So, the basics. This is from an old KTM/WP shock manual:
Springs and damping
To meet the high demands put on our suspensions, our suspension systems have a spring and a damping system. The spring absorbs the wheel movement, the damping system controls the spring movement.
The spring and the damping work together to fulfil all of the different suspension tasks: Comfort, Vehicle Balance and Safety.
When looking at the total stroke of the suspension, the suspension tasks mentioned above can be linked to different parts of the stroke. If the stroke is divided as following:
- The first 25 % of the stroke mainly takes care of comfort.
- The middle 50 % of the stroke takes care of balance.
- The last 25 % of the stroke ensures safety.
These three factors always work together, because they influence each other.
If all of the suspension components work together properly, the result is a comfortable, balanced and safe motorcycle.
Comfort
Comfort can best be seen as the amount of vehicle movement that is transmitted to the rider. Maximum comfort would mean that no movements are transmitted to the rider.
Balance
Vehicle balance is determined by the weight distribution between the front and rear wheel. The motorcycle is primarily balanced by its springs. Therefore it is very important to make sure, the springs are suited to the rider's weight and/or use of the bike.
Safety
In order to maintain a level of safety, the tire must be in contact with the ground at all times.
This means that at least a small amount of suspension travel should always be left.
If there is no suspension travel left (when the suspension 'bottoms-out'), grip will be severely compromised.
Springs
The tasks of the spring are:
- Supporting the bike's weight (at standstill and during driving).
- Absorbing the wheel movements.
- Preventing the fork / shock from being fully compressed.
- Returning the motorcycle to its 'neutral' position after being compressed.
Spring rate and preload
The springs mounted on the bike have a rate which suits the weight and area of use of the bike.
The first step in maintaining the bike's handling characteristics is making sure the springs are preloaded correctly and have the correct spring rate.
Spring rate
The spring rate is the force (N) needed to compress the spring a specific amount (mm.). The spring rate is measured in N/mm.
Spring preload
The spring preload is the amount (mm.) the spring is compressed by the preload adjuster/preload bushes.
NOTE:
The spring 'rate' and the spring 'preload' are two very different things! The spring rate is a fixed value which stays the same. The spring preload can be altered in order to set the correct ride height. Changing the spring preload does not alter the spring rate!
Suspension sag
Each bike needs a predetermined amount of 'sag'.
A certain amount of sag is needed to make sure the wheel is able to move down when it passes over a hole in the road.
If the bike wouldn't have any sag at all, the wheel would only be able to move up, which would compromise safety and comfort.
Sag is adjusted by altering the spring preload.
Correct sag settings are needed to ensure the bike has the desired ride height and steering geometry. The sag can be seen as the 'fine-tuning' of the spring preload.
The sag can be separated in:
Static sag
Difference between when the vehicle is entirely unloaded (jacked up) and when it is resting on the ground with only the weight of the vehicle (stationary on ground without rider)
If this value is too low, it will have a negative impact on vehicle response. It should be approx. 10% of the total suspension travel.
Riding sag
The difference between when the vehicle is entirely unloaded and when it is loaded with the rider in full gear.
Dynamic sag has the greatest impact on the adjustment of the spring preload. It should be approx. 1/3 of the total suspension travel.
NOTE: When measuring the sag, the measurement should always be taken between a fixed point on the frame and the wheel-axle. If the fork is equipped with a preload adjuster, the preload can easily be adjusted at any time. Without a preload adjuster, the fork must be opened and the preload bush must be changed.
Damping
The tasks of the damping are:
- Control of wheel movement.
- Control of spring movement.
Compression and rebound damping
The wheel can move in two directions (up and down), both directions require a different damping rate.
The damping can be separated into two different kinds of damping: Compression and rebound
Compression damping
When the wheel moves up, the suspension component is compressed (against the spring force). The damping that controls this movement is the compression damping.
The compression damping can be separated into two different kinds of damping: LOW-SPEED and HIGH-SPEED.
a. Low-speed damping
If the suspension is compressed slowly (low piston velocity), the movement will be damped by the Low-speed damping.
Low-speed damping regulates the damping up to a piston velocity of 0,5 m/s (shock absorber) / 1 m/s (Front Fork). Above this speed, the high-speed damping will take over, the influence of the low-speed damping will then be negligible.
The "low-speed" setting has the greatest impact on the change of the total compression damping - comfort or sport setting. When adjusting the low-speed damping, small changes in "comfort" can be felt as well.
b. High-speed damping
If the suspension is compressed rapidly (high piston velocity), the movement will be damped by the High-speed damping.
High-speed damping regulates the damping when piston velocity is above 0,5 m/s - 1,0 m/s. Maximum piston velocity lies around 3,5 m/s (shock absorber) / 7,0 m/s (Front Fork)
High-speed damping has the biggest effect on the "balance" of the motorcycle.
Rebound damping
When the wheel moves down again the suspension component extends (extension is forced by the spring). The damping that controls this movement is the rebound damping.