Rolling Resistance - Tyre Type and Pressure

[Colin Morgan]

Not all questions have a simple answer - or even any answer in some cases - and tyre performance appears to be one of them.

Tyre engineering isn't the only area to suffer from complications that go beyond the understanding of ordinary mortals that don't have the benefit of forty years experience. Richard Feynman was famously asked on a radio programme to explain how a permanent magnet works - he refused point blank. The more you know, the more you know you don't?

Meanwhile, I will be keeping the Longstone tyres on my Ruby pumped up to the recommended 26 psi - and enjoying the extra mph or two this might give me.

[Tony Press]

One of the problems of being a tyre engineer is that everyone and his dog knows more about them than you because they have got four of them. So will someone explain why the last reply is correct because I can’t. Yet it’s from a teacher so it must be correct mustn’t it?

If this answer is so wrong maybe some expert should correct it- because I know very little, I sometimes resort to Mrs Google (unfortunately being American it could be suspect I know) . There does not appear to be any refutation of the poor teacher so far.

[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?

[Terrytuned]

Photo of Alan, testing the limits of grip at Curborough a few years ago photo by Dennis Rushton.    

[Tony Griffiths]

Not really relevant to us, but an interesting take on what happens when you play about with different wheels and tyres on a modern car: https://www.caranddriver.com/features/a1...els-tires/

[Alan]

Thanks Terry. I really liked those tyres. They were so predictable, gripped well especially when it rained and best of all were only 24 quid a corner.

[Tony Griffiths]

That's interesting, Morgan. Here is what Copilot AI said when asked the same question:
Ah, what a wonderfully tricky contradiction—it’s the kind of thing that makes physics both fascinating and counterintuitive! Let’s unravel it together.
You’re absolutely correct that the basic principles of friction teach us that the force of friction (FfF_f) is equal to the coefficient of friction (μ\mu) times the normal force (FNF_N), and this relationship doesn’t explicitly depend on surface area. So, for a simple block on a surface, changing the size of the block’s contact area won’t directly affect the frictional force.
However, when it comes to tires and traction, there’s more nuance at play:
1. Real-World Contact Mechanics:
Tires aren’t rigid blocks—they’re made of a deformable material (rubber) that interacts with an irregular surface (like asphalt). Wider tires spread the vehicle’s weight over a larger area, reducing the pressure at the contact patch. This helps the tire grip better by allowing more of the rubber to conform to the road’s microscopic texture, increasing the effective friction.
2. Heat Dissipation:
Wider tires are better at dissipating heat generated during high-speed driving or cornering. Heat can reduce the effectiveness of friction (e.g., causing rubber to become slippery), so the improved cooling capability of wider tires indirectly contributes to better traction.
3. Dynamic Effects:
Traction isn’t just about static friction. Wider tires improve handling by offering greater lateral grip during turns, especially in performance driving. They also have more area to transfer torque, which helps with acceleration and braking.
4. Vehicle-Specific Factors:
Tires must work in harmony with suspension, weight distribution, and other vehicle dynamics. Wider tires complement these factors, making them popular among car enthusiasts for high-performance applications.
So while the engineer in you holds to the theoretical principle that friction is independent of surface area, the car nut in you appreciates how practical realities like material deformation, heat, and dynamics all interact to give wider tires a performance edge.
Doesn’t it feel like the universe is full of delightful exceptions to the rules?

[dickie65]

With My Type 65 I have run up to 36 PSI on 3.50 x 19" Blockleys
My car is faster at the higher pressure and steering is much sharper. We are talking about the 60 to 70 Mph area.
This is almost certainly down to reduced rolling resistance as the tyre does not deform at the contact patch which gets smaller at the higher pressure.
The ride gets a bit harsher and road bumps throw the car about a bit more.
I discovered this when I forgot to let the tyres down after winter storage at high pressure. I normally run at 26 to 28 psi.

[Alan]

Tony I can’t give you a simple technical explanation of why simple friction doesn’t explain why tyres grip. It would take longer than my attention span. However, trust me, I’m at least as reliable as Google when it comes to tyres. As a tyre engineer I know I can completely change a tyres grip characteristics by adjusting things like sidewall cord tension and angles, tread belt (ie the metal bit) construction, even tread pattern can have an effect. Simple friction theory says this shouldn’t be possible. It is. That’s why I said what I did.