why is it that when the high schools and even howstuffworks teach about cars and friction on the road, they always leave out surface area of tire in contact with road? i know that weight on tire, and coefficeint of friction matter, as well as static and dynamic friction are the main issue, but no one ever mentions the all important surface area??? if i remember correctly, even the high school physics equation leaves this variable out? why?
no doubt a drag car does much better with it’s wide tires, as opposed to 2 inch tires?!
here’s the “howstuffworks” page with no mention of it:
The reason is that, to a good approximation, surface area or…more properly…“apparent surface area” doesn’t come into the calculations. Basically, two surfaces unless essentially atomically smooth don’t touch each other in most of the region where they are in apparent contact. The regions of actually contact are much smaller and, to a reasonable approximation, I believe that they are proportional to the normal force.
Friction is a slippery subject though (pun intended)…And a “law” like frictional force equals friction coefficientnormal force is really a different use of the equals sign than saying F=ma. The latter is a real law whereas the former is more of an approximate empirical fact.
By the way, someone was recently telling me that in general it is narrower tires rather than wider tires that handle better in the snow although I am not exactly sure why (or even how true this is).
Surface area has a much smaller effect than weight and coefficient of friction. It’s a reasonably good approximation to leave out the surface area. Imagine a 1x1x2 ft box; depending on the orientation, the contact area with the floor is either 1x1 or 1x2 ft, but the weight and friction coefficient are the same. You’ll find that regardless of orientation, it takes roughly the same force to push the box across the floor.
guys, seriously, surface area matters a lot. let’s keep this discussion to one type of friction for simplicity sake: cars on the road. to keep this simple, why do drag racers want wide tires, as opposed to thin ones? (if you want to say because they rub stickem’ on the tires, which defeats my point, then why do corvettes/vipers come with wider tires? o gosh, you might say b/c they will last longer… ugh)
if a car has wider tires, will it be able to brake quicker if you lock the tires up, as opposed to thin tires locking up?
PLEASE ANSWER ALL THESE QUESTIONS!!! everytime i post multiple questions, people choose the one they know and forget about the others…
As has already been stated previously, no it doesn’t. With respect to friction, and to a very good approximation, seriously, surface area matters very little.
As tires roll, they continuously deform. This deformation produces friction and heat. When tires skid, this also produces friction and heat. Larger, wider tires have more mass into which to dissipate this heat. Thinner tires have essentially the same friction as wider tires, but are more likely to melt and/or fail. Corvettes and vipers have wide tires for similar reasons, but primarily for aesthetics.
Here is the missing part and why wider tires are so much better.
Heat dissipation.
At the edges of traction and beyond, some slippage is to be expected. Slippage with a small contact patch will quickly heat the tires to a temp beyond where they are effective.
Also, small contact patches require greater forces per unit area and volume. Soft compounds are stickier compounds. Soft compound tires must have large contact patches to reduce the forces at the contact patch enough not to shred/tear the tire surface.
So, wide tires are important to racing/performance driving because they dissipate heat better and spread to forces across a larger area. Both allows for an overall softer and therefore stickier tire to be used.
On preview, I see that robby has gotten some of this… But, still worth posting I guess.
Why does physics neglect the surface area ? Cos it does’nt matter in most cases.
Remember physics is an approximation:
If you remember from physics, when a cylinder rolls on a plane, it touches the plane it is rolling on in a line and the plane is tangential to the cylinder surface !! Now only physics makes that approximation because it serves their purpose. ** In reality if the cyclinder touched the plane only on a line, there will be infinite pressure on that line **
** Physics explanation on neglecting surface area **
The frictional force is dependent only on the nature of the two surfaces but not on the area of contact. This is intuitive because, consider a brick, on its larger side, if you think friction as proportional to the area of contact and also the pressure (stress) of contact. Then pressure times area is always the weight of the body hence it is independent of the area of contact.
** Real world dependence on surface area **
In the real world surfaces are deformed by the aforementioned pressure. And the frictional properties of two surfaces depend on the pressure. So a flat tire is not only offering more area of contact ** but its frictional properties are very different from a well inflated tire**
To Summarize If surfaces did’nt have different frictional properties at different normal stress (pressure) you can neglect area effects Frictional properties don’t change too much for the range of physics experiments you do at school - like keeping a brick on its broader side / shorter side hence the approximation works.
ALso when the car starts skidding the tire shears apart (small parts of rubber shear off).
The larger the tires the less force per sq in against the road (not talking about air pressure inside the tire). Less pressure means smaller shearing force per unit area. Less shearing force means that it may fall below the point where shearing happens and no skidding occurs.
For all non - skidding driving the friction is independent of tire size.
Yes snowtires are narrower/ allows deaper penetration, less floating.
Friction is caused by interactions between the surface molecules of the two objects IIRC. Now, the bigger the surface area, the more molecules there are to interact - but the weight (actually the component of the weight normal to the surface) gets distributed over the entire surface. So the bigger the surface, the less force there is to mesh the molecules together temporarily. So it works out to pretty much the same thing.
for giving a perfectly clear and concise explanation that identifies the misconception behind the question–or rather, the cause thereof: forgetting that “spreading out” also reduces the downward force per unit area, in a proportional manner.
so wide tires are better for skidding/braking and chirping/accelerating… because they dissipate heat more efficiently, keeping shape?
slippage will occur at same moment given a force, with either wide or narrow tire. with that given force continued, slippage is more likely to cease with wide tire, because of better dynamic friction (heat dissipation/minimal deformation)? thus wide IS better?
heat is not a BIG issue until slippage occurs?
can you guys verify these statements so i can make sure i am understanding? bare with me if i am off on some of these…
Thats the traditional thinking all physics textbook teach. But its true for “rigid” bodies and also within a very specific range.
