3.6 STABILITY AND BIOMECHANICS

PART 3

It is very important for instructors to be able to look at a rider and answer this simple question: “Is the rider stable?” This basic primer should help you quickly answer “yes” or “no.”

CENTER OF MASS (COM)

The Center of Mass (COM) is the balance point of an object. Our “object” is the bike and rider, and the Center of Mass is a point in space we will reference that’s just in front of the rider’s navel. The COM moves if either the rider or bike changes position.

BASE OF SUPPORT (BOS)

The Base of Support (BOS) is the contact patch where the bike tires touch the ground and the imaginary area between these patches. When only one wheel is on the ground, the base of support is the contact patch between that one tire and the ground.

LINE OF GRAVITY (LOG)

The Line of Gravity (LOG) is the straight path from the Center of Mass to the center of the earth. Because gravity does not change, the Line of Gravity is always vertical, no matter how the bike and rider are positioned. Generally, the LOG should fall through the bottom bracket.

The figures below show the elements responsible for a rider’s stability. Observe how the Center of Mass (COM), Line of Gravity (LOG), and Base of Support (BOS) interact for a rider on flat terrain and for a rider descending.

PRINCIPLES OF STABILITY

There are three basic principles of stability arising from the relationship between the Center of Mass (COM), Line of Gravity (LOG), and Base of Support (BOS).

Three things a rider can do to improve their stability.

• Lower their Center of Mass (COM).
• Increase their Base of Support (BOS).
• Bring the Line of Gravity (LOG) closer to the center of the Base of Support (BOS).

STABILITY INCREASES WHEN THE CENTER OF MASS IS LOWERED.

Examples:

• Crouching the body on technical climbs.
• Moving the hips down and back on descents.
• Lowering the shoulders toward the handlebars in tight turns and high-speed corners.

Which rider below is more stable?

The rider on the left is more stable because the Center of Mass (COM) is lower.

STABILITY INCREASES WHEN THE BASE OF SUPPORT IS INCREASED.

The Base of Support is largest when both wheels are on the ground and the bike is on flat terrain. As the Base of Support decreases, stability will also decrease.

Examples:

• When only one bike wheel is touching the ground, the Base of Support (BOS) becomes the contact area under one tire. The Base of Support (BOS) is very small and stability is hard to maintain.
• Increasing the width of the tires on the bike increases the Base of Support (BOS) slightly because wider tires have a larger area of contact with the ground.

Which rider below is more stable?

The rider on the left is more stable because the Base of Support (BOS) is larger.

STABILITY INCREASES AS THE LINE OF GRAVITY MOVES CLOSER TO THE CENTER OF THE BASE OF SUPPORT.

Riders have maximum stability when the Line of Gravity (LOG) falls in the center of the Base of Support (BOS). Maintaining a balanced position is crucial to a rider’s stability because it keeps the Center of Mass (COM) over the middle of the Base of Support (BOS).

Stability decreases as the Line of Gravity (LOG) moves away from the middle of the Base of Support (BOS). The bike and rider are unstable if the Line of Gravity (LOG) falls outside the Base of Support (BOS).

Which rider below is more stable?

The rider on the left is more stable because the Line of Gravity (LOG) falls closer to the center of the Base of Support (BOS).