Chubbs wrote:Conceptual wrote:Unfortunately, your argument only is true in a perfect testing environment.
so what are you saying with this? that slicks will offer more grip because there is more rubber to provide more variance? doesnt make sense :S
Allow me to pontificate (why else would you write in a forum?
) about "the dangers of the blinding equations". First, as this is one of my "infamous" long posts, a sum up:
A wider tire:
1. When working by adhesion generates more force.
1. It runs cooler.
2. it makes more efficient use of its contact patch by having a greater percentage adhering
3. it can run at lower inflation pressure and therefore actually have a larger contact patch
4. it can have greater lateral stiffness at a given pressure and therefore keep its tread planted better
5. it can use a softer, stickier, faster-wearing compound without penalty in longevity.
Now, if you're interested in reading this, go on. I'll try to explain why without making too many amateur mistakes. So, a rant on tires:
Here we have two perfect reasonable persons, Conceptual and Chubbs, and who knows how many people more, saying that the venerable Coulomb law for dry friction is logical.
Logic and science are not well connected, that's why engineering exists.
This law, as any other law, is an empirical simplification.
That is the essence of science: to simplify observations using an abstract language, the language of equations.
On the other hand, I postulate that
the essence of engineering is to accomodate the equations to the real world using the human experience and common sense.
I've changed the tires of my car and I've felt more grip. You put wider tires in the back and you get understeer. So, the engineer in me assumes something is wrong. I tend to believe my own eyes and my butt (which is what "feels" the grip, sorry to say it like this).
First, as Chubbs and Conceptual points correctly, grip SHOULD be equal with a razor sharp tire IF equations were correct. The fact is that
they are not. Their reasoning, if I rephrase it correctly, is simple:
1.
If you use a wider tire the contact area is the same. It is wider, but it is also shorter. Simple: what matters is the pressure. Conceptual and Chubbs did not use this argument, but I throw it for free.
2.
If you use a wider tire the force you can develop is the same. Old Coulomb law takes care of that: it's insensitive to area.
That's, to put it in a harsh way,
pure bull manure.
First and foremost, as I've posted many times after I learned about it (so many that I'm really ashamed to give the links again, please search for "Bo Persson" in this forum)
F1 tires don't even work by friction.
They work mainly by adhesion (or at least the majority of the force comes from its "stickiness"). Of course, that's not
entirely true for a normal tire. When you use adhesion, is clear that the area of the patch DOES influence on grip.
Second, and crucial, picture the interface for a moment: when you push the car down, the rubber interlocks with the surface. When you turn, the rubber is dragged around the asperities as it oozes past them, but its also sheared. You could think of two gears pushed together with such force that one of them "loosing" its teeth on the process. Of course, if you increase the area, you diminish the pressure between the teeth and they "shed" at a lower rate, giving you a more durable tire. You do not want to lose the compound at such a rate (as would happen in a razor sharp tire) that you are left running on the rim after a couple of laps.
Third, if you follow the threads in this forum, you'll see that the first mathematical description of the true forces in the interface was developed in this century (around 2000, when Ferrari started to dominate!). It works by evaluating the interlocking between minute irregularities at many scales AND the molecular bonding.
Molecular bonding follows Coulomb's law, more or less, if I understand it correctly. It's what happens between dry, clean, polished surfaces, or in "natural adhesion". It depends on the materials only.
Interlocking has a limit: if you followed the "meshing gear" paragraph above, you can imagine that the harder you push a rubber tire, more and more the rubber is pressed into the irregularities, increasing the interlocking and "explaining Coulomb's law",
until there is no more surface available because all asperities have been filled by the squeezed rubber.
Simple: if you "start" with a lower pressure, you reach that limit later.
Finally, there are other effects not taken in account by old Coulomb: a tire works in a thermodinamic way. It generates drag through an slip angle and this drag times the speed equals the power. That power is dissipated as heat.
On a wider tire there is a large dissipating surface to radiate heat: you do not want to overcook the tire, or you lose the sticky part of the rubber faster. The second effect is that the patch is "generated" because a minute part of the wall bends.
On a wider tire, the wall bends less to develop this effect.
This post is too large to talk about lateral stiffness, but I'll mention I've seen my friends racing in rallies, running at 50 psi, to increase control. There are many references of this effect.
