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Suspension Tech: How does high speed compression damping differ from low speed?

what is the difference between high speed and low speed compression damping
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In modern mountain bike suspension, compression damping is how you control the rate at which your shock compresses under force (an impact, you pressing down on it, etc.), helping you maintain control of the bike. It also helps limit compression under certain circumstances, usually under light forces, so it’s not bobbing up and down as you pedal.

To do this, there are two distinct damping circuits for compression, one for high speed and one for low speed. In most cases, these circuits control the speed of compression by routing oil through valves and ports or around shims. The design of such things and how to set them up are topics for another post; here we simply want to explain why you need both for proper suspension function. These control the rate at which your fork or shock compresses, either because it hits an obstacle or you’re pressing down on it as you pedal or move the bike under you.

High end forks often give you separate external control of both high and low speed compression damping.
High end forks often give you separate external control of both high and low speed compression damping.

WHAT’S THE DIFFERENCE BETWEEN HIGH & LOW SPEED COMPRESSION DAMPING?

Ace Draper, head technician at suspension tuning shop Manticore Bike, explains it like this: “Separate high and low speed controls typically use the velocity of the main piston (which is what’s pushing the oil through the circuits) to determine where the oil volume can pass the various ports inside the shock. Imagine swinging a tennis racket through the air. Now imagine swinging the same racket submerged in a swimming pool. Now imagine swinging a racket or paddle with no holes in it, submerged in a swimming pool. Many compression controls on the market today essentially restrict or reduce the flow of oil through ports or valves, causing the compression piston to slow down or feel firm upon compression.”

Without this damping, you’d simply blow through your travel, regardless of the type of hit the suspension is taking, and you’d have less control over the bike. There are two different circuits because different types of impacts move through the suspension at very different speeds. From here on, we’ll use “shock” to represent both the shock and fork as the principle is the same for both.

when should I adjust high speed compression damping versus low speed compression damping

WHAT DOES LOW SPEED COMPRESSION DAMPING DO?

Low speed compressions happen when you’re pressing down on the shock, like when you stand up to pedal. Or when you’re riding over gentle rollers, berms or dips in the trail. The shock is simply absorbing your weight against gravity in a slow, controlled manner.

The idea is to support the combined weight of you plus the bike so it’s not bobbing through half the travel as you pedal across smooth, rolling terrain. The low speed compression circuit is like your shock’s first line of defense against unwanted compression, and adding a few clicks helps create more of a platform. The flip side is that reducing the amount of low speed compression can improve small bump sensitivity.

“Recent trends have everyone shopping for shocks with the most sensitive small bump compliance possible. This can be a nice feature for many riders on certain terrain,” says Draper. “However when riding smooth, firm, high speed terrain it can lead to some undesirable sensations – involuntary chatters, washy unsupported feeling, etc. In this scenario many riders will benefit from a few clicks of low speed compression to stabilize the front end at speed and increase sure footed confidence. We also have many customers who wish to have a firm feeling when climbing, which can also be achieved through increasing low speed compression.”

As with most things in life, there are compromises to be made, but it’s also acting as one small part of a bigger system that involves spring rate, air volume, etc., which goes beyond the scope of this article.

WHAT DOES HIGH SPEED COMPRESSION DAMPING DO?

High speed compressions happen when you hit a root, rock or drop off a ledge and the shock has to compress quickly to absorb the abrupt impact. When this happens, the oil flows very quickly and with enough force to bypass the low speed circuit. Most shocks design the low speed circuit to get out of the way, or provide alternate flow paths for the oil to go around it during high speed compressions.

The high speed circuit is also there to control the rate of compression. Without it, you’d be bottoming out your shock on almost every hit. Or, at the very least, it would compress extremely quickly up to the point where your spring rate ramps up, leading to an abrupt change in shock movement that could reduce your control of the bike.

We’ll dive into how you should be setting up your compression damping in the future. In the meantime, Draper gives us this food for thought:

The only real mistake you can make is to buy a shock with both controls and never experiment with them!  Many customers purchase shocks with all possible controls, and immediately want to set them in one magic position for all conditions.

