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Topic Index > General Discussion > TAKING CURVES

TAKING CURVES

12/6/08-
TO: CAR-CARE PROFESSIONALS
RE: THE FOLLOWING QUESTION ORIGINALLY SENT TO THE "CAR GUYS" ON 11/27/08.
PLEASE SEND ME YOUR ANSWERS. THANKS.

ORIGINAL MESSAGE:
NOV. 27, 2008 * HAPPY THANKSGIVING !

Dear Tom & Ray-

Like everyone else feels- you guys, & the show, are great-- a real LIFT from the troubles swirling around us. Thanks, guys.

Question: When driving on a basically straight road at, say 55 MPH, and one encounters a fairly sharp, less than gradual, curve / bend in the road -no matter to the left or to the right- the tendency (physics ?) involved is for the vehicle to want to pull to the left in the case of a right bending curve, or to the right on a left bending curve- in particular if the speed of the vehicle is maintained.
Centrifugal force kicks in.

It has long been my practice in this situation to briefly accelerate -say 5 - 7 MPH- in order to create centripetal force so as to counter the centrifugal force and thereby bring the vehicle to a more balanced, centered, and stable continuous movement forward.

Of course, the spouse vehemently objects to this practice and claims that either remaining at the same MPH or s-l-o-w-i-n-g a tad is the more reasonable method to use.
(Slowing, in my experience, only causes the centrifugal force to intensify).

And I feel the strain on the suspension, etc. is increased -however minimally- thereby creating (minimally) greater & unnecessary wear and tear on the vehicle when one drives as my spouse 'instructs'. Please verify this point as well.

Now, a quick joke for you that I heard on 'LENO':
-An old geezer is pulled over & advised by the officer he'd been speeding for several miles. The officer further broke the news to the old gent that his wife had fallen out of the car five miles back but, fortunately was OK.
"Oh, thank God!" the old guy seemed relieved, "I thought I was going deaf !"

Thanks a million fellas. Best Regards.
Sincerely, Dan Cammaroto

12/6/08- RSVP

You've got it backwards - lower speed = lower centrifugal force, lower strain on the suspension/tires/everything. Your wife is absolutely correct.

I vote for the wife also,don't let those curves sneak up on you.

You are saying you come upon a curve, and you are going to fast for the curve and you speed up? Sounds scary.

But you can over do the slow down. I spun a early 911 by lifting off the gas to much while entering a curve,those cars were very prone to doing this, and I was very young(it was a 67).

Was this one of the old swing-axle 356's?

It was a 911 (6cyl). The worst thing is that it was a customers car,absolutely no damage,didn't hit anything,yes I had permission for a extended test drive. This Porsche had mechanical fuel injection and did not want to start after a extended drive.Are you familar with Topanga Canyon Blvd (in L.A) crosses the Santa Monica Mts. to the beach,the drop offs are hundreds of feet.You know I will say swing axel but I am not sure (VW started with IRS in 68 but I am not sure for Porsche,4450 will know)we were a VW shop mainly.It scared me tremendously,I don't know more, the fear of dying or because it was a customer car.

First, there is no such thing as centrifugal force. There's only centripetal force acting on inertia. I'll give you a D in physics. Now here will come the hornets out of the nest...........

The reason the car feels better when you accelerate in the curve is because of weight transfer. More speed actually causes more centrifugal force....I mean more inertia. Anyway, the typical FWD road car has more weight in the front than in the back. Shifting wme of the weight to the rear by accelerating in the curve creates better balance and the turn feels more in control.

Now to the final answer. Your wife is right. Even if your wife is wrong, your wife is right. Say "yes honey" and maintain a constant speed around the curves.

there is no such thing as centrifugal force

From Merriam Webster:

CENTRIFICAL FORCE:
the apparent force that is felt by an object moving in a curved path that acts outwardly away from the center of rotation — compare CENTRIPETAL FORCE

CENTRIPETAL FORCE:
1 : proceeding or acting in a direction toward a center or axis
2 : AFFERENT
3 : tending toward centralization : UNIFYING

The OP was using the correct term.

