School me on compression.

armada · May 10, 2011

the second one is wrong

buildyourown · May 10, 2011

Technically, the first def is right. The water has been muddied by companies using the terms in you second def to describe high and low because people were confused, is that clear?

So yeah, high and low speed in respect to the actual speed of the shock shaft, not the bicycle speed.

v6 4.3 · May 10, 2011

those are all ways that people try to explain high/low speed compression but they make much more sense once you know the basics...

first off..
compression = suspension going down. or compressing
rebound = suspension moving back to full travel.

ok now..
Low speed compression = ANY compression where the stanchion(s) is moving at a slow rate or speed, TOTALLY independent from how slow or fast the wheels are moving.

High speed compression = ANY compression where the stanchion(s) is moving at a fast rate or speed. TOTALLY independent from how slow or fast the wheels are moving.

Now, with the above said..
low speed compression is usually seen during climbing and also when hammering a super high gear at high speed. Both of these also known as pedal bob.

high speed compression is usually seen when going through a technical section and going over rock to rock at different wheel speeds and different rock heights.

In a perfect world low/high speed compression could be explained as the following: "low speed comps are rider induced and high speed comps are wheel induced." But this turn out to not be so true because when you bunny hop or pull up on the bars hard, your initial push down can be hard enough to go into high speed compression.

parking lot example:
If you have your fork's low speed compression dialed to FULL ON and the high speed to FULL OFF: then when you slowly bob or lean on your fork it should be rather stiff(lowpseed). but if you lift up your front wheel and slam the wheel and push the bars down together, it sink right down and be plush(highspeed).

if the high speed was dialed at FULL ON then it would still be stiff even if you slam the bars down quick.

All of the low/high speed settings should be set after you find a good psi or spring rate for yourself. Although you could adjust the psi or spring rate to a lower rate afterwards if you were interested in using the shocks high/low speed compression settings for increased damping control but it can get very finicky.

v6 4.3 · May 10, 2011

Freeridin' said:
High speed comp is your lower travel/ ending stroke
Low speed comp is your high travel/ beginning stroke

This is wrong but partially true because low speed travel usually happens when you have full travel (beginning stroke) and your just pedaling super hard and the bike is bobbing.
and high speed is blowing down your travel hard to soak up hits.

It doesn't matter what point in the travel you are in. The only setting that has control over a certin point in your travel is bottom out control where the last bit of travel can be set to be really stiff to avoid bottoming out super hard

Udi · May 10, 2011

v6 4.3 said:
parking lot example:
If you have your fork's low speed compression dialed to FULL ON and the high speed to FULL OFF: then when you slowly bob or lean on your fork it should be rather stiff(lowpseed). but if you lift up your front wheel and slam the wheel and push the bars down together, it sink right down and be plush(highspeed).

Not actually true in practise (in any LSC/HSC adjustable product I've tried anyway) because the HSC adjuster usually preloads a spring that preloads a shim or light shimstack. With no preload, there is usually very little damping, even if the low speed is closed off - because the HS circuit opens up far too easily (with the lack of preload).

So I think in reality it is useful to have the high speed wound in a reasonable amount before trying to set the low speed, and then backing it off if spiking occurs.

I know you were just giving an example but it might be confusing if someone actually uses it to tune.

v6 4.3 · May 10, 2011

Udi said:
So I think in reality it is useful to have the high speed wound in a reasonable amount before trying to set the low speed, and then backing it off if spiking occurs.

I know you were just giving an example but it might be confusing if someone actually uses it to tune.

Yeah, that's defiantly good to know for tuning. All products take getting used to and work differently, for sure.

Did you find that on an air of spring fork, Udi?

Udi · May 10, 2011

Doesn't matter if it's air or coil (no direct effect on working of compression damper).

Pslide · May 10, 2011

Udi said:
Not actually true in practise (in any LSC/HSC adjustable product I've tried anyway) because the HSC adjuster usually preloads a spring that preloads a shim or light shimstack. With no preload, there is usually very little damping, even if the low speed is closed off - because the HS circuit opens up far too easily (with the lack of preload).

So I think in reality it is useful to have the high speed wound in a reasonable amount before trying to set the low speed, and then backing it off if spiking occurs.

Thanks for this tip. I have been running my 2010 Boxxer WC with quite a bit of LSC and almost zero HSC. I'd still like more LSC, so I might bump up the HSC a few clicks.

BTW, I did experience this very thing with the older CCDB. Just didn't think of it applying to the fork as well.

jonKranked · May 10, 2011

Udi said:
Not actually true in practise (in any LSC/HSC adjustable product I've tried anyway) because the HSC adjuster usually preloads a spring that preloads a shim or light shimstack. With no preload, there is usually very little damping, even if the low speed is closed off - because the HS circuit opens up far too easily (with the lack of preload).

