less than) the force of the stanchion compressing. Is it essentially a "friction force?"I'm not sure what you mean, but possibly?

    Therefore, what this graph shows is that the faster a stanchion moves, the more the opposing damping force resists this movement... and that this relationship is "exponential" such that as the stanchion speed increases the opposing force increases disproportionately more. Then, at a given threshold speed, the opposing force falls off, becoming almost linear relative to further increases in stanchion speed for a while.Sort of. What your seeing is that at a certain threshold the oil can't pass through the orifice quickly enough, and the pressure from the oil trying to pass through the orifice bends the shims and escapes that way. That point where the shims start allowing oil through is the knee, and how much force it needs and how much oil it allows to slip through at a given pressure can be dictated by the person who assembled the stack of shims.

    Did I get that right?

    I'd understand the graph better with a single line plotted. The "adjuster" arrows make it clear they influence where the threshold occurs and the resulting plot line. The adjuster arrows show that the user can change the size of the orifice (low speed) and add preload to the stack of shims(high speed)

    Am I correct to infer there is no scenario where you'd ever see the force/velocity relationships graphed in the upper left quadrant? I.e., the plot-line tail spiking almost straight up that follows AFTER the adjuster threshold?almost anything is possible with a clever shim stack.

    Finally, why should the stanchion force be plotted as velocity — as opposed to force? Not being that great at physics, I'm wondering if that is purposeful? Is the opposing damping force actually a factor of stanchion speed or is it a factor of stanchion force? Aren't they just both forces pushing against each other?

    Could we take that further and simply display both axes as acceleration axes?I'm not equipped to answer these questions

    I tried my hand at a graph I would grasp easily... but I'm having trouble believing that changes to LSC and HSC (independently or together) would affect the curve the way the graph implies. So I think I got it wrong Yeah, thats wrong. low speed is bleed- like how you can't push water out of your camelbak any faster after a certain point. high speed is what the valve does when you reach that point. By reshaping the shim stack it can have a different curve or slope, but generally a high speed adjuster knob mostly affects the slope.

    The setting on the low speed also determines when the high speed tuning comes in to play- a really slow free bleed will create a lot of pressure to act on the high speed shims at a relatively low shaft speed.


    Sorry if I'm coming across as OCD or obstinate; I'm honestly trying to intuitively grasp what this curve communicates and how it's influenced.This stuff is hard, and i learn a lot when the really savvy folk discuss it too.Click image for larger version. 

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    affect befect cefect defect effect fect

  28. #28
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    Here is the Manitou ABS+ damper tuning guide with lots of graphs for same damper in different configurations:

    http://home.cyber.ee/arne/ABS+%20Tun...03-10-2011.pdf

    You can see concrete examples how the position of the bleed port screw (LSC adjuster) and different shim stacks (kind of HSC adjuster) affect the speed-force relationship.

    I think that the speed-force graph is the correct one to think about the damper. You should think about this relationship as "how much force does it take to move the shaft at given speed". Or "how fast will the shaft move when I use so much force to move it".

  29. #29
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    Damping is indeed "controllable friction"

    You can compare the exponential relationship of the LSC bleed to people trying to use a single door simultaneous. Even though more people are trying to get through that door, the throughput is hardly changing, because there simply isn't any more space.

    Quote Originally Posted by scottzg View Post
    high speed is what the valve does when you reach that point. By reshaping the shim stack it can have a different curve or slope, but generally a high speed adjuster knob mostly affects the slope.
    Thats not right... the slope is only changed by the general shimstack stiffness. The point at which the stack STARTS to flow oil is dictated by the HSC adjuster.
    So this means the position of the "knee" kan be manipulated in two ways... by changing LSC it changes in the horizontal plane (like you have drawn).
    But when the HSC increases, it doesnt change perfectly vertical, but it travels along the LSC curve upwards or downwards.

  30. #30
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    You could also look at friction as just a simple means of damping movement. Take away the oil and shims, and you still have some damping just from the friction of the shaft moving through the seals.

    Hydraulic damping allows for stronger damping force, more adjustment, and better consistency than just using simple friction alone.

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    Ok, I couldn't stop thinking about this and the flaws in my previous graph. All night. lol... After thinking it through carefully... this is now the best my brain can come up with...would this graph be correct?

