How to set up a rear shock

There are three main controls you’ll see on MTB suspension: preload, rebound, and compression. The latter is sometimes split into low- and high-speed compression on top-end shocks.

Preload

Preload is the resistance the fork gives against your weight. So the heavier you are, the more preload you’re going to need. For a shock with a coil spring this would equate to having a heavier, or thicker spring but for an air shock it’s simply a case of pumping in more pressure.

Compression damping

Compression damping comes from the internals of the shock and works by regulating the flow of oil through small holes. Compression damping only affects the shock when it’s compressing – this doesn’t affect the preload but can appear to have a similar effect on the rear suspension.
The more compression damping you dial in (+) the slower the fork will move through its travel. This
is good if you want a bike to pedal without bobbing, but the negative effect will be the limitation of the suspension’s movement when you hit a bump, making it feel a bit like it’s locked out (see below). In fact, suspension lockout out is simply an extremely high amount of compression damping.

Rebound damping

Rebound damping is a similar internal system to compression and only affects the shock when it’s returning to its natural position after an impact.
As you dial in more rebound damping (+) the fork will return to its natural position more slowly after an impact. A slower return – or more rebound compression – is required if the bike feels like it’s trying to buck you off, especially after corners or when you land a jump, but if repeated hits are causing the suspension to feel like it’s ‘packing down’ and not returning to it’s natural position, you’ll need less damping.

Lockout/CTD

Your suspension might feature settings that are designed to give it different characteristics for different types of riding. Lockout is the most common and when activated will use the compression damping system to effectively stop the fork from working.
Lockout is useful when you encounter prolonged climbs or flatter sections where you want to put some power down through the pedals. CTD or climb, trail, descend is a slightly more advanced form of lock out, which gives a ‘tune’ that’s better suited to the type of terrain you are riding on. CTD is a Fox-specific term, but other manufacturers have comparable systems.
Climb mode pretty much acts as a lock out, although typically will allow more movement; trail or ride mode is a slightly stiffened setting to allow movement, but with some resistance in order to give a good pedaling platform with a stiffer compression setting; while descent means the suspension is fully active and would offer little compression damping during use.

How to set up rear suspension

Before you start, make sure you have have your full riding kit to hand – you’ll be setting the shock up to work based on your riding weight.
It’s a good idea to get set up before you ride, but take a shock pump with you so you can make further adjustments on the trail.

1. Set preload

It’s a good idea to have someone to help, but it’s possible to set sag yourself by leaning against a wall or solid object. 
Start by measuring the inner shock shaft – the shiny bit that moves inside the larger outer can. Divide that by four to get your ‘sag’ measurement.
For optimum performance you need 25 percent sag. However, some more aggressive shocks like those found on downhill bikes can be set with up to 30 percent.

Most shocks offer optimum performance with the sag set at 25 percent
Most shocks will have a small rubber ring or foam bump stopper to allow you to measure the amount of sag – if yours hasn’t, you can tie a rubber band around it. Don’t be tempted to use a cable tie and certainly don’t leave it on the shaft; the dirt it collects combined with the hard plastic will scratch the surface, something that won't be cheap to remedy.
Set the compression damping switch to ‘descend’ or its open position, reset the sag ring, gently mount the bike (wearing your riding kit, or at least with it in your backpack) and assume the standing riding position. Try not to bounce the bike – you’re looking for a standing weight.
Carefully dismount and check the sag distance – how far the small rubber ring has moved. If it's more or less than a quarter of the way down the shaft, you need to adjust the psi. For most riders using air shocks, 150 to 200 psi will give 25 percent sag, so if you have no idea what pressure to run then pump in or release air until the gauge shows 150psi. Then, add or subtract 25 to 50psi at a time until you arrive at 25 percent sag.

Start at 150psi and work up or down from there
For coil sprung shocks, you can make minor adjustments using the preload dial, but might need a heavier or lighter spring depending on your weight – these should be available from any good bike shop.

2. Set compression and rebound damping

Depending on your shock model, you’ll have a CTD switch or a compression dial, as well as a rebound damping dial. If you have a CTD then just set it up according to your current terrain; if you have a dial then set it up in a similar way to your rebound damping, as follows:
Start by working out how many ‘clicks’ of range you have in the dial. To do this, wind the dial fully in clockwise, then wind it back out. As you wind it out, count the clicks.
If you aren’t sure what you need, or have a new shock, you’d be well advised to set the dials to the middle of the range. You can then experiment by one or two clicks at a time, either way, until the desired setup is achieved.
It’s quite rare that taking either compression or rebound to the extremes will have much benefit, so most riders will want to stay near the midpoint.
Remember different trails and terrain will have difference setup requirements, so it’s a good idea to get a feel for what the dials are actually doing in terms of suspension action.