Tech Help - RBC Glossary
Here at RBC, our research indicates that over 60% of riders with bikes featuring high-end suspension have never had it serviced. We're convinced that one of the reasons is that they don’t really understand suspension when they buy it. Their first priority, rather than spending time browsing technical diagrams, is to go out and ride! When they decide that they need or want to know more, they can be confronted with a wall of suspension ‘one-up manship’ which can be sometimes be deliberately confusing. Often this confusion is made worse by different people using different language to describe the same thing.
In this ‘jargon buster’ we aim to give some clarification of some of the key terminology being used, as well as explaining some of the key principles which if understood can help make riding more pleasurable – this is the reason you buy high performance suspension, after all!
Coil & Air Suspension; mountain bike suspension will feature either a coil or air spring. It is this that supports the rider's weight, and compresses and rebounds when in use on the bike.
Coil Spring; One way in which suspension supports the rider’s weight. Spring suspension is linear, which means it feels the same way throughout the travel – the resistance doesn't change until the unit bottoms out.
Air Spring; The other most common way of supporting the rider's weight. Air springs have progressive travel, which means that it is soft for the first section of travel, then stiffens up the more the suspension is compressed.
Bottom Out; the harsh feeling when you run out of suspension travel, the spring is fully compressed. Can be avoided by having the right suspension, well maintained and correctly set up (air pressure or spring weight).
Top Out; the harsh feeling when the suspension is fully extended at the ‘top’ of the travel. May feel like a knock or a bump as the suspension reaches its maximum extension. Again this can be avoided by having well maintained and correctly set up suspension.
Travel; this is the term used to describe the movement allowed by the suspension. With a fork, the travel will be expressed in terms of the maximum length (inches or mm) that the fork will compress under the rider weight. Shocks have their travel described by how much the shaft can compress. Frames are usually described in the rear-wheel travel length.
Compression; the movement of the spring as it is ‘squeezed’, by riding over bumps, etc.
Rebound; the action of the shock/fork extending having been compressed.
Compression Damping; this controls how quickly the forks are able to be compressed. This is used by the rider in order to keep pedalling efficiency up, and to make sure there is travel left for bigger hits.
Rebound Damping; this controls how fast the unit extends after being compressed. If the rebound damping is set too low, the fork will extend too quickly, resulting in a lack of control and grip. If it is set too high, the suspension will have a tendency to compress and remain compressed over multiple bumps, resulting in a harsh, bumpy ride.
High Speed Compression; this controls how easily the unit is compressed for large, fast movements, like drops or rock gardens.
Mid Speed Compression; this controls the unit once the initial compression damping has been used up.
Low Speed Compression; this controls how easily the unit is compressed for small movements. You should use it to keep the bike prepared for riding situations like berms, where the suspension is compressed but you might still need a little bit of travel in reserve.
Compression Control; a dial or lever on a fork or shock that enables the rider to adjust the ease with which the suspension will compress. This might be expressed as ‘Clicks from maximum/minimum’ or ‘Turns from maximum/minimum’ referring to the extent to which the compression control lever is rotated from one or other position. ‘Maximum’ will be the point at which the fork is very difficult to compress in all situations.
Rebound; the movement of the spring as it extends having been compressed.
Low Speed Rebound; this controls the unit extending after hitting small bumps ridden at all speeds.
High Speed Rebound; this controls the unit extending after hitting a large obstacle at medium to high speeds.
Rebound Control; a dial or lever on a fork or shock that enables the rider to adjust the ease with which the suspension will rebound. This might be expressed as ‘Clicks from maximum/minimum’ or ‘Turns from maximum/minimum’ referring to the extent to which the rebound control lever is rotated from one or other position.
Sag; this is the amount of travel used up by sitting on the bike when it is at rest, which allows the wheel to drop into bumps to give more grip and a more comfortable, controlled ride. A rider wants sag so that the suspension is active as soon as the bike's in motion rather than only when it is being compressed by bumps. This can be changed by adjusting the preload/ air pressure. Depending on your style, you may use 15 to 25% sag (XC/Trails) or up to 50% sag for downhill or freeride.
Preload; this is how much force the spring is under before you sit on the bike. Increasing preload results in less sag, and vice versa.
