Since the force of friction is just proportional to the normal force, and friction coefficient..... That would mean tire width wouldn't make a difference.. But yet I don't see anybody driving around on bike tires... What am I missing?

well,my theory is kinda... more rubber on the ground, will prevent you from spinning out (with proper inflation of corse)

wider tires and shorter sidewalls, means a tighter handeling car

my fiero with 215-60-15's rides a little softer, but when i put on the 225-50-16's it is a world of differance, the ride is a little stiffer because of the shorter sidewall, but the handleing differance is night and day

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Rich AIM: ONE FAST 2M8:
98 Black GTP 2 Dr
86se V6 FOR SALE $1,500
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ok, i'm in physics right now, so i actually halfway understand what you just said but i know one thing, surface area makes a difference

if you want i'll ask my physics teacher to figure it out, it'll be a nice half hour distraction... lol

Travis

Traction: (Wide is better) Gas Mileage: (Skinny is better) Looks: (Wide is better) I'm running 17x10.5 rims on the back with 17x9 on the front with 245/45 front tires and 315/35 rear tires looks great but the front wants to really follow the road and the rear tracks well I felt like I lost 50 HP when I put my rims and tires on the car with a stock six but on my V8 car I did not feel any difference. Once at hwy speed I do not notice a difference with either.

F=Nu is for non-elastic, non-deforming materials, which rubber is not.

The rubber in a tire conforms to the irregularities in the road surface. As the contact pressure increases, the rubber is pushed down into more irregularities, however, there is a diminishing returns principle at work. The first X psi of contact pressure fills the largest irregularities, but the next X psi doesn't fill nearly as many.

The result is that a tire has a non-linear coefficient of friction. The graph looks something like sqrt(x). This doesn't just apply to the tire as a whole, this applies to the rubber on the basis of contact pressure rather than total normal force. On a wider tire, contact pressure is lower and the tire operates further down on the coefficient of friction graph. Thus a given weight on a wide tire gives that tire more available grip than the same weight on a narrow tire... other things like rubber compounds being equal.

I think the reason wider tires handle better around corners is because the road and the tire are not smooth surfaces. They interlock somewhat. And the more surface area that interlocks the greater the cornering ability. I don't know how much this plays out for street vehicles, but I think the idea of interlocking surfaces came about when race cars started generating more than 1G lateral cornering forces.

What he said ^. Normally, you'd be right - the more contact patch, the more the load is spread over the larger contact surface, negating the effect of having more contact patch. Normally it's only Mu (the coefficient of friction between two materials) that matters. But again, unlike a non deforming material, rubber exhibits "grip". It fits into the little cracks and crevices in the street. Think of the rubber as grabbing onto the irregularities in the pavement like a rock climber grabs onto a handhold in a cliff. Where in a certain area where gravel, sand, or grit may prevent the rubber from hanging onto the actual pavement, more contact patch gives more surface to contact clean pavement as well. Therefore, as Pontiac says: "Wider is better".

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Chris

Blue 1987 GT Getrag 5 speed, 1" lowering springs, 225 50R16 rims and tires, Fiero Store 9 3/4" HD clutch
Silver 1986 2m4 THM 125c automatic
Black 1985 2m4 SE Isuzu 5 speed (no paperwork)

Edited:

I misspoke earlier. If the normal force and the coefficient of friction stay constant then the number of square inches makes no difference as was pointed out. It must be the other grey area factors that have to do with grip (not coefficient of friction) that make more square inches valuable, to a point, when talking tire width.

My mistake:
Since Coefficient of friction is unitless, dividing the force up between more square inches makes the force per square inch less but the total force remains the same.

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RickN
White 88GT 5spd
White 85GT Auto

[This message has been edited by RickN (edited 01-02-2005).]

What Will said. As the tire gets wider, the contact patch gets shorter and area is pretty much unchanged.

quote Originally posted by Will: F=Nu is for non-elastic, non-deforming materials, which rubber is not. The result is that a tire has a non-linear coefficient of friction. The graph looks something like sqrt(x). This doesn't just apply to the tire as a whole, this applies to the rubber on the basis of contact pressure rather than total normal force. On a wider tire, contact pressure is lower and the tire operates further down on the coefficient of friction graph. Thus a given weight on a wide tire gives that tire more available grip than the same weight on a narrow tire... other things like rubber compounds being equal.

That makes sense..... So less pressure per unit area = higher friction coefficient? <scratches head>

btw- they never inserted the disclaimer seen above, maybe that was part of my confusion.

[This message has been edited by ryan.hess (edited 01-01-2005).]

