MTB Suspension Glossary

Start researching suspension bikes and you'll encounter a huge range of terms that get thrown around by marketing heads, reviewers, forum-dwellers and riders, allegedly with the intention of describing and analysing the way a particular bike behaves or feels to ride. For the uninitiated, some of these terms might as well be in a foreign language for all the sense they make.

To make matters even worse there’s an overabundance of words that are either misused, misunderstood, or too vague to actually convey anything tangible anyway. In order to help you see through the often nonsensical jargon and extract meaningful information from someone talking about sagging from eye-to-eye whilst post mounting their damper to a full floater, we’ve compiled this not quite comprehensive list of common terms, as well as a not always serious description of what they do, could, or should mean.

Air Can or Air Chamber: The part of an air-sprung suspension unit into which the air is compressed by the piston. The higher the air volume, the more linear the suspension unit will feel; the smaller the volume, the more progressive it will be.

 

Air Spring: The spring formed when an airtight piston compresses air in a sealed chamber or air can. For everything but dedicated DH bikes air springs have all-but replaced coil springs as they are lighter and the spring rate can be altered by simply adding or releasing air. Air springs are inherently progressive, however by altering the size of the air chamber and/or how far the piston pushes into it, they can be made to feel more or less linear.

Anti-Squat: Not leg extensions at the gym, but rather the ability of a suspension system to resist the tendency to compress when you accelerate forward. Suspension ‘squat’ is caused by the rider’s weight moving rearward as the bike accelerates forward underneath them (because you are not a rigid, fixed part of your bike); this rearward weight shift is generally proportional to the degree of acceleration, so the faster you accelerate, the more your weight shifts rearward, and the more your suspension squats under power. This is one of the main contributing factors to energy wasting, rider-induced ‘pedal bob’, or the tendency for dual suspension bikes to move up and down as you pedal.

Some dual suspension designs overcome this phenomenon by using heavily a damped rear shock that resists compression; however this adversely affects bump absorption, even when the rider isn’t pedalling. There are also shocks that overcome this tendency by using an inertia valve design which has its own pros and cons. Many manufacturers design their bikes so that chain tension from pedalling tries to extend the suspension, and this force resists the squatting effect (hence the term ‘anti-squat’). The biggest benefit of using chain tension to control squatting is that the two opposing forces are proportional, so when you accelerate faster the anti-squat force is also higher. Additionally, this means that the shock doesn’t have to control pedalling forces, and can have a softer compression tune and hence be more supple and absorb impacts better.

The downside is that the anti-squat effect provided by chain tension will vary at different points in the travel, and will also vary with each different chainring/cassette cog combination. Because of this, dual suspension bikes will be optimised for a particular gear combination and percentage sag; the further you go from this ‘balanced’ situation, the more either suspension squat or extension will overpower its opposing force. As chain-induced anti-squat inherently relies on a degree of chain growth, at its most unbalanced this method will result in high levels of pedal kickback with a noticeable stiffening or lockout of the suspension.

Axle Path: Exactly as it sounds, this is the line or path that the axle follows as it moves through its travel. All axle paths are, overall, simple C-shaped curves, with the short-link four bar designs potentially showing the most variation from completely circular, and single pivot designs (linkage driven or otherwise) being absolutely circular.

All rear suspension designs can deliver paths which are either predominantly rearward, vertical, or forward; which of these occurs depends entirely on the precise location of the pivot points. Marketing departments often stretch the truth here;Santa Cruzonce claimed that their VPP axle paths were S-shaped but have since admitted that this was exaggerated, and others use qualifying terms like ‘nearly’ vertical, which could be applied to virtually any bike’s axle path due to the limited variation across all designs.

It is widely accepted that predominantly rearward axle paths offer the best bump absorption, however they also lead to increased chain growth and hence pedal kickback. Forward axle paths limit these two negative effects, but tend to produce less anti-squat when pedalling, and suspension performance tends to suffer, especially on mid-to-large square-edged impacts. Because of the need to balance these two conflicting characteristics, most real-world axle paths tend to fall into a fairly narrow design window, with different manufacturers (and sometimes different bikes within a brand) prioritising one or the other of these performance parameters.

