The following glossary defines common suspension-related terms and concepts as they are used in our course materials and presentations.
This is the virtual path drawn by the wheel axle as the suspension is compressed (ie. when it hits an obstacle). There are certain handling characteristics that are deﬁned by the axle path.
Also called top caps. Besides allowing external adjustments — including compression, rebound, threshold and travel adjustment among others — the top caps seal the top of the stanchions/air springs.
Depending on the fork, these can be for the main or supplementary air springs, the negative air spring or platform damping adjustment valve. Keep clean and check the valve core is tight if you get a leak.
Schrader valve used for adding or removing air from the positive air spring on air shocks. Normally marked with a ‘+’ on it. Keep clean and check valve core is tight if shock starts to leak.
This can be a regular 9mm quick-release, 20mm bolt-through or 15mm bolt-through. Now quick-release bolt-through axles, such as RockShox’s Maxle and Maxle Lite (20mm and 15mm for 2011 forks) and Fox’s QR15 (15mm), are increasingly being seen on cross-country and trail bikes.
A shock is said to “bottom-out” when the suspension reaches its maximum travel and no longer works as a sprung suspension, but a rigid structure.
Simple bumper to stop a harsh clang at full compression.
Brake jack is extension of the rear suspension. It can improve tracking of the wheel, but upsets geometry due to the rear extending and the front compressing, causing the fork to feel harsh.
Brake squat is compression of the rear suspension under braking. This can cause the suspension to feel harsher, but can actually balance the geometry of the bike as both ends compress together. Most bikes tend to squat to some degree.
Synthetic slider guides for smooth telescopic action between upper and lower legs.
Compressing the suspension usually causes the wheel to initially move away from the bottom bracket, causing the chain to lengthen (chain growth) which is taken up by the rear derailleur spring.
Metal coil spring on coil shocks. Can be steel or titanium (for light weight), straight rate or rising/falling rate. Spring weight (in ft/lb) will normally be printed on the side as well as the dimensions.
Dial or lockout lever that controls the compression damping circuit. See ‘Lockout lever’.
The speed at which the suspension goes from uncompressed to fully compressed. See ‘Suspension compression rates’.
An undamped fork or shock would simply take the energy stored in the spring and ﬁre it right back at you. Control is important, and that is achieved by forcing oil through ports and shims inside the stanchions to slow the action down.
Rebound damping is exactly that: as the fork or shock extends from a compression, it slows the rebound and turns the excess energy into heat as the oil is squeezed through the valves. Most forks and shocks will have an adjuster to precisely tune how fast you want them to rebound. Too much damping and the fork/shock will pack down (ie have less travel) over successive hits, too little and it will feel uncontrolled and bouncy.
Compression damping controls the fork/shock as it compresses, allowing it to react proportionately to different sized impacts. Slow-speed damping regulates movement such as brake dive, fork bob and excess compression in berms, while high-speed damping can prevent the fork/shock blowing through the travel (bottoming out) on big hits and drops.
Too much high-speed damping can be a bad thing, though — the oil pressure may build up causing a spike (when the oil can’t get through the ports or shims fast enough), which can be felt as a sharp knock. Sophisticated forks and shocks have shim stacks — thin washers that can bend out of the way — allowing the oil through and the fork/shock to move faster.
Lockout levers prevent the fork or shock moving, sometimes completely but many retain a little bit of travel to help traction. Lockout is useful on smooth climbs or road sections. Blow-off adjusters let you set the force of impact that will knock the lockout off to regain full travel. Not all forks and shocks have all these features, but the very least you need is rebound damping.
Traditionally forks used slotted 9 mm dropouts for quick-release hubs, but with the new 15/20 mm through-axle standards compatible forks have a hole rather than slot to slide the axle into.
6 mm or 8 mm hole at either end of the rear shock. Shocks come in various lengths measured from eyelet to eyelet (eye to eye length).
Falling compression rate
The compression rate speeds up through compression. See ‘Suspension compression rates’.
