The Motorcycle Frame / Chassis: Part I
The motorcycle frame or chassis provides a strong, rigid structure upon which to attach the components necessary to make up the machine. The size, weight, and type of frame contributes as much as any other factor to the "personality" of the motorcycle. Frame geometry determines, to a larger extent, the bike's handling characteristics. Also, the type of construction is a critical factor affecting frame stability and rigidity. Don't take the frame for granted. It's a very important part of your motorcycle and requires periodic inspection and maintenance.
Single and double downtube swing-arm frames have emerged as the most accepted type. The tubular frame is light yet strong, and provides very good stability at highway cruising speeds. It also seems to have more "eye appeal" to most riders because of its leaner, racier look.
Often both the tubular frames rely on the engine members instead of additional frame to add rigidity to the structure. While this design helps to reduce a machine's weight and cost, it often results in frame flex if the engine mounting studs are not properly torqued. With engine-based frame design carried to the extreme frame flex becomes a real concern. Some of these motorcycles have been likened to "riding a 150-mph ball joint". To a lesser degree, all frames that use the engine as a frame member may develop this problem if not properly maintained by tightening the engine mounting bolts every month or so. There are also a wide variety of angular metal frames including monocoques, mostly used in superbikes.
The steering head is the tube at the top of the frame where the front fork assembly is attached. The steering head incorporates two sets of ball bearings for easy steering and a special adjuster nut for tightening the bearings as they wear. The steering head angle is very important because it affects the high-speed stability and low-speed manoeuvrability of the machine a great deal. A steep or near vertical angle results in quick slow-speed steering reaction, while a more "raked" angle yields more high-speed stability but sacrifices precise low-speed steering. Steering head angle is set during manufacture and is usually in keeping with the intended purpose of the bike. Smaller displacement trail bikes tend to have steep steering head angles, but higher-speed road machines and some racing motorcycles incorporate more "chopper-like" steering head angles for high-speed control.
Triple clamp or fork-leg holders are two triangular brackets that mount the forks to the steering crown. The handlebar mounts are usually incorporated into the design of the upper triple clamp along with provisions for mounting gages such as the speedometer and tachometer. The lower clamp sometimes incorporates a steering headlock or part of a steering dampening device.
Steering Dampening Devices
There are two popular types of steering vibration dampeners. The simpler consists of a set of fiber washers and metal discs that may be pre-loaded to restrict turning motion. A more sophisticated vibration dampener mounts between the lower triple clamp and the frame. This unit is basically a shock absorber that dampens the unwanted jolts and oscillations from the front wheel.
Handlebars, Clamps, and Cushions
There are literally hundreds of variations on the handlebar theme, but they generally fall into two sizes and classifications as far as mounting provisions are concerned. Handlebars are usually 7/8 or 1 inch (2.23 - 2.54 cm) diameter tubing. They are either fixed solidly to the upper triple clamp or mounted in rubber to help dampen vibration. The clamping devices on handlebars range from sturdy alloy caps to steel V-bolts.