How To Use Low- And High-Speed Compression Adjusters

Ever had a time where you know you needed to make a compression adjustment, but when you went to the knobs and saw `low-speed and high-speed’ you didn’t know which one or way to turn? In this video we talk about the differences between low- and high-speed compression adjustments. While we use Dual Speed Compression (DSC) adjusters on a Jeep in the video below, you will really find this information helpful, regardless if you’re using GRIP 2 on your mountain bike, DSC on your truck, iQS on your sled, X2 on your UTV, or QS3 on your motorcycle.

How is the main piston assembly different from the compression adjuster?

The best way to quickly grasp the answer to this question is to watch “How Shocks Work”. The main piston assembly controls oil flow inside the shock body, whereas the compression adjuster controls displaced oil flow.

During compression, as the shock shaft pushes the main piston assembly through the shock body, oil flowing through the ports and compression valve stack is controlled.

But not all of the oil flows through the main piston assembly during compression; some of it is displaced by the shock shaft’s volume entering the shock body. This displaced oil flow is not controlled by the main piston assembly.

To control the displaced oil flow, base valves are used to meter the flow between the shock body and the reservoir. In most cases, this base valve is your compression adjuster.

How do you describe compression adjustments?

We talk about adjusting damping in two directions: decreasing and increasing. Remember the saying you probably learned as a kid: “Lefty loosey; righty tighty”? It applies to shocks, too.

Decreasing compression damping is when you turn the adjustment knob counter-clockwise. We commonly also refer to this adjustment as “loosening”, “backing out”, “speeding up”, or “opening”. Think “lefty loosey”.

Increasing compression damping is when you turn the adjustment knob clockwise. We commonly also refer to this adjustment as “tightening”, “bringing in”, “slowing down”, or “closing”. Think “righty tighty”.

What does the shock do when I make an adjustment one way or the other?

When you decrease compression damping by turning the adjustment knob counter-clockwise, you allow the shock to compress faster. It feels softer and more supple.

When you increase compression damping by turning the adjustment knob clockwise, you make the shock compress slower. It feels stiffer and more supportive.

What’s the difference between low- and high-speed compression?

Low- and high-speed compression refer to the speed that the shock shaft is compressing into the shock body. While this is often correlated with the speed you’re moving at, it isn’t a causal relationship.

As Mike said in the video at 01:30, even if you’re doing 70 mph on the freeway and you just have a gradual altitude change, that would be a low-speed shaft movement:

Another example of a low-speed shaft movement happening at a high speed is a mountain biker braking to set up for a high-speed bermed corner. Even with the cyclist moving 30 mph, the weight shift of the rider’s mass moving forward to compress the fork is a low-speed shaft movement.

FOX makes a variety of adjusters. What types of compression adjustments do they make?

DSC, X2, GRIP 2 – low-speed and high-speed dials;
FIT4 – low-speed dial with 3-position switch changing oil flow paths that affect high-speed;
GRIP – low-speed dial or 3-position switch;
QS3 – low-speed 3-position switch;
iQS – low-speed electronically actuated 3-position switch.

How do you get oil to flow one way during low-speed events and another way during high-speed events?

Oil will flow through every available valve, but first it flows through the path of least resistance. By engineering certain oil flow paths to have less initial resistance than others, we’re able to create speed sensitive valving: low- and high-speed oil flow paths.

In low-speed scenarios, the oil will flow through a valve that takes minimal force for the oil to open. Adding a low-speed adjuster gives you control over the amount of force it takes for the oil to open the low-speed oil flow path. In many cases, a low-speed adjuster will change the size of a port that the oil can freely flow through.

But as the velocity of the impact increases and the shock encounters a high-speed scenario, the path of least resistance is no longer able to accommodate the increased amount of oil flow on its own. The increased force of the high-speed oil flow opens other valves that allow for increased amounts of oil flow. Adding a high-speed adjuster gives you control over the amount of force it takes for the oil to open the other high-speed oil flow paths. In many cases, a high-speed adjuster will add or remove preload to plate or valve stack, thereby increasing or decreasing the force required to flow through the high-speed circuit.

When and why would I decrease or increase low-speed compression damping?

Here are some examples of low-speed scenarios:

Chassis movements during acceleration, braking, cornering, and pedaling;
Gradual altitude changes: roller coaster type trail or road; or flowing through a pump track;
Small bump sensitivity and washboard-type terrain;
Traction.

Here is when you should consider decreasing low-speed damping:

Ride generally feels harsh and you desire it to feel looser and more comfortable;
Vehicle or bike rides too high in travel; center of gravity isn’t low enough;
Small bumps and washboard-type terrain are harsh and uncomfortable;
Losing traction while cornering;
Losing traction in loose or slippery conditions.

Here is when you should consider increasing low-speed damping:

Ride generally feels too soft and you desire it to feel tighter and more responsive;
Back end excessively squats during acceleration;
Front end excessively dives during braking;
Chassis rolls too much while turning a corner;
Bike bobs up and down too much while pedaling;
Ride feels mushy and uses too much travel during gradual altitude changes;
Front end bounces off of small to medium sized obstacles (like rocks);
Back end bucks off of jumps and other bumps.

Wait a minute. If the back end of my vehicle or bike bucks, that’s a rebound issue; right?

Not necessarily.

It’s easy to assume that the back end bucking off of a jump or obstacle means that the rebound on the rear shock(s) is too fast. However, it could actually also be a couple other scenarios.

First, keeping in line with the rebound train of thought, the rebound on the front shock(s) could be too slow. This gives the perception that the back is too fast. We’ll give more insight into the nuances of rebound damping and adjustments in the next FOX Academy installment.

Second, and most relevant to the conversation today, is that it is likely that low-speed compression damping is too soft and should be increased. Why? Let’s break this down using the jump example:

As you move up the face of the jump, your suspension will slowly compress. This is because moving up the face of a jump is considered to be a “gradual altitude change” where a low-speed chassis compression occurs on your vehicle or bike;
If your shocks do not have enough low-speed compression damping, your suspension will compress more;
As your suspension compresses more, your springs store more energy;
As your springs store more energy, they have more energy to release;
As your rear wheel(s) leaves the jump’s lip, the extra energy stored in the spring is released;
The rear end now bucks; not because rebound is too fast, but because low-speed compression was too soft allowed the spring(s) to store too much energy.

When and why would I decrease or increase high-speed compression damping?

Here are some examples of high-speed scenarios:

Square-edged obstacles: potholes, rocks, roots, etc.;
High-frequency, medium- to high-magnitude impacts like whoops, rocks, or roots;
Bottom-out situations: landing jumps, drops, rock ledges, etc.

Here’s when you should consider decreasing high-speed damping:

You aren’t using full shock travel, even in high-speed scenarios, and you’d like more comfort;
You’re generally going to be driving at lower speeds (remember, high vehicle speed is correlated with high-speed shock movements, but doesn’t necessitate it);
Potholes on the road and other square-edged obstacles are particularly jarring;
Vehicle or bike bounces off of high-frequency, medium- to high-frequency impacts like whoops because the shocks aren’t compressing enough;
Landing jumps and drops feel harsh.

Here’s when you should consider increasing high-speed damping:

You’re using all of your shock travel and bottoming out too often in high-speed scenarios, and you want more comfort and control;
You’re generally going to be driving at higher speeds (remember, high vehicle speed is correlated with high-speed shock movements, but doesn’t necessitate it);
Square edged obstacles use more travel than they should and sometimes even bottom out shocks;
Vehicle or bike bounces off of high-frequency, medium- to high-frequency impacts like whoops because the shocks have packed down and have no more available travel to absorb the next impact.