Resources

Terms and Definitions

 

Damping

Controls the speed at which the fork moves, converting kinetic energy of the wheel into heat through friction. This provides the rider with increased comfort, control, and safety through rough sections of trail. Damping is usually accomplished by forcing oil through a small hole called an orifice or a port. Occasionally air is used as the damping medium, and on some very basic forks, sliding friction between the upper and lower tubes is used. Without damping, all the energy of impacting a bump would compress the shock very quickly and then shoot the fork open again (rebound) just as fast. In general, the more air pressure (or the higher the spring rate), the more damping will be needed to control the shock properly.

 

Many forks offer external damping adjuster knobs, while others can be adjusted internally. Internal adjustments include changing the oil weight/viscosity (higher weight/thicker oil will produce slower motion), changing orifice size, or changing the shim stack of the damper piston. In most cases turning external knobs clockwise increases the amount of damping. More damping results in a slower motion of the fork. Adjustments can be made to tune the damping so that it provides the best possible performance for a rider's weight and riding style. If in doubt, start with the adjustment somewhere near the middle and make small adjustments until you find a setting that feels good. Many adjuster knobs have detents which can be counted (ex: 5 clicks in from full counter-clockwise).

 

Rebound Damping

Controls the speed at which the fork opens or rebounds after it has hit an obstacle. Too little rebound damping (too fast) and the fork will open too quickly, possibly bouncing the wheel off the ground, throwing the rider off balance, or providing poor traction. Too much rebound damping (too slow) and the fork will not open fast enough to respond to the next impact and will give a harsh ride. It will not have reached full extension before the next bump and will move further and further into the travel until it gets to the end and has packed up. Try adjusting the rebound knob so that the fork is as fast as possible without feeling uncontrolled. Alternatively, have a friend watch you ride off a curb, seated, and adjust the rebound so that the fork bounces exactly once.

 

Compression Damping

Controls the speed at which the fork collapses or compresses as it encounters an obstacle. Too little compression damping (fork moves too fast) and the fork will go through all its travel on smaller sized bumps and bottom out. Too much compression damping and the fork will feel harsh and will not achieve full travel. Some forks offer this as an external adjustment while others are pre-set from the factory. Those forks without external adjusters can usually be adjusted internally if necessary. Additionally, many of the forks on the market intended for more aggressive riding will have two external compression adjustments: high and low speed.

 

High-Speed Compression Damping

Controls the motion of the fork during high shaft velocities such as large impacts or sharp/sudden impacts. This adjustment can be used to reduce bottom out (higher/slower setting) or reduce spiking during sudden impacts (lower/faster setting).

 

Low-Speed Compression Damping

Controls the motion of the fork during low shaft velocities such braking and small bumps. This adjustment can be used to reduce brake dive and wallowy feel (higher/slower setting) or make the fork more sensitive to small bumps and track better in loose conditions (lower/faster setting)

 

Lockout

The ability to turn a shock off or make it inactive. Typically controlled by the compression damper. Oil is prevented from flowing by blocking the compression valves. This setting is usually accomplished by turning a dial or moving a lever. Useful for riding on the road, especially during climbing, when the motion of the shock wastes energy. Most modern lockout systems have a blow-off valve to prevent damage to the damper when a large impact is encountered while in lockout mode. The fork will temporarily break free to absorb the bump, then return to the rigid lockout state.

 

Blow-off/Lockout Threshold

The amount of force required to bypass the lockout circuit. Some forks with lockout offer the user the ability to adjust the amount of force or size of bump required to activate the blow-off valve. This adjustment usually covers a range from very firm to almost no lockout at all. Some riders find it useful, or practical, to leave the fork in lockout mode at all times and adjust the blow-off threshold to correspond to the maximum size of bump that they feel will be uncomfortable. This will provide a firm, bounce-less setup for aggressive, out of the saddle efforts at the expense of some small bump absorption - used mainly by competitive riders. This setup can be thought of as a pedaling platform.

