Please answer along with your reason why. And no smartass answers please like "a brick wall" LOL.

 

Friction. It's natural force that man has been able to harness, as far as brakes, or binders. Brake fade is the result of heat on the surface's making contact. The ability to dis-burse the heat, via cross-dilling, venting, and even a forced air supply blowing on the 'fixed' and rotating mass helps reduce brake fade.Then there's always a big oak tree to carl,,,LOL

 

Well, its a whole chain of things.
I could say "friction" or "the brakes" or "the tires"
but one thing by itself will not stop the car (other than the brick wall)

The drivers foot pushes the brake pedal,
the lever action of the pedal pushes the brake master cylinder,
which pushes hydraulic fluid through the brake lines,
which pushes the wheel cylinders (calipers),
which push the brake pads,
which squeeze the brake rotors,
which slow the rotation of the wheels,
which slow the rotation of the tires,
which generate a force through friction with the road,
which is transmitted through the suspension links,
back to the car,
which slows down

If any link in the chain is broken the car does not stop

Actually I guess friction could eventually stop the car by itself when you run out of gas.

 

From a physics standpoint, HEAT stops the car.

 

Hmmmmm..... I'm looking for a basic simple answer. How bout this. Pick a component.

 

some sort of resistance to forward momentum whether it be rough ground, a flat tire, brakes, a brick wall, whatever.

just the simple fact that things rub when it moves forward (bearings, brake pads, tires on the ground, etc) will stop it eventually when you let off the accelerator.

What brought this question up?

EDIT- The one component on a car that stops it is the TIRES.

 

Ok, When you step on the brakes, what component stops the car? Hows that ?

What brought it up is it seems that people have a misunderstanding of what really stops a car.

 

Here's the way I look at it-

The stock brakes will lock up the wheels, but the size of your tires determines how quickly your car will stop.

I think that answers the question.

 

yea you could have the biggest brakes in the world but if you got little tires you're not going to stop very fast. the tires determine how much acceleration, positive or negative(decel), that the car can undergo before it starts sliding. the tires create a certain amount of friction, and if the force of accel or decel is greater than that, the car will lose traction. however, when braking, the front tires are what supplies most of the friction, so if you have a big muscle car with drag slicks on the back and little tires on the front, you will go fast, but slowing down will not be so easy. since rx7s are designed for handling and not straight line acceleration, when you are choosing your tires, dont undersize the front ones .

 

Good job guys! Tires stop the car.

 

Good to know - I'll remember to never leave home without my tires

 

Well, if you wanna go with the SIMPLISTIC answer...

 

How about...it's the (4) 3" X 9" patches of rubber contacting the Earth and the resulting weak atomic attraction force that binds that rubber to the Earth via assisting prssure from the Earth's gravitational field?

That is the actual correct answer.

Class is over now.

We are not evolved from chimps, they are our genetic cousins from a common ancestor in the past.

 

Interesting question...

It really depends on what level of the system you look at. From the highest system level view, the driver is what stops the car; it is the source of input that operates the other car-stopping systems that ultimately stops the car.

On the other hand, if you look at the lowest level the physical forces that act opposite the cars motion are either from the tire-road contact or the drag from the air.

If you want the go higher and look at the sources for the tire-road drag force or the aerodynamic drag, you can move up level after level.

A way to get another perspective on this subject is to ask the question "What does it mean to stop the car?"

Any moving object has a kinetic energy: 1/2*Mass*Velecity^2
A typical RX-7 w/driver moving at 10mph has 1.1kJ of kinetic energy, 60 mph has 405.4kJ, 120 mph has 1622kJ. Energy in classical physics can't be created or destroyed, so for the car to stop it has to convert that energy to something else (heat, light, sound, etc.). That is what it means to stop a car; to take the moving car's kinetic energy and converting it to some form of energy. Wankelguy was close, heat definately plays a role. Heat though is the byproduct of the car stopping, ie the form of energy that the car's kinetic energy was converted to. What really stops the car is the process that converts the car's kinetic energy to heat.

 

Well yes WG I do. People often fall into the trap, need big *** brakes to stop the car, bigger brakes........ Which will not help you if your overloading the tires. I just wanted to get everyone thinking, specially anyone who wasnt aware of this. Study the traction circle right?

Bigger brakes may be easier to modulate but wont ... excuse me CANT go beyond the tractive capacity of the tire in a horizontal plane (well sometimes they can, but thats another discussion). That would be breaking the laws of physics.

