There are a few simple tests you can do to determine whether your forks are in good condition.
By pulling the front brake lever and depressing the forks as far as they go and then, whilst holding the brake lever in, release your weight from the forks you can determine their state of play. Forks that are well maintained and in good order will return evenly to their upright position without yo-yoing. They should also not spring up too quickly or slowly.
General Front Suspension Issues
Use the following to trouble shoot general fork performance issues.
Problem: Front forks sluggish/nearly immobile, handlebars “jump” in your hand when accelerating and crossing ruts, front wheel bounces / chatters on poor road surfaces.
Cause: Spring pre-load too high, spring too hard, air chamber too small.
Modification: Reduce spring pre-load, fit softer springs or progressive springs with a lighter initial strength, increase air chamber (reduce oil level in front fork).
Problem: Motorcycle sags too deeply on the springs, too much negative suspension travel, suspension occasionally bottoms out, front forks shake when braking and travelling downhill.
Cause: Spring pre-load too low, springs too soft, air chamber too large.
Modification: Increase spring pre-load, fit stiffer springs or progressive front fork springs with a heavier initial strength, reduce air chamber (increase oil level in front fork).
Problem: Front forks sluggish/nearly immobile, shocks in the steering when accelerating, front wheel chatters when braking hard.
Cause: Ingoing (compression) damping too hard.
Modification: Reduce the ingoing (compression) damping, possibly use lower viscosity fork oil.
Problem: Front forks shock when braking and on uneven road surfaces, dips rapidly when braking.
Cause: Ingoing (compression) damping too soft.
the ingoing (compression) damping
possibly use higher viscosity fork oil.
Problem: Front forks spring back slowly, front wheel unable to cope with uneven surfaces, indirect steering behaviour.
Cause: Outgoing (rebound) damping too hard.
Modification: Reduce the outgoing (rebound) damping, possibly use a lower viscosity fork oil.
Problem: Front end is lively, bouncing and shocking on uneven surfaces, front forks spring back too quickly.
Cause: Outgoing (rebound) damping too soft.
Modification: Increase the outgoing (rebound) damping, possibly use a higher viscosity fork oil.
Problem: Front forks shock, front wheel shudders, not enough progressive hardening of the spring towards the end of travel.
Cause: Air chamber too large.
Modification: Increase the oil level in small steps of 5 to 10mm.
Problem: Forks hard, steering shock, front wheel bounces when braking due to hydraulic stop blocking.
Cause: Air chamber too small.
Modification: Decrease the oil level in small steps of 5 to 10mm.
Pre-load: This compresses the suspension springs and alters the amount of sag front and rear allowing the suspension to work in it's most effective range the middle two thirds of suspension travel. Too little preload causes the spring to work in the soft part of its range and so needs more damping. Too much preload causes the shock to top out over bumps and reduces the working range of the suspension.
Rebound Damping: Controls the speed that suspension springs return to their full length after being compressed. Without rebound damping suspension would bounce up and down uncontrollably. Too much rebound damping slows down rebound and causes the front of the bike to pump down over bumps. Too little rebound allows the front to bounce back too quickly reducing control.
Compression Damping: Controls the speed the suspension is compressed. The amount of feel and feedback from the front tyre vary with the amount of compression damping. Too much compression damping causes the wheel to be deflected by bumps and the tyre has to work harder absorbing the bumps itself. Too little compression damping allows the wheel travel too much over bumps and dive too much under braking.