Bearing: The majority of suspension pivots use sealed, radial cartridge ball bearings to allow movement between the links; these are just like the bearings in a skateboard wheel, only (usually) bigger. Some high-end bike designers use double row (i.e. two sets of balls in one casing) bearings, and/or angular contact bearings (where the inner and outer races are at an angle to the plane of rotation) to further increase durability and resistance to lateral forces. Another excellent variation for suspension use is full complement bearings (sometimes calledMAX bearings), where the internal spacing cage is removed to allow more balls to be added, hence decreasing the load on each individual ball. Although not technically ideal for oscillation applications, ball bearings have proven over time to be an effective solution and they usually keep the suspension components moving freely for extended periods. And that’s enough talk about balls…

Bottomless: Whilst nothing at all to do with how you feel after riding a hardtail for three hours on a rutted fire road, this is one of the most overused and potentially meaningless terms in suspension jargon today. Does it mean that the suspension is too firm and therefore all available travel was never used, or full travel was used but without any obvious sense of this occurring? Without a qualifying statement, treat this one with the contempt it deserves.

Bottom Out: Possibly what happens when you ‘step-off’ your bike and rip your knicks or shorts by sliding along the gravel, but more commonly used to describe using all of the available suspension travel. This one also needs some qualification; whilst you don’t want harsh bottom out (in either context), gently bottoming your suspension (feel free to snigger here) is something you should do fairly regularly, otherwise you’re not using all the suspension that your bike has to offer, and which you’ve paid for.

Brake Jack: a) What you shout to your friend Jack when he’s obviously going too fast for the next corner, and will probably take a late exit off the cliff edge; b) A term originally coined to describe the tendency of some early dual suspension designs to extend violently under braking, all too often resulting in a highly unpleasant OTB incident. In modern usage, it usually refers to any highly aberrant suspension behaviour when the brakes are applied, whether extending, compressing, or simply become stiff and ‘locked’.

Although profuse marketing efforts have led many riders to believe that certain types of suspension designs are always ‘fully active’ under brakes, whilst other systems are always prone to brake jack, in truth there is little or no scientific evidence to support this theory. Braking behaviour is inherently linked to a bike’s anti-squat value (more correctly called anti-dive in relation to braking), and any of the suspension systems on the market can, and often do, have the same anti-squat (dive) figures. It’s true that some bikes behave better under brakes than others, but this is a result of the specific details of pivot location and geometry, not the type of suspension system used.

Bushes/Bushing/Bush: a) where you often end up after overcooking a corner; b) an alternative to bearings for suspension pivots, bushes (aka journal bearings) are a thin piece of low-friction material that allows relatively easy movement of the linkage components over its surface. Theoretically these are the ideal solution to oscillating movement, and when executed well they can last for many, many years with virtually no maintenance. Bushes, however, rely on incredibly tight tolerances to work properly, and if things are out of whack (a technical term) they tend to be either very sticky, resulting in poor suspension sensitivity, or very sloppy, potentially resulting in premature wear of other components. For these reasons they are not particularly common on modern mountain bikes; c) let’s just not go there!

Cartridge Damper: A suspension design where the damper oil only controls the suspension performance—it doesn’t ‘double-duty’ as a general lubricant. The oil is usually held within a self-contained damper unit or in a sectioned-off part of the inner leg on a suspension fork.

Centre of Curvature (CC): The point around which the rear axle is rotating at any given instant in time. For any type of single pivot design this point is always located at the main pivot (along with the IC), whilst for Horst Link/short link four-bar designs it changes position as the suspension moves through its travel, although often not by more than 10-20mm over 150mm of suspension travel.

