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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, girder forks or make your own leading link 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.

 

What is Frame Rake (Rake Angle)?

Frame rake is the angle formed by the center line of the neck tube and a vertical line. As a rule of thumb, the greater (higher) the rake angle, the longer the front forks must be.

 

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 trike 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. However, the smaller the trail the better it will handle.  Those of us that make our own leading link front ends strive for a 1-inch 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.

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Just how does rake and trail affect a trike? Well, rake doesn't seem to have much of an effect with a trike, but that's not the case with trail. The more trail you have the front end will tend to steer harder at speeds over 30mph.

 

TOO LITTLE OR NEGATIVE TRAIL
With too little or negative trail (steering axle mark behind the front axle mark), the bike will handle with unbelievable ease at low speeds, but will be completely out of balance at high speed. It will easily develop a fatal high-speed wobble. EXTREMELY DANGEROUS!

 

NORMAL TRAIL

Normal trail is somewhere between 3 and 5 inches. The bike will handle easily at both high and low speeds. Flowing smoothly through curves without swaying or wobbling. , you should strive to keep the trail as close to 1 inch as possible.
 

TOO MUCH TRAIL

If the trail is more than 5 inches the bike will handle sluggishly at high speeds. It will seem almost too steady. You will have trouble balancing the bike at lower speeds or on winding roads. It will feel generally sluggish and clumsy.