High and low speed compression

PB Forum :: Mechanics' Lounge
High and low speed compression
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flag srv1
Posted: Oct 27, 2019 at 13:32 Quote
can someone please explain HSC and LSC in laymans terms without all the techy lingo.... Im soooo confused. thanks!!
flag R-M-R
Posted: Oct 27, 2019 at 16:42 Quote
A shock has a spring and a damper. The spring resists load by becoming stiffer as you compress it a greater distance. The damper, which is like a plunger in a tube of oil, resists motion; it provides resistance only when moving, unlike the spring, which provides resistance whenever the amount of compression is greater than zero.

The faster the plunger (it's actually a "piston") moves, the more resistance it provides. Unfortunately, the range of speeds at which your suspension compresses is too much for a simple piston to properly handle, so it has multiple flaps that open and close at different speeds to maintain an appropriate level of force over a wide range of speeds. Note: we're talking about the speed at which the shock is compressing, not the speed at which you're riding, though they're usually related.

Dampers go one step farther and have separate mechanisms that are active during the compression and during the rebound motion of the shock. So we have multiple modes for various speeds and for the direction of motion. The four main modes are:

• High-speed compression damping
• Low-speed compression damping
• High-speed rebound damping
• Low-speed rebound damping

Some shocks allow you to adjust all four separately. Most don't have adjustments for all four modes, but they still have all four modes - they're just not adjustable via dials.

The typical way the damper works is for the low-speed mechanism to engage, then, if the force becomes quite high due to rapid motion, flaps / valves of various sorts open up to reduce the force by allowing oil to flow more freely through the high-speed mechanism. When the motion slows down enough - and, therefore, the force does likewise - these valves close and the fluid returns to flowing through the more restrictive low-speed mechanism.
flag lifted-d
Posted: Oct 27, 2019 at 18:48 Quote
srv1 wrote:
can someone please explain HSC and LSC in laymans terms without all the techy lingo.... Im soooo confused. thanks!!

RMR has a good description going, I would also add. neither of them are any substitute for correct spring rate. they are fine tune adjustments for ONCE your correct spring rate had been determined. (air pressure PSI, or coil rate lb/inch or KG/mm of displacement)
bear in mind air pressures feel vastly different every 5 psi or so, and do take your shock pump off everytime you 'test' your set up, the extra air volume of the pump messes up the progression/spring rate you would feel.
also if your fork lowers havnt had oil changed/serviced lately get it done before too late!

PS I typically run my fork with zero or minimal compression damping at all, the more you add the harsher it will feel because the piston is resisting smooth motion and passing the force onto your hands/pedals.
flag gmoss
Posted: Oct 28, 2019 at 18:58 Quote
To put it simply, think of both as damping adjustment, more in is more damping. High speed refers to a high speed shaft movement, not how fast you going, like a sudden big root you hit or g out. Low speed is like when you are pedaling and the ahaft is moving back and forth. Once you understand that, should be fairly straightforward.
flag Branmuffin
O+
Posted: Oct 30, 2019 at 5:58 Quote
I am curious where the cutoff is for hsc and lsc. I have seen many videos and set up guides contradict each other on what type of features/obstacles should be considered for hsc or lsc.
I realize a chattery section could be mostly using lsc if you are moving at a slower clip, but then does it become more hsc oriented if you are going 30mph? I would think the answer is yes.
Or, is it generally easier to just relate lsc to climbing speeds and hsc to descending? I just took delivery of a Ibis Ripmo with a grip2 36 and Factory X2 so dialing in the front and rear is a work in progress but it’s very interesting to see how much a difference 1 click hsc can make.
flag gmoss
Posted: Oct 30, 2019 at 6:07 Quote
Hsc vs lsc has is less about bike speed and more about shaft movement. You can have high speed shaft movements at slow speeds. I think this is where some confusion lies.
flag bronco5
Posted: Oct 30, 2019 at 7:41 Quote
Branmuffin wrote:
I am curious where the cutoff is for hsc and lsc. I have seen many videos and set up guides contradict each other on what type of features/obstacles should be considered for hsc or lsc.
I realize a chattery section could be mostly using lsc if you are moving at a slower clip, but then does it become more hsc oriented if you are going 30mph? I would think the answer is yes.
Or, is it generally easier to just relate lsc to climbing speeds and hsc to descending? I just took delivery of a Ibis Ripmo with a grip2 36 and Factory X2 so dialing in the front and rear is a work in progress but it’s very interesting to see how much a difference 1 click hsc can make.