Chain Growth: As your bike’s suspension compresses (unless it has a single pivot that is concentric with the bottom bracket) the rear axle moves slightly away from the bottom bracket, which causes the effective chainstay length to increase and cause ‘chain growth’. Some chain growth is good as it provides anti-squat to improve pedalling efficiency, however too much chain growth will result in unpleasant levels of pedal kickback, and potentially cause suspension lockout. Most current bike designs tend to limit chain growth to 20mm or less; beyond this point it becomes much more difficult to balance these positive and negative effects.

Coil Spring: Just the way it sounds; a metal rod (usually spring steel, but sometimes titanium) wound into a coil shape. Coil springs are inherently linear, and have a fixed spring rate.

Crown: a) The beer that bogans drink to (unsuccessfully) try to look sophisticated; b) the part of your fork that has the steerer tube coming out of the top, and the legs coming out of the bottom. Traditionally, downhill forks have ‘dual crowns’, one above and one below the head tube to help resist the greater forces they are subjected to; trail and XC forks generally have a single crown which sits below the head tube.

Damper: The internal parts of a fork or shock that stop it from bouncing up and down like an uncontrolled pogo stick. They usually work by moving oil through a series of small holes (called ports) in a piston which moves back and forth inside the suspension unit. Top level dampers use flexible washers called ‘shims’ to open the ports more on larger/faster impacts. A damper will typically have a compression circuit that controls how the fork or shock compresses, and a rebound circuit that controls how it extends. This action of the damper is called ‘damping’; anyone talking about ‘dampening’ or a ‘dampener’ has their mind or hand on a garden hose or other watering device.

Eccentric Pivot: Yes, they are weird, but an eccentric pivot is so called because it rotates around a point other than itself. GT’s early I-Drive bikes used an eccentric pivot, Yeti’s new Switch suspension uses an eccentric main pivot, and the prototype Ibis Ripley uses two of them; many dedicated single speed bikes use an eccentric bottom bracket to allow tensioning of the chain, too. Functionally, eccentrics work like super-short suspension links, and give suspension designers another way to skin the proverbial cat (yet another eccentric habit).

Eye-to-Eye length: Nothing whatsoever to do with the width of your facial features; optometrists call that inter-pupillary distance. Eye-to-eye length is the distance between the centres of a rear shock’s two mounting bolts when the shock is fully extended. For a given eye-to-eye length there is often more than one available stroke length. Both eye-to-eye length and stroke length are required to correctly identify the size of a given shock.

Faux Bar: A suspension design which has the main pivot on the front triangle and a rear pivot on the seat stay, with another link pivoting off the front triangle between the seat stay and the shock (think Kona and Merida, for example). These are also called linkage-driven single pivots; purely from an engineering perspective they are still a ‘true’ four-bar link, however in the cycling world the term Faux Bar is used to differentiate this design from those which have a rear pivot on the chainstay. It has been suggested that the term was originally coined by the marketing department of a bike manufacturer who uses a Horst Link design, in order to cast aspersions on competing suspension systems; we’re only reporting what we’ve heard…

Floating Bottom Bracket (FBB): A suspension design where the bottom bracket is neither part of the main frame, nor the rear triangle, but instead ‘floats’ on a separate link between the two. The most common examples include GT’s I-Drive, Mongoose’s Freedrive, Maverick’s Monolink, and Lapierre’s Pendbox. FBBs can have very rearward axle paths without excessive chain growth, however they can potentially suffer from stiffness issues and be heavy.

Fork: The springy bit that joins your front wheel to the front of your bike. Remember that you don’t eat your post-ride pasta with a ‘forks’; one bike has one fork, and the plural only refers to what you’ll find on multiple bikes. After more than ten years, debate (and occasional fisticuffs) still rages over whether Cannondale’s Lefty is actually a fork, or in fact just one-tine.

Full Floater/Floating Suspension: A suspension design in which neither end of the shock is attached to the bike’s main triangle, but instead both ends are attached to moving links (hence the term ‘float’). Contrary to what some may think this design doesn’t reduce stiction; however it does allow suspension engineers to manipulate the leverage ratio curve more than if one end of the shock is fixed. Trek, Commenćal and Pivot (amongst others) all have examples of this design.

