Basic Suspension Tuning
Boing, boing, boing.
Pick up a musical instrument and play a few notes. Chances are it'll sound terrible. You'll find stray harmonics, wavering beats and notes which just sound bad.
If instead you had taken a few minutes to tune that instrument the results would have been different. Notes would be in tune, no unwanted harmonics, no wavering beat.
The same principles apply to suspension. From the factory forks and shocks are set to fit the average rider. A person who doesn't actually exist but would weigh 60-70kg, ride at speeds of around 15-25km/h and on trails of modest roughness.
Our purpose here is to work through the basic priniciples of tuning, to gain an understanding of what each adjustment does and to get our suspension working as best it can.
Every suspension system contains a spring of some description. It may be metal coiled, in a leaf or torsion bar, a pressurized air chamber or it may be plastic foam in the form of Micro Cellular Urethane (MCU).
Regardless of the spring on a bike there is a large difference in the weight of the bike and the weight of the bike and rider. With the bike alone the suspension is usually fully extended, against the topout stops front and rear.
When a rider climbs on board the suspension sits in or Sags a certain amount depending on the riders position and the firmness of the springs.
Sag is necessary for good suspension performance, to allow the suspension to float up and down over small ripples in the ground. Too much sag uses up the positive travel which the suspension needs for bump control but too little gives a harsh ride over small bumps and ripples.
Sag is useless if your rebound damping isn't set fast enough to use it.
The ideal amount varies from rider to rider, the general rule says 10-25% of your total travel on each end for XC use and up to 33% each end for downhill use. Many racers run forks and bikes with no sag, a design which capitalises on this is the Giant NRS.
More aggressive riders should run a firmer fork (less sag) than their weight alone would indicate.
Air forks and shocks often run less sag than coils due to the high preload inherent in air springs.
Sag depends on the firmness of the springs and the amount of preload on the spring. Get as close to ideal as you can by swapping springs or air pressure then use the preload knob or collar to wind out excess sag on a soft spring. Beware that excess preload can cause topout problems and spring failure, more than half a dozen turns of preload means you should be looking for the next spring up on a rear shock.
Damping robs energy. It turns motion which isn't wanted into heat in the damping medium of air, oil or friction.
Friction damping doesn't really work and normally refers to a non damped fork or shock but a few mongrel designs do exist which rely on rubbing strips and friction rings to deaden the movement.
Air damping is found in lightweight forks and shocks. The problem here is air damped components are usually also air sprung. The damping causes the air inside the shock to heat. The heat causes the pressure to rise and your spring rate increases.
This normally only occurs in extreme cases of intense use over a short time where the heat cannot be dissapated to the atmosphere.
Oil damping is the best way to control suspension. It has a large heat capacity, expands little with temperature, is available in a multitude of viscosities (thicknesses) and lubricates as it damps.
Most forks and shocks which are oil damped also have an externally adjustable freebleed. This knob controls the amount of oil which is allowed to bypass the main damping piston on both compression and rebound. Closing the freebleed (knob clockwise) increases the damping force on both compression and rebound but has the greatest effect in slowing the rebound speed. This is why the freebleed is often referred to as the rebound adjuster.
The correct amount of rebound adjustment depends on the riders preference, the terrain ridden, springs installed and speed travelled.
The correct amount is also the least amount possible.
To set the rebound damping start with the adjustment all the way out, find a bump which can compress the suspension most of the way and ride at it repeatedly.
Every time the fork kicks back increase the damping a notch or two. Stop when the kickback becomes bearable.
Too much rebound damping will make the fork feel dead. It will pack up on repeated bumps (not return enough) and feel harsh at speed.
Too little rebound damping can kick the front wheel off the ground after the suspension has been compressed.
Compression damping is similar in function to rebound damping. It converts excess energy into heat and takes some of the energy encountered in a bump away from the springs.
The majority of compression damping is usually handled by the shims and piston inside the shock. Generally factory set and not easily changed.
Only on higher priced shocks and forks is the compression damping externally adjustable. This is usually acheived by a throttle on a second piston or orifice which oil must travel through at a slower speed after passing the main damping piston.
Like all damping the correct amount of compression damping is the least possible. When you can get full travel from your fork and shock while only bottoming out once or twice a ride your compression damping is about right.
Too much compression damping feels harsh on all bumps and doesn't let the suspension use it's travel. Too little compression damping lets the suspension dive through it's travel and bottom out harshly.
Fork and rear shock oil is available in a range of different thicknesses (viscosities). The common term is oil weight and numbers of suspension oil weight range from 2.5 on and up to around 30.
Most forks and rear shocks use stock oil ranging from 5wt to 10wt. This gives the tuner a range of oils both thicker and thinner to choose from.
Changing to a thicker oil increases both compression and rebound damping, thinner oil decreases both compression and rebound damping.
Beware though that shim stack type dampers (manitou, most rear shocks) don't rely as heavily on oil weight as orifice type dampers do (marzocchi, RS hydracoil). Changing to a lighter weight oil in a shim stack damper will have a large decrease in low speed damping but only a small decrease in high speed damping.
Conversely changing to a thicker oil in a shimstack damper will increase low speed damping and may increase high speed damping to the point of spiking (soo much damping that the fork stops dead on compression).
Many forks these days are open bath design. This means they have a bath of oil inside each leg of the fork which rises as the fork is compressed. This traps a volume of air against the topcaps of the fork which compresses and adds to the spring rate as the fork nears the end of it's travel.
The amount of air present in the fork determines how progressive it gets at the end of it's stroke, this is governed by the height of the oil surface in the fork.
Adding more oil to the fork raises the level in each leg. This gives the air less space to compress into at the end of the travel and increases the pressure inside and the progressiveness of the fork.
Too much oil in the fork can prevent full travel from being acheived, too little oil will let the fork bottom out harshly. These forks often do not have bottomout bumpers, the metal on metal impact could remove false teeth.
Even a few mm's in oil height can make a big difference, make only small changes and document each one.
On a full suspension bike it is also important to get the front and rear suspension working in harmony. Otherwise your bike rocks from front to back over bumps, control is compromised and your good health can be threatened.
Balance is simply a matter of matching the feel of the front and back suspension until they handle bumps in a similar fashion. It is best to back off the end which feels harsher than to wind up the end which feels soft.
This is difficult with bikes which have largely different amounts of travel from front to back. In this situation either the short end must be set up softer to compensate or the long end must be set up firmer to match the feel.