Hornet #1.

Yeah, I know, it's in the dictionary and commonly used. But in physics there is no such thing. The op was using the correct common usage term, but was incorrect to the world of physics. And, if you'll notice, the definition says "apparent" force. There is no ACTUAL force.

We stirred up a hornet's nest a few years back with this very subject. Back then I took the position that since the term was commonly accepted one could argue that it could be called a "force". I was thoroughly trounced. I was incorrect and I knew it.

In truth, centripetal force is any force acting on the tendency of a moving object to continue in a straight line via inertia and causing it to move in an arc around a central axis. That which is called "centrifugal force" is actually the inertial force trying to push the object in a straight line.

Agreed. It's about the only thing I remember from my physics class 15 years ago. "There are no angels beating their wings" is the way my professor put it.

I agree. There is no centrifugal force, only centripital and your description is correct. Remember that forces are not scalar quantities, but directed (vector) quantities. If the positive dierction is towrds the origin of the arc, the negative direction is away from the origin.

DEC/11/08-
DEAR 'jtsanders'- THANK YOU FOR YOUR RESPONSE AND AFFIRMATIVE STANCE ON MY POSITION. MOST RESPONDERS FEEL I'M WRONG, HOWEVER. CAN YOU DO ME A FAVOR AND EXPLAIN -A BIT MORE IN LAYMAN'S TERMS- JUST WHAT YOU'RE SAYING IN YOUR RESPONSE.
GREATLY APPRECIATED. THANKS- DAN

You are truly grasping at straws - JTS is not agreeing with your 'speed up for corners' position, just commenting on a fine point of the physics.

Yeah, I know, it's in the dictionary and commonly used. But in physics there is no such thing.

Try telling that to my college physics professor at Ohio State University. OK maybe physics has changed since the 1970's. I so does the Barns and Noble college outline physics book (also from that age.) as does the current book "Physics for Dummys" That was all the reference books I had around the house.

My guess that is what you were taught. I can remember some comments about it even back in the 70's. However I would guess "centripetal" has become real in modern physics. I would guess a lot of what I and you were taught on the sub atomic level has changed.

You might take a look at

http://en....ugal_force

Which by the way has some diagrams displaying the subject of this thread.

Updated: 12/07/2008 06:18:51 AM

Centrifugal force is a fictitious force, as you feel you are being pushed into the window in a turn it is actually the window pushing on you, but that does not mean it is not a useful shortcut for describing events. Einstein thought gravity was a fictitious force also. If your kids have gotten used to the podcast of car talk you can threaten them with having to read and figure out this page
http://en....ious_force

It take the professional race car drives a long time to learn that trick and they practice it a lot. It also will change from car to car. Doing it wrong could cause a crash.

If you goal is safety, then I suggest slowing down before entering the curve.

DEC/11/08- THANK YOU 'Joseph_E_Meehan' FOR YOUR COMMENT. ARE YOU SAYING THAT, THIS 'TRICK' IS JUST AS VIABLE ON CITY / HIGHWAY DRIVING AS IT IS ON THE RACETRACK ?
-CLARIFICATION: I DRIVE A LOW PROFILE, '04, LOW TO THE GROUND, COMPACT PSUEDO SPORTY-TYPE CAR. AS I SAY IN MY ORIGINAL LETTER, I OBEY THE SPEED LIMIT, BUT WHEN ENCOUNTERING A SHARP BENDING CURVE I ACCELERATE A BIT -SAY APPROX 7 MPH- IN ORDER TO HOLD THE ROAD AND KEEP FROM DRIFTING INTO THE ONCOMING TRAFFIC. THIS PRACTICE ABSOLUTELY WORKS FOR ME AND I'M QUITE PERPLEXED AS TO THE CONTRARY POSITIONS I'VE BEEN RECEIVING FROM MOST OTHERS. THANKS- DAN

Let me get this straight, you speed up 7 MPH to avoid drifting into another lane? Am I the only one who sees a flaw in that reasoning?

