Steering dampers can work magic on a motorcycle's stability, but there's really no magic involved in their design and construction. In essence, a steering damper is simply a shock absorber that provides a controlled resistance to the movement of the front end as it rotates back-and-forth in the frame. They come in two basic types, through-shaft dampers and rotary dampers, and both work by forcing oil through various orifices, thereby creating what is called fluid friction.
Rotary dampers are straightforward in their installation and usage. This is one way to mount such a damper to the Kawasaki ZX-10R.
Through-shaft dampers (TSD) are the most popular type. They're effective because they dissipate heat effectively, offer a long stroke, can be made to have equal compression and rebound damping characteristics, and are simple to mount. TSDs are long, however, with more steering angularity equating to a longer damper rod. So, while they may be simple to mount, compromises often need to be made for them to work adequately. Due to differing lever and motion ratios, some TSDs may even need different settings for turning the front end to the left than to the right.
Rotary dampers suffer no such packaging or mounting foibles, because they mount directly inline with the steering axis. In general, rotary dampers don't work well in high-stroke, high-heat applications--such as rear-suspension dampers located near exhaust pipes (remember Suzuki's TL1000S?)--but they work exceptionally well as steering dampers. And companies such as Sachs, which makes rotary dampers for Formula 1 cars, are producing units that are much more durable and consistent.
The Scotts Damper
The Scotts damper is a rotary type that controls the rotational movement about the steering axis by using a blade-style wiper to force oil through internal orifices. By varying the location and size of the orifices, the damping characteristics can be adjusted to suit rider needs and riding conditions.
The underside of the Scotts damper. The link arm and shear pin are easily visible.
The Scotts damper has three areas of adjustability: the low-speed circuit; the high-speed circuit; and a unique sweep circuit that allows the damper function to disengage, depending on how far the handlebar is turned. For the serious racer, the metering devices that control the different damping circuits can be custom-made, and different oil viscosities can be used, thereby tailoring the damper's effective range of adjustability and characteristics.
The low-speed circuit (also called the base circuit) controls the overall resistive force and is effective all the time. This circuit is adjusted by turning the knob with the black pointer (on the left in our application on a 2004 Kawasaki ZX-10R). Rotating the adjustment knob to the right increases the damping, and turning the knob to the left decreases it. The high-speed circuit (usually covered with a press-fit plastic cover) functions in much the same way as the low-speed circuit, but operates through a much higher rotational acceleration range. The two circuits overlap each other; conceivably, you could set the base circuit stiff enough to make the high-speed circuit ineffective.
The variable sweep control-a feature unique to the Scotts damper--allows the rider to adjust the damper's effective range of motion. The rider can set the damper to function throughout its full range of motion, or within a narrow band in the middle of the range, or in two other settings in between. Once the handlebar is rotated past the sweep control's limits, the damper becomes ineffective and the handlebar is free-floating. Your first experience with this sudden absence of resistance can be a bit disconcerting, particularly if you have a stiff base-valve setting.
The reasoning for this feature is simple: With a stiff base-valve setting, the steering at parking-lot speeds suffers, particularly when the handlebar has to be rapidly moved to full deflection. With the sweep control, a rider can have a high base-valve setting but still not have to exert a lot of steering force for those tight, slow-speed maneuvers.
Units such as the Honda Electronic Steering Damper (HESD) pictured here are indicative of the future in chassis and suspension technology.
The future of the rotary damper is looking bright. For the first time, a major manufacturer, Honda, has employed a high-tech rotary damper as stock equipment on one of its motorcycles. The Honda Electronic Steering Damper (HESD) is standard on the CBR1000RR and provides the benefits that a rotary damper can offer, but with a uniquely Honda twist: damping that is both speed- and throttle-position-specific. At slower speeds, the damping function cuts out, allowing for free and unimpeded bar movement. As you speed up, the damping rate increases, imparting greater stability.
As sportbike chassis design continues to evolve with ever-more-increasing power and ever-more-diminishing weight, other manufacturers will begin to exploit the virtues of these simple yet effective dampers and include them as standard equipment on new models. Until then, Scotts and others have rotary units you can install yourself. For the enthusiast whose sportbike has more than a little wiggle in its walk, they're certainly worth serious consideration.