Tires

It's All About Traction

Tire Pressure

The amount of force, in pounds, exerted on the sidewalls of the tire per square inch – or PSI

PSI = \frac{force}{area} = \frac{{weight}_{rider + bike}}{tire patch}

The area of the tire that touches the ground, or the TIRE PATCH, creates a frictional force called TRACTION with the surface. Tires with too high or low of pressure do not have the optimal size tire patch which causes problems with surface traction.

Properly inflated tires are firm and evenly distributes the weight. — **Figure 33:** Proper Inflation

Overly inflated tires bulge out due to excessive air and will wear the tread in the middle. These can lose traction because the shape’s been deformed by excessive air pressure. — **Figure 34:** Over Inflation

Rolling resistance
damage: popped tube
little comfort

Under inflated tires have a sagging look to them and will show wear on the outside edges of the tread. — **Figure 35:** Under Inflation

+ rolling resistance
+ damage: pinch flat
- responsiveness

Graph of bike tire pressure according to rider weight. For a mountain bike, the increase of weight to tire pressure is a very slight incline while a road bike has a steeper incline. — **Figure 36:** Line graph of bike pressure in relation to rider weight. Riders should adjust their tire pressure based on their **WEIGHT** to optimize the tire patch size. Starting at 40 pounds, tire pressure for a road bike should be about 80 PSI and for a mountain bike, should be about 30 PSI. The road bike has a steeper, steady incline towards 240 pounds with about 135 PSI. The mountain bike only slight increases towards 240 pounds with about 50 PSI.

Adjust tire pressure to accommodate for:

Road or trail conditions
Weather conditions
Tire type
Temperature
Altitude

Tread Pattern

The pattern along your tires’ rubber changes the force of friction along the road or trail.

smooth tread diagram showing closely pacted pattern. — **Figure 37:** A smooth tread pattern is more compacted and closer together, leaving very small gaps in the tread.

A smooth pattern, like on a road bike, decreases resistance with pavement

knobby tread diagram showing a loose pattern with more gaps between the tread. — **Figure 38:** A knobby tread pattern has more gaps in its design

A knobby pattern, like on a mountain bike, helps keep traction with a trail

Tire Size

A tire with a smaller diameter accelerates faster, however a larger diameter tire will clear obstacles easier due to the IMPACT ANGLE.

Bicycle Gyroscope

Is the Wheel Defying the Laws of Physics?

Have you ever noticed that it seems difficult to balance your bike when moving at very slow speeds? When you pedal, you increase your speed and create angular momentum which helps you maintain a stable, upright position.

Slow Speed:

Low angular momentum

Pedaling at a slower speed, there is less stability and therefore low angular momentum.

Faster Speed:

High angular momentum

Pedaling at a faster speed, high angular momentum is created and helps maintain a stable, upright position.

A rotating bicycle wheel acts as a gyroscope. It is thought that the gyroscopic effect plays a major role instability and control when riding a bike. A gyroscope is based on the principle of angular momentum.

$angular momentum = momentum of inertia \times angular velocity$

moment of inertia: describes an objects tendency to resist angular acceleration

$moment of inertia = mass \times {radius}^{2}$

Thus,

$moment of inertia = mass \times {radius}^{2} \times angular velocity$

Mass is evenly distributed on the front wheel and as it spins, the axis rotates and in turn develops angular velocity. — **Figure 40:** Gyroscope Diagram showing the angular velocity, mass, radius and axis of rotation.

When a wheel is spun, it develops angular momentum which causes the wheel to resist changes in orientation as explained by the law of conservation of momentum. This states that the angular momentum must remain constant in both magnitude and direction unless acted on by an outside force.

angular momentum _initial = angular momentum _final

Although the gyroscope tries to maintain its orientation, outside forces such as friction act on the gyroscope causing it to eventually lose its angular momentum as the wheel’s speed decreases.