Extremely important fork end bolts that hold the internal spring/piston rods in place, stop damping oil leaking out, and the whole lower leg assembly falling off. Sometimes called ‘Foot Nuts’.
The part of the frame forward of the seat tube.
Instant centre (IC)
This term is used in suspension theory for linkages — in our case — four-bar linkages. Unlike a single pivot system, a four-bar linkage does not have a constant center of rotation, instead this point is moving.
The instant centre of rotation, or IC, is a virtual point in space: for a split moment in time it can be said that all other points are revolving around the IC for that instant. The IC migrates as the bike moves through its travel, and will be at a different spot at different points in the travel.
We can calculate the IC of the linkage at a given travel position by plotting the imaginary intersection of the two outermost linkage bars.
This is the ratio of wheel travel to the shock compression. A 2:1 ratio with 4 inch wheel travel at the axle gives 2 inches of shock shaft travel. With many designs the ﬁgures vary through the travel. Values between 2:1 and 3:1 are used currently. Higher leverage ratios equal higher stress on the shock and less sensitivity to small bumps.
Linear compression rate
The compression rate remains constant through compression. See ‘Suspension compression rates’.
A small part between two sets of bushings or bearings.
An external control that stiﬂes oil and air ﬂow through the compression or rebound circuits to lock a fork rigid — handy for climbing or spinning on the ﬂat. Most have a blow-off (sometimes adjustable) valve to stop unexpected hits snapping your wrists.
Lockout / low-speed compression
A damping circuit that resists compression from low-speed inputs like pedalling forces.
Helper spring that opposes the main spring to help overcome seal resistance.
Rubber ring on shaft showing the ‘tidemark’. Helps accurate sag set-up by showing how much travel is being used when sitting on the bike.
Extra damping or air spring expansion chamber mounted parallel to the main body. Used mostly on all-mountain and downhill long-travel air shocks.
A threaded collar found only on coil shocks that can be used to add additional load to increase the initial spring resistance of coil and elastomer forks for a stiffer start. If you’re using more than 2.5 turns from contact, consider the next spring weight up.
Bob is the feeling of bobbing the rider experiences due to the pedaling/drivetrain inﬂuence on the rear suspension, and how our mass reacts when we accelerate. It’s exacerbated by a choppy pedaling style and dynamic rider weight shifts. Certain pivot locations use the chain tension and drive forces to counteract the tendency to bob.
Pedal kickback occurs when the rear axle moves further away from the bottom bracket. The top run of chain wants to get longer (chain growth) so something has to give: the tension pulls at the cranks as if trying to turn them backwards.
You can feel this through the pedals and it is magniﬁed in certain gear combinations. For many people this is an undesirable effect and the simple solution would be to put the pivot very close to the bottom bracket. This does reduce the chain growth, but introduces another factor: Pedal-bob.
Anywhere two parts are connected by a rotating bushing or bearing.
Platform shocks are the saviors of certain single pivot and ﬂoating pivot designs that have a natural tendency to bob, ie. those with pivots near to the bottom bracket. These shocks use slow-speed built-in compression damping to overcome the very active nature of these designs. They work at the expense of some small-bump sensitivity. Other shocks can be fully locked out so they don’t move at all (for steady climbing), while some platform shocks — such as the Fox RP23 — have three-stage platform settings (light, medium and full) so you can tailor their feel to suit.
Progressive or Rising compression rate
The compression rate slows down through compression. See ‘Suspension compression rates’.
The part of the frame rearward of the seat tube.
External adjuster for the rebound damping circuit that controls the rate at which the fork/shock extends after compression. May be on the top of the fork leg or at the base and is normally red. On a rear shock, it is normally colored red, but can sometimes be blue. Increasing rebound damping slows down the speed at which the fork leg or shock returns after each hit; decreasing rebound damping speeds it up.
