The most basic of modern damper-rod forks is all about holes and oil.
The most common front suspension on a motorcycle is the telescopic fork. It’s designed to absorb the shocks produced when the front wheel rolls over bumps, and does this by compressing and extending to allow the wheel to move up and down without transferring the majority of that movement into the chassis.
Enabling a fork to telescope are three main components: the fork tube, the slider and the spring within. On a conventional fork, the tube is the smaller-diameter steel cylinder that mates to the larger-diameter aluminum slider. The slider connects to the wheel, the tube to the triple clamps.
Left unchecked, when a fork compresses after hitting a bump, the spring will extend it back to its original position, and it will rebound back down again, doing this for several oscillations, possibly causing nausea in the rider.
It is the oil within the fork that “damps” this oscillation and keeps you from upchucking that Timmies panini you had for breakfast. Note that fork oil also lubricates and protects the various bushings, while also reducing friction and heat that would otherwise create inconsistent suspension compliance.
The most common and least expensive type of fork is the damper-rod fork. This has a hollow rod – located at the bottom of the fork tube and submerged completely in fork oil – that has fixed-diameter holes drilled crosswise into it, through which oil passes as the fork is compressed. It’s the movement of the oil through these holes that slows the bouncing movement of the fork. The number and size of the holes determine just how much the bouncing will be damped; oil viscosity also affects the damping rate – the thicker the oil, the stronger the damping force.
When dealing with the multitude of road surfaces out there, however, it’s a very compromising design. Not all bumps are the same: some cause the front wheel to move vertically very slowly, while others jolt it upward violently. A damper rod’s fixed-diameter holes allow oil to flow freely at low-speed compression of the fork (low-speed damping), providing little damping, and thus a plush, maybe even sloppy, ride over small, smooth bumps.
However, as the speed of the fork compression increases, as would happen over larger, sharper bumps, the increased speed with which the oil passes through the holes greatly affects the damping force. In this “high-speed damping” situation, every time the fork compression speed doubles, the damping force of the oil is quadrupled (it’s physics). That means the harder the bump, the harsher the ride will be.
Bike manufacturers try to find a middle ground when designing a damper-rod fork, but any way you look at it, an improvement in one area of the damping range will mean it will worsen somewhere else.
You can, nonetheless, fine-tune your suspension to better cope with your riding style. If the bike is very sloppy at low speeds and over smaller bumps, increasing fork oil viscosity will improve the suspension compliance under those conditions, and help eliminate that wallowing movement you feel through long, sweeping turns. Note, however, that the suspension will feel harsher when riding over bigger bumps at speed.
A damper rod with continuously varying-diameter orifices would provide the ideal damping over a variety of bumps, opening up on bigger bumps and tightening up on smaller ones, but that technology doesn’t exist. You can alter the size of the holes, either by drilling them out or filling them in, but this is a trial-and-error undertaking that might involve several fork teardowns and lots of testing. I once blocked one of the damper-rod holes with epoxy to firm up the damping on a road-race bike, though it was ridden in very consistent conditions on a racetrack; I did not have to deal with potholes and such.
The amount of air within the fork also has an effect on the spring rate. The more that air is compressed, the higher it is pressurized. This high pressure assists the spring to do its job, and many bikes in the 1980s even had Schrader valves in the fork caps to add air pressure. This practice has been mostly abandoned, as it was discovered that the increased air pressure also caused the fork seals to squeeze tighter on the fork tubes, thus causing stiction, and preventing the fork from telescoping freely, providing a harsh ride.
You can effectively increase the spring rate of your fork if it is too soft by increasing the volume of oil in the fork, which reduces the volume of air and increases its pressurization rate. Just measure the distance from the top of the fork tube to the top of the oil in the fork, with the spring removed and the fork compressed, and adjust it accordingly. And because there is very little pressure in the fork when it is in its relaxed position, there will be minimal stiction.
Technical articles are written purely as reference only and your motorcycle may require different procedures. You should be mechanically inclined to carry out your own maintenance and we recommend you contact your mechanic prior to performing any type of work on your bike.