I don't understand why you are perplexed. There are some extremely well-written explanations of how and why your reasoning is flawed.

I can't believe you are asking Joseph to validate this behavior after he clearly stated "If you goal is safety, then I suggest slowing down before entering the curve." What part of that don't you understand?

Those ALL CAPS are hard for most people to read, please ....

What I am saying is the professional drivers may well use a trick something like what you are explaining. However they are driving a far different car than you are, and at far different speeds (I hope :-)

It is not something that would be easy to do, even if it would work on a road car. In most cases it will only make the car less stable. I am not sure it is really used by race car drivers for that matter. Also remember that their goal is to go through the corner taking the least amount of time and to come out of the turn going as fast as possible.

I would not be surprised if you are confusing the way to drive through a corner and start accelerating before they come out of the corner. I don't believe they start accelerating as the enter the turn.

My wife has a PhD in Physics but she is sleeping so I'll have to do without and just report a little anecdote from growing up and getting the driver's license in Germany. There it is legally required to do at least 25 or so hour-long lessons at driving school (and spending 2000$ in the process) before you are even admitted to the exam. That allows you to spend some quality (and often humbling) time with a driving instructor that calls you out on all your mistakes. Coming from a rather mountaneous area I vividly remember him instilling into me the practice of decelerating before a turn and accelerating in the turn. The argument is (I paraphrase) to "pull" the car out of the turn and through acceleration give it more "side guidance" (don't know the English word). This coming from a driving instructor in german of all palces makes me think that this is the DMV equivalent's thing that they want students to know there. This being Germany makes me think that they have pondered it well.

Then again they may have the cars for that and doing this in a bouncy Buick would wreak havoc ;-)

So in the situations you describe with your wife, my course of action would be to lift the foot from the pedal just a little before entering the turn (which will make her feel better) and take advantage of the "pull" by re-accelerating to the previous speed in the turn. Going through a turn at a higher speed than in a straight seems counterintuitive.

My college degrees are in physics and I have competed in amateur road races.

A vector is a quantity that acts in a specific direction. Therefore, any force is a vector. The same is true of velocity. A scalar as a quantity that doesn't have an associated direction. Examples are density and temperature.

Centrifugal force is real. The equation for it is F = m * v^2 / r. That is, the force equals the mass multiplied by the square of the velocity and divided by the radius of the curve. So is centripetal force. Left to itself, a body such as a car will keep going in the same direction (Newton's first law of motion). A force applied at right angles to the direction of motion causes the body to move in a curve. This is acceleration. If the magnitude of the force remains constant and the direction of the force keeps changing so that it is always at right angles to the direction of motion, the curve is a circle.

In road racing, the basic cornering technique is to enter the corner at a reduced speed and accelerate through it so that you exit at a higher speed. You use all of the road so as to make the radius of curvature as large as possible. On the street, if you need to slow down for a corner, it's best to slow down a little more than necessary so that you can accelerate gently through it. Your car is more stable under steady power.

Updated: 12/07/2008 11:59:12 AM

It troubles me to see that a person with a degree in physics says that there is centrifugal force, when my physics text specifically admonished the use of this concept. I never used that concept in my mechanical engineering course work.

While you can feel something like a "centrifugal" force, it is really a reaction to centripetal force. Inside a vehicle, they balance each other out and that's why you don't fly around inside. But to an observer in an inertial frame of reference outside the vehicle, there is only centripetal force and the occupant inside is constantly accelerating toward the center of the circle. If such observer see some mythical "centrifugal" force balancing with centripetal, the occupant and vehicle would be going in a straight line, which means there wouldn't be any centripetal force at all. Because we arrive at this contradiction, there cannot be any "centrifugal" force in the inertial frame of reference, a frame of reference where force and acceleration vectors may be analyzed.

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TAKING CURVES