Rolling Resistance - Tyre Type and Pressure

[Goldnrust]

I believe there's oversimplification going on in that explanation of the physics.

Trying to dredge up my A-level physics memories, the concept of Ff=μFn is familiar (I remember the endless calculations of will the object on an inclined plane slide, roll or stay still...), but like many mathematical concepts that we know are true in a 'perfect world' we know are considerably more complex in reality.

Firstly theres static fraction, which I belief the above calculation is for, between two stationary bodies and secondly theres kinetic friction between two bodies sliding over each other. In the case of car tyres things get a lot more complex as the 'grip' we feel as a driver is generated by the slip angle of the tyre and the reactionary forces back through the tyre distortion to the wheel. It's considerably more complicated.

Put your fingertip on the desk in front of you, drag it across the desk with the weight of your hand pushing down, then do the same with your whole hand flat on the desk and tell me the surface area increase didn't make it grip more. Like the tyre your hand deforms and interacts with the table top in a much more complex way the the simple equation suggests.

Back on topic though... Not with an Austin 7 but in my '65 Fiat 500 (similar weight and power to a Ruby I believe?) I've also experienced just how much difference tyre pressure can make on top speed. I'd been experimenting with lower tyre pressures as many folks say it helps make the car less twitchy at speed, which it did, but it was knocking 5-8mph off my top speed. I opted to be able to cruise at 50-55 instead of 45-50 and to live with the twitchiness!

[Tony Griffiths]

Mrs. Google tells me:
 
The relationship between frictional force and the area of contact is a nuanced topic in physics. Here's a breakdown of the key points:

1. Frictional Force Basics: The frictional force (Ff) between two surfaces is generally given by the equation:
   Ff=μFn
   
2. where μ is the coefficient of friction (which depends on the materials in contact) and Fn is the normal force (the perpendicular force pressing the two surfaces together).
   
3. Area of Contact: Intuitively, one might think that a larger area of contact would lead to a larger frictional force. However, in many cases, the frictional force is independent of the contact area when the normal force is constant. This is because while a larger area may distribute the force over a larger surface, it also reduces the pressure (force per unit area) on any given point of contact.
   
4. Pressure Consideration: The frictional force is more strongly influenced by the pressure between the surfaces. For a constant normal force, increasing the area decreases the pressure, which can lead to a decrease in the microscopic interactions that contribute to friction.
   
5. Exceptions: There are some scenarios, particularly in materials with very high surface roughness or in cases involving adhesives, where the area of contact can play a more significant role in determining the frictional force.
   
6. Conclusion: In general, for most practical purposes and under normal conditions, the frictional force does not depend on the contact area for rigid bodies in dry friction scenarios. The key factors are the nature of the surfaces and the normal force applied.
   

Indeed -  the chemical (rubber compound) and mechanical actions between the tread and road surface are important - and in practice, wider tyres do, in general, give greater grip under 'normal' conditions, though, of course, on snow and standing water, wide tyres are rather less happy.  I once had the front tyres on a UNO Turbo replaced, drove away from the garage on dry roads and was appalled at the instant and massive understeer. Upon checking, the garage had mistakenly fitted tyres far narrower than those specified. Check YouTube videos of two identical BMWs fitted with summer and winter tyres of the same size accelerating away on a wet track. It's all very interesting....

[Malcolm Parker]

Let's not get too carried away with advanced physics!
When it comes to these matters Alan knows his stuff.     I have always relied on road testing and made my own mind up as to what tyre pressure suits the Austin 7 best.   My  RL saloon on Longstone 3.50 x  19 's is happiest at 24psi, with the Avons I used to run at 22psi.

[Duncan Grimmond]

"Let's not get too carried away with advanced physics! I have always relied on road testing and made my own mind up as to what tyre pressure suits ...."

I agree wiv' 'im!

[Chris KC]

I'm by no means a tyre expert, but in the world of vehicle braking we use the formula F = µR.
Let me also explain that in the world of modern vehicle braking, stopping distance is all about tyre grip :0)
F is the friction force (that is to say, the attainable force which can be used for braking and/or steering), µ is the coefficient of friction (a constant to all intents and purposes, for a given tyre compound and given road surface), and R is the reaction force (which is simply the weight on the wheel or axle).
You will note contact area and tyre width are nowhere to be seen in that formula.
That's not to say that area of a tyre on the road - or of a brake shoe on a brake drum for that matter - cannot have an influence; but it is generally a negligible one.

[Tony Press]

More science- I know that today everyone knows more than the person who has scientific training - (look at the US)

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?
Asked by: Mark Secunda

Answer
This is a good question and one which is commonly asked by students when friction is discussed. It is true that wider tires commonly have better traction. The main reason why this is so does not relate to contact patch, however, but to composition. Soft compound tires are required to be wider in order for the side-wall to support the weight of the car. softer tires have a larger coefficient of friction, therefore better traction. A narrow, soft tire would not be strong enough, nor would it last very long. Wear in a tire is related to contact patch. Harder compound tires wear much longer, and can be narrower. They do, however have a lower coefficient of friction, therefore less traction. Among tires of the same type and composition, here is no appreciable difference in 'traction' with different widths. Wider tires, assuming all other factors are equal, commonly have stiffer side-walls and experience less roll. This gives better cornering performance.
Answered by: Daryl Garner, M.S., Physics teacher MacArthur High School, Lawton, OK

[Alan]

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?

[andrew34ruby]

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?

I can't see how that last reply is correct. It seems to be saying that a softer, grippier, tread needs to be wider in order to support the weight of the car.

[Steve Jones]

Alan and I have known each other for a fair few years. I know his level of expertise when it comes to tyre engineering right to the level of him being an international expert witness. He knows his subject. Sadly, it seems Mr. Google and Mr. AI now know more

Steve

[Chris KC]

Where a larger contact patch might help is in reducing the rate of wear. 
Naturally a soft composition tyre will wear a lot faster than a "hard" one. 
The impressive width of those track tyres may just get them through the race.

Once again, emphatically not a tyre expert! I'm ready to be corrected (and Alan, I know how you feel. Everyone knows more about cars than the people who design them! Especially mechanics at dealerships and lads on the production line).