I'm on a Ibis HD4 with a X2 / Fox 36. I'm 130-140 lbs. I have found that I like to run a few clicks of LSC (6-7 ish) on both the shock and fork, but run no additional HSC (or HSR). The LSC seems to add a little bit of platform/stability for pumping, pedaling, etc. I originally rode with similar 6-7 clicks of HSC (or whatever Ibis recommends) and the bike rode fine, but I found at my weight that I prefer less HSC especially on chatter, brake bumps, small stuff but that there was still plenty of support on bigger stuff.

I remember reading about some Ripmo riders blowing through travel too quickly when riding the bike aggressively. I would think that's a case for adding HSC.

As far as your question about where the cutoff is between LSC and HSC... I don't think there really is one. All the settings affect each other (including rebound and spring rate). You might be able to get to the same place with different settings. LSC changes are going to be most noticeable at slow shaft speeds (pedaling) but will also affect all shaft speeds. HSC changes will be most noticeable on square edge hits, rock gardens and other fast shaft scenarios, but adding HSC will still have impact on slower shaft speed events.

I think one common thing I have noticed with a lot of friends is that they often run too much (in my opinion) rebound damping. In a parking lot test it makes the bike feel more planted/stable, but when you start pushing the bike through rough sections the suspension can't reset fast enough and the suspension is only effectively using a fraction of its travel.
flag Branmuffin
O+
Posted: Oct 30, 2019 at 10:40 Quote
I am pretty much in the same boat with my Ripmo, blowing through the travel very easily in the rear. I have worked it out mostly by adding a bit of HSC. But still fairly surprised how much travel is being used on smaller hits, small jumps etc. my Jeffsy was very hard to bottom out but the suspension curve is clearly more progressive.
flag gmoss
Posted: Oct 30, 2019 at 11:29 Quote
I was not blowing thru the stroke on my Hightower LT, but I was using more of it more often than I liked. I have been experimenting with the volume reducers and a small change there makes a big difference in the feel of the rear. Something to also consider.
flag R-M-R
Posted: Oct 30, 2019 at 12:28 Quote
Branmuffin wrote:
I am curious where the cutoff is for hsc and lsc. I have seen many videos and set up guides contradict each other on what type of features/obstacles should be considered for hsc or lsc.
I realize a chattery section could be mostly using lsc if you are moving at a slower clip, but then does it become more hsc oriented if you are going 30mph? I would think the answer is yes.
Or, is it generally easier to just relate lsc to climbing speeds and hsc to descending? I just took delivery of a Ibis Ripmo with a grip2 36 and Factory X2 so dialing in the front and rear is a work in progress but it’s very interesting to see how much a difference 1 click hsc can make.

gmoss wrote:
Hsc vs lsc has is less about bike speed and more about shaft movement. You can have high speed shaft movements at slow speeds. I think this is where some confusion lies.

A damper doesn't know how fast you're going; dampers only respond to force, which is a product of shaft speed. For example, a super smooth flow trail is probably one of the faster riding speeds you'll experience, yet much of the suspension motion will be in the LSC range as you pump and compress through rollers and berms. You don't often activate a properly set up HSC circuit when climbing because even a square-edged impact may hot produce enough force to blow open the HSC valving. It should, but it can't, due to not enough mass above the suspension ("sprung mass") to anchor against the damper force. But that's another conversation ...

Think of it this way:

• LSC is for "terrain", like rollers, g-outs, and berms (and lurches due to rapid brake application). It's also the one you feel when pumping, jumping, and "working" the bike.
• HSC is for "roughness", like roots and rocks at normal speeds, bumpy dirt at high speeds, and landing jumps.


Branmuffin wrote:
I am pretty much in the same boat with my Ripmo, blowing through the travel very easily in the rear. I have worked it out mostly by adding a bit of HSC. But still fairly surprised how much travel is being used on smaller hits, small jumps etc. my Jeffsy was very hard to bottom out but the suspension curve is clearly more progressive.

