How do damper rod forks work

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Home Year How do Forks Work? Year Features February Tips. Share on Facebook. Please enter your comment! Please enter your name here. You have entered an incorrect email address! Get genuine BMW Motorrad parts on our online store! It is almost as if the oil flow—and therefore the fork velocity—reaches a speed limit.

This is not absolutely true, of course, but because the damping force becomes very high the maximum velocity the fork reaches, in practice, becomes limited. Another drawback of orifice damping occurs in the low-speed range of movement. When braking, the front end dives rather slowly in comparison to the high velocities created on an abrupt hit of a square-edged hole or rock at high vehicle speed.

At these low damper velocities there is not much resistance to flow and the forks feel mushy and dive relatively rapidly. In fact, when going through long duration dips or gullies—even if they are not very deep—the forks can bottom. This change in orifice size is illustrated in the compression damping curves in figure 3.

The larger damping hole will be mushier, though it will be better at high speed. This style of damping seems to provide the worst of both worlds—harshness and bottoming. Any fixed orifice will do: square, triangular, oval, and so on. First the forks extend as the spring pushes on the damping rod. The rebound check valve then closes and chamber B gets smaller, raising its pressure on rebound to the highest level in the fork. There are two ways oil can get out of chamber B: first through the rebound orifices routed to the inside of the damping rod and, second, between the inner diameter of the check valve and the outer diameter of the damping rod directly into chamber A.

Rebound damping is the resistance to this flow. Chamber A on the other hand, is getting larger on the rebound stroke, and therefore has the lowest pressure. This low pressure in chamber A causes oil to be sucked back in, refilling it. Just like on the compression stroke, rebound resistance on a damping rod fork is created through orifice damping.

Rebound damping, however, is a much simpler job than compression damping. Rebound only has to control the force of the fork spring, whereas compression damping has to deal with whatever forces the road or track dish out.

Maximum compression velocities often range from two to six times greater than those during rebound. Thus the limitations of orifice damping are less critical for rebound than for compression. A major potential problem of a damping rod fork is cavitation see figure 3.

Cavitation is the formation of vapor bubbles in a flowing liquid caused by a decrease in pressure. This occurs specifically in areas where the pressure of the liquid falls below its vapor pressure. This is the same phenomenon as boiling, but in this case it is caused by a decrease in pressure rather than the addition of heat. This creates two problems.

Second, when the void in the oil rapidly collapses, it produces a shock wave that can damage and pit the surface of the parts this second issue is much more of a problem in shocks than forks. The potential for cavitation increases at lower pressures and higher temperatures. On the rebound stroke, chamber A is getting larger and sucking oil back into it. The greater the resistance to flow is at the compression orifices smaller holes or thicker oil , the greater the potential for cavitation.

Stiffer springs and hotter oil also increase the potential problem. In fact, orifice-style damping is the most progressive type of damping there is. At low speed it has a lot more compression damping. This will make the action much firmer when hitting the brakes, thereby controlling dive the front end compressing during braking. It will also help bottoming because on every compression stroke, no matter what maximum velocity is reached, the velocity begins and ends at zero.

This means the damper sees low-speed damping twice per stroke, so any increase in low-speed damping tends to improve bottoming resistance. This maximum velocity is partially dependent on the size and shape of the bumps being hit, the vehicle speed, the mass of the bike, and the suspension setup.

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. Why are forks so effective? What kinds of springs are there? What difference does all this make? So cartridges are better? And upside-down forks? Recommended articles.