 

Inertia Valve

Lockout system that is controlled by the trail instead of the rider. The shock has a piece of metal blocking the compression port which prevents oil flow and keeps the fork locked out. When a bump is encountered, the metal piece is knocked out of the way, allowing oil to flow and the shock to absorb the bump. After the bump has been absorbed, a spring pushes the metal piece back into place to block oil flow again. Adjusting the bump threshold knob controls how big of a bump is required to move the metal piece and start oil flowing. With an inertia valve, the shock has a firm lockout feel, yet absorbs bumps to provide a smooth ride. An inertia valve fork will provide a more comfortable ride compared to a locked out fork set up with a low to medium threshold setting while remaining bob-free under pedaling.

 

Pedaling Platform/Platform Valving

A form of low-speed compression damping used to limit unwanted movement/bouncing of the shock during pedaling. Can be thought of as an efficiency or stability mode. Pedaling platforms reduce the amount of oil that can flow through the compression circuit, which makes it more difficult for the shock to compress and reduce bouncing. When the shock hits a bump, the platform setting is bypassed, oil is allowed to flow as usual, and the shock can absorb the bump. It then returns to the stable mode. It does take a certain sized bump to blow off the platform valve, so small bump sensitivity is affected somewhat. Different riders will notice this change to different degrees. Try riding with and without the platform or with the platform at various settings to find which works best for you. Some riders never turn the platform on, some never turn it off, while others climb with it on and descend with it off. Platforms are set in a variety of ways from different manufacturers - twisting a knob, adjusting air pressure, or flipping a lever. Check you shock's manual to find out whether your shock has a platform and how to adjust it.

 

Travel & Stroke

Travel refers to the maximum distance the wheels can move as the shocks compress. The more travel a bike has, the larger the bump it can effectively absorb. Wheel travel, which is most commonly discussed (ex: 5" / 125mm travel bike), can be best thought of by imagining that the frame is held in place and the wheels are lifted off the ground, compressing the shocks. When the shocks are fully compressed, the distance the wheels are off the ground is the travel. It is worth noting that while a wheel might move 5'' the shock itself may move less. The distance a shock moves is commonly referred to as stroke.

 

With telescoping forks, the fork moves the same distance as the front wheel. In this case the fork's stroke is the same as the front wheel travel. For this reason, suspension forks are often rated by their travel (4" / 100mm fork). With rear shocks or shocks in linkage forks, the travel of the wheel is usually greater than the stroke of the shock (ex: 2.5'' / 63mm of shock stroke produces 6'' / 150 mm of wheel travel). The linkages of the suspension system act as multipliers to allow the wheel to get the desired amount of travel from limited motion of the shock. Rear shocks are rated by their stroke, along with an overall length or eye to eye length (distance between the bolt holes at each end of the shock). When replacing a rear shock, it is important to know both the stroke and eye to eye length. Otherwise, the shock may not fit, you may not get the full amount of travel, or you may get too much travel and damaged the frame.

 

Sag

The amount the shock compresses when the rider sits on the bike, usually referred to in terms of distance or as a percentage of the overall travel (ex: 12mm, ½ ", 20%). Having a shock set up with sag allows the wheels to fall into depressions and maintain contact with the ground. This will maximize the rider's control. Additionally, by having the wheel fall into the depression instead of the whole bike and rider, less momentum is absorbed by the depression and the rider is able to carry more speed more comfortably.

 

Sag is controlled by spring rate or air pressure. For a given bike and rider, a stiffer spring (more air, higher spring rate) will produce less sag, while a softer spring will produce more sag. Increasing sag typically produces a more comfortable ride and the suspension responds better to small bumps. Decreased sag typically makes the bike more firm and can improve acceleration. Too much sag will make the shock bottom out too easily. Cross country bikes are typically set up with 15-20% sag, trail bikes 20-30%, and downhill bikes 25-35%.

 

Springs

Used to open the shock after it has hit a bump and compressed. The spring can be a physical piece of material (metal, rubber, foam) or compressed air. Air springs are used predominantly for cross country and trail riding applications, while metal coil springs are used predominantly for aggressive/gravity riding. Air shocks offer the easiest range of adjustability and minimum weight, while coil shocks offer maximum durability and most responsiveness.