The leverage curve of the Ripmo is flat to slightly digressive from about halfway through the travel to full droop (fully compressed), so it's not surprising you're blowing through the travel. Solutions, in order of priority:

• More progressive spring, i.e. add volume reducers in your shock.
• Less sag than with other bikes. Maybe 25%.
• More HSC damping.
flag gmoss
Posted: Oct 30, 2019 at 13:04 Quote
Susp is a game of opposites, many times. Coming from a dirt bike background and heavy in the susp part of it, understanding it so the bike works as desired, sometimes what you think you should change is not what you should change, and even if you do, in the wrong direction.
flag lifted-d
Posted: Nov 1, 2019 at 12:45 Quote
Branmuffin wrote:
I am pretty much in the same boat with my Ripmo, blowing through the travel very easily in the rear. I have worked it out mostly by adding a bit of HSC. But still fairly surprised how much travel is being used on smaller hits, small jumps etc. my Jeffsy was very hard to bottom out but the suspension curve is clearly more progressive.

this what i'm referring to. open up both lsc/hsc and set your air pressure first so you get good support, bottom out resistance then tune lsc/hsc much later once you have tinkered with air pressures on your best favorite trail with some roots and g outs, drops, jumps.
flag gmoss
Posted: Nov 1, 2019 at 13:16 Quote
Air pressure is your spring. It's job is to hold you and the bike up with proper sag. There is a range thst will work, but opening up the adjustments and playing with spring rate is not the way to go about tuning susp. To have controlled performance, you want the valving to control the movement of the fork/shock. If you run full open, you are bypassing the valving on compression. This can and will make the susp wallowy and harsh. Air pressure needs to be set for weight with proper sag, within range, and then you need to dial in one circuit at a time.

If you want to affect the air spring, I would reduce or increase the volume reducers in the fork instead.
flag R-M-R
Posted: Nov 1, 2019 at 14:37 Quote
lifted-d wrote:
Branmuffin wrote:
I am pretty much in the same boat with my Ripmo, blowing through the travel very easily in the rear. I have worked it out mostly by adding a bit of HSC. But still fairly surprised how much travel is being used on smaller hits, small jumps etc. my Jeffsy was very hard to bottom out but the suspension curve is clearly more progressive.

this what i'm referring to. open up both lsc/hsc and set your air pressure first so you get good support, bottom out resistance then tune lsc/hsc much later once you have tinkered with air pressures on your best favorite trail with some roots and g outs, drops, jumps.

I'm afraid this is incorrect. You do not want sufficient support and bottom-out resistance solely from the spring: a large fraction of the support comes from the damper, so a spring that provides sufficient resistance is far too firm.

This is why the concept of "sag" exists. Set the spring to provide appropriate sag, which varies a little from bike to bike, then tune the support with damping and spring ramp-up, if necessary.
flag lifted-d
Posted: Nov 1, 2019 at 16:36 Quote
R-M-R wrote:
lifted-d wrote:
Branmuffin wrote:
I am pretty much in the same boat with my Ripmo, blowing through the travel very easily in the rear. I have worked it out mostly by adding a bit of HSC. But still fairly surprised how much travel is being used on smaller hits, small jumps etc. my Jeffsy was very hard to bottom out but the suspension curve is clearly more progressive.

this what i'm referring to. open up both lsc/hsc and set your air pressure first so you get good support, bottom out resistance then tune lsc/hsc much later once you have tinkered with air pressures on your best favorite trail with some roots and g outs, drops, jumps.

I'm afraid this is incorrect. You do not want sufficient support and bottom-out resistance solely from the spring: a large fraction of the support comes from the damper, so a spring that provides sufficient resistance is far too firm.

This is why the concept of "sag" exists. Set the spring to provide appropriate sag, which varies a little from bike to bike, then tune the support with damping and spring ramp-up, if necessary.

sag should be in normal range i agree, but without any comp damping coming into play. dampers dont support the load, merely slow/control speed of the damper. don't take my word on it though, they are the words of Bob Fox yes that guy, (paraphrased) from an interview on common suspension set up mistakes that i read years ago. his advice hasn't failed me yet.

if your not convinced try it your way, then Bob"s way
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