Taking curves

#1

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



#2

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

#3

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).

#4

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.

#5

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.

#6

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.

#7

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.

#8

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

#9

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.

#10

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.

#11

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.

#12

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 :wink:

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.

#13

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.wikipedia.org/wiki/Reactive_centrifugal_force

Which by the way has some diagrams displaying the subject of this thread.
#14

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.

#15

If it’s FWD, then what you are doing can work so long as you don’t exceed the traction available to the front wheels and gradually increase in speed through the curve causing them to “pull you through it”. If RWD it can hurt you quickly as adding more fuel just makes the situation worse.

Your wife is being “reasonable” and in the vain, is right, but dull. It is a lot more fun to do what you do (I tend to do that too in my sports car). Straight roads are made less boring with an occasional curve or chicane. I think it should be mandatory. :slight_smile:

So when she is with you, drive like a truck driver. When she isn’t - have fun!

#16

Plus, if your car has ESP tell her that any craziness on your end would be electronically corrected.

#17

I’ll try to keep the english a little less “sciency”.

A lot of people think that it takes energy to go around a turn. Actually the only thing that takes energy is to enter a turn, and that is BARELY any energy.

Assume you are going straight at a constant speed, then enter a turn. It must take some kind of energy to do that, right? Well barely any. The only energy required for turning is to create the initial rotational inertia- that is, once the car starts to rotate (i.e. go around a corner), the car continues to turn due to STATIC forces.

To put it in plainer english, if you get 25 MPG in a straight line, you’ll get 25 MPG driving in a large circle (assuming you aren’t doing skidpad testing or something).

If you zig-zag (i.e. changing the rotational direction of the vehicle frequently and harshly), then you might notice a decrease in MPG.

If you’ve ever had a car on a single-post lift, you’ll notice that you can rotate the car with the force from your pinky finger. This is all of the energy required to enter a (regular) corner- barely any!

If you want to take the corner in the smoothest way possible, follow the outside-inside-outside approach. That is, start the corner as far outside as possible, 1/2way through the corner you should be at the inside of the corner, then end up on the outside of the lane. This is the same thing racecars do.

#18

You can ask any race car driver. You accelerate though the curve, not into the curve. In fact, you decrease speed before entering the curve, accelerate gently in the curve (or maintain a constant throttle), and give it more power as you exit the curve.

In your case, on public dry roads with normal curves, coasting into the curve or maintaining a constant speed into the curve works best, depending on the angle.

Accelerating as you enter a curve increases the likelihood you will either lose traction or be forced to brake inside the curve, neither of which are desirable.

#19

You sound like you enjoy driving a car - now it’s time to learn how. Here’s a web page (there are many, just Google it) that discusses the “slow in, fast out” technique, as many have mentioned: http://phors.locost7.info/phors17.htm
You might even enjoy some driving lessons at a local race track.

#20

I feel it depends upon your drive train…generally FWD cars tend to plow. RWD cars tend to over steer. LET up on the gas if the object is to get through a corner safely with FWD. Exception, when we raced SAABs you could induce a little oversteer by playing with the parking brake on the rear wheels.

As a general rule FWD cars are poor handlers compared to balance RWD cars.
NEVER accelerate in a FWD car in a corner if you are not sure of traction conditions. Loss of traction means IMMEDIATE lose of control…then it’s into the other lane on right turns and the ditch on left.