Skip to main content


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 = force area = 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.

Figure 33: Proper Inflation

Properly inflated tires are firm and evenly distributes the weight.
Figure 34: Over 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.
  • Rolling resistance
  • damage: popped tube
  • little comfort
Figure 35: Under Inflation

Under inflated tires have a sagging look to them and will show wear on the outside edges of the tread.
  • + rolling resistance
  • + damage: pinch flat
  • - responsiveness
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.

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.

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.

Figure 37: A smooth tread pattern is more compacted and closer together, leaving very small gaps in the tread.
smooth tread diagram showing closely pacted pattern. 
  • A smooth pattern, like on a road bike, decreases resistance with pavement 
Figure 38: A knobby tread pattern has more gaps in its design
knobby tread diagram showing a loose pattern with more gaps between the tread.
  • 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.

Figure 39: A tire with a diameter of 26 inches will have an impact angle of x ° . A tire with a diameter of 27.5 inches will have an impact angle of x ° - 4 . A tire with a diameter of 29 inches will have an impact angle of x ° - 6
Impact angle for various tire sizes in inches from 26, 27.5 and 29 are respectively x, x-4 and x-6.

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 × angular   velocity

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

moment   of   inertia = mass × radius 2


moment   of   inertia = mass × radius 2 × angular   velocity
Figure 40: Gyroscope Diagram showing the angular velocity, mass, radius and axis of rotation.
Mass is evenly distributed on the front wheel and as it spins, the axis rotates and in turn develops angular velocity.  

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.