# My first ride in a Supercar

Discussion in 'American Cars' started by SirShiftAlot, Oct 17, 2004.

1. #51
Neither does the mclaren F1.

2. #52
Last edited by a moderator: Apr 25, 2016
Ahh, kinetic and static friction, by george I love real science. Finally somebody who speaks my language.

First, let us do a breakdown of the link you posted for laymans terms.

Static friction is the holding force. This is the forces applied between two objects at rest. Or an object at rest on another surface. Example is your car parked on a hill. The static friction is that of your tire to the road. Should the force of gravity acting on the vehicle exceed that if the static friction, the car will slide. Since the car is then in motion, it becomes kinetic friction. Often kinetic friction is less than that of the static friction since the interlocking of rough surfaces is eliminated. However if one surface is smooth or near frictionless (such as ice), kinetic and static are the same. I will extrapolate on the importance of this later.

To stop something, there must be a frictional force greater than that of an applied force. For the example of the car on say an icy hill, if the gravitational force remains higher than that of the kinetic friction, the car will accelerate until the kinetic force increases (such as a dry spot)

When braking, there are two points of frictional forces. That of the tire on the road, and that of the brakes on the rotor. A rolling object will not slide due to the static friction. It is static friction since the point in contact with the ground is in fact not moving. When you press the brakes, you have a kinetic friction applied to the rotors. This energy transfers the kinetic motion of the vehicle to that of the tires static friction, thereby slowing the vehicle down. Should the kinetic friction of the brake on rotor be greater than that of the tire, the tire will then slip and turn into kinetic friction.

Ahh, and in here lies the point that somewhat relates to how an almost locked tire can stop a bit better than that of a locked one. since as stated, the static friction before an object begins motion is slightly greater than that of the kinetic friction once in motion.

This means that just before lock, the rotor's kinetic friction is near equal to the tire's static friction. once the tire locks, it becomes the tire's kinetic friction that is stopping which is less than that of the tire's static friction.

You are right, I didn't mention the difference between a locked and turning tire. Because when friction on one surface is limited, such as ice, etc. The kinetic and static are near identical. When ABS lets off, the kinetic friction between the rotor and the brake is less than that of what the kinetic friction between the road surface and the tire.

Now, I was talking some scientific constants and from theory, one would thing that it would make a big difference on dry. However, the scientific truth of the matters are that the difference is negligible enough to not have a noticable difference between the kinetic and static difference between a tire. This is even stated in that link:

"One experiment with trained drivers asked the drivers to stop a vehicle on signal by 1) locking the wheels and 2) stopping as fast as possible without locking the wheels. On dry flat concrete, the stopping distances were very nearly the same. Both test yielded coefficients of friction near 0.8 for tires with new tread on this surface. The coefficient of kinetic friction is anticipated to be considerably less on a wet surface where the water can act as a lubricant. It is also likely that worn tires will have a considerably smaller coefficient of kinetic friction than static friction."

This is where some theory can come into play for the second half of that paragraph. I did state that water can act as a lubricant, a sort of film between the tire and its counter surface. When at tire is rolling, it will be pressed down and displace the water (you can see if when you see vids of aqua tread tires and is the purpose of the channels to allow the water to displace) The reason that it can act as a lubricant when sliding is that it will hydroplane slightly on the water. If there is only light water however this will be reduced. since the road surface still maintains it's texture. On something like ice that does not have a water coating, this would not be the case since is is a solid and cannot be displaced.

As for a more worn tire, I can understand this one too. A tire is a softer compound so it will conform to the surface of the road slightly. This is why racing tires get such good grip without any tread grooves. They are soft. This is also why when a race car loses traction, it cannot regain it until it is almost stopped. because when sliding, the motion causes the tire to essentially pull apart and anything that grips the road gets pulled off (big black marks, tire is gripping, however the molecular bonds arent). The tire grooves however will grip and "bite" in the same way the teeth on a saw will. Groves also allow the tire some movement on the tread surface. These motions (sorta like jello) allow the tire, even though is sliding, to still have static friction. That is the whole point of having tread depth, not only to give the tire life, but maintain that static friction.

Ahh, so how does this relate to an ABS system. Well, for a wheel to unlock, or re-establish static friction, it has that same static to kinetic spike to overcome. This means that it has to let off enough so that the static energy is less than that of the kinetic energy of the the tire sliding. lowest friction is what slips. So, what does this mean? To prevent a wheel from locking, the brakes must be having less kinetic braking force than what of the kinetic friction of a locked wheel. Less friction means lower rate of deceleration.

And thus is why ABS brakes can take up to 20% longer to stop than a non ABS car under certain conditions.

Ta Da! I have now confused the 99% of the members who have now read this.

Umm, ownage again?

3. #53
I wasn't referring to ABS at all with that post, I was just pointing out that a car will typically stop faster when its tires are at the threshold of braking (I'm out of it, but I think that's the term), rather than past it (as in locked wheels).

