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Old 03-04-2003, 03:46 AM   #1
AWD-T
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Default .99g on the skidpad.....explaination?

i've seen these numbers (not really .99) around a lot and was just wondering what this number meant? i've been searching for an answer but gave up after 15 minutes of website-hopping.
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Old 03-04-2003, 05:27 AM   #2
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I'm not sure what your asking?
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Old 03-04-2003, 07:58 AM   #3
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It is the maximum amount of lateral, or outward force that a tester could make the car get while turning around in a circle. With better tires and suspension, you will get a stronger force. They will run it in a circle just to the point where the tires give and measure centrifigual force.
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Old 03-04-2003, 09:55 AM   #4
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To add to that, "g" is representative of the force of earth's gravity. If a car is pulling 1.00g on the skidpad, then the outward, or lateral, force will be exactly equal to the force of gravity. The better a car's tires and suspension, the more lateral force it can withstand without breaking away.
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Old 03-04-2003, 11:00 AM   #5
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i believe it's done around a 200 ft. dia. circle
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Old 03-04-2003, 11:08 AM   #6
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Just to blow your mind F1 cars can get as high as 3.5 g in a turn. that means 3.5 times your wight is pressing you against the side of the car. If I was turning 3.5gs it would be 1000lb of force (yes I am not a small guy).
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Old 03-04-2003, 01:47 PM   #7
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Quote:
Originally posted by DetroitWRX
Just to blow your mind F1 cars can get as high as 3.5 g in a turn. that means 3.5 times your wight is pressing you against the side of the car. If I was turning 3.5gs it would be 1000lb of force (yes I am not a small guy).
your F1 comparison isn't equal however. the skidpad is a level surface, but the F1 cars are pulling over 3 G's in banked curves.
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Old 03-04-2003, 01:51 PM   #8
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Quote:
Originally posted by bjlee
your F1 comparison isn't equal however. the skidpad is a level surface, but the F1 cars are pulling over 3 G's in banked curves.
Banked curves? Whatchu talkin bout, Willis?

He said F1, not NASCAR. Besides Indy, how many F1 tracks have truly banked turns?
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Old 03-04-2003, 02:43 PM   #9
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the reason F1 cars can pull so many g's is that they can get up to 3.5x's their weight in areodynamic downforce...
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Old 03-04-2003, 02:52 PM   #10
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Quote:
Originally posted by Kaiser


Banked curves? Whatchu talkin bout, Willis?

He said F1, not NASCAR. Besides Indy, how many F1 tracks have truly banked turns?
They don't need to be heavily banked turns to greatly add to lateral grip. Very few corners on tracks are truly flat, most are "tilted" (amounts of camber) one way or the other. Even simple elevation changes make a big difference to the amount of grip available.

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Old 03-04-2003, 08:34 PM   #11
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Just to add to the confusion, you have to be careful with the F1 comparison because it isn't on a skidpad. That is a peak value. Your car can reach over 1g of lateral acceleration, but it cannot sustain this acceleration. That's what gives your car (most likely) <.90g on the skidpad. Similarly, an F1 car would probably not be able to get 3.5g on the skidpad, but it would still be insane.
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Old 03-05-2003, 12:00 AM   #12
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I am sorry I have made us stray.
OK nothing to do with F1 cars now I brought it up just to tell an extreme end of handling.

OK the facts
1.The skid pad is a test on a large flat even surface that a car goes around in a circle. Typically this circle has a 200’ dia.

2.They measure the G force the car is pulling during this test.

3.G force is the measure of centripetal force or pull to the outside of the circle. When you take the onramp fast and you get pushed to the door of the car that is centripetal force. If you weigh 100lb and 100lb of force is being pushed into the door that is 1g if 50lb it’s .5g and so on. Speed is a factor the faster you go in a circle the more force you have. I forgot the actual equation but knew it one time.

4.When a corner is banked like a NASCAR speedway it used the cars own centripetal force to help it corner. On a 45deg angle ½ of the force is being used to hold the car on the track. If it was 90 all of the force would hold the car to the track. Like a slot car going though a loop on the track.

I hope this helped and answered your questions, I am no means an expert this is just what I remember from general physics in college, hence I can’t recall the equation but can tell you 35 different Canadian beers.
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Old 03-05-2003, 01:19 AM   #13
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car and driver uses a 300' skidpad right?

correct me if i'm wrong, but is the skid pad number a measure of lateral acceleration when the tires break from the ground? so in metric a "g" is roughly 9.806 m/(sec^2) and in english (english units, even though we're the only ones that use it now) a "g" is 32.174 ft/(sec^2) ? is that right?
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i need to graduate, this civil engineering is killing me. i swear, i was cool in highschool
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Old 03-05-2003, 10:11 AM   #14
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You are correct, C&D uses 300 feet
http://www.caranddriver.com/xp/Caran...ng.xml?&page=3

I don't think radius matters. A larger circle just means you have to drive faster to pull the same amount of g's.

