What is UpSide Down Fork or USD Suspension?
The term USD stands for UpSide Down. It is a telescopic fork but fitted inversely or in an upside-down position. So, in short, it's an upside down fork. In this design, the upper tubes (fixed fork tube) have a larger diameter than the lower tubes (immersion or slider tubes). This increases the torsional stiffness while preventing torsion in the fork bridge at the same time.
Some modern sportbikes and most adventure bikes use sliders that have a spring/damper unit at the top. They are clamped to the yoke while its foundations are at the bottom. This is mainly to reduce the unsprung weight by suspending the heavier components. In this design, the strong large-diameter sliders are clamped to the yokes. This also improves the strength and rigidity of the components.
The inverted system is referred to as an upside-down fork or in short "USD". One disadvantage of the USD design is that the entire storage of damping oil is above the slider seal. So if the oil drains out because of leakage, damping becomes ineffective. Manufacturers use aluminum for making the Tube and Fork Bridge. This ensures that the contacting surfaces seamlessly interlock under pressure.
Additionally, unsprung weight is not lower than the conventional fork. In USD forks, the interior immersion tubes are a smaller diameter. Besides, the manufactures use heavy-duty steel to make them. As a result, their weight is higher than with conventional aluminum or even magnesium forks. The key advantage is the very long guiding of the immersion tube through the fixed tube.
The upside-down fork uses anodized aluminum fixed fork-tubes and manganese alloy steel tubes. It provides the motorcycle with sensitive response characteristics and maximum stability even under high loads. The outer tubes are torsion-resistant. The bridges guide them with a greater diameter. Greater leverage forces act on them which are very useful for off-road riding.
Manufacturers use protrusion of the fork immersion tubes to suit the motorcycle height in the front section area. Thus, for a 15 mm travel, the front section can be lowered by up to 5 mm and raised by up to 10 mm. The upside-down suspension comes with cartridge inserts on the inside. This is a separate hydraulic piston-cylinder system. It has adjustment options for preload and compression/rebound stages. For e.g., for total spring travel of 120 mm, 75 mm is positive while 45 mm is negative.