Register to reply

Wider tires, Friction questions

by masscal
Tags: area, friction, independent, tiers, tire, tires, wider
Share this thread:
masscal
#1
Mar4-14, 07:21 PM
P: 28
I was reading my textbook, and I came across the old formula F friction = mu N . It also explicitly stated that force of friction is independent of area. Of course I immediately wondered if this were true then why do wider tires have move grip, So googled searched for the answer. I tried a couple of different searches, and read about 2 dozen different resulting pages. Here's the problem. They all had different answers, and every answer had another response that completely contradicted it.
After all that reading I actually became less certain about what exactly friction is and how it works, so here is a list of questions I have.


1. Is the force of friction independent of area for all different types of friction

If Friction is caused by the molecular forces between two objects in close contact, and said objects are relatively FLAT than why wouldn't an increase in area cause greater friction.( more molecular bonds/ attractions)

If the friction is caused by the interlocking of teeth of the two objects together, and the only way to start moving is to knock down the teeth (instead of lifting or sliding one object over the teeth of the other) then shouldn't an increase in area cause a great increase in friction? An even better example would be Velcro, surely an increase of the area of Velcro would cause a greater force of friction.

2. If the force of friction is independent of area, then Why do wider tires have more friction or better grip?

I searched for the answer but I found many different responses. Most of the responses were contradicted by a different answer given by someone else. Some claimed the tire didn't actually have more grip, but instead would wear down less and thus provide better traction. Some said the tires when hot act like glue or scotch tape, and thus increasing the surface area would be like adding more glue, others claimed that the Contact patch didn't even increase with larger tires. I can't even list all of the different answer here.
So, I'm hoping you guys can shine some light on this .
Phys.Org News Partner Physics news on Phys.org
New approach to form non-equilibrium structures
Nike krypton laser achieves spot in Guinness World Records
Unleashing the power of quantum dot triplets
masscal
#2
Mar4-14, 08:16 PM
P: 28
So how do you edit the title of a thread...
AlephZero
#3
Mar4-14, 09:05 PM
Engineering
Sci Advisor
HW Helper
Thanks
P: 6,931
Your textbook is not talking about real-world friction. It is talking about a simple mathematical model of friction, proposed by Coulomb about 250 years ago, which it is fairly accurate for hard materials (wood, metal, glass, etc) sliding at fairly slow speeds. That is the sort of friction experiment you would do in a ihigh school physics lab. The big advantage of Coulomb's "law" (or model) is that you can use it to learn how to solve mechanics problems, without needing a computer to crunch the numbers.

There seems to be a lot of mis-information because some people have never learned anything except Coulomb's law, and think that is "everything" there is to know about friction.

I'm not going to try to "explain" friction of car tires, but they certainly do NOT obey Coulomb's "law" of friction.

The contact area of a tire does not depend on the size of the tire. It mainly depends on the air pressure. The total contact area of all the tires, times the air pressure, = the weight of the car. If you want to demonstrate that, just let some air out of one of your own car tires (but make sure you pump it up to the correct pressure again afterwards!!!)

berkeman
#4
Mar4-14, 10:15 PM
Mentor
berkeman's Avatar
P: 40,653
Wider tires, Friction questions

Quote Quote by masscal View Post
So how do you edit the title of a thread...
I took care of it for you.

BTW -- any time you want a thread or title edited, just click the Report button and ask the Mentors for help.
masscal
#5
Mar4-14, 11:00 PM
P: 28
Quote Quote by berkeman View Post
I took care of it for you.

BTW -- any time you want a thread or title edited, just click the Report button and ask the Mentors for help.
thanks
cjl
#6
Mar5-14, 10:33 AM
P: 1,008
Quote Quote by AlephZero View Post

The contact area of a tire does not depend on the size of the tire. It mainly depends on the air pressure. The total contact area of all the tires, times the air pressure, = the weight of the car. If you want to demonstrate that, just let some air out of one of your own car tires (but make sure you pump it up to the correct pressure again afterwards!!!)
This is true, however the shape of the contact patch (which affects grip available) and the ability of the tire to withstand high heat loading (from high performance driving) are both affected by the size of the tire, which is part of the reason why wider tires tend to be preferred for high performance cars.
LURCH
#7
Mar5-14, 12:28 PM
Sci Advisor
P: 2,510
Quote Quote by AlephZero View Post

I'm not going to try to "explain" friction of car tires, but they certainly do NOT obey Coulomb's "law" of friction.
I'm sure a full explanation would get far over my head pretty quickly. However, is there a simplified version that would help a layman understand the difference between the forces on a real car tire and those that would be predicted by Coulomb's?
russ_watters
#8
Mar5-14, 06:29 PM
Mentor
P: 22,234
Tires deform, which changes the surface structure and therefore how much grip they have.
FactChecker
#9
Mar5-14, 06:45 PM
P: 278
An extreme example is the tire friction of a top fuel dragster. They can accelerate much faster than 1G even before the aerodynamic down-force begins to help. They burn the tires to make them sticky. But there are new things being discovered even now. Google gecko feet adhesive if you are interested. It has something to do with getting a lot of pad area very close to the surface.
SteamKing
#10
Mar5-14, 07:02 PM
Emeritus
Sci Advisor
HW Helper
Thanks
PF Gold
P: 6,279
This article discusses briefly the physics of tires:

http://www.physics.sc.edu/~rjones/ph...efriction.html
masscal
#11
Mar8-14, 12:07 AM
P: 28
Quote Quote by SteamKing View Post
This article discusses briefly the physics of tires:

http://www.physics.sc.edu/~rjones/ph...efriction.html
Interesting, In that link it states that the contact area for wider tires increases (assuming all else is equal). I've heard that stated before, but I have also heard the opposite (the contact area doesn't increase for wider tires). The very first reply to this thread states the opposite.


Register to reply

Related Discussions
Wider tires experience more friction than narrow tires? Introductory Physics Homework 11
Friction and wide tires. Classical Physics 8
Tires and friction General Physics 1
Friction in tires Introductory Physics Homework 1