Correct me if I'm wrong, but my understanding is that in general, your HSC adjuster only controls the point at which (or, the amount of force required) the high speed portion of the compression damping will open up. And to actually change the high speed compression damping, you'd have to open it up and adjust the shim stack.

slyfink · May 10, 2011

Great thread... I've been playing with and toodling with my HSC/LSC dials on my 36 TALAS for the past 5 rides, and only just started to figure this out.

Anyways, Udi, could you describe/explain "spiking"? And I too would really like to know the answer to JohnKranked's question. I'm not finding that adjusting the HSC is doing much for controlling the depth of travel or the speed of the stanchion moving...

Udi · May 10, 2011

jonKranked said:
Correct me if I'm wrong, but my understanding is that in general, your HSC adjuster only controls the point at which (or, the amount of force required) the high speed portion of the compression damping will open up. And to actually change the high speed compression damping, you'd have to open it up and adjust the shim stack.

Yes, although it's important to note than the more force that's required to open the HS valve, the more damping you'll have across the board. If you just had a ported damper, the damping curve would be quite progressive, and only when the HS valve starts to open will that curve start to regress. So if the curve starts regressing later (via more HS adjuster clicks / more stack preload), the progressive curve will be followed for longer (more damping force) and the regression in the curve will start later (more damping force).

This pic (stolen from PVD) makes it easy to visualise - curve A is a ported-only damper, and curve B (in stage 2) shows the HS circuit opening.

This one (also stolen from PVD) shows how increasing stack preload (1-5) increases damping.

slyfink said:
could you describe/explain "spiking"? And I too would really like to know the answer to JohnKranked's question. I'm not finding that adjusting the HSC is doing much for controlling the depth of travel or the speed of the stanchion moving...

Spiking is basically when the damper can't flow enough oil to successfully absorb a bump. I'm sure this is a subjective term since we'd all have different ideas on what is considered satisfactory absorption of a bump (eg. an old 888RC would do a great job of that, but offer very little chassis stability), but excessive compression damping is an easy way to make dampers spike - which is why a lot of companies limit how much of it is on tap.

If your suspension followed curve A in the first image I linked, it would almost definitely suffer from spiking, because in that case, as shaft speed increases, damping increases progressively. Thus when you hit a square edge or some braking bumps (high shaft speed), you'd basically hit a wall of damping, and most of the bump would get transferred to your body - increasing fatigue and decreasing traction.

A high-speed blowoff (via floodgate, shimstack, whatever) helps alleviate this spiking by opening up more port area as shaft speed increases. But since this is still user controlled (via external adjusters, or internal revalving), there's still potential for problems due to setup.

big-ted · May 10, 2011

jonKranked said:
Correct me if I'm wrong, but my understanding is that in general, your HSC adjuster only controls the point at which (or, the amount of force required) the high speed portion of the compression damping will open up. And to actually change the high speed compression damping, you'd have to open it up and adjust the shim stack.

Depends on the damper. Old MoCo dampers definitely worked as you describe, hence the "gate" adjuster being named as such, rather than a HSC adjustment. I find Fox dampers to work very much in this way also, despite the naming. The newer MiCo dampers seem to give be more of a true HSC adjustment, although one still needs to run a few clicks of HSC to get sufficient LSC in my experience.

Huck Banzai · May 10, 2011

low speed path is through port whose size is increased/reduced by adjusting the needle in or out (providing relative resistance).

When forces are strong enough to overwhelm this path, pressure is placed on shims which respond based on stack config allowing alternate path for oil. Speed sensitive adds increased displacement for higher shaft velocities.

Simple, but not.

So if the fork moves fast enough to overwhelm the port, oil is forced through the shim stack - which is far more tuneable than the simple port.

Holiday Inn Express..no doubt.

Udi · May 10, 2011

big-ted said:
The newer MiCo dampers seem to give be more of a true HSC adjustment, although one still needs to run a few clicks of HSC to get sufficient LSC in my experience.

The Mico damper design is no different to the Fox (or most anything else), the HS adjuster just preloads the largest shim on the stack.

It's probably just differences in implementation or stack configuration that makes you feel they're different.

Udi said:
bla bla boring post

I want to add.. when it comes to reality though, the other super important thing to stress here is that most stock suspension products, especially forks, don't really offer a range that causes much concern in terms of spiking.

RS mission control forks for example (including boxxer) are so lightly shimmed that the compression damping does little even if cranked. The Fox 36/40 forks were no different from 05-08 or so, and from 09-11 (noticeably more range), you're still not likely to have issues unless you're almost closed on both adjusters. Obviously the 06-09 boxxers are an exception here (compression all the way to lockout).

