Measuring the Force

Understanding Tire Pressure and Contact Patch

Tire pressure has a direct effect on the contact patch, which is the area of the tire that is in contact with the road. As tire pressure increases, the contact patch decreases, and as pressure decreases, the contact patch increases.

Formulas for Calculations

Convert PSI to kg/cm²

To convert tire pressure from PSI to kg/cm², use this formula:

kg/cm² = PSI × 0.070307

Example: If the tire pressure is 32 PSI, then the pressure in kg/cm² is 32 × 0.070307 = 2.25 kg/cm².

Calculate Required Contact Patch Area

To calculate the contact patch area (in cm²) based on the weight supported by a single wheel (in kg) and the tire pressure (in kg/cm²), use this formula:

Contact Patch Area (cm²) = Weight Supported per Wheel (kg) / Tire Pressure (kg/cm²)

Example: If a wheel supports 300 kg and the tire pressure is 2.25 kg/cm², then the contact patch area is 300 / 2.25 = 133.33 cm².

Watch and discuss how cyclists need to consider the overall impact of tyre width and pressure when it comes to riding fast. How do you think this might relate to a race car using slicks vs wet tyres?
Normal rubber tyres wouldn’t last being heated and cooled by 100’s of degrees in the sun and shade. Do you think we should also be using metal wheels on Earth?

Understanding Tire Pressure, Contact Patch, and Ride Comfort

Tire pressure directly influences the contact patch, which is the area of the tire in contact with the road. A higher pressure reduces the contact patch, while a lower pressure increases it. This relationship affects how the tire interacts with the road and can be explained using Newton’s laws of motion.

Newton’s Laws and Tire Contact

According to Newton’s third law of motion, every action has an equal and opposite reaction. When a car’s weight pushes down on the tire, the road pushes back up through the tire with an equal force. This upward force is applied across the contact patch—the area where the tire touches the ground. As tire pressure changes, so does the contact patch area, impacting how effectively the car’s weight is distributed across the tire surface.

In simplified terms, the contact patch area (cm²) can be calculated using this formula:

Contact Patch Area (cm²) = Weight Supported per Wheel (kg) / Tire Pressure (kg/cm²)

Example: For a wheel supporting 300 kg and tire pressure at 2.25 kg/cm², the contact patch is 300 / 2.25 = 133.33 cm².

Low-Profile Tires and Ride Comfort

Tire profile, or sidewall height, also affects the ride comfort. Low-profile tires have a smaller sidewall height relative to the tire width, meaning there’s less rubber between the wheel and the road. This reduced cushioning causes the vehicle to feel bumps more sharply.

In contrast, tires with higher sidewalls have more rubber between the wheel and the road, which acts as a cushion. The extra rubber absorbs some of the road impact, providing a smoother, softer ride. This is why low-profile tires typically feel rougher on uneven surfaces than tires with a taller profile.