Horst Link (aka FSR/Four-bar): A suspension design which has a main pivot on the front triangle and a rear pivot on the chainstay, below the rear axle and inside the rim circumference, with another link pivoting off the front triangle between the seat stay and the shock (think Specialized, Norco, Cube, and numerous others). It is named after Horst Leitner who originally patented the design; when this patent expired, a US patent (FSR) was granted to Specialized, however this second patent is not recognised outside the USA. Rocky Mountain’s ‘Smooth Link’ suspension is very similar to a Horst Link, however the chain stay pivot is slightly above the rear axle, and hence does not infringe on the FSR patent.

Inertia Valve: A type of one-way valve (basically a weighted spring) used in Fox (Terralogic) and Specialized (Brain) suspension and designed to limit pedal bob by only allowing the suspension to activate from upward force (i.e. from the ground). The theory is great, but so far most riders agree that it doesn’t feel quite as plush as traditional designs; not yet, anyway...

Instantaneous Centre (IC): The point around which the imaginary ‘centre’ of the entire rear triangle is theoretically rotating at any given instant in time. For any type of Single Pivot design this point is always located at the main pivot (along with the CC), whilst for Horst Link/Short Link Four-Bar designs it changes position as the suspension moves through its travel. The position of the IC can have a huge effect on pedalling and braking behaviour, and in some cases it moves dramatically throughout the travel in order to fine tune a particular bike’s ride characteristics.

Leverage Ratio: How far a bike’s wheel moves relative to how far the shock moves. Although an overall ratio can be calculated by dividing total wheel travel by shock stroke, for virtually all bikes the leverage ratio will vary at different points in the travel, and plotting the specific ratio at every point in the travel will result in a leverage ratio curve. Suspension designs where the shock moves more for every millimetre of wheel travel (or, thought of a different way, the wheel moves less for every millimetre of shock travel) as the suspension compresses are called progressive designs, those that remain fairly constant are called linear designs, and those where the shock moves less for every millimetre of wheel travel are regressive designs.

Typical leverage ratios will range from about 4:1 to 2:1 (wheel:shock); at higher ratios the travel often feels more plush but the shock has to work harder and will usually require a higher spring rate to control the wheel, whilst lower leverage ratios tend to resist unwanted movement better, but are predisposed to having a stiffer feel.

The wheel rate, or how the suspension actually feels at the wheel, is a combination of both the suspension design and the spring design; for example, a regressive suspension leverage ratio combined with a very linear coil spring may feel regressive, but the same design with a high volume air can may feel quite linear, and with a small air can may actually be progressive.

Linear: A suspension action that feels essentially the same from the beginning of the stroke all the way to the end; pretty self-explanatory, really! This feel is created by the wheel rate.

Link/age: a) A single rigid piece (e.g. a chainstay or seat stay) of a bike’s rear triangle, which is connected via pivots to other linkages to allow the rear wheel to move up and down; b) a bicycle suspension analysis program downloadable from the internet (www.bikechecker.com) which allows riders to plot the suspension design of any bike they want and analyse its characteristics, or choose from hundreds of previously uploaded bikes. Also allows new designs to be entered and uploaded, so you can devise your own perfect bike. Hours of fun to be had!

Lockout: a) A lever, switch or inertia valve which renders a suspension unit immobile, generally by closing the compression circuit in the damper; b) when excessive amounts of anti-squat combined with a concerted pedalling effort make it difficult for the rear suspension to compress upon impacts.

Lowers: Commonly refers to the lower part of a suspension fork that slides on the stanchion tubes. They are usually cast from magnesium to save on weight, so it’s best to keep away from naked flames!

Main Pivot: The largest pivot on a dual suspension bike, located closest to the bottom bracket, which is subjected to the greatest force. Its exact location will have a major impact on suspension behaviour, especially axle path.

