Physics - friction class...

[Externet]

Hello.

It was perhaps eight grade, and the friction formulas and the explanation of being independent of the contact area; asked the teacher why were then, the tires on racing cars wider.

After his babbling of being 'different case' would like to know what he meant, over half a century later.🤔

[swansont]

If the tire skids, the friction is doing work, which will heat the tire. This will possibly change the coefficient of friction, and also possibly damage it. A larger tire minimizes the temperature increase. A larger tire can also have a lower pressure; a temperature increase could also cause problems, possibly rupturing the tire.

And the equation F= uN is likely an approximation, so there may actually be an increase in friction for such high-performance tires.

[Sensei]

1 hour ago, Externet said:

After his babbling of being 'different case' would like to know what he meant, over half a century later.🤔

This is experimental physics - you take a tire, perform an experiment and get results that are put on a graph. And a few such repetitions and averaging them produces a graph that extrapolates to the rest.

 

BTW, Formula One drivers like warm tires:

https://www.google.com/search?q=formula+one+driver+like+warm+tires

"warmer tyres are grippier on the tarmac and that is essential when driving a car which can be chucked around corners at 190mph. When lining up on the grid before the start of a race, the tyres are wrapped up in covers – acting almost like a blanket – in order to preserve as much heat in them as possible."

28 minutes ago, swansont said:

a temperature increase could also cause problems, possibly rupturing the tire.

A typical F1 driver uses 3 sets of tires for the main race.

"On a standard weekend, drivers are given 13 sets of dry weather tyres, four sets of intermediates and three sets of full wets. An extra set of softs is reserved for those who reach Q3, while all drivers must use at least two different slick compounds in the race, providing the track is dry."

"Street car tyres will last about 15,000 Km, but the life of an F1 tyre is between 60 and 120 Km. Since they are built for extreme performance at high speeds, the rubber wears off in no time. This is why team personnel rack their brains to change the tyres at the appropriate moment of the race and in minimum time."

 

[Externet]

Those many sets of threaded, gummy, slick tires for wet, dry, cold, hot track are now all very wide with more tire contact area with the pavement.  Never narrow tires, which should present muuuuch better aerodynamics. Does not go well with a formula with absence of the area factor.

The school taught formula  F = uN  leaves voids.  May be just approximate; or experimental; but that is never told by the teacher.  Instead, I got the babbling , stuck in retina since the sixties. ☹️

In the fifties, seems started with double tire :

 

-Images borrowed from the web-

[Genady]

1 hour ago, Externet said:

The school taught formula  F = uN  leaves voids. 

It is quite straightforward to see why there is no area in this formula. Start with some area, weight, and frictional force. Now, make the area twice larger. The weight per square cm becomes twice smaller. But there are twice as many square cm. So, the frictional force per square cm becomes twice smaller but there are twice as many square cm. Thus, the total frictional force remains the same.

Edited July 24, 2023 by Genady

[studiot]

14 hours ago, Externet said:

Hello.

It was perhaps eight grade, and the friction formulas and the explanation of being independent of the contact area; asked the teacher why were then, the tires on racing cars wider.

After his babbling of being 'different case' would like to know what he meant, over half a century later.🤔

Embarrased or not, your teacher was not so far wrong. Car Tyres are indeed an entirely different case.

Not only that but there are further complications I will try to elaborate.

 

The friction theory they teach at elementary level only refers to the contact of dry solids.

Some textbooks and teachers make this point, some do not.

A car tyre is in general neither solid nor dry !

The study of forces involved between bodies in contact is called tribology.

 

Furthermore a car is a dynamical system, with constantly changing velocities, directions and accelerations.
More of this later.

Here is a well produced table by Professor Sir Charles Inglis of Cambridge University.

 

Note very carefully what he says about reactions and contact area.
You were taught only the left hand column.