Shocks in my opinion are more likely to have issues if you start cranking things right up, because they are designed to cater for multiple leverage ratios, and thus have larger ranges of damping.

Anyway the reason I bring this up is that often I see people blaming the damper for issues that are caused by other things. Incorrect spring rate, excessive chassis stiction (guilty of this one personally), even tyre pressures.. all these aspects will control bump absorption / traction / ride characteristics, and it's easy to forget about them and just blame the damper.

slyfink · May 10, 2011

Udi said:
Anyway the reason I bring this up is that often I see people blaming the damper for issues that are caused by other things. Incorrect spring rate, excessive chassis stiction (guilty of this one personally), even tyre pressures..

This is precisely where I think my problem lies. I think I have too much stiction to achieve proper sag, and so I'm running a lower spring rate than I should, and then trying to control the motion of the damper via compression adjustments. But that just doesn't work (well). Only problem is, I've got maybe 14 hours of riding time on my fork since I last changed the oil and cleaned the wipers. I don't know how to durably get rid of that friggin stiction in my fork. :confused:

thanks for all the info.

ebarker9 · May 10, 2011

slyfink said:
This is precisely where I think my problem lies. I think I have too much stiction to achieve proper sag, and so I'm running a lower spring rate than I should, and then trying to control the motion of the damper via compression adjustments. But that just doesn't work (well). Only problem is, I've got maybe 14 hours of riding time on my fork since I last changed the oil and cleaned the wipers. I don't know how to durably get rid of that friggin stiction in my fork.

thanks for all the info.

Stiction shouldn't really impact the sag measurement as long as you're using the "bounce around and find where the suspension settles" method, which usually requires a helper. If you're just sitting on the bike (or, better, standing) and then going based on what the O-ring or other indicator shows, you probably will be off.

Slightly off-topic, but since we have some folks who are actually knowledgeable about suspension. I always read that preload only impacts the very first part of the stroke, which doesn't make any sense to me. Assuming a generally linear spring rate, don't you effectively add on the difference in force from the preload throughout the full stroke of the shock?

tacubaya · May 10, 2011

ebarker9 said:
Stiction shouldn't really impact the sag measurement as long as you're using the "bounce around and find where the suspension settles" method, which usually requires a helper. If you're just sitting on the bike (or, better, standing) and then going based on what the O-ring or other indicator shows, you probably will be off.

Slightly off-topic, but since we have some folks who are actually knowledgeable about suspension. I always read that preload only impacts the very first part of the stroke, which doesn't make any sense to me. Assuming a generally linear spring rate, don't you effectively add on the difference in force from the preload throughout the full stroke of the shock?

Preload allow forks to resist a higher initial force input (thereby reducing sag) nevertheless it does not modify spring rate.

ebarker9 · May 10, 2011

tacubaya said:
Preload allow forks to resist a higher initial force input (thereby reducing sag) nevertheless it does not modify spring rate.

I understand that the spring rate doesn't change (slope of the line and all), but I would read that chart as showing that the spring exerts ~18kg more force at any given point of the shock's stroke.

Edit: I should mention that the fact that springs are not actually linear, especially as they're compressed to a greater percentage of their un-loaded state, will also have an impact.

tacubaya · May 10, 2011

ebarker9 said:
I understand that the spring rate doesn't change (slope of the line and all), but I would read that chart as showing that the spring exerts ~18kg more force at any given point of the shock's stroke.

Edit: I should mention that the fact that springs are not actually linear, especially as they're compressed to a greater percentage of their un-loaded state, will also have an impact.

Lol yes.

Steve M · May 11, 2011

ebarker9 said:
I understand that the spring rate doesn't change (slope of the line and all), but I would read that chart as showing that the spring exerts ~18kg more force at any given point of the shock's stroke.

Edit: I should mention that the fact that springs are not actually linear, especially as they're compressed to a greater percentage of their un-loaded state, will also have an impact.

You are correct, but the real world result is simply that you ride however much higher in the travel as the preload has added to the spring; in the case of that diagram (assuming this is a fork spring), 35mm of spring preload would simply change your sag point/ride height by 35mm. It'd also make your fork top out like crazy. It is essentially a pretty useless adjustment, the only reason it should be used in my opinion is to make sure there's no slop in the travel (ie that the spring never loses contact with the top cap or the spring collar on a rear shock).

miuan · May 11, 2011

Socket said:
It is essentially a pretty useless adjustment, the only reason it should be used in my opinion is to make sure there's no slop in the travel (ie that the spring never loses contact with the top cap or the spring collar on a rear shock).

especially on older forks where the coil springs are worn and became a little shorter in time.

slyfink · Dec 7, 2011

I've been re-reading this thread as I'm still playing with my fork's settings. The fork is a 2009 36 TALAS RC2 on a 2005 Specialized Enduro. For a while this summer I was running the fork a little undersprung, with lots of low-speed compression (maybe 3 clicks from full-in) and my high speed compression a little less than half way in. The fork felt OK like this, but the bike didn't feel balanced with my rear suspension (which is a Monarch RT3 bought and tuned directly by Push) unless it was in the pro-pedal off position. I felt like the fork was diving in little dips, whereas the shock was riding higher in its travel, so I felt like I was riding the front all the time and needed to really pull back to ride decently. I use this bike entirely for xc/trail riding.