More Traction. It would be fun to watch Jeff Gordon run around the Daytona track with bicycle tires on his car.
Don

Here's what Milliken has to say.
"The rubber is stuck to the road by a variety of mechanisms including mechanical "gearing" to the texture of the pavement and molecular adhesion to the surface.These mechanisms are not totally understood."
Carroll Smith agrees with the "not fully understood bit" and adds "This molecular adhesion only comes into play at very high loads and (friction) coefficients and is the reason we are able to leave impressive black marks on the track when we are neither spinning nor locking the wheels nor sliding the vehicle."
Got that?
What I have been able to to gather is that the SHAPE of the footprint has a bearing on how the tire is able to generate traction forces. A "long" footprint is better for acceleration/deceleration, and a "wide" footprint is better for lateral acceleration. Probably because of the way that the tread footprint itself is distorted and deformed under load.

Ahh... I guess so:

btw - yes, I understand wider tires = more traction... What I was saying is if you had steel tires and the streets were steel...... The difference between a 1" wide tire, and a 10" wide tire should be immeasurable.

Looking at a simple point of view. Wider tires have more contact surface to the ground that a skinny tire. So saying that both tires have the same compound and the street or surface contact are the same, the wider tire(2 times wide for example) will equivalen to have twice the grip due its total material(x2) reaching the ground vs the skinny tire material(x1) area reaching the ground.

Dre

[QUOTE
btw - yes, I understand wider tires = more traction... What I was saying is if you had steel tires and the streets were steel...... The difference between a 1" wide tire, and a 10" wide tire should be immeasurable.[/QUOTE]

If the surfaces were polished, I'd expect that to be true, theoreticaly at least. In the real steel world, a wider steel tire on a real steel street would probably grip better due to surface imperfections, until it rained--steel pellets.
Everybody here has done a good job explaining why rubber tires on tarmac are different than steel on steel, answering your questions.
Something missing?

Surface tension.

Wide tires weigh more, so gravity sucks wide tires more.

quote Originally posted by ryan.hess: Since the force of friction is just proportional to the normal force, and friction coefficient..... That would mean tire width wouldn't make a difference.. But yet I don't see anybody driving around on bike tires... What am I missing?

doesnt take a phsyicsmajor to figure it out.. wider tires equals greater grip on the road. picture if you will.. 8 guys trying to lift a heavy pipe..
8 guys can barely get it up, but if they add two more guys, they can get it easily..

The other answer is that the wider the tire the lower the pressure you can run at which equates to more grip and comfort. If you had bicycle tires they would have to be at 1000 psi in order to hold up the car and it would be almost like riding on steel wheels.

Hmm...seems like many need a high school physics refresher.

Will and Blue Shift explained it the best.

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My Web page | The Turbo Super Duty Build.

There are a lot of factors that vary. At some point a tire becomes too wide and loses grip in certain road/driving conditions.

Bottom line (no pun intended), skinny girl = less grip, fat girl = more grip.

Thanks, I'm honored. Blue Shift - that's a physics reference as well (that, and my car is blue and a 5 Spd, I figured it'd work).

I have a book that actually mentions what we're talking about here. It reads:

"It says in some science books that coefficient of friction can never be larger than 1.0. This "fact" led people to predict that dragsters could never beat a 200 MPH speed or 9 seconds in the quarter mile. Drag racers proved otherwise, as they broke these "impossible" limits and went well above 200 MPH. They did it with grip. [drawing] The drawing shows interlocking of the tire tread with the road surface. The softer the rubber copound, the more the interlocking and the more grip is increased. A rubber tire on pavement interlocks with small holes and bumps in the pavement. Grip between tire and road can be increased by increasing the total area of rubber in contact with the road because there are more interlocking points. Thus the wide and low pressure tire came into use. In general, the larger the contact patch of a tire, the greater will be its grip at the limit of adhesion. Under a sliding condition, the interlocked rubber is actually ripping off the tire, and this is what leaves skid marks on the road." P. 13-14, "How To Make Your Car Handle"

BTW, for comparison I thought you guys might find this interesting as well:

Coefficient of Friction

Surfaces
Static Friction
Kinetic Friction (sliding)

Steel on steel (dry)
0.6
0.4

Steel on steel (greasy)
0.1
0.05

Teflon on steel
0.041
0.04

Brake lining on cast iron
0.4
0.3

Rubber tires on dry pavement (why smoking tire isn't the fastest way to launch or brake)
0.9
0.8

Metal on ice
0.022
0.02

Rubber tip of crutch on rough wood
0.7
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quote Originally posted by Blue Shift: Coefficient of Friction . Steel on steel (greasy) 0.1 0.05 -

Good thing we don't drive with steel wheels on greasy steel roads.

The entry level physics make a lot of simplifying assumptions and factoring out surface area is one of them. In the real world size does matter.

Wider tires theoretically will also dissapate heat better, and last longer, under race conditions for any given load.

