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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/ Iím 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/ Iím 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.
    Iím 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 Iíve gathered. I donít 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, Iím 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 ďBoost PressureĒ apparently worked similarly to activate valve resistance for increased high speed damping increasingly in deep travel. I donít 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.
    "Bikes aren't fast--people are fast. Bikes are overpriced. It's an important distinction."---BikeSnob NYC

  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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  16. #16
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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.
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