Suspension.
FORKS
The forks on most bikes do two jobs
1. hold the wheel to the rest of the bike, and
2. control how the wheel moves up and down to absorb bumps.
Each fork leg contains a spring to support weight and an oil damper to control how fast it extends or compresses.
DAMPING
If suspension only had springs the bike would continue bouncing up and down after hitting a bump. To stop it bouncing, the shock and fork legs have dampers inside which slow there movement.
Compression damping slows down the wheels upward movement; rebound damping slows down the wheels downward movement.
Fancy suspension damping is fully adjustable while on cheaper units normally only rebound damping is adjustable. Sometimes compression and rebound damping are linked so one adjuster increases and decreases both at the same time.
Some set-ups are not adjustable for damping at all.
SHOCK
Connected at one end to the bikes frame and at the other to the swing-arm via linkages. It lets the back wheel move up and down in relation to the rest and controls how far and fast the wheel moves for a given bump.
SPRING
The spring pushes the two halves of the suspension unit apart to support weight on it. The more weight on it the more it will compress and the lower the bike will be.
Preload is the amount the springs are compressed when there’s no weight on them and is adjustable. If 20kg compresses the spring 0.5cm, then 40kg will compress it 1cm. If the spring is preloaded by 2cm, it will support 80kg without compressing any further.
SUSPENSION: its job
Suspension exists so the wheels can move up and down, roll over bumps and into dips, while the rest of the bike stays level - it isolates you and the majority of the bike from shocks and keeps the tyres in controlled contact with the tarmac. Without the tyres on the ground you’ll have no control; even poorly controlled contact gives you less grip.
Without suspension, tiny bumps would rocket you into orbit. And if you managed to stay on the ground, the jolts would turn you into a jabbering wreck and shake the bike apart. Riding over gentle crests would have the wheels airborne and levelling out from riding down hills would squash the tyres flat.
SUSPENSION: its travel
When the wheels move up too far the suspension becomes fully compressed and bottoms out. When the wheels move down to far the suspension tops out and the wheel leaves the ground. This is suspension travel and either way the wheels can’t move any further in that direction, which effectively means you now have no suspension. Any more force upwards can’t be absorbed and gets jolted through the bike and on to the rider.
When riding we need the wheels to be able to move up and down without effecting the rest of the bike. If the combined weight of the rider and bike isn’t enough to compress the suspension at all, when you ride over a dip the bike will drop in to it rather than just the wheel.
To absorb dips, the weight of the bike and rider must compress the suspension, making it sag somewhere between its compressed and extended length - this is the bikes ride height. The rule is that with the bike loaded as normal - this can include pillion - the sag takes up the first third of suspension travel.
To adjust the sag you alter preload. Increasing preload will decrease sag, and decreasing preload will increase sag. This is probably the most important and affective adjustment you can make.
