Why do fat tires offer better grip on the road than thin tires? *SOLVED*

[Triumph]

Originally posted by: Skoorb
You're quite right and it's what I think most of us agree upon. It doesn't change simply because as surface area expands so does u. It changes in this case because of what exactly that surface area expansion is doing behind the scenes, and in this case it's introducing the asperities mentioned throughout the thread.

Yeah, I didn't really bother to read the entire thread. From what I always understood, though, was that asperities were the cause of all friction. Microscopic surface imperfections coming in contact with each other and exerting a lateral force on one another. I do know that science does not know everything about what exactly causes friction.

 

[tcsenter]

REPOST!

 

[DrPizza]

Clearly if, as the pure textbook physics answer states, u is independent of surface area the "coefficient of friction is the same for all objects made of the same material.".

Actually, the coefficient of friction isn't a property of just one material, but rather is a property of the interface between TWO materials.
That is, you cannot say "the coefficient of rubber is .90" What you COULD say though is "the coefficient of static friction between rubber and dry concrete is .90"

The coefficient of static friction between rubber and asphalt is .85
or

the coefficient of kinetic friction between rubber and ice (sliding, not static - after you lose control) is .15

Note: there are no units on the coefficient of friction.
Skoorb has been correct all along in that generally, the frictional force is INDEPENDENT of the surface area of contact. Any difference in "grip" of the tires is due to other considerations (see my post earlier in this thread) such as wider tires being able to "take advantage" of more imperfections in the road's surface.

Also, F=uF is for relatively smooth surfaces. In the case of tires, you have the rubber being pushed into the small nooks and crannies in the road's surface, so in addition to frictional forces, you have to consider sheering forces on the tire. The entire issue is incredibly complicated beyond the level of basic physics. If fact, if it weren't, tire companies really wouldn't need R&D departments. But, back to the physics level, the coefficient of friction DOES NOT matter on the surface area of contact. BUT, the interaction between road surface and tires isn't one strictly of just friction.

 

[Fiveohhh]

Now before I think too hard about this. I want to verify this statement. You have a pyramid shaped piece of material and no matter the size of the surface area facing the ground. it has the same amount of friction...

 

[RayH]

Originally posted by: DrPizza

I'd like to question, though, do wider tires really provide that much more traction? Or is this something everyone is assuming?
I don't know how many times I've seen "A car moving at 60mph requires x number of feet to stop"
Never have I seen "A car moving at 60 mph requires x number of feet to stop with narrow tires, and 2x feet to stop with wider tires"

You can look at 1/4 mile tracks and compare 60' times of awd and rwd cars of similar weight. There are alot of awd cars on street tires with lesser horsepower approaching 60' times of rwd cars with slicks. On a 1/4 mile track there's a certain level of adhesion going on where contact patch can make a huge difference (think gecko feet).

 

[StageLeft]

Originally posted by: Fiveohhh
Now before I think too hard about this. I want to verify this statement. You have a pyramid shaped piece of material and no matter the size of the surface area facing the ground. it has the same amount of friction...

Yep that is right

 

[Carapace]

Originally posted by: Triumph

Originally posted by: Carapace

Originally posted by: Skoorb

Originally posted by: DurocShark
It's the side-to-side torque that's affected. In a straight line, there's no difference (other than rolling resistance).

Well then why do drag cars have fat ass tires?

Because the tire expands as it goes faster, in all actuality about 1/4 of the tires width is making contact with the surface. Also, fat tires are not always better. For snow and ice you want more pounds per sqaure inch spread out over a smaller area, thus giving you the pentration through the snow and ice.

Tire width absolutely does have an affect on straight line acceleration. There is no disputing that. The top fuel dragster's don't have fat tires *because* they expand. That's some kind of circular reasoning. The fat tires will provide more width when expanded than a thin tire would provide when expanded. More width = more grip.

Sorry, my point all along was traction...... I just thought it didn't need to be spelled out.

 

[Triumph]

Originally posted by: DrPizza
BUT, the interaction between road surface and tires isn't one strictly of just friction.

I disagree. The interaction between surfaces is basically the definition of friction, defined as such by us humans. The equation F = uN always works because u is a calculated value. Whatever you are thinking that is affecting the grip of a tire besides friction, is really just rolled up into the value of u.

 

[DrPizza]

Originally posted by: Triumph

Originally posted by: DrPizza
BUT, the interaction between road surface and tires isn't one strictly of just friction.

I disagree. The interaction between surfaces is basically the definition of friction, defined as such by us humans. The equation F = uN always works because u is a calculated value. Whatever you are thinking that is affecting the grip of a tire besides friction, is really just rolled up into the value of u.

go back to the analogy earlier in this thread about 2 interconnected cogs turning each other... is that a matter of friction? I'm implying that to a small extent, that's also happening with the tires.