Advances in full-suspension design and technology have meant more and more riders benefiting from the advantages that shock-absorbing rear travel can provide, with effective full-suss bikes and frames now available even at the budget end of the market.
At the heart of any full-suspension frame is the rear shock absorber (shock), which plays a crucial role in getting the best from the bike over rough terrain.
If you are thinking of replacing an old shock for something more up-to-date, or upgrading your current shock to improve performance, shed weight or add adjustability, our guide will help you choose the right one.
If you don’t want to read our in-depth guide on rear shocks, you can go directly to the relevant product pages via the links below:
Rear shocks consist of two telescopic tubes which slide into each other, a spring of some type and an eye at either end through which the whole mechanism is affixed to the frame. A small bushing (a type of simple bearing) in each eye enables the shock to pivot smoothly when the suspension is active.
Every shock will feature some kind of spring mechanism which compresses under force, and all but the most basic will contain some kind of damping system which regulates the rate at which the spring compresses, as well as how it rebounds.
An undamped shock, for example, will tend to shoot through its full travel with an unpleasant “clunk” as it bottoms out (compresses fully), and will bounce back uncontrollably giving a “pogo-stick” effect. Only the most budget shocks will therefore be undamped – perhaps a reason to upgrade if you are looking to improve performance on an entry-level bike.
If you are looking to replace or upgrade your rear shock your choice will largely be determined by two things – sizing (i.e. the stroke and eye-to-eye length of the shock, in order to fit your particular frame) and spring type (air-sprung or coil-sprung).
Both sizing and type are interrelate and depend to some extent on riding discipline – lighter full-suss bikes designed for XC and marathon riding will typically use shorter-travel air shocks, while more burly gravity machines will require longer shocks, sometimes coil-sprung.
Air vs. Coil
Rear shocks are divided into two basic types depending on what kind of spring mechanism they have – air or coil.
• Coil-sprung shocks use a large metal spring – normally steel – positioned outside the telescopic tubes that make up the shock body.
• Air-sprung shocks meanwhile feature a compressed air spring inside the body (“can”) of the shock.
Whether you choose an air or coil shock will largely depend on the type of riding you are doing, as well as on your budget. Coil-sprung shocks are cheaper than air units so they can typically be found on budget bikes – an air-sprung shock, being much lighter is often a desired upgrade for riders looking to shed weight (although even budget XC and trail bikes will now mostly feature basic air shocks).
As well as being lighter, air shocks are also more versatile than coils. Adjusting a coil shock for different rider weights or riding preferences may often involve changing the entire spring for one with a different compression rate (a ‘heavier’ or ‘lighter’ spring). Air shocks can be easily adjusted however by simply increasing or decreasing the air pressure in the shock with a shock pump
However in riding disciplines where light weight is not such an issue – Downhill (DH) racing and Freeride (FR) – high-performance coil shocks still have plenty of fans. The linear compression rate of coil springs – they behave the same way through all stages of being compressed, compared to air springs that can stiffen up in the latter stages of their travel – is a plus for hard-hitting riders who make frequent use of a shock’s full travel.
For high speeds and air time over very rough terrain, coil shocks have traditionally been regarded as smoother, plusher and more “bombproof” than their lightweight cousins. Coil shocks can also shed weight by using a titanium spring instead of steel, but this is expensive.
As shock technology progresses the gap is closing, with air shocks now longer-lived and more reliable than ever under demanding riders – but the price and performance advantages of big-hit coil shocks will ensure they always have a place.
Replacing a shock
When replacing a shock it is essential to make sure that it is compatible with your frame. If you want to simply replace a “blown” shock with the same model, it’s a straight swap, but if you are looking to upgrade there are several elements to take into account.
A shock is measured not by the amount of the bike’s rear travel – which is largely determined by the type of suspension design used on your frame – but by its stroke length and eye-to-eye measurement.
• The stroke length is the amount by which the shock compresses when fully loaded. Again this is not the amount of travel available – a shock with a stroke length of 2” may for example be used on a bike with 4” or 5” of travel, depending on the suspension design.
• The eye-to-eye measurement is the length of the shock from the centre of one mounting eye to the centre of the other.
Also remember to measure the size of the connecting hardware, including the width of the end bushings and the size of the bolts used to attach shock to frame (normally 5mm or 7mm).
