A Matter of Moment

A Matter of Moment

Weighty Problems In Pounds and Inches

WILLIAM MARON

MOTORCYCLE HANDLING IS affected by weight distribution. As weight is moved toward the rear axle a motorcycle becomes more unstable and less controllable. The extra weight of a passenger makes this readily apparent. Placing a load behind the rear axle drastically changes the feel of the machine. Each pound behind the rear axle alters the handling characteristics until a dangerous condition exists.

The reason is simply a matter of moment. Moment is the tendency, or measure of tendency, to produce motion about a point or axis. The front and rear axles of a motorcycle, Ligure 1, constitute two points. When the load (machine, rider, and fuel) is between the axle centers there is almost equal weight on either wheel. Additional load, such as a passenger and camping gear, toward the rear

reduces the weight on the front wheel. Thus steering effectiveness is decreased.

If part of the load is behind the rear axle, the front wheel contact with the road is sharply reduced. The basic diagram, f igure 2. illustrates this tendency.

Moment is calculated by multiplying the weight by the distance from the point (axle). If the distance is expressed in inches, the answer is in in.-lb. (inch-pounds); if in feet, the answer is in ft.-lb. (foot-pounds).

The effect on handling can be best understood by assuming a load of 100 lb. concentrated 15 in. behind the rear axle. This results in a moment of 1500 in.-lb. tending to litt the front wheel from the ground. Steering becomes extremely light, which leads to instability and overcontrolling.

The rear axle becomes the fulcrum and, like a see-saw, greater weight on one side makes the lighter side rise. To balance a see-saw, the lighter person sits farther from the point on which the see-saw rests. I his is not possible with a motorcycle. What is needed is some way of decreasing the 1500 in.-lb. of moment that is lifting the front. The only way is to remove part of the load or shift some weight forward by putting small, heavy items in a bag atop the fuel tank.

If the load on the tank is located 30 in. forward of the rear axle, calculation will show that 1 lb. put on the tank is equivalent to removal of 2 lb. from the rear, f or example, 30 lb. on the tank equals 900 in.-lb. This reduces the lifting force which tends to raise the front wheel by 900 in.-lb. Transfer of 50 lb. to the tank would completely neutralize the effect of 100 lb. on the rear.

There is an additional benefit, for each pound taken from the rear and put on the tank, a better overall balance is attained. An example will make this clear. Assume the rider normally travels with a passenger, plus 40 lb. behind the rear axle. With this total load, the handling of the motorcycle may be within acceptable limits. Now, it 20 lb. maybe a tent, stove, utensils, and food is added to the 40 lb., there is an additional 300 in.-lb. lifting the front wheel from the ground and control becomes a problem.

By redistributing the load so that 15 lb. is on the tank. 450 in.-lb. is moved forward. This has the same effect as removing 30 lb. from the rear. Handling is greatly improved, because the front wheel now carries greater weight, and the CG (center of gravity) has also moved forward.

The use of a tank bag which enables a better distribution of load makes it possible to carry more gear without adversely affecting the handling. It is simply a matter of moment.

The nomogram, f igure 3, provides means to determine in.-lb. values without the need of calculation. The nomogram gives in.-lb. values for weight up to 100 lb. and distance from the rear axle (forward or aft) up to 40 in. To use the nomogram place a straightedge from the figure which indicates the weight ot the load on the left scale, to tire distance from the rear axle on the right scale. The point at which the straightedge crosses the center scale is the value in in.-lb. f or example, assume a load of 50 lb. placed 12 in. behind the rear axle. Place a straightedge from 50 on the lett scale to 12 on the right scale, as shown by the dashed line. The center scale is crossed at 600, which is the value in in.-lb.