Oil Bath/Open Bath: We’re not talking about the freshly-scented stuff you might use for a romantic evening at home, but rather the (generally) small amount of suspension oil that splashes around in the bottom of your fork or in the air can of your shock to keep the seals lubricated and free from stiction. Please don’t mix your two bath oils up, or you’ll wind up with seized suspension and smelling like a motor mechanic’s workshop, both of which are decidedly unpleasant.

Pedal Bob: a) What you shout to your friend Bob when he’s being pursued by an angry Taipan; b) The tendency of dual suspension bikes to move up and down when the rider pedals, particularly evident when out of the saddle. The inconsistent up/down weighting of the pedals is a contributing factor, as is squat. This movement dramatically reduces efficiency and energy transfer to the rear wheel, so suspension designers use firm damping, anti-squat, and inertia valves to try and banish it from our bikes. A good bike is a bob-free bike (sorry, Bob).

Pedal Kickback: Caused by chain growth, this isthe force that the pedals exert on the soles of your feet as the suspension compresses. Pedal kickback is most obvious when pedalling up rougher, square-edged climbs, and at its worst it can become quite unsettling. Small amounts of pedal kickback are inevitable if a suspension design is to have any anti-squat; how much you’re prepared to tolerate is a matter of rider preference.

Pivot: The bearing, bushing, or eccentric about which two links are connected and move.

Plush: Not the sofa company, although plush suspension may be a similar sensation to collapsing on your favourite pile of pillows. Plush-feeling bikes generally use their travel quite easily, especially on small obstacles and trail chatter, and there is often a sense of ‘floating’ over the trail. Too much plushness (is that even a word?) can sometimes lead to inefficient pedalling performance, harsh bottom-out, and feeling disconnected from the terrain.

Post Mount: Where your brake calliper is bolted to two stubby ‘posts’ (duh) on the frame or fork, with the bolt axis in the same plane as the brake rotor. First used for suspension forks, post mount rear brakes are becoming more popular on modern frames as well, due to their lighter weight and higher stiffness than the older style I.S. mounts, where an adaptor bolted to the frame/fork perpendicular to the plane of the brake rotor, and the calliper bolted to the adaptor. Just be careful not to over tighten the mounting bolts, as the posts are an integral part of your fork lowers or frame!

Progressive: Generally, this refers to a suspension action which becomes firmer as it goes deeper into its travel. Progressive bikes tend to be very plush on small to mid-sized impacts, and when well designed will use full travel on large impacts without bottoming out harshly. The soft initial travel can sometimes lead to some pedalling inefficiency, so progressive designs are more commonly seen on longer travel trail and all-mountain bikes than on XC race bikes.

Alternatively, sometimes the word ‘progressive’ is used in a more general context to describe a bike that adopts new technology or geometry before it has become more commonplace.

Ramp Up: Essentially another term for progressive, although often used more specifically to describe a shock which gets substantially firmer towards the very end of its travel.

Regressive: Generally, this refers to a suspension action which becomes more plush as it goes deeper into its travel. Regressive bikes tend to run less sag, pedal extremely well, but bottom out easily on larger impacts. This design is becoming less popular with time.

Sag: Sag is the amount a suspension unit compresses under the static weight of the rider, and it’s a wonderful thing! Sag is absolutely essential to getting the best from your suspension, as it allows the wheels to follow all the dips in the trail surface as well as absorb bumps. Typical sag (measured at the shock/fork) ranges from 10% (for XC types on certain suspension designs) to around 40% for downhill bikes, with typical trail bikes running 20-30% sag to optimise their performance. Some suspension designs require very specific levels of sag to work properly; if you’re unsure, check with the manufacturer.

Shock: a)What your significant other goes into when they discover the cost of your new bike; b) The rear suspension damper and spring unit on your bike. Its performance can make or break how a bike rides, so get a good one and look after it.