The suspension travel caused by the weight of the rider sitting on the bike when it is stationary. You usually set this to 20—30 percent of the available travel.
Multiple wipers and lubricating sponges vital for keeping insides in and dirt out. Clean and check for damage often.
Wiper seals designed to keep the insides in and the outsides out. Check for any splits, embedded grit or other damage before it scars the shaft, and clean regularly if the shock can be dismantled (most air shocks can).
The moving part of the rear shock made of cast magnesium or carbon ﬁbre. Big generally means stiff (freeride/aggressive trail), skinnier generally means lighter/more ﬂexible (cross-country).
A device used to absorb impact.
Normally just a sandwich of alloy spacers but can be a spherical rose joint to reduce sideways stress. Check for wear/rattle regularly and replace immediately if this occurs.
Air-tight can that contains compressed air acting as a spring. Some are adjustable to change travel or internal chamber dimensions, and therefore the compression rate.
The moving part of the fork made of cast magnesium or carbon ﬁbre. Big generally means stiff (on freeride/aggressive trail forks), skinnier generally means lighter/more ﬂexible (cross-country forks).
Provides the basic up and down motion. The spring stores the energy created when the shock or fork compresses. In cross-country forks it’s usually compressed air or it may be a metal coil.
Air springs (light and easily adjustable) can be precisely adjusted to the rider’s weight and are lightweight, but air sprung forks can suffer from stiction (when the stanchions don’t slide smoothly in the lower legs because of ﬂex in the fork) and resistance from increased sealing.
Coil springs (ultra smooth and reliable) have an adjustable preload to set the ride height and sag, but the adjustment window is usually very narrow and often a change of spring is needed to get the desired results.
Sprung weight is everything on your bike that doesn’t move (in theory at least) when your bike hits a bump. Stick your full suspension bike in a good bike stand, move the back wheel up and down and observe everything that doesn’t move — handlebar, stem, main frame, seat post and seat, cranks (probably), pedals etc. Reducing your bikes sprung weight allows it to go uphill faster.
Squat refers to how the rear end of a suspension bike sinks under acceleration in response to rider pedaling.
Slippery upper leg so the lower leg can slide smoothly over it. Occasionally steel, mostly aluminium and ranging from 28—40mm diameter, increasing in stiffness and strength as width increases. Watch for scratching or corrosion as this will rapidly ruin seals.
Suspension compression rates
There are three types of compression rates: progressive (also known as Rising), linear and, less commonly, falling. Progressive compression rate suspensions stiffen up at the end of the shock travel; this is a typical cross-country bike setup. Falling compression rate suspensions feels super-plush at the end and are much easier to blow through the travel (ie. bottom-out), while a linear setup feels the same throughout the travel.
The distance covered on the shock shaft as the suspension goes from uncompressed to fully compressed. This movement is measured vertically or on the path of the movement. By default it is interpreted vertically. When the suspension reaches its maximum travel, it is said to have “bottomed-out”.
Fork or shock travel is typically between 80mm and 200mm (3—8 in). More travel allows a softer spring rate and gives the fork/shock more time to deal with an impact, increasing the control over energy absorption. The amount you need depends on the intended use and what your bike is designed to handle, given its geometry. Using an incompatible fork/shock will adversely affect the bike’s handling and lead to invalidated warranties.
Unsprung weight is everything that moves when your bike hits a bump. In most designs it includes the whole chainstays, seatstays, rear wheel, disc brake and caliper, tire, shock absorber, and all suspension linkages and their associated bearings. Reducing unsprung mass has a far more significant effect than simply making your bike lighter, it dramatically improves the performance of your suspension; improving control, comfort, and most importantly speed — uphill and down.
Reducing the weight of all unsprung parts is thus highly desirable. Lighter wheels, rear derailleur, tires, and lighter brakes will all contribute to better suspension performance.
Virtual pivot point
A type of suspension design. The suspension appears to rotate around a point in the frame where there is no real pivot — hence a virtual pivot point.
