Rolling Resistance - Tyre Type and Pressure

[Colin Morgan]

For interest, I put the answer given above to the question:

 "As an engineer, I know that friction does not depend upon surface area. As a car nut, I know that wider tires have better traction. How do you explain this contradiction?"

into Chat GPT and asked it comment on the validity or otherwise.  This is its answer:

"Let's break down the explanation in relation to each statement:
1. "This is a good question and one which is commonly asked by students when friction is discussed."

• Comment: This is an introductory statement that acknowledges the complexity of the question. It sets the context for a detailed discussion on friction and traction, which is helpful for understanding why the apparent contradiction exists.
  
• Relevance to the Argument: It helps to frame the discussion, indicating that the topic is a common point of confusion, especially when discussing friction in engineering contexts.
  

2. "It is true that wider tires commonly have better traction."

• Comment: This is a correct statement. Wider tires generally provide better traction in certain conditions, as they can increase the contact patch, which helps distribute forces like braking, cornering, and acceleration more effectively.
  
• Relevance to the Argument: It helps to set the stage for the explanation of why wider tires are often thought to provide better traction, despite the friction model suggesting otherwise.
  

3. "The main reason why this is so does not relate to contact patch, however, but to composition."

• Comment: This is an incorrect or at least overly simplistic explanation. While tire composition (the materials used in the tire) is crucial to traction, the contact patch still plays a significant role in traction. The surface area of the contact patch influences how much grip the tire can provide, especially under high load. The argument that composition is the main reason for better traction neglects the role of contact patch and surface area.
  
• Relevance to the Argument: This is misleading because it suggests that the width of the tire doesn't matter, which isn't accurate. While composition is important, the contact patch (which is influenced by tire width) still affects traction.
  

4. "Soft compound tires are required to be wider in order for the side-wall to support the weight of the car."

• Comment: This is partially correct but requires clarification. Softer tires, often used for high-performance or racing, do tend to need a stronger sidewall to support the forces during driving. However, the primary reason tires are made wider is not just for supporting the weight, but to improve traction by increasing the contact patch. A wider tire can deform and grip the road better, particularly in soft compound tires.
  
• Relevance to the Argument: This helps explain why some tires are wider, though it misses a key point—wider tires provide better grip due to more surface area in contact with the road, not just to support weight.
  

5. "Softer tires have a larger coefficient of friction, therefore better traction."

• Comment: This is correct. Softer tires, due to their composition, generally have a higher coefficient of friction, which leads to better traction. The softer rubber deforms more easily, allowing it to better grip the road surface.
  
• Relevance to the Argument: This statement supports the notion that softer tires (regardless of width) offer better traction because of their higher coefficient of friction. However, it should be noted that softer tires tend to wear out faster.
  

6. "A narrow, soft tire would not be strong enough, nor would it last very long."

• Comment: This is correct. A narrow, soft tire may not provide adequate support, especially under high-performance conditions, and it will wear out faster due to the increased heat generated in the soft rubber. Tires need to be designed with both traction and durability in mind.
  
• Relevance to the Argument: This statement highlights why tire design balances factors like width, composition, and strength—showing that even with a soft compound, a tire needs the correct design to balance performance and durability.
  

7. "Wear in a tire is related to contact patch."

• Comment: This is correct. The contact patch influences how much rubber is in contact with the road, and thus how much wear occurs. A larger contact patch will generally result in more uniform wear over time, though it can also increase wear if the tire material is soft.
  
• Relevance to the Argument: This supports the idea that the contact patch plays a role in wear, which is an important consideration when thinking about tire design and longevity.
  

8. "Harder compound tires wear much longer, and can be narrower."

• Comment: This is correct. Harder compounds are more durable and are typically used in tires designed for longevity, like those on passenger vehicles. These tires can be narrower because they don’t need to be as wide to maintain traction, and the harder material reduces wear.
  
• Relevance to the Argument: This contrasts with the idea of soft compound tires, which wear out faster but provide better traction. It emphasizes how different compounds affect both durability and performance.
  

9. "They do, however, have a lower coefficient of friction, therefore less traction."

• Comment: This is correct. Harder tires have a lower coefficient of friction compared to softer tires, which results in less traction. The harder rubber doesn’t deform as easily and therefore doesn't grip the road as effectively.
  
• Relevance to the Argument: This clarifies the trade-off: while harder tires are more durable and wear less, they provide less traction due to their lower coefficient of friction.
  

10. "Among tires of the same type and composition, here is no appreciable difference in 'traction' with different widths."

• Comment: This is partially correct but requires context. If two tires are of the same composition and type (e.g., both are soft compound tires), the width may not drastically affect traction in ideal conditions, as the friction primarily depends on the material properties of the tire. However, in practice, a wider tire generally provides better traction because it can handle larger forces (in terms of braking and cornering) and can deform more under load.
  
• Relevance to the Argument: This is a bit of a simplification. While the material and tire type are key factors, the contact patch still plays a role, so a wider tire will generally provide better traction in real-world conditions.
  

11. "Wider tires, assuming all other factors are equal, commonly have stiffer side-walls and experience less roll. This gives better cornering performance."

• Comment: This is correct. Wider tires often have stiffer sidewalls, which reduce tire roll and help maintain stability during cornering. This is one of the reasons why wider tires improve handling and cornering performance.
  
• Relevance to the Argument: This is an important point because it explains how tire width affects handling performance, especially during high-speed cornering. The stiffer sidewalls reduce the tire’s tendency to deform, improving overall performance.
  

Conclusion:

• Correct Statements:
  • Softer compound tires provide better traction due to their higher coefficient of friction.
    
  • Narrow, soft tires may not provide enough strength and durability.
    
  • Wear is related to contact patch size.
    
  • Harder tires wear longer and can be narrower.
    
  • Wider tires provide better cornering performance due to stiffer sidewalls.
    
• Incorrect or Oversimplified Statements:
  • The claim that traction differences due to width don't exist among tires of the same type and composition overlooks the fact that width can still affect traction in real-world conditions through factors like tire deformation and load distribution.
    
  • The suggestion that contact patch doesn't matter for traction is misleading, as the size of the contact patch does influence traction, particularly in performance scenarios.
    

This overall explanation helps clarify that traction is influenced by both the tire composition (softness or hardness) and the physical characteristics like width and contact patch size. The original friction model needs to be expanded to include real-world factors like tire deformation, sidewall stiffness, and material properties."

Does that help?