Once again car tyres are neither solid nor dry.
They are actually a mixture of all three types (type 3  = partially lubricated)  - about as complicated as you can get.

Why do I say all this ?

Well the forces of interest for car tyres and the road surface are collectivelly known as the grip.

Here is a simple summary from https://www.racecar-engineering.com/tech-explained/tyre-grip/

Quote

This is where the roughness of the road’s surface excites the rubber material. As tyre rubber is viscoelastic, it distorts and adapts to the texture of the road surface as it slides over it. Therefore, as the tread block strikes a bump in the road, it deforms, but due to the hysteresis of rubber it does not immediately return back to its original shape. This asymmetrical deformation of the rubber generates a reaction force which opposes slippage – in other words it generates a friction force (grip).

 

Michelin. (2001). The Tyre – Grip. [Online]. Available at: http://www.dimnp.unipi.it/guiggiani-m/Michelin_Tire_Grip.pdf (Accessed 02/01/20)

 

Note that they describe tyres as having viscoscity.

Solids don't have viscoscity, liquids do.
Viscoelastic refers to a mixture of characteristics.

But even this does not tell the complete story.

You are asking why early racing cars and modern road cars have thinner tyres.

 

Well road cars have a dry coefficient of friction of 0.8 dry and zero to 0.6 wet.

Modern racing cars enjoy a coefficient of 1.4 to 1.7, on their fatter tyres.

 

Cars are not like your diagram of a solid block with full or nearly full contact area and a simple C of G load point.
The loads are distributed to the four extreme points.
They are dynamical systems (when running) which means that the loads are also constantly changing with the overall motion of the car as it goes not only forwards but twists and turns as well.
So the parts suffer accelerations not only due to the car's overall motion, but also due to load transfer.
With modern cars this transfer is modified by being transferred through springs.

For example a racing car's coeffiecient of friction can drop to 1.2 when also suffering sideways loads from cornering.

As noted in the linked article the tyres themselves have an internal dynamical structure which also modifies the friction laws.

 

There is even more to the subject as the tyres distort and recover and use energy, heating up in the process, leading to the so called rolling resistance or coefficient of rolling friction.
Also the wider tyres give a safety margin against slipping  and locking which is needed for the higher speeds and stresses involved in F1.

 

 

[Externet]

Thank you, studiot.  +10 

Never saw the 'right side' column, even at the university in neither fluid dynamics nor physics classes.  Well explained and thorough.

[sethoflagos]

On 7/23/2023 at 10:09 PM, Externet said:

It was perhaps eight grade, and the friction formulas and the explanation of being independent of the contact area; asked the teacher why were then, the tires on racing cars wider.

Much of what has been previously posted (@studiot in particular) is of course true.

However, I believe the main reason for using wider tyres is to gain maximum possible cornering forces without exceeding the limits of adhesion of any given tyre.

During safe, controlled cornering, the contact patch of the tread, which is aligned with the instantaneous direction of travel, is rotated relative to the main body of the wheel by what is called the 'slip angle'. The elastic forces created by this rotation sum to generate the 'cornering force' which provides the lateral acceleration profile necessary to rotate the vehicle without skidding out of control. Within limits and all other things being equal, the cornering forces are proportional to both the slip angle and the width of the contact patch - ie. tyre width. As excessive slip angles lead to sudden loss of adhesion, the highest safe cornering forces could be generated by wider tyres providing the consequent reduced frictional force per unit area problem could be overcome for these wider contact patches.

Since the 1960s, the incorporation of significant aerodynamic downforce into sports car design mitigated this loss of contact pressure by adding greatly to the gravitational downforce (hence restoring the higher available friction force per unit area) without unduly compromising acceleration and braking performance.

ie the available traction force for a sports car is not proportional to weight but to weight + aerodynamic downforce. Post '60s this has tilted the balance in favour of wider tyres despite the increased mass and drag.

 

 

 

Edited July 26, 2023 by sethoflagos
better wording