Base Valve
Base valves (BV) are typically found at the bottom of the fork leg or on a shock’s reservoir. Their primary function is to manage high-speed compression damping. These valves are fixed, and they work in conjunction with the mid-valve (MV), which is attached to a piston on a shaft. Both forks and shocks can incorporate both base valves and mid-valves in their design.
Bottoming Out
Bottoming out occurs when your suspension fully compresses, reaching the end of its travel upon impact. While occasional full compression is acceptable to utilize the suspension’s full range, frequent bottoming out can be detrimental. Repeatedly bottoming out can put excessive stress on the suspension system, potentially leading to damage or failure of components.
Closed Cartridge Dampers
Closed cartridge dampers represent a suspension design where the damping oil is isolated within a sealed cartridge tube. Unlike open bath dampers, the oil in a closed cartridge system does not circulate into the fork leg itself. In this configuration, additional oil is added to the fork leg to lubricate seals and bushings, ensuring smooth and low-friction operation of the suspension components.
Compression Damping
Compression damping is the mechanism that dictates how your bike feels over bumps, influencing its plushness or stiffness. It regulates the speed at which the suspension compresses when encountering an impact. Excessive compression damping (“stiffness”) prevents the suspension from compressing quickly enough to effectively absorb bump forces, leading to a harsh ride. Conversely, insufficient compression damping results in a soft, “mushy” feel, where the suspension compresses too readily through its travel with minimal resistance.
Fork Oil Level
Fork oil level refers to the volume of oil contained within each fork leg. It’s typically measured in cubic centimeters (cc’s) with the fork fully compressed and the spring removed. The oil level is a tuning parameter that affects the amount of air space inside the fork. Since air is compressible and acts as a spring, increasing the oil level reduces the air volume, creating a more progressive spring rate—meaning the suspension becomes firmer deeper into its travel.
Free Sag
Free sag, also known as static sag, is the amount the suspension compresses under the bike’s weight alone, without the rider. While less critical for tuning on modern lightweight mountain bikes compared to older designs, free sag remains a factor to consider. It provides a baseline indication of how the suspension responds to the bike’s inherent weight distribution.
High-Speed Damping
High-speed damping (HSD) controls suspension movements that occur rapidly. This type of damping comes into play during fast, demanding trail sections, such as rough and technical terrain, G-outs (compressions in the trail), and hard landings from jumps or drops. It’s important to note that “high-speed” refers to the velocity of the suspension shaft movement, not the rider’s speed. HSD is optimally controlled by a dedicated high-speed oil circuit, often located within the base valve of the suspension unit.
Low-Speed Damping
Low-speed damping (LSD) manages slower suspension movements, such as those experienced while climbing, navigating slower-paced trails, or riding through rolling whoops. A practical example of low-speed suspension movement is slowly rolling over a large rock and descending its backside. In such scenarios, the suspension compresses fully but at a slower rate, engaging the low-speed compression circuit. Similar to high-speed damping, “low-speed” refers to the suspension shaft speed. Effective LSD is typically achieved through a low-speed oil circuit and/or a shim stack within the damping system.
Mid-Valve
Mid-valves (MV) are positioned on the piston shaft of the damper and are active during the middle portion of the suspension stroke. They play a significant role, especially in long-travel forks, in preventing excessive “diving” or excessive compression too easily through the travel. Mid-valves function as oil flows through them from one side of the damper chamber to the other. Importantly, the rebound valve is also integrated into the mid-valve assembly.
Negative Spring
A negative spring is designed to control the extension of the suspension as it reaches its maximum length. It manages the “top out” feel when the suspension rapidly extends. Negative springs can be implemented using various methods, including air springs, coil springs, rubber bumpers, or combinations of these. They also contribute to a smoother initial compression feel, enhancing small bump sensitivity. Adjustable negative springs offer a wider tuning range to accommodate different rider weights. For instance, heavier riders require stiffer main springs to prevent excessive compression under load, but this increased spring rate might overpower a non-adjustable negative spring, rendering it ineffective at controlling top out.
Open Bath Dampers
Open bath dampers are characterized by a damper cartridge design that is not fully sealed. This allows damping oil to circulate freely between the inside of the damper cartridge and the outer fork leg. In open bath systems, the oil serves multiple roles: damping, lubrication, cooling, and contributing to end-stroke “ramp-up” or progression (increased resistance towards the end of travel). However, a drawback of open bath systems is the added weight associated with the larger volume of oil required to fill the entire fork leg.
