Smaller shims affect the higher end of the speed range while bigger shims affect the lower end. Most damping systems are velocity dependent. What do adjusters do?
Compression and rebound valves each have a small secondary passage, which are controlled by external adjusters, or cone shaped needles. They work by raising and lowering themselves through the passage. The further the needle is wound in, the more it plugs the hole, restricting the oil flow. The more it is turned out, the more of the hole is open, allowing the oil to flow more freely. What does re-valving do? With standard forks, the only thing that can be improved is the internals.
A commonproblem is fork dive. To slow the dive rate down, damping can be increased to provide a stiffer feel. This is commonly done via the shim stacks. A lot depends on the original piston size.
The bigger the piston, the bigger the shims that can be used. Generally, if you wantastiffer set-up, changing the shims closest to the piston face will help. Re-valving starts with changing the shim stacks. Sometimes new pistons will be needed to fine-tune the feel still further. The less fluid there is the hotter forks will run, leading to fork fade. Theremust alsobe enough oil present to lubricate the mechanism properly, including seals. Changing the spring rate by using springs suitable for your weight can also help.
Why do damping curves matter? Adamping curve is the measured force of the shims opening. Most riders prefer a set-up that is firm, plush, with resistance to bottoming. For example, if you want good lowspeed feel, mid-turn stability yet still soak bumps up, a digressive damping curve that climbs sharply to start off with, followed by a more gradual rise, should do the job. What are closed cartridges? A completely closed cartridge system means having the lubricatingoil for the inner and outer tube totally segregated from the damping cartridge itself.
The benefits are no oil contamination no brake dust, particles breaking off from inside surfaces etc , so the whole system will last longer and operate more consistently and therefore efficiently. What about fork diameter? The number quoted on forks, be it 41, 43 or 50mm, refers to the lower slider or inner tube diameter.
The reason diameters vary is that the bigger the tube, the more rigidity there is, which can give more front-end feel. However, larger tubes can be too stiff for some riders, as it affects howmuch they flex from side to side — some flex is still needed. With the smaller fork diameter, the curve will rise very quickly, and with the larger diameter the curve will be much more gradual. How this translates to feel is with the 41mm forks, the air pressure will build up quickly, so in a brake-hard, turn-quickly situation, the front end could be lost.
What exactly are BPFs? Developed by Showa, Big Piston Forks BPFs effectively convert the whole fork leg into the cartridge, allowing the use of a much bigger main piston that incorporates both rebound and compression damping within it.
This allows much more oil to flow through at lower pressures greater volume means lower pressure, meaning that the oil travels at lower speeds. This improves low-speed damping control, such as when the forks start to compress under braking, reducing fork dive. When the forks rebound, oil flows through a different set of holes, again damping the movement - usually to a greater degree, because you want less compression damping than rebound just hit a kerb to find out why.
The problem with the damper rod system is as that as the forks move faster hitting that kerb, for example , damping increases by the square of the speed. This is a Showa Separate Function Fork from a KX motocrosser, with one side housing the spring and preload, while the other houses the damper. Yes, lots. They use a stack of different size shims to control oil flow within a cartridge.
At low speeds the oil bends the weakest shim and leaks through a bit, gently controlling the movement. If you hit that kerb, the sudden jolt forces the oil to bend all the shims and you get a massive flow, letting the forks move up fast.
Cartridge forks also have the advantage that they can have adjusters to control the damping on the compression usually on the bottom of the fork leg and rebound on the top strokes, and even vary the high and load speed damping rates.
On forks with no adjustment, you can only vary damping rates by altering the viscosity of the oil, or the quantity. How it works: Forks. The most common suspension systems found on motorcycles use a coil spring and hydraulic damper setup. Air springs and other types of suspension exist, which will be covered more in-depth in another article.
Springs allow a motorcycle wheel to move independently from the chassis, and dampers control and manage movement of the spring. A motorcycle riding only on springs would bounce continuously and dangerously after every road impact. Springs are coiled steel wire that compress or stretch when acted upon by an external force.
Spring rate is the measurement of force required to compress it a certain distance, which is typically measured in pounds per inch. Spring rate varies with material thickness and number of coils. Heavier duty springs will have relatively thicker coils spaced further apart from one another. In its most basic sense, a damper slows and controls spring action. Dampers control spring action using hydraulic fluid, which travels through a series of passages and restrictions.
A piston with a precisely measured passage orifice travels within the shock body in a bath of hydraulic fluid. When a motorcycle encounters a bump, dampers slow spring compression and rebound as the fluid slowly travels through the passages within the shock body.
Kinetic energy from spring movement turns into heat energy within the damper, and the hydraulic fluid dissipates the heat. Rear motorcycle shocks generate much more heat than front forks, due to the additional loads they support. Compression damping is the intentional slowing of spring compression hitting a bump travel.
Rebound damping is the intentional slowing of the spring expansion as it resumes to its natural state. Some motorcycles will have both high and low-speed adjustments to compression and rebound damping. Sport bikes and off-road motorcycles typically offer greater adjustability than entry level, or cruiser style motorcycles.
High and low-speed damping refers to the speed of the suspension travel, rather than the speed of the motorcycle. High-speed damping affects suspension behavior when hitting a sudden pothole on the street, or an individual rock on a trail.
Low-speed damping affects behavior such as braking related dive or cornering changes. In the image below, you can see how the shims bend and flex as oil travels between the upper chamber A and the lower chamber B. Motorcycle springs are always under tension, even when stationary.
Vehicle weight causes compression at all times. Add a rider or two and luggage, and the suspension compresses even further. Sag is the percentage of suspension travel utilized while stationary. If the suspension sags too much when at rest, the bike may bottom out when encountering bumps once underway. Too little sag can cause a stiff, harsh ride.
Some motorcycles offer suspension preload adjustability. Preload is the amount of tension on the springs when the bike is at rest. Increasing preload will decrease sag, and vice versa.
Since a single motorcycle is often used for solo riding, riding with a passenger or riding with luggage, preload adjustment allows a degree of adaptability for multiple use cases. Although not recommended, adjusting preload can increase ground clearance for off road travel or decrease seat height for shorter riders.
While not ideal, this is a common practice, as changing springs is expensive and labor intensive. The image below shows a typical rear suspension preload adjuster. By turning the bottom adjuster collar, more or less preload force is applied to the spring, while not changing the overall length of the spring. Increasing preload will result in less suspension sag once under rider load. Up front, motorcycles use suspension forks.
In some systems, both springs and dampers work together within each fork leg. In others, one leg contains the spring and the other contains the damper separate function forks. The front wheel axle mounts to the lower end of the suspension fork, and the triple tree secures the top end. Out back, it is common to see a heavy-duty coil wound around a damper, with external adjustments for preload and damping. Motorcycle rear suspension carries more weight than the front, so heavier duty springs are common.
On some motorcycles, dual rear shocks mount between the frame and the swingarm directly. On others, a linkage and single shock system handles suspension needs.
Setting up and tuning motorcycle suspension is a common technician task. In fact, suspension tuning is often a standalone business in this industry. Riders who enjoy track days, dual sport riding, adventure riding, off-roading and touring all have very different suspension needs. If this sounds like an exciting career to you, completing a motorcycle training program can be a great place to start.
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