The_7

anyone who knows about physics knows that one of the laws is that friction is independent of surface area - it depends on the material and the force - so the grip of a tire depends on what it is made of and how much the car weighs - so why do sports cars have wide tires. well i can see that wider tires would work better in rain, snow, debris, etc. but what about dry clean pavement like a drag strip. the dragsters have ULTRA wide drive tires - but according to the laws of physics that doesnt help traction - so why do they have them?

The_7

i know that question strays a little from first gen 7s but this is my fav forum....also the only reason i can think of for dragsters to have those really wide tires is to prevent the tires from overheating because the larger tires and more surface area would dissapate heat quicker...am i right?

j9fd3s

iTrader: (3)

well wider tires only help up to a point (car dependent), but perhaps a 205size tire is smaller than optimum? road and track actually has a good article about f1 tires this month (ferrari enzo on the cover)

mike

nickj

A wider tire mainly changes the shape of the contact patch, not the size. Tire pressure changes the size of the patch. Support a 2400 lb. 50/50 car with tires at 40 psi and the contact patch of each tire will be, in theory, 600/40 = 15 square inches regardless of the tire width.

anthrax

That is for a granular particle, so look at it this way there are many many particels that make up the surface area of the tire so the particel forces combine to generate greater friction.

MarkPerez

In the case of drag slicks, A 14x32 slick is a normal size
slick for funny,top fuel drag race cars. A 14x32 is the biggiest slick made. thoses cars produce well over 1000
horse power. 14 inch wheel x 32 inch's wide, watch how
these cars smolder those slicks, they stretch real tall
and become only 12 inch's wide. the slicks are screwed to the wheels so the wheel doesn't spin inside the slick.
and don't forget to include gravity to the physics equation that is a force often over looked.

anthrax

Good Point gravity on a heavyer car allows better tracktion to a point.

But the effects of gravity are cirtinly not neglagable in this situation

bliffle

I think wider tires just make the contact patch larger and therefore less susceptible to a pebble of given size.

B

fatboy7

The 7, your correct, for most materials. Rubber, and rubber like materials exibit a viscoelastic effect that essentially makes the rubber change its effective coefficient of friction based on size of contact patch, as well as slip velocity among others. So in effect, the more tire on the ground the better. Street tires usually have a coefficient of friction around .5-.7, depending on size and tire pressure. R compound tires, and more so with wide full race slicks, the coeffiecent can reach 1-1.3, and in some cases as much as 1.5 with the right amount of wheel spin.

tmiked

Friction is independent of the surface area for solids. Rubber is not exactly a solid and actually extrudes into the road surface. Hence to some extent the force developed by this action comes from shearing the surface or the rubber (leaving skid marks) and that force is proportional to the area.
Even with metal to metal, with no lubrication and enough normal force, the two metals, especially if they are identical or similar will fuse and the friction force increases in a non-linear fashion. This is commonly called galling. Friction welding is a commercial use for this efffect.

fatboy7

yea, somewhat like tmiked said, rubber has a certain property, adhesion, and flexibilty that makes it able to grab the surface. Its pretty complex, and I don't fully understand the nitty-gritty details of how and why it does what it does.

O 16581 72452 5

I'm not sure that this is correct but this is an opinion... take sand paper for example, a small piece and rub it across something... pretty easy... apply the same force to a huge piece of sandpaper and it gets much harder, what i'm guessing is you can apply more energy to a small piece of sandpaper than a large piece... same with tires, wide tires are going to spread that energy out over a larger area so traction loss isn't as easy.

hamster

adhesion

The_7

so fatboy7, youre saying that because of the odd properties of rubber, the friction DOES increase with surface area because it changes the cof. id like to learn more about this viscoelastic effect.....do you know of any websites that would explain it in detail?

Cody

Wider tires allow you to have fatter girlfriends.