Packing
Packing is a suspension issue caused by excessive rebound damping. When encountering a series of consecutive bumps, if the rebound damping is too high, the suspension does not extend quickly enough to prepare for the next impact. This results in the suspension compressing further and further with each bump, leading to a harsh ride feel and reduced traction and control. The wheels lose their ability to effectively track the terrain contours.
Platform Damping
Platform damping refers to a mechanism that provides resistance to initial suspension compression, primarily to counteract unwanted suspension movement induced by pedaling or rider weight shifts. Various techniques can achieve a platform feel. Restricting oil flow through the low-speed compression circuit is generally considered the most effective method for minimizing suspension “bobbing” caused by pedaling forces. Depending on the specific suspension technology, low-speed circuit restriction can be achieved through adjustable pressure springs on the shim stack, bleed needle depth adjustments, or variable-sized orifices controlled by a slider. These are common methods used to create a platform damping characteristic that improves pedaling efficiency.
Rebound Damping
Rebound damping manages the rate at which the suspension extends back to its original position after being compressed by an impact. It is crucial for maintaining tire contact with the ground and ensuring traction. Excessive rebound damping prevents the suspension from extending quickly enough after a bump, causing the wheel to lose contact with the ground on the downside of the bump – this is known as “packing.” Insufficient rebound damping, on the other hand, results in a “bouncy” or “hopping” sensation, also leading to loss of traction and control. Proper rebound damping is as vital as compression damping for optimal suspension performance and requires careful tuning. The rebound valve is optimally located on the mid-valve (MV) and typically utilizes a tapered shim stack to control oil flow. Damping systems that rely solely on orifices or small holes for damping control often lack the precision needed to manage the dynamic demands of high-performance suspension.
Spring Preload
Spring preload is adjusted using a preload ring or collar, which compresses the suspension spring (either on the shock or fork). Preload adjustment effectively changes the spring’s initial length and is used to set the suspension within the correct operating range of its travel. Increasing spring preload raises the bike’s ride height, while decreasing preload lowers it. Preload primarily affects sag and initial ride height, not the spring rate itself.
Spring Type
Mountain bike suspension systems commonly use either coil springs or air springs. Both types effectively resist initial compression under the rider’s weight and are independent of the damping system. The spring rate—the force required to compress the spring—is crucial. A spring rate that is too stiff will result in a harsh and uncontrolled ride feel. Conversely, a spring rate that is too soft will cause the suspension to sag too deeply into its travel, feeling “mushy” and prone to bottoming out easily. Selecting the correct spring rate for each rider’s weight and riding style is essential. Setting the “sag” is the primary method to determine if the spring rate is appropriate.
Stiction
Stiction, or static friction, describes the friction generated when parts rub or slide against each other. In suspension systems, stiction occurs as the stanchion tubes glide against bushings, O-rings, seals, and other components during both compression and rebound strokes. These points of contact create friction, and bending loads can increase stiction, potentially causing the suspension to bind or feel “notchy.” Stiction can significantly increase when seals become dry, leading to a noticeable reduction in small bump sensitivity.
Twin Tube Dampers
Twin tube dampers are designed to combine aspects of both open bath and closed cartridge damper technologies. In a twin tube system, the damper cartridge is housed within an outer tube (hence, “twin tube”). This design allows oil to flow between the cartridge damper and the outer tube, providing increased oil flow and enhanced damping performance without the weight penalty associated with the excessive oil volume required in traditional open bath fork legs.
Valving
Valving refers to the internal mechanical components within a suspension damper that create compression and rebound damping forces. Valving systems are a combination of check valves, orifices, ports, shims, springs, and other elements. The most sophisticated and effective valving designs typically utilize pistons with tapered shim stacks. Shims are thin, high-quality steel washers that, when stacked together, provide a smooth and predictable flex pattern as oil flows around them. Shims offer resistance to oil flow at varying speeds. Fewer shims generally indicate a less refined damping system. Systems with only one or two shims can feel harsh on high-speed impacts. Unfortunately, many suspension designs, even those marketed as high-performance, use a limited number of shims and/or rely on small holes (orifices) to control damping forces. If your damper’s adjustment clickers have a limited effective range or feel ineffective, it is often a sign of an inadequate piston and shim stack design.