As well as ensuring that your replacement shock is a correct fit for your frame, it makes sense to match the type of shock with the type of bike you are on and the style of riding you are doing.
As discussed above, lightweight and easily adjustable air shocks are best suited to XC , trail and Enduro bikes which have to be pedalled uphill, while heavier coil shocks are best matched with hard-hitting DH and FR rigs under demanding riders who appreciate their characteristics.
Some suspension frame designs also exclude the use of ‘piggyback’-type shocks – big-hit coil and air shocks which hold extra damping oil in a separate cylinder attached to the main can – as they can strike the frame, so check before you buy.
Adjusting your rear shock – preload and compression damping
The first step in correctly adjusting a bike’s suspension is to set the sag, the amount by which the rear suspension compresses under a rider’s weight alone. Recommended sag settings vary, but around 10% of travel is a good rule of thumb.
The amount of sag is determined by the amount of preload the spring is under. With air springs, the preload can be easily adjusted by pumping in or releasing air, while coil shocks usually feature an external preload adjuster knob, the winding or unwinding of which compresses or releases the spring.
With coil shocks, adjusting the preload will allow a small amount of fine-tuning for rider weight, but major adjustments (for a significantly heavier or lighter rider) will require a spring change.
As already mentioned, most shocks will also feature some kind of damping mechanism to control the rate of compression and the rate of rebound, with oil generally used as a damping medium inside the shock.
When a spring is active the oil is forced to force to flow through holes in a piston, with the damping rates then controlled by making adjustments to the size of the holes, and therefore the speed that the oil can flow through.
Shocks at the mid- to-upper end of the market will allow riders to adjust the compression and rebound damping, usually with knobs or dials on the top of the shock cylinder.
Platform-damped shocks are probably the biggest development in shock technology since rear suspension was first developed.
Engineers of suspension design have over the years tried to eliminate the phenomenon of “pedal bob”, where a bike’s suspension becomes active under pedaling forces. Put simply – the bike bounces up and down when you pedal, sapping energy and affecting power delivery. This has traditionally been a problem on older suspension designs such as single pivot, but platform-damped shocks have given a new lease of life to such designs.
A platform shock is intended to become active only when a certain impact threshold is reached. This means that it will effectively ignore pedaling forces, but start to work when it hits rough ground.
Many modern platform shocks offer a range of adjustment via a calibrated dial, from ‘fully locked out’ (no compression) to ‘fully open’ (no platform damping) with a selection of intermediate options that offer riders on-the-fly tuning for different terrain types and riding styles.
Can: Cylindrical chamber housing the shock’s air spring.
Bushes: Simple bearings between the shock’s eye and frame mounts that allow the shock to pivot properly. Can become worn and require replacement – stiff bushings will affect the performance and life of the shock.
Stroke length: Maximum movement of the shock from unloaded to fully compressed (not the same as travel length, which is the maximum amount the bike’s rear axle will move when the suspension is fully compressed)
Spring weight: Springs for coil shocks are rated using two numbers, eg 450 X 2.25. The first number is the weight (in pounds) needed to compress the spring by an inch (lighter riders needing lower-rated springs, heavier riders needing higher-rated), and the second is the travel length of the spring (in inches). Again, this is not the same as the travel length of the bike’s suspension, which is determined by the suspension design used.
Damping: The means by which the compression and rebound of the shock is controlled. Compression damping controls how the shock compresses under impact, by forcing oil through series of valves, while rebound damping uses the same method to control how a shock returns to its uncompressed state.
High-speed damping: Prevents a shock ‘blowing through’ its full travel on very big large or high-speed impacts.
Low-speed damping: Controls how much a shock moves in response to minor forces such as pedalling or braking torque.
Platform damping: Platform-damped shocks require a certain amount of load before they start to work. This is to stop unwanted movement from the shock under pedaling/braking forces.
Sag: The amount by which suspension compresses under a rider’s weight alone.
Preload: The adjustment made to the spring to alter the amount of sag.
Your rear shock is an integral part of your bike’s active suspension and parts will wear and require replacing through time and hard riding.
Keep your rear shock running smoothly with our range of replacement parts including bushings, seals, springs, remote lockout kits, valves, adjuster knobs and more.