Motorcycle Front Ends
Chopper Fundamentals 101
Styles of Motorcycle Forks
When it comes to creating a custom chopper or customizing a motorcycle, the front forks (front end) of a motorcycle will have a significant impact on both the ride and aesthetics. The primary function of forks, however, is to attach the front wheel to the motorcycle. Motorcycle forks also provide the suspension and framework for mounting brake components and fenders. Understanding the characteristic differences between tube forks, springer forks and girder forks should help make choosing the right set easier when creating a custom chopper.
Tube Forks (Hydraulic Forks)
Inverted Tube Forks and Fork Dimensions
Tube Forks (Hydraulic Forks) Explained
Tube forks incorporate an internal shock and spring in one sealed unit. The smaller tube slides in and out of the slightly large tube that is sealed around it. Internal springs and hydraulic fluid serve to dampen the movements of the tube forks to smooth out the ride, just as on an automobile, providing the active suspension for the front wheel. Traditionally the rigid portion of the tubes are joined to the frame, via the triple trees, while the wheel axle is fastened to the active (live) forks. Hydraulic fluid levels must be checked periodically to insure proper operation.
Tube forks are available in conventional (traditional) and inverted configurations. Conventional tube forks have the smaller tubes fixed to the triple trees, while inverted tube forks have the larger tubes mounted to the triple trees. Both configurations essentially work the same way. Inverted tube forks have been used on sport bikes for years. Tube forks provide good dampening qualities for most applications where rake (angle from vertical) does not exceed 35 degrees. When used in higher rake applications the dampening affect may be reduced by side-loading stresses (pinch points) exerted on the tubes.
Tube forks are manufactured in several diameters, widths (distance between fork centers) and lengths. 41 and 39 millimeters are the industry standards for tube diameters, though other offerings are available. Fork widths vary from 10 inches (Harley Davidson Wide Glide) to narrower (Harley Davidson Sportster) styles. Some manufacturers have introduced the Mid Glide widths that fall in between the wider and narrower styles.
Springer Forks use two sets of parallel forks. One set, mounted to the frame, remains rigid while the other set, the active set, absorbs the movement of the front wheel. The lower ends of the active forks are connected to the rigid forks through links called rockers. The rockers provide the pivot points by which the front axle is allowed to move. The upper ends of the springer forks are connected to the rigid axle through the suspension components, usually springs, shocks or a combination of both. When a shock absorber is added to the suspension components, vibration and harmonics are reduced. Both axles are then mounted to the frame through the (typically) integrated trees.
Since springer forks use a pivoting rocker, they are ideal for use where longer forks are needed. The overall movement of the rockers is relatively unchanged no matter the length of the forks. This is great for rakes exceeding 35 degrees. Because of their elegantly simple design, springer forks are comparatively lighter than tube forks and require less maintenance.
If an old school look is desired, than springer forks may be the right choice for a custom chopper, or customizing a motorcycle. In either case, allowances will have to be made for mounting fenders and brake hardware.
Though not as well known as tube forks or springer forks, girder forks have been around a long time. Girder forks are similar in looks to springer forks but are constructed quite differently. Girder forks are rigid; each side being welded together at the top and at the point where the axle is mounted. The entire assembly is then fastened to integrated triple trees through the active suspension. All active movement takes place at the trees. A four bar (parallel link) mechanism provides the method for mechanical movement. The suspension components usually consist of a spring, shock, or combination of the two. Adding the shock reduces the affects of harmonics and vibration.
Mounting points for hardware, such as brake components and fender, are usually found on girder forks. Because the front axle is mounted on rigid forks allowances for wheel travel do not have to be factored into hardware mounting, unlike that of the springer forks. Girder forks are ideal for greater rake angles and situations where longer fork length is required.
Below you will find a comparison chart that gives side-by-side characteristic pros and cons to each type of fork.
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Reference: Chopper Fundamentals 101 © 2007