Motorcycle Geometery - Measurements Explained
Your front suspension geometry is defined by the following six variables which are defined as:
OFFSET: Centerline of the top steering neck to the centerline of the top of the fork tubes.
RAKE: A motorcycle rake (frame neck angle) is the fork angle measured from an imaginary 90 degree line. The rake of a bike has a direct influence on the trail.
FORK LENGTH: The distance between the top of the fork tubes to the centerline of the axle.
DIAMETER: The diameter of the front tire.
TRAIL: The distance between the the vertical line from axle to ground and the extended line of the fork angle to ground.
WHEEL BASE: Defined as the distance between the center the front and rear axles of a motorcycle while the front wheel is positioned straight ahead. Wheelbase is a function of motorcycle frame length, steering axis angle, and fork offset. Wheelbase has a major influence on the longitudinal stability of a motorcycle. Sportbikes with a shorter wheelbase have the quickest handling and cornering characteristics. However, motorcycle’s which have a longer wheelbase have a tendency to have more straight line stability at high speeds, hence cruisers and tour bikes dont perform as well in high speed corners.
RAKED TRIPLE TREES: Manufacturers make triple clamps with angled steering stems which can increase the stock angle of the fork tubes without cutting and re-welding the neck. These triple trees can be purchased in 3, 5 or 7 degrees of rake.
HOW TO MEASURE CORRECT TRAIL
Raise the bike to an upright position, using a tape measure, hold the tape straight down from the front axle to the floor. Put a mark on the floor at that point. Then place the tape parallel to the steering neck, following the angle of the steering neck all the way up to the floor. Put a mark here also. Now measure the distance between the two marks and you have your trail measurement. It should read between 2 and 4 inches. Note: If your bike is equipped with a rear suspension, have someone sit on the seat when you make the measurements to simulate your actual riding condition.
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 is somewhere between 2 and 4 inches. The bike will handle easily at both high and low speeds. Flowing smoothly through curves without swaying or wobbling. If you use a very fat rear tire, you should keep the trail as close to 4 inches as possible.
TOO MUCH TRAIL
If the trail is more than 4 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.