4. #54
Ok man, Just because you've ridin in a high end car, doesn't mean is a supercar. The Viper is a great car, but to me, when there is the S7, Enzo, Murcielago, Carrera GT, ect. I dont classify the Viper as a "Supercar".

5. #55

6. #56
That post was to BuickGNXkills

7. #57
People who act like hard asses on the internet suck. A lot.

8. #58
Let us break this down again the correct way.

Friction is a force that acts perpindicular to a normal force. The normal force is normally gravity. When a wooden block is resting on a table the normal force is provided by gravity. The equation for the force of friction is the normal force times the coeffecient of friction. This coeffecient of friction can be divided into two categories, static and kinetic.

The static coeffecient of friction is what is needed to calculate how much force it would take to push that block from rest. The kenetic coeffecient would be used to calculate the force it takes to keep the block moving across the table. Static Coeffecient of friction is always greater than that of kenetic.

When a tire is rolling, the tire surface of the tire contacting the ground at any time is not moving relative to it. Since it is not moving relative to it, static friction is what is going on. This is good, because this allows a greater force of friction with the tires. Once the tire locks up, the tire slides against the ground so we use kinetic friction. This means less force so the tire can not slow the car down as much.

So a locked tire will not stop a car faster than a rolling tire. So keeping a tire just at the edge of lock is the best way to brake. This is what ABS does. ABS measures the speed of each wheel. When it senses that one wheel has suddenly is stopped when the others are moving, the car will reduce the brake force for that wheel. The computer senses this and reacts in miliseconds. No driver would be able to react as quickly or be able to control the force for each wheel. This allows an ABS equiped car to stop faster.

Since the point of lockup changes depending on speed the driver would need to modulate the brake the entire time, the computer will do this better.

Notice the where ABS is allowed in racing the cars use it. In F1 they all used it until it was outlawed.

Your tire will have the most grip in the wet when new, but they will have the worst dry grip when new. This is because you do not want a huge amount of tread, this causes the tread to squirm under large loads. This causes the tread to buckle and loose grip (This am citing Paul Haney's excellent book "The Racing and High Performance Tire"). Notice that in every racing series that mandates treaded DOT tires like some Miata spec classes, they "shave" the tire. Even the Tire Rack will provide this service if you request.

So in summary an ABS car will stop in a shorter distance than an equal car without it. This is because a computer can react much quicker than a driver ever could.

9. #59
Last edited by a moderator: Apr 25, 2016
you did miss one key point. Remember how static friction has more force than non static? that is why you can stop faster on near lock. However, the window of having less friction than locked and locked is so negligible that even the slightest modulation of braking force will cause it to fall into lock or less force. If you want to talk theory, it is all fine and dandy, but theory only is any good if there is practicality to it. to which in cars, is impossible.

you also failed to comprehend one thing. In braking, there are two frictional points. Tire and road, and brake and rotor. The ABS senses when the wheel is locked. Therefore, in order for the wheel to unlock, the brakes must release to less friction than that of the tire to the road. Since a locked tire's kinetic friction is less than it's static friction, the brakes must let off even more than that. Therefore the braking friction on the rotors is less than that of even if the wheels were locked. it is a proven fact that with exception of water, ABS takes up to 20% longer to stop than at lock.

That is the second part of science, experimentation. did you look at the NHTSA site? they even stated the fact.

F1 has outlawed many things asside from ABS. The reason is that they were losing popularity in droves, many europeans even started to choose nascar over F1. Reason is that there was so much technology, driver skill was virtually nil. They took ABS out to as a bit more of a variable to the sport. With ABS, the driver no longer has to worry about locking the tires in corners, they can mash the brake all they want, that is the advantage.

Next time you agree with everything I say (I said most of what you said in my post, read all the data) be sure to get applicable real world application and use a complete modeling structure to fully analyse all the data. In DOT tire racing, it is all about the compound that they ban for more than tread. Compound has a much greater effect than tread does for handling on ashphalt for the reasons of static friction.

i am getting info to futher substanciate my statements, something of interest I find is that accidents that occur when a vehicle leaves the road actually increased 15-20%, why I do not know as of yet. Time to post links!

http://www.nhtsa.dot.gov/cars/rules/regrev/evaluate/808206.html
the accident increases

Theory as to this, In poor traction, your tires cannot bite into any surface (such as dirt or rough pavement) since the tires will roll over any minor formation. Not a good thing to me. It does state that pedestrian and wet accidents reduce, which is understandable, water is a lubricant, no sliding, no lubrication, better stopping and pedestrians, well, you can swerve.

Note in this next link that it states 2 wheel ABS is better.
http://www-nrd.nhtsa.dot.gov/vrtc/ca/lvabs.htm

because you get controlled and good emergency stopping.

Anyways, I am out of time, more later