Motorcycle Front Ends

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)

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

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.

Girder Forks

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.

This information is taken from:

http://hubpages.com/hub/Motorcycle-Front-Ends

motorcycle fork geometry

Fork Length, Rake and Trail

Motorcycle Fork Geometry and Measurement

One of the most crucial components when building a custom chopper, or customizing a motorcycle, is the front forks. Whether you choose tube forks, springer forks or girder forks the right front end will significantly affect ride comfort and handling.

Certain geometry and measurements should be understood before purchasing the front forks for your motorcycle or chopper. The first to be discussed will be fork length, followed by rake and then trail.

Measuring Fork Length

Fork length is measured from the base of the frame neck to the center point of the axle mounting hole. For any off-the-shelf motorcycle this standard measurement for the front forks is considered stock length. When buying forks other than original replacement parts, their lengths are compared to the stock length. Forks that measure longer than stock are listed as Over-Length (OL). Those forks that measure shorter are listed as Under-Length (UL). If the forks are the same length as original the fork length is listed as stock. It is important to note that when the rake angle is changed, fork length may need to change in order to compensate for the change in rake angle.

Changing Rake Angle

Rake angles can be changed in two ways. Changing the neck angle on the frame, or adding angled triple trees.

Changing the neck angle involves cutting the neck from the frame tubes and lengthening or shortening the frame tubes to get the desired angle, then welding the neck tube back into place. This is not a job for a novice. Any change in frame geometry can result in serious handling changes. Consult a local fabricator or motorcycle shop. Custom frames can be purchased with a variety of rakes.

The other option is to mount the front forks with angled triple trees. Angled triple trees use mounting holes that have been angled in order to add 2-5 degrees of rake to the front forks without changing frame geometry. These are very useful for adjusting trail when minor changes to fork length, and wheel sizes have been made.

A quick math example, along with the illustration to the right (Affect Of Angled Triple Trees), should demonstrate the overall affect of using triple trees.
If the Rake Angle is 35 degrees and the triple trees are angled at 3 degrees, the Overall Rake Angle is 38 degrees.

Measuring Trail

One of the handling characteristics of longer front forks and increased rake angle occurs at lower speeds. The effect is called “flop,” and it refers to the front wheel wanting to fall to one side or the other when turning. It makes the handle bars hard to control, and if flop is great enough, it can actually make the chopper dangerous to maneuver at low speeds. One of the ways to reduce flop is to adjust the trail.

Trail is a horizontal measurement, that describes the distance between a line drawn vertically from the center of the front axle to the ground and where the neck center line intersects the ground. It sounds complicated to explain, but is more easily illustrated to the right. Though no exact measurement could be reached from my interviews and research, it seems the general consensus is that 3 to 5 inches is the desired range for proper trail. Any trail measurement outside that range will result in undesirable handling characteristics.

Trail Adjustment

Trail can be adjusted in a few ways. Before trying any of these options check with a local mechanic or motorcycle shop.
Changing the front wheel diameter or tire profile is one way. This will only result in minor changes in the trail however.
Changing the fork length is another option.
Adding angled triple trees will change the rake angle and will significantly impact the trail measurement.
Change the geometry of the frame. This should be done as a last resort, by somebody experienced in frame modification.

This information is taken from:

http://hubpages.com/hub/Motorcycle-Front-Ends

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