    What I've tried to illustrate is:

    1. A maximally stiff suspension would behave like a hard-tail; using force-vs-force as my axes, this means maximally stiff would be a perfect 45° line extending from the origin.
    2. The 45° locked out "horizon" line cannot be crossed. The "no-man's land" on the other side implies a damping force that actively exceeds the force of the stanchion.
    3. The low speed compression damping only operates up to a "crossover" point (not adjustable on most suspensions).
    4. At extremely low forces, the system is essentially undamped regardless of settings, since the low oil flow will easily pass through even the smallest LSC damping aperture.
    5. You can't really modify the curve of the LSC or HSC damping... you can modify —upward or downward— how close to the locked-out horizon each comes, or how close to "completely undamped" each comes... however, the actual resulting curve is a function of the physics of the system design.
    6. You generally want LSC damping to be fairly stiff and closer to locked-out than fast and loose.
    7. As stanchion force increases, the user-selected aperture of the LSC damping circuit is eventually overwhelmed, oil cannot flow any faster, and, as a result, the damping becomes ever more forceful. The LSC circuit becomes exponentially stiffer, the higher the force.
    8. At forces higher than the crossover, the HSC shim is opened under pressure. Therefore, damping response quickly falls off at the crossover point as higher oil flows are restored.
    9. Presumably the suspension manufacturer has designed the LSC damper to reach its max exactly at (or slightly past) the crossover. Otherwise, you would get near-locked-out behavior at forces much higher than the LSC aperture can handle but still lower than the crossover force. Due to points 6 and 7, there will be a "discontinuity" in the response curve around the crossover. Both before the crossover as the LSC response starts to shoot up toward the locked out horizon, and after the crossover as the HSC circuit starts to open up to relieve the pressure.
    10. Presumably, past the crossover, the HSC shim opens even wider under even higher pressures, maintaining a semi-linear response for a while thereafter.
    11. Eventually, at even higher forces, the HSC shim can open no wider, so the oil flow starts to be obstructed. Damping force begins to increase exponentially relative to the stanchion force.
    12. Finally, at the highest values, the spring bottoms out and the system becomes rigid / locked out, like a hard-tail; it remains linear and locked out for anything higher.
    Attached Thumbnails Attached Thumbnails High speed and low speed compression adjustments.. what do they affect?-damping-circuit-curves-2-01.png  


  32. #32
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    Quote Originally Posted by arnea View Post
    Here is the Manitou ABS+ damper tuning guide with lots of graphs for same damper in different configurations:

    http://home.cyber.ee/arne/ABS+%20Tun...03-10-2011.pdf

    You can see concrete examples how the position of the bleed port screw (LSC adjuster) and different shim stacks (kind of HSC adjuster) affect the speed-force relationship.

    I think that the speed-force graph is the correct one to think about the damper. You should think about this relationship as "how much force does it take to move the shaft at given speed". Or "how fast will the shaft move when I use so much force to move it".
    That was a very interesting resource.

    I still have trouble wrapping my brain around velocity to force though. It helps to have concrete numbers as in the Manitou graphs.... but still I can't stop thinking it's force vs force and not force vs velocity.

  33. #33
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    Damping force is (ideally, and approximately) proportional to the speed. Or velocity, but since the velocity is always parallel to the shock's axis, it makes just as much sense to simple call it speed.

    We're basically talking about how much a fluid resists being forced through a tight passage. Low speed, low resistance - high speed, high resistance.

    So, the force-vs-force graph isn't capturing the key variables. High force on the shock tends to create high speed - but the speed that results from a given force is a function of (among other things) damping, which is the thing you're trying to predict, so that complicates things.

  34. #34
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    Quote Originally Posted by NWS View Post
    So, the force-vs-force graph isn't capturing the key variables. High force on the shock tends to create high speed - but the speed that results from a given force is a function of (among other things) damping, which is the thing you're trying to predict, so that complicates things.
    So you are saying that for any given speed of x m/s, the shock might subject to a totally different force? My physics is weak, but I figured the force required to move the shock at a given speed, assuming a consistent mass (rider+bike), would always be the exact same force.

    In other words, if 1 Newton (in the direction of the shaft) produces a shaft compression speed of 1 m/s, then every time you apply 1 Newton in the direction of the shaft it will move at 1 m/s... and every time it moves at 1 m/s a force of 1 Newton caused it.

    To my mind, the shock movement is creating a force (pressure) on the spring and the fluid within. It is this very pressure that creates an opposing force (and which maxes out the LSC aperture... or the single door people are trying to flow through).

    What you're saying, if I understand correctly, is that a given speed of shaft movement can —and does— result in different internal forces each time... and therefore it makes no sense to compare force to force. Yes?

    I have an enormous amount of trouble accepting that. Or is there a whole other dimension to this I'm still not grasping?

  35. #35
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    Our bike shock absorbers have two parts; the spring and the dampers.
    The spring and dampers are two totally different things. On a fork, one is on the left leg and one is on the right leg of the fork. I'll assume you've got the spring part understood.

    There are two types shock movement damping. One for slowing compression=shock getting shorter like on a hit, and one for controlling rebound=shock getting longer after a hit. The two dampers are only working to control velocity of the shock getting longer and shorter. They only react to the pressure of the oil through flowing through the damping circuits.

    If you want to understand the concept of pressure (power) = force x velocity, you need to acquire some basic knowledge of physics.