BRealistic

This tire size vs friction has been debated on many forums. Unfortunately for me, math is hard.
I agree with the statement about rubber (actually tries aren't REAL rubber. but let's not talk about that)- that tire rubber is not true solid. But to further describe the friction between the road and the tire, you must look at the road surface. The friction is dependant on weight and material, but not surface area would only hold true if the road and tire surfaces were perfectly smooth. Say, find a perfectly smooth concrete area like a parking garage, and get very hard slicks that won't get hot and sticky when heated up, and do some experiments. That would probably be the closest to tire traction being independant to tire size.
But normal paved road surfaces are not smooth, their surface is always changing and basically the tire rides on the bumps. Every bump extends slightly into the rubber of the tire, ar the tire blocks grad the side of the road bump. Now having a larger 'contact patch' alllows more road bumps to protrude into the tire, which is more of a shear strength than actual friction surface.
Oh, one other thing- the blocks of the tire want to bend with the loading. Smaller blocks help with wet traction, but small block also move more under load. By spreading out the side loads through a bigger tire and more tread blocks, each block transmits less force and is less likely to deform under load= more traction.

fatboy7

I wish I had a good reference to give you..... I understand I've picked up from several different books and SAE articles. Go to the library, and pick up a book on vehicle dynamics. Almost any book with that subject will have a chapter on tire dynamics. To further help you visualize how rubber works (at least how I understand it) here's another example:

Rubber, as it contacts a surface sticks to the surface, much like a rock climber sticks to a cliff. It streches and gets in between the irregularities (on a microscopic level) in the material. Much like a rock climber really doesn't have enough friction (cause he's got little or no force pushing him against the cliff), the actual rubber has a fixed coefficient of friction just like every other substance. But like the rockclimber it is able to resist movement because he's stuck his fingers and toes in the cracks and crevises of the surface. Likewise rubber sticks its "fingers and toes" into the surface. The more surface area, the more "fingers" the rubber has stuck in the irregularites.

Rubber also has hysterisis, frequency dependant deformation, that makes it slip like a catapiler walkling in reverse. Parts of it slide while another parts really dig into the surface, these areas of slip and grab move as waves across the surface. If you pull the rubber across the surface at the right speed, it actually will really grab a surface. Ever noticed that when your windshield is dry and you turn on you wipers, that they start from a stop quickly, get going fast, but then slow down? Its actually due to this effect. Pure rubber rubbing against glass ( with virtually no irregularites ) at the right speed can have a coefficent of friction greater than 2!! That means it takes more than twice the force holding it to the surface to slide it along the surface!!

excitingleopard

Yes. Adhesion. Staying on the road is more important than going fast. If you had thin tires, you would go faster on the straight, but the minute you hit a curve, you'd have problems.

Spieder

Here ya go:

Dependent on area
Static and kinetic friction are independent of the area in contact. This is also somewhat true for rolling friction caused by deformations. But the part of rolling friction that is cause by adhesive forces is very dependent on the area of the surfaces in contact. The equations for the adhesive part of rolling friction are highly complex. Often the coefficient of adhesive friction is determined empirically, through experimental measurements.

In conclusion
The coefficient of rolling friction is much less than that of sliding friction, especially when hard surfaces are involved. Deformation of the wheel and other surface cause the wheel to "plow through" the surface and is a major factor in rolling friction. Soft surfaces increase the friction. Molecular adhesion is another faction in some materials, and it is proportional to the surface area.

DBOGGSRX7

you guys made this way too complicated. take into account the fact that dragsters do a burnout to heat up their tires. they heat up their tires to make them sticky. Now image puttun a foot long strip of scotch tape on the ground (sticky side down) and putting a weight on it. no too hard to pull on the end of the tape and move the weight. no imaging using shipping tape (same tact just 2" wide instead of 3/4" wide) it would be much harder because there is more area of "sticky ****" on the wider material.

The_7

thanks for all your input guys, i understand how it works now. last year i had physics and we mostly delt with sliding friction and didnt learn about the adhesive properties of substances like rubber so i didnt think about that. next year i got ap physics so well probably talk about it then, but thanks again.

BRealistic

But normal street tires don't get sticky.

alien_rx7

Eh? Maybe you're thinking of muddin' tires The best snow tires (Hakka's) for cars are actually thinner than street tires, putting more weight on a smaller spot in the snow or ice where the wider tires would want to ride up on the snow. Same thing for driving in the rain. You would want a smaller contact patch so it moves through and pushes away the water easier instead of wanting to roll up on top of it (why do you think INDY/NASCAR have to stop when it rains and CART/F1 don't?).

The_7

hmmm....yea your right....my bad but what about when its dry and theres debris on the road - then wider tires would be better

alien_rx7

Wider tires tend to drift (not track to well) on a paved surface with debri across it