And no, that number is a measure of the maximum amount of lateral acceleration a car can sustain while driving in a circle WITHOUT breaking away

You just made me dizzy with those numbers.
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Old 03-05-2003, 11:02 AM   #15
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Just wanted to point out the difference between centripetal force and centrifugal force. The term centripetal was used both correctly and incorrectly above. Centripetal force is the "real" force. It is effectively what causes things to move in a circle. In the above example, "When you take the onramp fast and you get pushed to the door of the car that is centripetal force." it is not really centripetal force that pushes you to the door, it's centrifugal, however you would never even feel this force if the door weren't pushing back on you as centripetal.

Basically, "an object in motion tends to stay in motion, in a straight line" and it takes centripetal force to make said object go in a circle. centrifugal force isn't really a "force" as much as it is a resistance to being pushing into the center of the circle.
My god, that wasn't so basic after all...
anyway, if you didn't follow that, I'm not a teacher :-)

Also, radius SHOULDN"T matter, but I believe it would as the cars alignment would not be optimally the same at all turning radii. This is regardless of the dynamic changes due to compression of the suspension. Actually compression and dynamic changes are a HUGE factor in what we are measuring, but in the case of error, I believe a car could generally pull alot more g's at 500' than it could at 100'.
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Old 03-05-2003, 11:13 AM   #16
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Quote:
Originally posted by Snowphun
They don't need to be heavily banked turns to greatly add to lateral grip. Very few corners on tracks are truly flat, most are "tilted" (amounts of camber) one way or the other. Even simple elevation changes make a big difference to the amount of grip available.

Paul
Snowphun: you are talking about dif things. First of all like many have said skidpad tests are used for testing tires and suspension components. When a curve is banked even to a slight degree much less friction is used to turn the car. Instead of using the friction force the "weight" of the car or also known as the “normal” force is used to provide the center seeking acceleration. if there was no A(centripetal) the car or any object would follow a straight path which is tangent to the circle ( like you see when the tires give out and the car just doesn’t turn )

example: if an expressway off road ramp which is designed for 50km/h and has a radius of 50m is banked 22 degrees NO FRICTION would be required to make the turn ( whcih means if you are going exactly 50km/h on pure ice you would still make a perfect turn.. speed up and you are in the trees heh)
Quote:
Originally posted by Slowpoke
You are correct, C&D uses 300 feet
[url]

I don't think radius matters. A larger circle just means you have to drive faster to pull the same amount of g's.

And no, that number is a measure of the maximum amount of lateral acceleration a car can sustain while driving in a circle WITHOUT breaking away

You just made me dizzy with those numbers.
You are correct to some extent because the acceleration is given by your ((velocity)^2)/radius but its best to keep the circle optimized because as you know suspension and tires behave differently under different conditions. (example would be friction forrce of your tire under low and high temperatures) that’s why they have standarts for tests.

and comparing F1 cars isnt fair because they are designed to produce max down force ( and our cars arent ) but a cool thing to look at... even though the F1 cars are super aerodynamic they drag they create is BETTER at slowing down a car then our disk brakes

ok im done hope i didn’t waste anyone’s time and sorry for the physics lesson.
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Old 03-05-2003, 11:22 AM   #17
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Quote:
Originally posted by mrbell
Just wanted to point out the difference between centripetal force and centrifugal force. The term centripetal was used both correctly and incorrectly above. Centripetal force is the "real" force. It is effectively what causes things to move in a circle. In the above example, "When you take the onramp fast and you get pushed to the door of the car that is centripetal force." it is not really centripetal force that pushes you to the door, it's centrifugal, however you would never even feel this force if the door weren't pushing back on you as centripetal.

Basically, "an object in motion tends to stay in motion, in a straight line" and it takes centripetal force to make said object go in a circle. centrifugal force isn't really a "force" as much as it is a resistance to being pushing into the center of the circle.
My god, that wasn't so basic after all...
anyway, if you didn't follow that, I'm not a teacher :-)



Also, radius SHOULDN"T matter, but I believe it would as the cars alignment would not be optimally the same at all turning radii. This is regardless of the dynamic changes due to compression of the suspension. Actually compression and dynamic changes are a HUGE factor in what we are measuring, but in the case of error, I believe a car could generally pull alot more g's at 500' than it could at 100'.
hehe nice ! you said everything i missed
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Old 03-05-2003, 11:59 AM   #18
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Quote:
Originally posted by NTlncer
Snowphun: you are talking about dif things. First of all like many have said skidpad tests are used for testing tires and suspension components.
Nope, you didn't read what we were discussing further up. I was offering an explination of how a car can pull higher cornering g's. An F1 car (as mentioned earlier) cannot pull 3.5+ g's on a skidpad, but can do so through a banked turn.