 

People mostly do brake upgrades for the look. But bigger brakes usually have more mass to hold in the heat that the braking "makes." So they aren't bad to do. Modulation is also very important too. If I have to mash the brake pedal to the floor to lock up the tires, the brakes aren't adequate IMO.

 

I don't care about stopping, if I wanted to just stop I would hit the nearest brick wall.

Slowing down controllably is more important. Easiest way to controllably slow down is to controllably convert the kinetic energy into heat. That's what the brakes do (friction) and to some extent the tires (more friction) because we are loading the brakes via the tires.

Now, if you're generating too much heat for the brakes, you either make the brakes handle higher heat loads (better/larger pads, more rotor mass) or you increase cooling to the brakes (better ducting/airflow, internal vents) or you decrease the amount of friction needed to work against the tires (larger diameter rotors, which also conveniently adds rotor mass).

If you brakes are overloading the tires, well, you know the drill - grippier tire compounds or larger contact patch, or making sure the braking is best allocated to the tires (if the front or rear lose traction first).

Of course we are only talking about driving on nice smooth non-deformable surfaces like asphalt. Drive on dirt or gravel and there are other ways of slowing down, like pitching the car sideways to move the road surface. Equal and opposite reaction slows the car down because it loses kinetic energy in giving kinetic energy to the dirt/gravel being thrown at the cheering spectators.

 

There was the misunderstanding about the tires stopping the car, but i have spotted another one.

Tire size has about as much to do with slowing down (and acceleration) as brake size. It is really all in the tire compound. It will make some real world difference but from a basic physics standpoint none.

I realize that there is a smaller contact patch with a smaller tire, but there is also more pounds per square inch on that smaller contact patch, so that all evens out. Does everybody remember there basic friction labs in their physics courses? Surface area has nothing to do with the amount of force necessary to break the static friction point. So it is all in the compound of the tire, wich determines the coefficient of friction, whit determines the ability to slow down and accelerate.

There is an article on a website somewhere, but I can't find it.

-Marques

 

I started a thread about this a while ago. Normally friction is independent of surface area. However, some substances (rubber being one of them) have a unique property that causes its coefficient of friction to change as the surface area (contact patch of tire) does. So because of the properties of rubber, a wider tire of the same compound will have more traction than a smaller one.

 

mwatson184 is correct. It is now time for a thought exercise to expand on peejay's observations to put all this in quick perspective. Imagine a 10 diam tire with a tire width of only 8 inches....yes, I know width doesn't matter, but the mind needs some parameters to get a good "gut" instinct for a solution...anyhoos,,,,

Now, imagine a spacer on the wheel that allows a disc brake rotor of any size to be mountedout of the rim but controlling that wheel....

If the rotor is 6" in diameter with the pads correspondingly small to fit the usable swept area of that disc, what kind of temps and pressure would it take to haul down a 2400 lb. load?

On the other side, a 15" diameter rotor with its bigger pads would exert what kind of pressure and build up how much heat to do the same task?

And if you want to make it really just a mental picture to draw the point, how much work would a 3 FOOT "inboard" rotor with its monster pads have to do to accomplish the same task.

Still, it all really boils down to the weak atomic force and the resulting attraction of asphalt molecules to rubber molecules...

 

You go mario !!

 

Actually, barring extreme situations like super stiff carcasses or having the rim touch the ground, the size of the contact patch is a function of the weight on the tire vs. the pressure in the tire. Notice that I did not say "larger tires", I said "size of contact patch".

So when you are running 35psi in your 185/70-13's, and then you bolt on some 225/50-15's and run 35psi, you have the same contact patch size. Because 35psi in 225/50-15's is simply waaaaay too much pressure for something the size of an RX-7.

Ever wonder why you put 50psi in a temporary spare and 5-10psi in a drag slick... ?

In the first case, you need to crank up the PSI because the tire is so small that trying to run a 35psi-sized contact patch will result in too much carcass flex in order to get that little tire to spread out that much.

In the second case, you want as much contact patch on the ground as possible. So you get a tire that is large enough to withstand such low pressures and not be overloaded, and has a soft enough carcass to be able to flex so that the tire's freedom to spread out is not hindered.

Next time you look at the sidewall of your tire (every morning, right?) note the weight rating. It will say max load XXXX pounds *at a certain pressure*. If you had a tire book to look at you would find pressure/load charts to let you know what pressure is needed to "optimally" have a given load with a given size tire.

Tires is complex things.


You think THAT'S all bad, we haven't even touched on how the suspension geometry, tuning, and even basic design affect braking performance

 

Ok carl. Next ?

 

damn long posts, make me so slack to read

 

Haha, you guys are good! I hope everyone here reads and gets this.