Motorcycle suspension is a very complex topic and so much can be written about it, but for now I am going to give a brief explanation on how motorcycle shocks and forks work. There a quite a few variations of motorcycle fork and shock types so for this purpose I am going to use an upside down cartridge fork and mono tube shock with remote gas reservoir, as these are the most common used on sports motorcycles.
Suspension consists of two main components, the spring and the damping. The spring basically supports the weight of the motorcycle and rider, whereas the damping controls how the spring compresses and extends when braking, accelerating, riding over bumps etc. To write about springs alone would take up a whole article so I am just going to focus on how the internals of the shock and fork work.
The pictures to the right show the main internal parts and the main piston valving of a monotube shock with external gas reservoir. The shock absorber is filled with oil and the reservoir contains a bladder which is filled with pressurised Nitrogen at around 8-10Bar, which in turn pressurises the oil. The piston has ports that pass through it from top to bottom and shim stacks then cover the openings of these ports. There is a rebound shim stack on top of the piston and a compression shim stack on the bottom of the piston. These shim stacks are as the name says, stacks of shims. These shims are very thin steel discs around 0.10mm-0.30mm thick and of various diameters, typically stacked on top of each other as shown in the images. As the rear suspension compresses the shaft and piston move up inside the shock body forcing oil through the ports in the piston and lifting up the compression shim stack. The force required for the oil to lift this shim stack is what creates the compression damping. As the shock extends the shaft and piston move back down inside the body and the oil is now forced through the second set of ports in the piston, lifting up the rebound shim stack and giving rebound damping. The shim stacks can be modified and different pistons can be used to change the damping characteristics on the motorcycle suspension. In the first image you can see at the top of the shock there is the compression adjuster and oil passage. As the shaft moves up into the shock body, it displaces oil which is pushed though the oil passage and compression adjuster into the reservoir. The compression adjuster contains a tapered needle which, when you turn the adjuster on the outside of the shock, moves in and out of the oil passage either restricting or opening up the flow of oil. This allows you to control the level of compression damping by controlling how easily the oil flows through the passage. The rebound adjuster works in a similar way but the needle and oil passage are in the middle of the shaft passing through the centre of the piston as seen in the second image.
On the right, you can see an Ohlins shock stripped down. The rebound shim stack is the row of discs to the left, with the piston and the compression shim stack in the next row. The four round holes on the top of the piston are the rebound ports which the rebound shim stack covers. The larger triangular holes between these four rebound ports lead though to the compression ports on the underside of the piston.
Below is a piston with shim stacks assembled on the shaft.
The image to the right shows the internals of a front motorcycle cartridge fork. This actual image is of a right way up fork leg with a cartridge but the internals are the same design as an upside down fork leg cartridge. The cartridge looks similar to a bicycle pump with the end of the shaft connected to the fork top cap and the bottom of the cartridge tube fixed to the bottom of the fork leg. Inside the cartridge, on the end of the shaft is a piston containing a rebound shim stack and at the base of the cartridge is a compression shim stack and piston. Both pistons have oil passages through the centre of them containing adjustment needles allowing adjustments of the damping to be made, just as the rebound in the shock. As the fork compresses the oil is forced through the compression shim stack and as it extends the oil travels through the rebound shim stack giving you the damping.
Below is an image of the valving from inside pair of fork cartridges. The two on the left are the rebound assemblies that would screw onto the shaft seen just behind them, along with the adjustment needles. On the right are the compression assemblies that would screw into the end of the cartridges and then be fixed to the bottom of the fork legs. You might notice that the two valving assemblies closest to the camera are different to the two behind. This is because the closer ones are an upgraded K-Tech SSK piston kit. These K-Tech SSK piston kits massively improve the front fork performance over the standard valving due to different designed pistons, needles, oil passages and shim stacks.
When setting up the suspension on a race bike or carrying out a suspension set up for a road bike, all adjustments are fine tuned to suit the rider and riding conditions. For a road bike suspension set up first the spring preload would be adjusted to give the correct static and rider sag (the distance the motorcycle sags under its own weight and with the weight of the rider), so that the motorcycle is sitting correctly and at the right geometry. Then the compression and rebound adjusters will be fine tuned to give the right level of damping for the intended use of the motorcycle. A suspension set up can be beneficial for many reasons such as: resolving handling issues, setting a bike up for a track day or making a bike more comfy for touring. On race bikes the setting up of suspension gets a lot more in depth. Spring weights and preload can be changed to give different feel as well as adjusting the sag and geometry. Compression and rebound adjusters are used to control how the bike handles at different stages of cornering as well as keeping the bike stable over bumps and undulations in the track. As well as compression, rebound, spring rate and preload adjustments mentioned, other suspension and chassis parameters can be changed to fine tune the handling on a race bike (for example fork oil quantity, shock gas pressure, shim stacks, ride height, wheel base, tyre pressure, tyre compound and sprocket sizes). As I said at the start, this is just a brief insight into how the suspension works on your motorcycle so I’ve tried to keep it short and not get too technical as there’s so much involved with it. If you want to find out how your suspension can be improved on your motorcycle please feel free to get in touch on 07728505762 or firstname.lastname@example.org.