Physics will also tell you that there is no rolling friction in rolling without sliding, but that never happens. Because surfaces deform on contact and there is no such thing as pure rolling.
Friction is indeed caused by the surface molecules, but how these molecules interact changes with pressure which is proportional to the pressure applied only for a small range of pressure. When the region of non-linearity sets in (drive a car with a flat tire) this is no longer true.
The linearity is specially comes into play when there are not so “rigid” bodies interacting. And you need not so “rigid” bodies because of their damping properties.
I’ll do my best, Fuel. Although I disagree with some of the other posts, I’ll be respectful of them.
Drag racers use wide slick tires because they grab harder. It’s not a simple thing, though. Years ago, drag slicks may have been simply wide tires with the tread sanded off on a lathe. Now, though, drag tires are made with a sticky, quick-wearing kind of rubber. They’re also made to be run at a very low air pressure and intentionally deform on acceleration. You can see the wrinkled walls in a slomo video. If you ran these on the street, they’d wear out quickly, if not come apart from the heat. They don’t need to dissapate the heat; they’re only run for a few seconds. The goal of all that tricky stuff, though, is to lay down a bigger footprint.
For the moment, I’m ignoring the issues of snow and rain. This is on dry pavement.
Second, Corvettes and Vipers. There are several reasons. The bottom line is better acceleration and stopping. All cars, including those two, have a weak spot in the go-and-stop chain. All power and braking is done through a pretty small footprint. The average car’s tire has a contact patch about the size of your handprint. Actually, it’s smaller than that because pavement is not truly smooth. It’s pebbly, because, well, it’s made of pebbles. Nearly every car on the road can break the tires loose on acceleration. Every car, no matter how fat the original tires, can squeal ‘em on braking (assuming you can disconect the antilock system.) This doesn’t mean that size doesn’t count. It only means we’re not willing to pay enough to buy big enough tires. If you could plunk down enough cash to put Viper-size tires on a teensy car that came with tires no wider than your own foot, you couldn’t possibly chirp them on starting. You’d be able to stop much shorter, too. With those huge tires, you might approach the point where the tire’s friction on the street exceed the brakes’ friction on themselves.
The other reasons? They look so cool. Folks who can afford to pay that much for a car don’t mind paying thousands for nearly adequate tires. I say nearly because Vettes and Vipers have extravagantly ferocious engines which can smoke even trash-can size tires, and they beg to be driven fast enough to make quick stops dangerous even with those big shoes. Do they last longer? The way these cars are typically driven, no.
For documentation, I’d have to dig through a lot of old Car&Driver magazines for tire testing. I shall not do that. It’s Christmas Eve, and Mrs. Nott is tapping her foot, waiting to see a movie.
Okay, so if I have a rectangular block, it’ll have, to a very good approximation, the same amount of friction whether I place it large side down or small side down. But which way will have slightly more? And how much difference are we talking about? One part in 100?
(If you need specifics, such as actual dimensions, speed of sliding, and surface composition, in order to answer these questions, feel free to supply them yourself.)
Now, whether putting it on the smaller side or larger side will give you more friction will depend on the material. Again, I’m not an mechanical engineer, so I’ll give an analogy from Chemical Engineering fluid friction (viscosity). Pseudoplastic fluids, also known as shear-thinning fluids, decrease in viscosity as the shear rate increases. Examples include paints, shampoos and water suspensions of clay. Dilatant fluids, also known as shear-thickening fluids, increase in viscosity as the shear rate increases. Examples include corn starch in water, titanium dioxide, and wet sand.
I am guessing there are solid material showing similar behaviour.
That’s simplified - or idealised - physics. It works very well for most objects. After all, all scientific theories are approximations of nature. The better a theory, the closer it is to the real thing. The better a theory though the more likely it is more mathetically complicated, so depending on your application you may just need a more idealised model. Like the Ideal Gas Law - no gas in nature is ideal, but it works quite well for a lot of applications.
Hm, not sure if this is true. I have not specifically studied cars with a flat tire to see if the friction generated by the flat tire is in accordance with classical physics. However if you have a cite I would like to look at it.
The dampening part does not need to be the part that causes movement.
Still, wider is better. The lower amounts of force per unit area allow the use of softer/stickier tire compounds. Compounds you could never use with an extremely narrow tire.
This is the way I see it: friction is not very much related to area so long as you are in a given range. Just like a spring deforms linearly but only in a given range. Pull more and you damage the spring.
A wider tire of the same material would not be much better but a wider tire allows the use of a softer material which would have higher friction coefficient.
Note that racing cars will heat their tires before they start because cold tires adhere less than warm tires. The rubber then is pretty soft and adheres well to the road. But those tires last a day and you could not have tires like that in normal cars where people expect them to last 40,000 miles.