Stay tuned for an explanation of high and low speed rebound damping next week (hint: it’s the same, but different)!


The fun never ends. Stay tuned for a new post each week that explores one small suspension tech, tuning or product topic. Check out past posts here. Got a question you want answered? Email us. Want your brand or product featured? We can do that, too.

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15 Comments
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Jake Terrell
6 years ago

Is this still necessary for normal people who do cycling just for fun?

Tyler
6 years ago
Reply to  Jake Terrell

Aren’t we all just Cycling for fun?

But seriously, yes, because it makes the ride more enjoyable. When your suspension is working at its peak, you can focus more on enjoying the ride rather than controlling the bike.

Tom
Tom
6 years ago
Reply to  Jake Terrell

The smoother it is, the funner it is!

qblambda
qblambda
6 years ago
Reply to  Tom

People only riding for fun usually can’t afford high end dampings.
True story

infinitesmooth
6 years ago
Reply to  qblambda

Greatest BS I’ve every heard. What does riding for fun have to do with your ability to afford high end components?

Wooyek
6 years ago

It’s a common thing to hear that the low speed compression damping can limit small bump sensivity. But aren’t small bumps the same as larger obstacles, hence more dependant on high speed compression? It seems not to make huge difference to shaft speed, whether you hit 2 cm rock or 10 cm rock?

Tyler
6 years ago
Reply to  Wooyek

Sort of. In theory, you’re right. But because the compression circuit works in tandem with air pressure and inherent friction in the system, there’s a minimum threshold that needs to be overcome. The tire takes up some of that, so there’s less total force reaching the form from a 2cm hit, and the low speed’s blowoff/bypass threshold might not be met, and so the fork may not have much of a reaction.

Ol' SHel'
Ol' SHel'
6 years ago
Reply to  Wooyek

Every high-speed piston motion begins as a slow speed one, so when you have more low speed damping, the harshness increases over the choppy stuff.

Steve
Steve
6 years ago

we need more of this kind of articles! Thanks!

Carl
Carl
6 years ago

So, the next article will be about rebound damping, then I assume there will be a discussion on how it all combines on trail? One thing that isn’t clear here and may be a bit misleading is that most people are riding on suspension that only offers one compression and one rebound adjustment. Is this single adjustment going to affect high or low speed damping? At which point, is there even high speed damping?

Mr. P
6 years ago
Reply to  Carl

If there is “low speed damping”, then there is “high speed damping” no matter what. It is just part of the damping. What form that takes is the question.

If one talks about a “high speed damping circuit”, that means high speed damping will have a separate flow to control the oil, adjustable or non-adjustable (no knob).

I’m pretty sure adjusting the low speed damping will always effect the high speed damping.

JBikes
JBikes
6 years ago
Reply to  Mr. P

Technically correct as LSC remains active during HSC blow-off, although I honestly don’t know how much one could detect as the the HSC bypass volume is probably significant compared to the LSC circuit flow.

I would be very surprised if someone could increase HSC in a detectable manner by altering only LSC and not have very detrimental effects on low speed fork compliance

Michael
Michael
6 years ago

Thanks! been looking for something like this for awhile.

Leandro
Leandro
6 years ago

Nice article. I just bought a Fox x2 shock for my Bronson, i’m still dialing it up but it’s driving me crazy. Maybe for the fork, lsc/hsc (which i dont have right now) makes more sense since it’s pretty straight forward. But for the rear i could barely notice it. Especially lsc. Could you enlighten me a little bit, if i set my lsc/hsc to full open (softest) what are the obvious things I should be expecting vs lsc/hsc to the firmest?

Rustam
Rustam
5 years ago
Reply to  Leandro

Not a suspension guru but with the rear shock if its fully open you will have some pedal bob. How bad it is will be dependant on you linkage type/design. As you add lsc clicks you should start getting rid of pedal bob but you also will reduce smal bump complience. The shock will feel very harsh on fully close lsc. I have never experimented with hsc ad i never had one but as far as i understand it it is used when you bottom out a lot. It might be hard to feel the difference at first but as you ride your bike more you will start feeling the adjustments.

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