The hydraulic fork is already an integral part of any modern motorcycle. We cannot imagine how a motorcycle would be with a stiff, non-damping front end.

The telescopic fork was pioneered by the Scott Motorcycle Company in 1908. However, it used only springs without hydraulic damping. The first motorcycle to utilize hydraulically damped forks was the 1935 BMW R12. Since then, motorcycle forks have evolved into marvels of engineering that we see today.

Working principle

The most basic principle involves inserting a spring in each fork leg. For many years, the spring was the only component to play the role of preload (keeping the suspension from bottoming out), damping (absorbing shocks), and damping (pushing the front tyre’s contact patch onto the road surface).

The earliest attempts of controlling damping was by adding another spring. That is achieved by inserting a smaller diameter spring and surround it with a larger diameter one. The bigger spring is longer than the former, so as the fork compresses more due to a larger bump or shock, the shorter, more tightly wound spring comes into play to put up more resistance. This creates a rising rate suspension.

However, springs will oscillate when they re-extend, so dampers were created to counter that.

Hydraulic damper rod

The hydraulic damper is the solution for spring oscillation.

Fluid (in this case, oil) is pushed through orifices on a damper rod to create damping. The rod is inserted into each fork leg, on top of the spring.

 The damping rate – how the quickly the suspension reacts – depends on the size or sizes of the orifice or orifices. The larger the opening, the less damping the fork provides and vice-versa.

Cartridge forks

Instead of using damper with orifices, a cartridge consists of different sized shims. Oil is then forced past these shims. A soft shock needs the oil to bend the weakest shim to allow oil through. Harder shocks will for the oil to bend more shims to allow the wheel to move up at a faster rate. This means damping is more precise. One of the biggest advantages of the cartridge fork is that you can replace certain shims for different damping characteristics.

Separate function forks

As the name suggests, one fork leg holds the spring or springs, while the other size holds the damper mechanisms. The one with the spring controls preload, while the other checks damping. This way, the forks could be made to cater to their specific purposes. They can also be lighter as the components are not duplicated. Having different functions on each side doesn’t give off different feedback when the bike is turned to either side as the forks are tied to the same triple clamps.

Electronic suspension

These systems can control preload and damping, or separately depending on the bike. Instead of having the rider adjusting the parameters with tools (plus sweat), it’s all done through a button on the handlebar.

However, the principles remain the same. What’s different is that sensors on the fork and swingram provide real-time ride height and damping data to the suspension ECU. The ECU then determines the correct strategy i.e. setting, depending on the selected mode. The ECU then sends signals to servos to alter the parameters. 

Fork oil

Damping works by converting kinetic energy (moving fluid) to heat. This is why oil is commonly used as it could absorb the heat plus has low flow resistance. Changing the oil to difference viscosities or amount will also alters the damping characteristics.

But bear in mind that it must be replaced at every 20,000 km.