    But understand that your damper(s) in your shock only react to how fast the piston inside it is moving, and that should be more than enough to help you tune your shock.

    If we didn't have high speed compression circuits, big shock movements would be very harsh because the 'normal'(lsc) circuit would ramp up it's damping so high and so fast that the piston would not want to move hardly at all, resulting in something that felt like you've bottomed out.

    The extra hsc circuit is there to 'open up' the damper to let the piston travel in extreme situations, by letting more oil flow. The hsc adjusts where in the realm of fast piston travel situations that extra oil is allowed to flow. Those situations can vary from hitting a street curb at 15 miles/hr or hucking off a 12ft drop to flat.

  36. #36
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    I totally get the concepts of what LSC and HSC are supposed to achieve.

    What I don't get (and it's admittedly totally beyond the scope of MTBR) is why, in the context of a shock, we can't "reduce" the relationship of shaft velocity vs. damping force to shaft force vs damping force. It's not important that this be explained; I trust what you guys have said about it not being valid. Still, it defies intuition for me. Something is not clicking.

    What I also still don't get is the original graph that two-one posted:

    Looking at that, I feel like the only lines I really care about are the green curve... up until it hits the orange curve, and then the orange curve to the right. Everything else is throwing me off.

    I especially don't see how you could ever achieve the damping force at the shaft velocity that is implied by the green line shooting upward after the intersection with the orange curve. This graph implies that you might have any of two different damping forces at the exact same shaft velocity. Nothing in the diagram explains what criteria would result in which of those two paths either. So, once I exceed the crossover velocity A... am I going to have damping forces dictated by the orange line or the green line (which still extends to the right of A)?

    Next... why does this graph suggest that changes to the LS adjuster results in a significant change of crossover velocity A? No shock or fork I've ever used has allowed me to change A. I get to change the behavior below A and the behavior above A, but not A itself.

  37. #37
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    It is pressure (power) you are dealing with, which is a result of force x velocity. You can't separate force and velocity. Force with no velocity =0 power. Velocity with no force=0 power.

    But to get back to your original post, I agree that there can be some terminology that gets "lost in translation" between the suspension engineers and the tech writers / marketing people as they all try to explain to the end user what the knobs do.

    Case in point: Vivid Air 2014 manual and knob labeling.

  38. #38
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    Quote Originally Posted by calzones View Post
    So you are saying that for any given speed of x m/s, the shock might subject to a totally different force?
    What I was saying in the paragraph that you quoted was that for a given force, the resulting shaft speed depends on the damping (ignoring the spring rate for the sake of discussion of course). Since damping is proportional to shaft speed, the force-vs-force graph is an unnecessarily complicated way to illustrate how damping works.

    You've basically got a feedback loop hidden within the variables that you've chosen, and the thing that you're trying to illustrate is actually in that hidden feedback loop.

    It would be clearer to show damping force on the Y axis and shaft speed on the X axis. That's the way things are normally done... below is one of the first things that comes up in a Google image search for "compression rebound damping graph."

    Name:  dyno5.JPG
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    I suspect that the knee in the green curve at 2.5 in/s shows where a high-speed damping circuit kicked in.

  39. #39
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    Quote Originally Posted by calzones View Post
    Next... why does this graph suggest that changes to the LS adjuster results in a significant change of crossover velocity A? No shock or fork I've ever used has allowed me to change A. I get to change the behavior below A and the behavior above A, but not A itself.
    I strongly suspect, but cannot say for sure, that all (or almost all) of the shocks in the MTB industry are only allowing you to change A, but they're labeled in a misleading way.

    Think about how these circuits would work.... You've got a little orifice through which oil flows at low shaft speeds. You could easily change the effective size of that orifice with a needle valve or something similar.

    And then you've got a bigger orifice through which oil must flow at high shaft speeds, but you need to keep that orifice sealed until some shaft-speed threshold is met. You could easily do that with a ball-and-spring sort of pressure valve. And then you could make the spring pressure adjustable with a screw. And then you could label that screw "high speed damping adjustment" and very few people would notice that what's actually changing is the threshold at which the high-speed circuit opens (the position of the A in the graph) and not the amount of damping provided by the high-speed circuit for a given shaft speed (the slope of the curve to the right of A).

    Edited to add: These Manitou guts would seem to support my theory:
    AIr shock's - why no HSC ?

  40. #40
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    Quote Originally Posted by NWS View Post
    I strongly suspect, but cannot say for sure, that all (or almost all) of the shocks in the MTB industry are only allowing you to change A, but they're labeled in a misleading way.

    Wow... this is a major paradigm shift. But it makes sense. Thanks.

  41. #41
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    "High shaft speed free-bleed blowoff gate threshold" won't fit on the knob. HSSFBBGT isn't very catchy either.

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