In the case of steering forces, you have a tyre twisted around the axis of steering. In the case of the longitudinal forces, the carcass also twists around the axis of propulsion. You don't need a suspension in a car for it to move.
I agree: most of what we see in this world are the electrons. For example, in soil mechanics small particles adhere to each other by electric forces. Same goes for the colours of the things we see: light interacts with the electrons. So, the tyres interact with the track by electric forces, true, altough we suppose all the time in friction theories that the "significant" force is the weight of the car.
If I can make a summary, that's what I've learnt about tribology (the theory of friction):
The first known theory of friction was written by Leonardo Da Vinci around 1450. He studied a lot of things about friction, including wear, bearing materials, plain bearings, lubrication systems, gears, screw-jacks, and rolling-element bearings. Almost two centuries before Amontons' Laws of Friction were introduced, he had discovered them. Unfortunately his writings were lost and unread for two centuries. After Leonardo, the first guy (and almost the last...) to explain friction was Amonton, around 1650. He rediscovered Leonardo's principles of:
- friction proportional to weight and
- friction independent of the area of contact
Coulomb, around 1750, introduced the idea of:
- kinetic friction independent of speed of displacement
These three laws can be summarized in this graph, which, unfortunately is most of what the majority of people learn about friction:
Some engineers never go beyond this graph. Sad.
Then after another century, Reynolds (and a russian guy whose name I cannot remember) came up with an equation (unchanged since 1880 or so) of:
- friction in fluids proportional to sliding velocity and bulk viscosity and inversely proportional to thickness of film
A few years later Stribeck (not sure about the name) came up with the Stribeck curve that explains that when the film is very thin, Reynolds equation fails. He stated that:
- the area enlarges because the contact surface deforms elastically and the film, with a larger area, can support the weight.
Then, after 50 years (we're getting some speed here...
), Hardy, around 1920, came up with the idea of very thin films like this:- asperities coming in contact, breaking and then reacting chemically with the lubricant, thus creating a tenacious layer of lubricant and small chips of material that supports the weight and prevents further wear (I swear I'm not making this up...
).Hardy findings about films inspired some people to try to understand what happens in dry friction. Finally, a few years later, a guy named (I think) Bowden came up with the concept of friction by adhesion. Another guy called Desangulier in Coulomb's time had the same idea but nobody heard him because of Aumonton 2nd law (friction independent of area), so people devoted to purely geometric explanations (interlocking of asperities). Bowden discovered that Miguel is right and that:
- friction is created by adhesion of solids because of electric charges. It might sound incredible, but asperities in solids deform above a critical shear strength, which depends on the adhesive forces of the two surfaces in contact. That adhesive force, the one that "crushes" the asperities, is created by electric charges.
Yes, I know, it sounds like science fiction, but I'm dead serious. The relationship with the load is not lineal but is:
F = L^2/3 (that is, friction is proportional to load elevated to two thirds)
The inconsistence with Aumonton's first law (friction proportional to load) is explained because the real contact area varies under load. Why nobody noticed in five centuries beats me.
Once electronic force microscopes were developed, around 1950, Bowden (and Desangulier) were vindicated, because measurements were precise enough to validate their theory. Thus, Miguel is right. That's most of what I know about friction. and you can stop reading now.
If someone is interested in visualize (that's what distinguishes engineers!) what's going on, please, come with me for a couple more paragraphs and imagine what happens if you become really tiny: your weight decreases as the cube of your height does (a person half as tall, half as wide and half as deep, weighs eight times less!), right? However, if you're half as tall, your surface is only one quarter of the original one. For example, let me tell the story about Gauss famous observation on the size of things.
Gauss teacher told the class (in primary school, according to legend) that the Universe could grow slowly to one million times its actual size and nobody would notice. Gauss answered that it was not true, because all the picture frames would fall from the walls. Their weight would increase to the cube, but the area of the strings holding the pictures would increase to the square and they would not be resistant enough to hold the pictures to the walls. Incidentally, that's why ants and spiders have such thin legs and elephants have sturdy ones, but I digress.
So, if you're a really small asperity, you have a very large surface compared with your weight. If your ten thousand times (2^13) smaller than a person (around 0.2 mm), your weight/surface relationship is 10.000 times larger (I think). Persons are taller than wider: this means that electric charges migrate to the pointy parts (by electromagnetic laws: that's why lightning rods are pointy). Like this:
Small clay particle with positive electric charges in the pointy parts
Thus, any point in a microscopic asperity of the tyre (charged positively) adheres to the sides of the microscopic asperities of the track (charged negatively). Its weight is ridiculously small compared with the surface, thus the electric forces are immense compared with gravitational forces at these scales. Tyres adhere to asphalt by electricity. QED.
Finally, in the last ten years friction has been explained by quantum theory. In the improbable case that someone is interested in "The Master Equation" of friction, there you go: http://www.sbfisica.org.br/bjp/files/v27_214.pdf
Friction Quantum Theory Master Equation. If someone can explain it in simple terms, be my guest!
As usual, sorry for going OOT and sorry for the length of the post, but I've been tempted to write it a thousand times. That's the same "lesson" I give in class when I explain the plasticity of clays originated in electric forces. Man, I love this forum... perhaps someone will read al this voluntarily! On the other hand, my poor students have no choice.
