I occasionally mention specific books in my articles.
The reason is simple: I want people who are actually looking for that kind of information to be able to find them.
I’m not trying to tell people who say, “I like cars, but I don’t want to read difficult books…” that they have to read them.
However, for someone who thinks, “I want to understand that part in more detail…”, they can’t really move forward unless they can read the books that the explanation was based on.
Cars are complicated. When you’re talking about complicated topics, having solid sources matters. That’s why I’m grateful for books that clearly explain the background and reasoning behind the things they discuss.
By the way, one of the first walls people run into when they start learning vehicle dynamics is this question:
“Are wider tires better, or are narrower tires better?”
If you’re the kind of person full of enthusiasm who says, “I don’t really know, so I’ll just try everything!” then that’s actually a perfectly valid approach. Just test different tires and choose the ones that give you the best lap times.
But for people who want to understand what principles are actually at work, it’s not that simple.
It is often said that narrower tires tend to lack grip, while wider tires can be disadvantageous when it comes to top speed and rolling resistance.
So the usual conclusion is: choose the right size.
But the important thing here is one fundamental law of physics related to friction.
Friction Force F = Coefficient of Friction μ × Load P (Amontons–Coulomb Law of Friction)
Here, the coefficient of friction μ changes depending on things like the properties of the rubber and whether the road surface is dry or wet.
The load, meanwhile, changes depending on the vehicle weight, aerodynamic forces, cornering G, acceleration and braking G, and the direction in which the force is applied. It is the vertical load acting on each tire at that moment.
Now, there is something interesting about this equation.
The contact area between the tire and the road does not appear anywhere.
If contact area is not included, then when we consider only this law, tire width should have nothing to do with tire grip.
Wait… what?
Really? It doesn’t matter?
But in reality, it definitely feels like wider tires grip better.
Or does it just feel that way?
The laws of physics do not always match our intuition.
It’s similar to how the speed of an object falling in a vacuum does not depend on its mass.
So let’s think about a simple example.
First, prepare a desk and an eraser.
Press the eraser firmly against the desk. It grips the surface quite well, doesn’t it?
Now place 10 books on top of the eraser.
Maybe around 5 kg (11 lb) or so.
If you try to move only the eraser, it won’t move easily because it is gripping the desk firmly.
Next, replace the eraser with a thin rubber sheet.
Let’s say the rubber sheet is just slightly larger than the books.
Place the same books on top of it.
Now, if you try pulling the rubber sheet, it still won’t move easily because it is gripping the desk firmly.
If the eraser and the rubber sheet have the same coefficient of friction, and the weight of the books placed on top is the same, then the friction force will also be the same.
…That is what F = μP means.
The eraser has a smaller contact area than the rubber sheet, but the friction force does not change.
If we apply only the Coulomb law of friction like this, we would have to conclude that tire width has nothing to do with grip.
But people who actually drive on circuits and chase lap times probably won’t be satisfied with that answer.
When something doesn’t make sense, I keep thinking about it until I can understand why.
It is important to have the fundamentals of physics firmly in your mind, while at the same time always keeping the perspective of: “Am I overlooking something?”
Many people have questioned the relationship between tire width and grip, and there have been many discussions about it over the years.
However, reading Bridgestone’s book Fundamentals and Real-world Application of Automotive Tires solves the mystery immediately. (This book is only available in Japanese, so I can’t link you to an English edition — but the same contact-pressure dependence is well documented in the broader tire and rubber-friction literature. See, for example, Persson’s research on how rubber friction depends on load.)
Automotive tires have something called contact pressure dependence.
The coefficient of friction changes depending on the contact pressure.
Therefore, wider tires provide more grip.
That’s the basic idea.
Simple, right?
Well, strictly speaking, it isn’t quite that simple, but this is the general idea.
In short,
・A tire is not just a lump of rubber. It is a complex structural material that uses components such as steel belts and is filled with air.
・A tire’s coefficient of friction is affected by the pressure at the contact patch, and higher contact pressure tends to reduce the coefficient of friction.
Because of these factors, when the load is the same, lower contact pressure at the contact patch results in greater grip.
In other words, a wider tire can reduce the pressure per unit area, which is why it can produce more grip.
For the full details, you’ll have to ask Bridgestone.
When I first read this book, I thought: “Wait a minute! Something this important should have been in an elementary school textbook!”
Because I had seriously struggled with this question and just couldn’t make sense of it.
Of course, expecting elementary school students to understand contact pressure dependence would be asking a bit too much.
In reality, high-grip tires become sticky as they heat up.
This stickiness is not just friction. It also involves adhesion.
And just like adhesive tape, adhesion becomes stronger as the contact area increases.
So this is another reason why wider tires tend to be advantageous.
I’ll leave the effects other than friction aside for this article.
If we trust Bridgestone’s research results, then the question of tire width doesn’t require endless debates.
The answer was already there: “Just read this first.”
People say, “Seeing is believing.”
But sometimes, reading one good book can be worth more than thinking about something a hundred times.
Something that someone like me, just an amateur enthusiast, could spend years struggling with can sometimes be solved by a single book.
Of course, that doesn’t mean we should blindly believe everything written in books. It is still important to think for yourself and decide whether the explanation really makes sense. However, in the case of Bridgestone’s book, we’re talking about information published by a world-class tire manufacturer based on many years of research. Compared with the assumptions and guesses that amateurs like us come up with, it’s naturally a much more reliable source.
I’m grateful that they’ve made even a small part of that research available to the public.
To me, this is what reading books and learning the fundamentals are all about.
The important things are already out there. We just haven’t reached out and taken them yet.
But that’s perfectly normal. Most people can live their everyday lives without ever needing to know these things.
When you become curious and want to understand more, that’s when you can simply reach for the bookshelf.
That said, relying only on books isn’t enough. There is also value in trying to work things out by yourself, using your own experience and intuition.
Creating new ideas, of course, requires your own thinking. But even understanding someone else’s achievements is impossible without thinking for yourself.
Steady, consistent effort will eventually pay off.
…Although sometimes, after struggling for years to reach an answer, you discover that it was already common knowledge decades ago. That happens more often than you might expect.
Either way, our lives are short, and there is only so much one person can accomplish. That’s why the achievements built up by the many researchers who have spent years building our knowledge are truly remarkable.
However, no matter how amazing the facts written in a book may be, simply knowing them is not what makes someone impressive.
The people who deserve the credit are the ones who spent years researching and building that knowledge. I just read their work, so I haven’t done anything special.
That’s why clearly showing the source is so important. It means saying: “This is the person who discovered it.” Or: “You can learn the same knowledge by reading this book.”
Good books do the same thing. Whenever they build on previous research, they clearly list their references.
Everything we have today is built on the work of countless people who came before us. I think we should always show respect for that.
A book contains a piece of someone’s life.
Reading difficult books is not easy. But simply by reading them, we can receive so much from the people who wrote them. That’s what books are all about. And I think that’s a wonderful thing.
That’s what I believe knowledge is.