On the wider tire subject...I've always wondered why "skinnier" tires with smaller rims are better in the snow! I know they are but wouldn't most think wider tires on bigger rims would be better!

quote Originally posted by RACE: Wider tires theoretically will also dissapate heat better, and last longer, under race conditions for any given load.

Hmmm.... that's an interesting thought, I guess the tire is able to reject more heat to the road. Interesting opposite... put the vehicle on a chassis dyno and the tires get hotter because they the heat of friction gets spun around and returned over and over, one issue repair shops have first time they attempt a long drive cycle or go looking for an intermittant on a chassis dyno is getting to see tires smoke or even catch on fire. On the road you keep leaving the hot spot behind you, on a dyno it's constantly returning and building.

quote Originally posted by Gokart: On the wider tire subject...I've always wondered why "skinnier" tires with smaller rims are better in the snow! I know they are but wouldn't most think wider tires on bigger rims would be better!

Because, snow (and water for that matter) adds an extra element between the tire and the road. The rubber of a wider tire won't get to the road as easily as the rubber of a narrower tire for the same reason a snowshoe works. The wider tire will stay on top of the snow (or water) and not grip the road as well as a narrower tire that would cut through either and actually contact the pavement.

Little simplified since I'm not into physics, but that's the gist of it. I'm sure someone else could explain it better

-Randy

quote Originally posted by Gokart: On the wider tire subject...I've always wondered why "skinnier" tires with smaller rims are better in the snow! I know they are but wouldn't most think wider tires on bigger rims would be better!

Narrower tires have higher contact pressure, which pushes down through the snow or slush to the pavement.

Narrower tires also give better steering feedback and handling precision, although they have less grip...

I have a degree in Physics and think all the bases here have been covered.........gas mileage, control and all.........up to a point the more surface area you have on the ground....the better for control.........and then the asthetic takes over.....we all like the looks of bigger ...wider tires.......I love the control my pilot tires give me....stiff sidewalls are what I want for tunning very curvey mountain roads.....where I really enjoy my Fiero.........

It's a combination of things, you will have a wider center of gravity makeing it harder to roll.Also better traction for less tire slipage.my 2 cents.

Bigger tires just look better

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I mostly agree with Will. On snow & ice, wider the tire, less grip you get. Thats one reason old Beetles went thru foot deep snow with no problem....they had motorcycle tires Wide tires grip better in acceleration and cornering on a DRY surface. Also rain is no good for wider tires, they surf much better (hydroplane) than skinny tires. Again, like WIll says, there is much more weight per square inch pushing down on skinny tires. I had 275/40 Khumo tires on one vette and they were a terror on a wet freeway. It was like driving a sprint car. Tread pattern also affects the water disapation.

[This message has been edited by rogergarrison (edited 01-03-2005).]

The friction formulas in your physics book are for ideal surfaces, i.e. one perfectly smooth surface moving on the other perfectly smooth surface with friction acting "normal" to the interacting surfaces. Tires and roads aren't ideal surfaces. Tires conform to irregularities in the road surface and thereby provide force vectors at angles other than the "normal", that is at right angles to the surface, so friction (call it traction) is not accurately expressed by textbook physics. This is why dragsters can exceed the theoretical limit of 1g acceleration. The wider the tire, the more surface there is acting at other than the "normal" to the road.

The answer to you question is, THEY ARE NOT.

Which all depends on what you mean by, BIGGER , OR LARGER.

It is possible to go TO BIG, which defeats the purpose of a larger contact patch on the road.
Actually, i have found, ( after 106k original miles on my Fiero) that the best tires are the sizes, that were called out in the original factory spec. Now of course you could go get , shitty cheap ass tires, that size, and you would be getting what you paid for. There is a reason why, high preformance tires, come with a larger price tag. I have high end Michelins ( the only tire i would buy to race or street drive on) factory size, my car handles almost like the day it was new.

did I mention that my 106,577 miles Fiero, is 99.9% all original.

How bout concrete tires on a rubber road?

Why are wider tires better? If you have to ask why? Your thinking too hard.

back to heat dissapation and tire compound, race car weaving to warm up tires, drag cars-burnouts. too little glue holding 2 peices of wood togather VS more glue...

quote Originally posted by madcurl: Why are wider tires better? If you have to ask why? Your thinking too hard.

yeah i think Curly nailed that one....

larger tires have a larger contact patch and more resistance or friction that helps them from going in a direction that they're not pointed in, meaning better grip. Do you really want to know the physics behind it?

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still plays with cars..

Good thing our pistons and cylinder walls are steel on greasy steel :-)

quote Originally posted by jscott1: Good thing we don't drive with steel wheels on greasy steel roads. The entry level physics make a lot of simplifying assumptions and factoring out surface area is one of them. In the real world size does matter.