Air Assist; this is a means of adding extra preload to a coil spring by adding air to the chamber as well. This is used as a cheap shortcut by some manufacturers to allow one fork to be suitable for all rider weights, in theory. It is also supposed to help ease the fork into its travel.
Platform; this is eliminates the need for a lock out system. Initially, the fork is difficult to compress, which means that pedalling is made more efficient. Big bumps aren't affected, though, as the platform will open up really wide when the bump is big enough, allowing you to use all your travel.
SPV; stands for Stable Platform Valve. This is a pneumatic piston which closes the oil ports during pedaling, this then opens when you hit bigger stuff. Again, this translates to more efficient pedalling, while still allowing full use of travel.
CVT; Control Valve Technology, same as SPV but named differently.
Lockout; this stops the unit from compressing easily, which makes for more efficient pedalling and a stiffer ride. It is not a substitute for having your travel shortened, and therefore should not be used for dirt jumping.
IFP; this stands for Internal Floating Piston. This piston separates the nitrogen from the oil inside the damper body. The reason for this is to allow the oil to expand when the shaft enters the shock (think of it like getting into a full bath, the water rises as you get in and spills over the top), this is necessary to allow the shock to get full travel. Also, when oil gets hot, it can expand, so having a chamber that can take up this expansion is very important.
SSD; otherwise known as SSV, this stands for Speed Sensitive Damping, and is used by some manufacturers to say that the damping increases at faster speeds. In reality all shocks are like this. At slow speeds oil that isn't under pressure can flow through the holes without force. At faster speeds there is a lot more pressure as the oil tries to force its way through the hole, meaning the fork is harder to compress. This can be likened to putting your finger over the end of a hose, then turning up the pressure.
Linear Rate; this means that the unit's travel feels the same all the way through to bottoming out, and is common in spring suspension systems.
Progressive Rate; this means that the unit stiffens up towards the end of the travel, and is common in air suspension systems.
Falling Rate; this is the opposite of a progressive rate unit – it is difficult to compress at first, then resists less as you use up more travel.
Bottom Out; the harsh feeling when you run out of suspension travel.
Bottom Out Resistance; this increases compression at the very end of the shock stroke. The resistance can come in the form of a hydraulic system, or it can be mechanical, such as a urethane bumper.
Shim Stack; a series of thin shims which regulate the oil flow through ports in a piston. These can be orientated in many ways and can have quite dramatic effects.
Damper Piston; this can come in many guises, but it is the doorway for oil to pass through separate rebound and compression sections. The piston has holes in it, called ports. On either side of the piston you can find shims or a variant of platform valving to slow down oil through these ports.
Shaft; this is the part of the shock that disappears into the main body of the shock under full compression.
Damper Body; this is similar to the shaft but is a term used for air shocks.
Air Can; this is the shroud/case for an air shock which holds the air that you pump in. This can be unscrewed on some shocks to carry out periodic maintenance.
Air Valve; this is the bit you attach your pump to when pressurising the shock unit.
Valve Cap; this is simply the cover for the air valve.
Eyelet; these are the loops at either end of a shock. They hold mount kits, and are part of the system which keeps the shock attached to the frame.
Shock Bush; this is the hula-hoop shaped item pressed into the eyelet of the shock. They are designed to wear instead of the eyelets of the shock, and are therefore replaceable. They may also be called glacial bushes.
Fork Bush; these are cylindrical rings in the lowers which the stanchions sliding smoothly up and down, and in line with each other- if they are too tight, they can make the fork feel sticky , but if they're too loose, the stanchions will have play.
Mount Kit; this is the assembly that presses into the shock eyelet. They can come in a huge variety of dimensions to allow the fitment of any shock to any frame.
Heavy Duty Mount Kit; this is a variant of mount kit and the only kind RBC use or sell. They consist of a stainless steel axle, with a polymer bushing to reduce friction on the eyelet. This eyelet is surrounded by two rubber sealing rings to keep out dirt and grime. Aluminium spacers are placed on either end of the axle to keep everything tightly fitted. These kits spread the force of the shock over a greater area, therefore increasing durability.
