Some motorcycles allow adjustment of pre-load by changing the air pressure inside the forks. Decreasing pre-load decreases the initial force in the spring thereby increasing total sag. Increasing pre-load increases the initial force on the spring thereby reducing total sag. Topping out occurs when the suspension extends fully and cannot mechanically extend any more. "Bottoming out" occurs when the suspension is compressed to the point where it mechanically cannot compress any more. Total sag is set to optimize the initial position of the suspension to avoid bottoming out or topping out under normal riding conditions. The difference between the fully extended length of the suspension and the length compressed by the weight of the motorcycle and rider is called "total sag" or "race sag". Pre-load is used to adjust the initial position of the suspension with the weight of the motorcycle and rider acting on it. Motorcycle suspensions are designed so that the springs are always under compression, even when fully extended. USD forks are typically found on sportbikes, though the Honda Valkyrie featured USD forks. Two disadvantages of USD forks are: (i) they are more expensive than conventional telescopic forks and (ii) they are liable to lose all their damping oil should an oil seal fail. This USD arrangement has two advantages: (i) it decreases the unsprung weight of the motorcycle and (ii) it increases torsional stiffness, which can improve handling. The slider bodies are at the top, fixed in the triple clamps, and the stanchion tubes are at the bottom, fixed to the axle. "Upside-down" (USD) forks, also known as inverted forks, are installed inverted compared to conventional telescopic forks. Some fork tubes, especially on early roadsters and off-road motorcycles, are enclosed in plastic protective "gaiters. The fork tubes must be mirror-smooth to seal the fork oil inside the fork. On conventional telescopic forks, the lower portion or fork bodies ( "fork sliders" in the UK), slide up and down the fork tubes ( "fork stanchions" in the UK). The bottom of the forks is connected to the front wheel's axle. The top of the forks are connected to the motorcycle's frame in a triple tree clamp (known as a "yoke" in the UK), which allows the forks to be turned in order to steer the motorcycle.
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