When we brake, the forces try to rotate the tyre contact patch around the IC, its position determining how braking forces inﬂuence the suspension. Certain positions can compress the suspension (brake squat) or extend it (brake jack).
Braking causes our weight to move forward, extending the shock. So, squat can be useful to maintain even geometry to counterbalance this effect, but it can also make the suspension feel harsh and lose traction, while a net extension may upset the geometry but increases the available traction.
The suspension designer can place the IC in such a way as to give a desirable balance of forces. If you take two designs, they may both have the same virtual pivot point for the rear axle, and the same axle path, but different ICs.
The designer can tune how the suspension behaves under braking independently from how it behaves under acceleration with a four bar. This is not possible with a single pivot bike, as the IC is always where the main pivot is, but a four bar allows the designer further ﬂexibility, and was the original reason the Horst Link was created.
Pedal-bob and anti-squat
Sir Isaac Newton once said “every action has an equal and opposite reaction” and this is true when we step on the pedals and accelerate: the bike moves forward and our weight moves further towards the back of the bike. This constant stop-start effect and weight shifting can compress and extend the shock rhythmically, which we call pedal bob.
There are two solutions to this. One is to introduce compression damping or platform damping/lockouts in the shock to resist the compression; the other is to locate the pivot in such a way that the chain tension and drive forces when pedalling want to extend the suspension.
This balances the tendency for the shock to compress as we pedal and is called anti-squat. 100 percent anti-squat is perfect balance of the forces. Squat itself is how the rear end sinks under acceleration as you pedal. Both methods can be effective, but not without problems.
Extra shock damping and platforms can stiﬂe the suspension performance over smaller bumps, while high levels of anti-squat bring back our friend pedal kickback, due to the pivot position required. Most designers have to trade off pedal kickback and bob with their pivot locations and resulting axle paths.
If four-bar suspension designs behave the same as single pivot designs when pedaling — then — all else being equal, it would make sense to go for the less complicated single pivot design. However, having the shock driven by a linkage, as in a four-bar setup, allows the designer to tune how the shock is compressed through the stroke and its resulting compression rate.
Chain growth and pedal-kickback
As bikes utilize chains in their drivetrain, and it is in connection with the engine (the rider), any changes regarding to the chain feeds back to the rider. Compressing the suspension usually causes the wheel to get farther from the bottom bracket, thus the chain is required to “get longer”. This is caused by the geometry of the suspension linkage, and taken up by some chain-tensioning device (or the rear derailleur). Since most bike have a rear hub with a clutch mechanism, which not allows for free forward rotation, the chain lengthening will cause the hub to turn forward (if it can). So the chain length change can be balanced either by wheel rotation forwards, or the cranks turning backwards or the suspension not moving. In real life, if a rider rides over a bump and the suspension is compressing, he might feel his pedals turning backwards to some extents. Or if he’s strong enough to withstand this, either the wheel will have an “extra” rotation forwards or the suspension will not compress that much, as it would without the chain (or drive train). All of these effects work at the same time in different amounts, degrading suspension performance and rider comfort.
Wheel rotation caused by suspension compression
There is another effect affecting wheel rotation besides chain lengthening. For most designs, suspension compression also makes the wheel contact point with the ground getting more rearwards from the main frame, thus if the wheel is not sliding, the rear wheel will turn backwards. Also, as the suspension is compressed and the wheel’s ground contact point remains the same (imagine a stationery position), it is turning backwards compared to the main frame. These wheel rotations cause pedal-kickback by the clutch mechanism and the tensioned chain — at the ratio of the rear and front cogwheels. Thus you will feel less pedal-kickback in larger gears.
You can check these previous effects (chain growth, pedal kickback, wheel backwards rotation) on your bike as well. Just pull the front brake and compress the rear of your bike. The wheel and the cranks will turn backwards. Of course, this also happens when you ride over bumps — it’s just hard to differentiate this effect from everything else that goes on when you are riding.