One of the most neglected pieces of equipment on most planes is the shimmy dampner. Mounted on the nose gear, the shimmy dampner is designed to act as a shock absorber of sorts, keeping the wheel from violently moving back and forth while it is turning. Sometimes a violent (and dangerously distracting) shake will occur at certain groundspeeds when the dampner/steering linkage interface is out of wack. Other times, the symptoms are not so obvious. A good preflight should include a visual inspection of the shimmy dampner.
When you look at the dampner, check the unit for leaks from either end. The amount of fluid in the average shimmy dampner is minimal, so any leaks need to be addressed quickly or the unit will not function properly. Warning: Rocking the nose gear from side to side to allow the dampner’s piston to react is not recommended, since it will help wear a flat spot into the tire.
Another good visual check to make is of the shimmy dampner rod. Take a good look at the piston rod and check to see if it is straight. A bent rod means the shimmy dampner is damaged, and will not function properly. Worse yet, since the rod is bent, the piston will not fit squarely into the cylinder. When the nose gear is turned to turn the airplane, the cylinder of the shimmy dampner can be scored and severely damaged.
The shimmy dampner prevents problems such as excessive nose gear tire wear and extreme wear to the nose gear steering linkage components and bearings. If a damaged shimmy dampner is not caught quickly by the pilot, the wear caused by the damage will make the dampner unrepairable — which translates to the purchase of an expensive replacement. Also, if a damaged unit is allowed to remain in service, the resulting forces will cause the nose gear parts to wear abnormally quickly — which will lead to additional expensive repairs.
BOTTOM LINE: Keeping an eye on your shimmy dampner will help keep your plane in tip-top shape, and will help to prevent excessive and costly wear to the parts that the dampner was installed to protect!