Reducers; these are the aluminium ‘top hat’ spacers that press into the bushes on a standard shock mount kit. These are generally weak and tend to wear out fast.
High Volume Air Can; air shocks can ramp up excessively toward the end of the stroke as the air can runs out of volume. The result is a lack of travel and a harsh feeling at the end of the stroke. A high volume can literally has larger volume to squeeze a bit more travel out at the end. Side effects are less support mid-way through the stroke as the air wallows in a now much larger can.
Nitrogen Charge; this is used to fill a separate chamber to allow the oil chamber to expand when the shaft enters the shock displacing the oil. Nitrogen is used because it has no moisture, unlike the air we breathe or pump into our shocks. (Moisture contaminates oil and the two should be kept separate) Another advantage is that nitrogen will not expand when heated up; if it did, it would restrict travel and give the shock a gapped feeling.
CSU; stands for Crown/ Steerer/ Upper. The upper assembly on most forks, these are usually bonded together so individual parts can't be replaced.
Lowers; these are the lower assembly of the forks. They are thicker than the stanchions (upper legs), and are usually covered in manufacturer's stickers.
Stanchions; these are the upper legs which slide into the lowers. These are anodised for a harder wearing finish but can wear out if the fork is not correctly maintained. If the stanchion is allowed to wear, the fork will perform poorly as oil will be allowed out, and water and dirt will get in.
Steerer Tube; this is the long tube which extends from the top of the forks. It goes through the headset and the stem is clamped to it.
Triple Clamp Forks; these are downhill forks that are braced above and below the headset, allowing greater strength at the cost of increased weight.
Single Crown Forks; these are more common than triple crown forks – typically, they offer less travel, but are lighter. They can be used for almost all applications in one form or another, depending on their design, amount of travel etc.
Bolt-Through Axle; this is a type of fixture to secure the wheel to the lowers. 20mm axles are common for this type of fixture, and usually have pinch bolts or a wedge system to stop the axle rotating. They are stiffer than quick release skewers, but take more time to attach or detach the wheel.
Quick-Release Axle; this is the other most common method of wheel attachment. The common sizes here are 15mm and 9mm. Advantages include easy tightening and quick wheel removal, but it's important to make sure they're done up properly every ride.
Shock Reservoir; the shock reservoir is either fixed onto the shock, or connected to it with a hose. It holds oil and a gas (usually nitrogen or air), separated by a piston.
Piggyback; this refers to the attachment of a reservoir to a shock. This gives more oil volume for the size of the shock. Most commonly seen on coil shocks, these are also present on some air shocks, like the Fox DHX Air and the Rockshox Vivid Air.
Travel Indicator O-ring; this is an o-ring on either the stanchion of a fork, or the shock shaft, which can be used to show how much travel you're using - as the suspension compresses, the o-ring is pushed along the stanchion/shaft.
Wiper Seal; this is the usual way of keeping dirt out of the fork lowers. As the forks are compressed, the wiper seal scrapes dirt from the stanchions, stopping it from contaminating the oil and damaging the internals of the fork. If these are allowed to degrade, the lowers will become contaminated with dirt and water.
Oil Seal; these keep the oil where it should be, in the shock/fork. Some forks can be run without these, which reduces stiction, but means the fork must be maintained more frequently.
Foam Ring; these are fitted between the oil seal and wiper. Sometimes there are only wiper seals and foam seals. The foam seals are soaked with oil, so that every time the forks are compressed, the stanchions receive lubrication, minimising wear.
Spiking; this occurs when the forks are compressed too fast for the oil to travel through the pistons and shim stacks. Imagine people walking through a door. One at a time, they can pass through okay, but when they all try to run through at once they jam up the doorway and shake the walls. The name 'spiking' refers to how this feels at the handlebars.
Ramping Up; this occurs towards the end of the stroke and is commonplace in air forks/ shocks. It is a feeling of increased resistance towards the very bottom of the suspension travel. This can also be an engineered bottom out resistance on some units to help with excessive bottom-out situations.
Elastomer Stack; these are plastic springs or bumpers used by manufacturers to cushion the blow of topping/bottoming out suspension.
Fork Brace; this is the curved arch which connects the lower legs of the fork over the front wheel. It increases the stiffness of the fork.