Short-Link Four-Bar: A family of suspension designs which use two very short, stiff links to connect a one-piece rear triangle to the main frame. This design allows for the highest degree of manipulation of suspension parameters, and hence there is a wide variation in how different adaptations ride. Short-link four-bars are generally heavier than many other designs, and require a very high degree of accuracy to manufacture. Common examples include Giant’s ‘Maestro’,BMC’s ‘APS’, Santa Cruz/Intense’s ‘VPP’, and several DW-Link variants, amongst others.

Squat: See Anti-Squat.

Stanchion: The inner leg of a suspension fork, usually attached to the crown.

Swingarm/Single Pivot: A suspension design where a single link making up the chainstays and seat stays rotates in a circular path around a single pivot point on the bike’s main triangle. A swingarm single pivot has no other pivot points or linkages, and drives the shock directly from the movement of the rear link; the Santa Cruz Superlight is probably the best-known example of a swingarm single pivot. This design is potentially the lightest and lowest maintenance suspension system possible, however for single pivot designs the exact main pivot location is absolutely crucial, as it must single-handedly balance the often conflicting attributes of axle path, chain growth, pedal kickback and brake jack. See also Faux Bar for a different type of Single Pivot design.

Splash Oil: Thelubricating oil found within an Oil Bath.

Spring: Every suspension unit has one; otherwise you’d bottom out your suspension as soon as you put any weight on it. Traditionally metal coil springs were used, made from either steel or, for the weight-weenie crowd, titanium. These days coil springs are generally reserved for gravity rigs, with most trail riders preferring the lighter weight and easier adjustability of air springs. Every spring has a spring rate; for a coil spring this rate is pre-set and fixed, with major suspension adjustments requiring a new spring, however air springs can be adjusted by simply changing the air pressure.

Spring Rate: How resistant a spring is to being compressed. Higher numbers/pressures mean a stiffer spring. Note that coil springs have an inherently linear spring rate, whilst air springs are inherently progressive.

Stiction: A term used to describe seal friction in a suspension unit. High levels of stiction result in poor sensitivity to small bumps and an overall harsh ride. Stiction is much more of an issue with air-sprung rather than coil-sprung suspension due to the extra seals required in the spring system; travel adjustable suspension is also generally worse. Regularly lubricating external seals and servicing your suspension is your best bet at keeping things stiction-free.

Stroke: How far a particular shock/fork can compress. For a fork this will be the same as the amount of travel available, however for rear suspension it is very different, and is most easily calculated by removing all air from the shock (or removing the spring if it’s a coil shock) and measuring the total change in the shock’s eye-to-eye length from fully extended to fully compressed. If you divide the bike’s total travel by the stroke length you will have calculated the overall leverage ratio of the suspension.

Suspension: Some form of (ideally) controlled spring which ‘suspends’ the rider above the ground and prevents them from feeling every dip, rise, and bump in the trail surface.

Travel: a)How far the wheel can move vertically relative to the bike’s frame; b) what many of us inAustralia have to do in order to ride quality trails.

Unsprung Weight: The parts of your bike between the ground and the springs; think wheels, tyres, brakes, rear swingarms, and even some internal suspension parts. Why care? The lower your bike’s unsprung weight compared to its total weight, the more easily the suspension will work, due to a variety of inertia and momentum factors. Remember, you are part of the sprung weight because there is a spring between you and the ground.

Virtual Pivot Point (VPP): a) When the IC of a bike remains in an essentially fixed location, that point is referred to as a virtual pivot point; this point can be situated outside the physical dimensions of the bike, and increases the engineer’s ability to fine tune suspension characteristics; b) An acronym for Santa Cruz’s patented suspension design; c) misused as a generic term for short-link four-bars, because of the false assumption that they all have a VPP.

Wheel Rate: How the suspension action actually behaves at the wheel. In simple terms, this is a combination of both the linkage design’s geometry and the spring properties. A regressive leverage ratio on the suspension system will feel regressive when combined with a very linear coil spring, but the same design with a high volume air can may feel quite linear, and with a small air can the net result may actually be progressive.