 

Spring Rate

The amount of force required to compress a shock a given distance. In metric units, it is commonly given by Newtons per meter and in the US it is commonly given by pounds per inch. With a linear spring, such as a coil spring, the spring rate remains constant throughout the travel of the shock so that compressing the coil the final few millimeters requires the same additional force as compressing the first few millimeters. With a progressive spring, such as an air spring, the spring rate increases throughout the travel so that it requires more additional force to compress the shock a given amount the farther into the stroke the shock is. The larger the spring rate, the stiffer the spring. Spring rates are selected based on body weight to give the rider the desired amount of sag. Spring rates on air shocks are adjusted by changing air pressure. Spring rates on coil shocks are adjusted by changing the coil springs, which are available in a variety of rates from the manufacturer.

 

Air Springs

Works like a piston with a plunger (piston head) compressing air inside a sealed chamber. The stiffness of the spring is controlled by the air pressure, which is adjusted via a shock pump. Air springs naturally have a progressive spring rate (the more the shock is compressed, the harder it is to compress it further) but shock manufacturers have done much to reduce this and provide a more linear rate. The volume of the air chamber controls how progressive the shock is. More air volume will provide a more linear spring, while less air volume will provide more ramp up at the end of the stroke to reduce bottom out. Air volume adjustments can be made by varying the amount of lubricant oil in the air chamber or by changing adjustable volume air chambers.

 

Coil Springs

A piece of metal wire wound into a coil shape. Coil springs are available in steel and titanium. Coil springs have a very linear spring rate. The stiffness of a coil spring is controlled by the diameter of the wire and its length. Thicker or shorter wires create stiffer springs. Coil springs are selected to give the desired stiffness. They are labeled with a spring rate (in pounds per square inch from US manufacturers) and a stroke length.

 

Spring Preload

The distance a spring is compressed when the shock is unweighted. Equivalently, preload is referred to by the amount of force initially applied to a spring. Mostly used in reference to coil shocks. Spring preload is used to adjust sag on a shock; the more preload the less the sag will be. Increasing preload also increases the amount of force required to start the shock in motion which reduces small bump sensitivity. Large amounts of preload are therefore discouraged. If proper sag can not be achieved with minimal preload, the coil should be swapped. Additionally, large amounts of preload can result in the coils touching each other during compression (coil bind), which can damage the shock. On air shocks with separate positive and negative air chambers, preload can be achieved by reducing negative spring pressure relative to positive pressure. The larger the difference in pressures, the greater the preload will be.

 

Negative Spring

A small spring that tries to compress the shock. Mostly seen in air shocks but also in some coil shocks. The seals in a shock, especially the air seals have a lot of starting friction (stiction) which makes it difficult for the shock to react to small bumps. The negative spring pushes against the main (positive) spring and helps get the shock in motion. An air shock without a negative spring typically has a very harsh feel as it takes a medium to large size bump to get the fork moving. Some manufacturers use a separate air chamber for the negative spring while others use a coil.

 

The negative spring rate (pressure) should never be higher than the positive rate or it will overcome the positive spring, compress the shock until they become even, prevent the shock from fully extending, and limit the amount of travel. Setting the negative spring rate even with the positive rate will allow the shock to start moving on small bumps and give the most sensitive or plush performance. As the negative rate decreases relative to the positive, it will become increasingly difficult to activate the shock and give an increasingly harsh ride.

 

To set up a shock with a negative air spring, always deflate the negative chamber, set the positive pressure to achieve the desired amount of sag, and then set the negative pressure to match the positive pressure. If you feel the fork is too bouncy or too plush, try decreasing the negative pressure in 10 psi increments until you achieve the desired feel. If you set the negative pressure first, the fork will not get full travel.

 

(from Cycle Monkey, http://www.cyclemonkey.com/suspension_101.shtml)

 

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