I'm thinking of going back to more spring (higher pressure), backing off the LSC almost all the way, and adding HSC almost all the way. And adding a few clicks of rebound damping.

Last weekend I added about 20 psi to the air spring, backed off a few clicks of LSC (so it was about half way) and put the HSC about half way in also, and the bike felt much more balanced. I feel like I lost a bit of small bump compliance, but the fork wasn't diving in corners and through little dips, which meant I was in a much higher position to pull out of said turns and dips, and therefore carried more speed. Slowing down the rebound seemed to help with traction.

So, my question is whether it would inadvisable to run HSC all the way in and LSC all the way out? I like the more balanced feel, but I'd like to regain a little of the small bump compliance.

Oh, and I'd like to confirm that by running the HSC all the way in, I'm actually adding damping throughout the entire range of the fork (if I'm reading post #12 correctly).

Udi · Dec 7, 2011

First things first - the pre-2011 RC2 dampers don't have a whole lot in the way of compression damping to begin with, and unless you have the HS (compression stack preload) adjuster wound in a fair way, any LS damping you wind on will get blown off too easily to be of much use.

I think Fox dampers work quite well the way you've thought about trying - HS fully closed gives a reasonable amount of mid-speed support without the small bump harshness that excessive low-speed damping causes. I'd close off the HSC like you suggested, and after starting with the LSC all the way out, wind it in a little at a time if you feel like you need more support. On those old dampers I would probably run at least a little on the LS as well, just because the total available damping isn't that much.

If you really want the best out of it (you seem to be into this stuff), I'd consider buying the 2011+ RC2 damper for it (will be the same as 2012 to my knowledge). You can also revalve the existing ones, but IMO it's a waste of time as the old dampers aren't very reliable. The new damper has a lot more compression range available which means you can strike a better balance of what I described above, instead of having to crank things up so much to compensate for the stock valving being too soft.

Anyway, try out your planned settings first and see how it goes. Using the Fox Green oil in the lowers and the new SKF seals together will work wonders for those forks too if you aren't already.

Konabumm · Dec 8, 2011

v6 4.3 said:
those are all ways that people try to explain high/low speed compression but they make much more sense once you know the basics...

first off..
compression = suspension going down. or compressing
rebound = suspension moving back to full travel.

ok now..
Low speed compression = ANY compression where the stanchion(s) is moving at a slow rate or speed, TOTALLY independent from how slow or fast the wheels are moving.

High speed compression = ANY compression where the stanchion(s) is moving at a fast rate or speed. TOTALLY independent from how slow or fast the wheels are moving.

Now, with the above said..
low speed compression is usually seen during climbing and also when hammering a super high gear at high speed. Both of these also known as pedal bob.

high speed compression is usually seen when going through a technical section and going over rock to rock at different wheel speeds and different rock heights.

In a perfect world low/high speed compression could be explained as the following: "low speed comps are rider induced and high speed comps are wheel induced." But this turn out to not be so true because when you bunny hop or pull up on the bars hard, your initial push down can be hard enough to go into high speed compression.

parking lot example:
If you have your fork's low speed compression dialed to FULL ON and the high speed to FULL OFF: then when you slowly bob or lean on your fork it should be rather stiff(lowpseed). but if you lift up your front wheel and slam the wheel and push the bars down together, it sink right down and be plush(highspeed).

if the high speed was dialed at FULL ON then it would still be stiff even if you slam the bars down quick.

All of the low/high speed settings should be set after you find a good psi or spring rate for yourself. Although you could adjust the psi or spring rate to a lower rate afterwards if you were interested in using the shocks high/low speed compression settings for increased damping control but it can get very finicky.

this is a great explanation! Thanks

slyfink · Dec 8, 2011

Udi said:
great information and great explanation....

THANK YOU! I'm glad to be getting an indication that I'm understanding this... at least a bit. And that information is super helpful.

It's funny you should suggest I get a new damper assembly, caus' at the end of 2010, I installed the RC2 assembly in my R fork. If next summer I'm still not happy with the damping range, I'll try and find a newer RC2 assembly from 2011 or newer. I just received some of the new SKF seals last week, but that will probably be an over-the-winter job. I think my riding season is winding down quickly.

thanks again!

School me on compression.

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