Paul
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Old 03-05-2003, 01:12 PM   #19
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Quote:
i need to graduate, this civil engineering is killing me. i swear, i was cool in highschool
I graduated three years ago and realize I will never be cool I am a Civil Engineer. Cool was than not now, not ever again. OK I tried to simplify things and I just opened up new arguments. Oh well by the way Sport Compact Car uses 200 ft skid pad. To each their own.
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Old 03-05-2003, 01:37 PM   #20
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actually, to be even more technically correct, there is no such thing as centrifugal force... that is actually INERTIA.

to whit: Centripetal force is the force exerted on an object to alter it's motion from a straight to a circular line. For example, a ball attached to a string is spun in a circle. The string is exerting centripetal force on the ball to make it go in a circle, instead of going in a straight line. The intertia of the ball wanting to go in a straight line is mis-named "centrifugal" force.

http://www.slcc.edu/schools/hum_sci/...mass_circular/ lists a good answer to the question "What is the difference between the centripetal and centrifugal force?"

"The difference between centripetal and centrifugal forces is quite simple - centrifugal forces do not exist while centripetal forces do. " The answer is quite a bit more involved than that (browse over and see for yourself).

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Old 03-05-2003, 02:36 PM   #21
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Quote:
Originally posted by shadowcat
actually, to be even more technically correct, there is no such thing as centrifugal force... that is actually INERTIA.
...
shadowcat
right... that's what i meant by saying it was a "real" force... but "centrifugal force" is a specific application of intertia. The same way that torque is a specific application of force. And it works nicely to consider it a force because, if something is continuing in a circle infinitely, the forces of the system will be balanced. and since you have one force pushing towards the center, it's convient to think of having an equal and opposite force pushing away from the center. It's not really real, but it helps in conceptualization...
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Old 03-06-2003, 09:24 AM   #22
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Uhm... specific application of inertia?... inertia is a property, it is not something you apply. You apply force, you don't apply inertia. Inertia is a property something possesses. Saying you are "applying inertia" is like saying you are using something's inherent weight (and NOTHING else) to MAKE the object move in a straight line on a level surface from a standstill. Take a pebble sitting on a level surface in a vacuum. It has inertia. It wants to stay still. There is no applying inertia to it. It already possesses inertia. To make it move in a straight line, you apply force to it. To make it move in a curve, you don't apply inertia, you apply centripetal force. Its inertia will make it want to continue moving straight, barring any other outside force (friction, etc).

As for the balance of forces, the centripetal force is balanced by the mass of the object, which has a direct relationship to the amount of intertia it possesses. Think of it another way: if the centripetal force was greater than the mass of the object, it would pull the mass tighter and tighter into the circle (a decaying orbit, for instance, although that is a bad example as a decaying orbit has more to do with atmospheric friction than increased gravity). A perfectly circular orbit at a constant velocity, the amount of centripetal force will be equal to the amount of force exerted by the object due to inertial mass. There is nothing pushing against the object to make it move in a curve. In fact, the mass of the object makes it want to move in a straight line. The force being applied is a negative vector, a pull, to make it go circular. That force is centripetal force. You are applying the equal and opposite reaction thing incorrectly. That refers to an action, not an application of force. I understand your point about conceptionalizing it, but it's not really the correct way to see it.

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Old 03-06-2003, 10:39 AM   #23
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I didn't mean application of the inertia itself, really, just the way it is applied in formulas. Something has inertia, so when it interacts with something else(like whatever is applying force to it to push it into the center), it imparts a force. That's what I meant by application of inertia. It was bad terminology on my part.

And it may not be the correct way to conceptualize things, but it worked for me in my AP physics classes in HS, and 2 years of physics in college...

Edit: I thought I should also add that this was all at least 4 years ago and i've not used any of it since, so I could be completely backwards... but it makes sense to me.

Last edited by mrbell; 03-06-2003 at 10:44 AM.
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Old 03-06-2003, 12:38 PM   #24
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Correct me if I'm wrong but I thought the size of the skidpad does matter. I thought that the handling characteristics of a car change as speed rises because of suspension loading and inertia. A car that has understeer tendencies at low speeds can oversteer at high speeds. Doesn't the inertia of the vehicle change at higher speeds depending on weight of the vehicle and how high the center of balance is in the car?
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Old 03-06-2003, 01:17 PM   #25
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The size of the skidpad will matter if you measure different cars on different radus tracks, if things like caster, camber, relative tracking (rear tires following front wheels) make a difference, plus downforce differences. This would be most noted if you ran a 10' radius and then did the same thing on a 500' radius. The 10' radius would show almost no downforce effect, while the 500' radius would show much less camber/caster/tracking/bumpsteer effect. The way around this is to always test on the same size track....and ideally on the same exact track at the same temp. You will sometimes see magazines putting forth excuses as to why a Vette in November showed much worse results comparted to their same test of the same car in July. Same track, colder temp. But the first test might have been V8 RWD cars and the 2nd, 2 seater cars. Each test would compare the cars done on that day, but the 2 tests won't correlate.

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