Once upon a time mountain bikes were simple devices. Chuck suspension into the mix, though, and things get more complicated. A lot of people don't get on with their full-suspension bikes, and sometimes that's simply because they're not set up right. But fear not, it's not a black art... We've covered front suspension before, and here we're running through rear shocks.
As with so many things, suspension set-up is often a compromise. You've got quite a few conflicting requirements - you want suspension that'll soak up small bumps without being overfaced on big ones, supple but not mushy, controlled but active. But with a few minutes' effort you can get most of the way there.
There are no hard and fast rules for suspension set-up. What works for one combination of bike, rider and trails won't necessarily work for the rest. Different manufacturers have different ideas about the best approach. Riders like different things. What we'll give you here is a brief guide to getting to a good basic set-up that'll give you the confidence to tune things more closely to your preference. If you know you can get back to something that works, there's no harm in trying to improve it...
First off, here's the bits of the shock that we're interested in:
This is a coil shock - you may have an air shock, which is fundamentally the same except that it uses air instead of a steel coil spring.
Setting sag
Like forks, most full suspension designs are intended to run with some sag. That is, when you're riding along a smooth surface the suspension is slightly compressed. This allows the wheels to drop into hollows in the trail, keeping the tyres in contact with the ground and improving traction and control. Sag is controlled by spring rate. The harder the spring, the less sag you'll have. Different bikes and riding styles favour different amounts of sag, but somewhere between a quarter and a third of the available travel is usual. Less sag gives a tauter, faster feel, more sag is plusher but can be more wallowy.
The best way to measure sag is by measuring vertically from the ground to a fixed point on the bike like the underneath of the saddle or something. You can measure it at the shock too, but there isn't necessarily a linear relationship between wheel travel and shock travel so it might be slightly out. It's easier to measure at the shock, though - you really need two people to measure from seat to ground.
First you need to work out your target sag. You need to know the total travel of your bike. If you've got 100mm of travel and you want 25% sag, that's 25mm of compression under your weight. Measure from seat to ground with you off the bike, then get on it and measure it again. The second measurement should be 25mm (or whatever figure you came up with) less than the first. If it's sagging too much, tighten the preload collar or if you have an air shock, add some more air. If it's not sagging enough, undo the preload collar or release some air. Repeat until you hit the mark.
If you're a lot lighter or heavier than average and have a coil shock, you may find that you can't get the sag right by just twiddling the preload collar. If that's the case you may need to swap the spring for a lighter or heavier one. Air shocks don't have this problem.
Rebound damping
With the sag set correctly, turn your attention to the damping. Rebound damping is what stops the suspension just bouncing uncontrollably when you hit a bump, but it has to be adjusted correctly - too much rebound damping and the shock won't extend fast enough to absorb successive bumps, leading to a phenomemon known as "packing down" where the shock gets more compressed by each bump until it runs out of travel.
To start, turn the rebound adjuster (often a red dial, sometimes labelled R) anti-clockwise as far as it'll go. If you've got a compression damping adjuster (often blue) do the same with that. This is fully open, that is, minimum damping and a fast shock action. Roll along a smooth surface (a road will be fine), stand up and give the suspension a bounce. Just stand up straight and then quickly push down on bars and pedals. The suspension should compress, re-extend slightly beyond the "resting" position and then settle back to where it started. Turn the rebound adjuster clockwise a click at a time until it behaves like this.
Once it feels good, go for an off-road ride and pay attention to how the rear end's acting. If it feels too bouncy and tends to get out of control or feel like it's pitching you over the front on bigger hits, add more rebound damping. If it packs down over successive bumps, reduce the rebound damping. Eventually you'll find a setting you're happy with. Make a note of where it is (count the clicks to fully open).
Compression damping
Most shocks don't have adjustable compression damping. If yours does, leave it fully open unless you find yourself bottoming the suspension out even though the spring rate's correct.
Go forth and fiddle
That should give you a pretty respectably performing rear suspension. From there it's a question of tweaking things to suit your own trails and riding style. If you like a fast, tight-feeling bike, run less sag. If you're into big drops, run more rebound damping. Just make a note of your basic settings, fiddle around, learn what's what and if it all goes wrong you've got something to fall back on...
