rec.autos.simulators

by **J. Todd Wass** » Tue, 19 Nov 2002 11:46:53

Imagine a car that's reached its top speed due to aerodynamic drag acting
exactly at the center of mass of the car, hence no torques exist on the body
from this force. The car's center of mass is 1 foot above the ground, and the
longitudinal force at the wheels is 1000 lb. The acceleration of the car is 0
ft/sec^2.

Is there rearwards weight transfer or not? It seems that D'Alembert forces
indicate that there is no torque on the chassis, but that conflicts with the
standard Newtonian approach... Is there 1000 lb*ft torque, or is there 0? (My
vote is for 1000 ;-)) It seems that RCVD indicates in a few places that the
answer is actually 0, but maybe I'm misunderstanding this.

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://www.racesimcentral.net/

My little car sim screenshots:
http://www.racesimcentral.net/

by **Haqsa** » Tue, 19 Nov 2002 12:17:34

In cases like this it is best to throw the terminology out the window
and draw a free-body diagram. You have a tractive force acting on the
contact patch, and a drag force acting on the center of mass. From a
force balance standpoint they cancel out, but they are not collinear,
therefore they form a moment. Assuming that the car is not rotating in
pitch, there must be some other moment opposing it. This other moment
is the change in the normal force distribution on the tires, aka weight
transfer.

by **Doug Hoo** » Tue, 19 Nov 2002 13:11:16

I would like to thank the two of you for making me feel even more stupid
than I thought I was.

> In cases like this it is best to throw the terminology out the window
> and draw a free-body diagram. You have a tractive force acting on the
> contact patch, and a drag force acting on the center of mass. From a
> force balance standpoint they cancel out, but they are not collinear,
> therefore they form a moment. Assuming that the car is not rotating in
> pitch, there must be some other moment opposing it. This other moment
> is the change in the normal force distribution on the tires, aka weight
> transfer.

> > Imagine a car that's reached its top speed due to aerodynamic drag
> acting
> > exactly at the center of mass of the car, hence no torques exist on
> the body
> > from this force. The car's center of mass is 1 foot above the ground,
> and the
> > longitudinal force at the wheels is 1000 lb. The acceleration of the
> car is 0
> > ft/sec^2.

> > Is there rearwards weight transfer or not? It seems that D'Alembert
> forces
> > indicate that there is no torque on the chassis, but that conflicts
> with the
> > standard Newtonian approach... Is there 1000 lb*ft torque, or is
> there 0? (My
> > vote is for 1000 ;-)) It seems that RCVD indicates in a few places
> that the
> > answer is actually 0, but maybe I'm misunderstanding this.

> > Todd Wasson
> > ---
> > Performance Simulations
> > Drag Racing and Top Speed Prediction
> > Software
> > http://PerformanceSimulations.Com

> > My little car sim screenshots:
> > http://performancesimulations.com/scnshot4.htm

by **Dave Henri** » Tue, 19 Nov 2002 13:15:28

Doesn't it make your head hurt just LOOKING at that stuff??

dave henrie

by **J. Todd Wass** » Tue, 19 Nov 2002 13:48:17

Ok, thanks for verifying that. That's how my stuff has been working so far,
but reading some mentions of D'Alembert inertia forces lately got me thinking
maybe I was missing something.

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com

My little car sim screenshots:
http://performancesimulations.com/scnshot4.htm

by **Doug Millike** » Wed, 20 Nov 2002 06:29:28

Agreed.

Some of the stuff in RCVD is heavily tied to the axis system definitions,
for example, the aero axis system has origin at ground level. Maybe this
is what was confusing you? BTW, we keep correcting errors in RCVD, so
if you think you have found an error, please let us know.

-- Doug Milliken
www.millikenresearch.com/olleyfl.html <-- gift for the ***<grin>

by **Doug Millike** » Wed, 20 Nov 2002 06:33:22

> > I would like to thank the two of you for making me feel even more stupid
> > than I thought I was.

> Doesn't it make your head hurt just LOOKING at that stuff??

That's nothing, you should hear the calls we get from the users of our
analysis software, or even our dinner table conversation...

Once you know that you are ignorant there isn't any more excuse<grin>.
Time to sign up for a physics course and learn how the world works.

by **J. Todd Wass** » Wed, 20 Nov 2002 08:50:20

I'm not quite sure what it was exactly. I read your book quite often (comes
to the bathroom with me 70% of the time too ;-)) and have occasionally run
across reference to D'Alembert's inertia force there. For some reason, I was
left with the impression that even a free body diagram (like I always have
used) might not be entirely correct. Needless to say, that would throw a
wrench into my new dynamics code, so I wanted to be sure that the good old
fashioned way was still correct. I'm relieved to hear that it is :-) My brain
did a little stutter there when I read that. If this is how it worked, there
would suddenly be a lot more serious problems with high speed oversteer.

I wouldn't jump to the conclusion that this was an error in RCVD though. This
was just an impression I got off a few sentences, most likely without reading
enough, and my imagination ran off with the idea while I blankly continued
"reading". I'll check it again and drop you a note if I think I found a
problem.

Thanks

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://www.racesimcentral.net/

My little car sim screenshots:
http://www.racesimcentral.net/

by **Haqsa** » Wed, 20 Nov 2002 09:21:33

Funny, that's exactly how I think of physics - not as science, but
simply as how things work. I guess that might be one of the reasons why
it makes sense to some people and not to others.

by **Doug Millike** » Wed, 20 Nov 2002 09:33:35

....

From <http://www.racesimcentral.net/;:

"...alternative form of Newton's second law of motion, stated by the
18th-century French polymath Jean le Rond d'Alembert. In effect, the
principle reduces a problem in dynamics to a problem in statics. The
second law states that the force F acting on a body is equal to the
product of the mass m and acceleration a of the body, or F = ma; in
d'Alembert's form, the force F plus the negative of the mass m times
acceleration a of the body is equal to zero: F - ma = 0. In other words,
the body is in equilibrium under the action of the real force F and the
fictitious force -ma. The fictitious force is also called an inertial
force and a reversed effective force. "

by **Haqsa** » Wed, 20 Nov 2002 11:30:10

I like the term inertial force better, so much easier to remember. ;o)

by **Haqsa** » Wed, 20 Nov 2002 11:32:30

Don't be fooled, we just make all this stuff up as we go. ;o)

by **Nick** » Wed, 20 Nov 2002 21:12:18

Remember also about rolling resistance, which is a force acting at the
contact point of the tyres with the road, and so *will* create a torque
opposite to the logitudinal force at the wheels, unless your longitudinal
force of 1000lb is a resultant force of forward drive and rolling resistance
added together.

by **Ruud van Ga** » Wed, 20 Nov 2002 22:19:07

On Tue, 19 Nov 2002 04:08:32 -0500, Dastardly Dan

>>Don't be fooled, we just make all this stuff up as we go. ;o)

>Yea really. Who's the dufus that decided 1+1=2 anyway?

That was before 1+1 became 10.
I guess we're stuck with 2 though, even though I really think the rest
of the world should learn to think in hexadecimal. ;-)

Ruud van Gaal
Free car sim: http://www.racer.nl/
Pencil art : http://www.marketgraph.nl/gallery/

by **Ruud van Ga** » Wed, 20 Nov 2002 22:20:47

Wasn't d'Alembert the acceleration described as a force?
That'd mean it's just there to get a system with F_sum=0, using the
net acceleration of a body actually as a force, cancelling the rest
out.

Need more pencils... :)

Ruud van Gaal
Free car sim: http://www.racer.nl/
Pencil art : http://www.marketgraph.nl/gallery/

rec.autos.simulators

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