CP vs CG

[Interested Observer]

“weight  n.  ...attraction of a material body by gravitational pull toward the center of the earth”

It is clear that JL222 prefers to operate in his own universe, apart and different from everybody else’s.

You can lead a horse to water . . .

At this point, I’m with Sumner.

[jl222]

“weight  n.  ...attraction of a material body by gravitational pull toward the center of the earth”

It is clear that JL222 prefers to operate in his own universe, apart and different from everybody else’s.

You can lead a horse to water . . .

At this point, I’m with Sumner.

  AND the flattening of top fuel tires at speed is from what?

  And the video is wrong too?

  Why don't you answer the questions that Summer won't?
                 JL222

[tortoise]

 If you think aero adds weight not force, does your parachute then have to be sized for the extra 5000 lbs you need to stop?  No it doesn't because there isn't actually 5000lbs more weight.

Whether jl222 wants to call downforce "weight" isn't too important. Speaking casually, a lot of racers, even engineers, might do the same. But they'd understand your point, and know you're correct.

Jl222, do you?

[64avanti]

It is unfortunate that we use the same terms for external forces and weight.  That is why some people here think that an external force such as aerodynamic force and weight are the same as they are measured in pounds.

As several have pointed out CG is the center of mass acted on by gravity.  External forces do not change the center of mass or the CG.  lbs force are not the same as lbs weight.  The fact that both are measured in lbs does not make them the same.

In the metric system people tend to talk about weight in kilograms (kg) and forces in Newtons (N).

[desotoman]

  AND the flattening of top fuel tires at speed is from what?

  And the video is wrong too?

  Why don't you answer the questions that Summer won't?
                 JL222

In the video I thought it explained mechanical vs. aerodynamic force pretty well, and I know it did not talk about CP, but it was the best video I could find that I thought might help this discussion.

I really like FORCE as the word used to describe what is going on aerodynamically, as IMO it differentiates from the word weight, and is easier to get a Grip (no pun intended) on the forces involved.

I posted the video because I am a visual guy and I thought it explained what Sumner and others had pointed out and explained.

JL222 also thanked me for posting the video.

IMO we all agree on the video. Now it would seem to me that there is an interpretation or word that might be throwing a wrench in the works.

I think using the word weight instead of Force is the problem.

In physics, a force is any interaction which tends to change the motion of an object. In other words, a force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described by intuitive concepts such as a push or a pull.

I also think GRIP is a good word to use.

So the wing on a T/F car at a given speed uses Force to increases the Tire Grip on the surface of the drag-strip, with out adding mechanical weight to the car which would slow down its acceleration and stopping ability.  

Tom G.

[Sumner]

I think we have lost sight about the intent of this thread which was...

....then he said I'm only going to tell one thing because you're probably going to kill yourself. Make sure the CG is ahead of the CP! And bang the phone slammed down. Pretty sage advice. Anyway, that was one the driving factors that led us to build the Danny Boy as a front motored, front wheel drive car and eventually with a huge vertical stabilizer....

The title is "CP vs CG" .  The thread is/was suppose to get that message out.  Maybe someone should start a new thread on all the different forces that can effect a car but that don't effect the car's CG,

Sumner

[wobblywalrus]

It is a complicated issue. Hard to isolate to just two topics.  The CG stays the same at all speeds, excepting the loss of fuel or coolant.  The CP varies in its location, and the magnitude and the direction of the forces acting on it change greatly depending on the circumstances.  Ultimately, the forces acting on the tire to salt contact patches are what matters and this is a combination of many. 

[entropy]

The title is "CP vs CG" .  The thread is/was suppose to get that message out.  Maybe someone should start a new thread on all the different forces that can effect a car but that don't effect the car's CG,

Sumner

I read the entire thread and learned a bunch of stuff, but i'm with Sumner.
I thought that the static mass distribution of the car results in the location of its center of mass.
On earth we most easily determine the the vertical component of the center of mass by measuring the force of gravity on its 4 wheels (weights)
Then we calculate the center of mass and call it CG

As the car goes down the track, all the forces that act on that mass dynamically push it around.  
Wings, traction, parachutes, wind, steering input, etc generate forces which will cause that mass to move. F=Ma
It will move differently according to its specific distribution of mass, but generally having Cp aft of Cg results in greater stability.

This seems too simple, i must be missing something.
Karl

[Sumner]

... Ultimately, the forces acting on the tire to salt contact patches are what matters and this is a combination of many. 

I agree totally that is what matters in order to maintain traction and that should be the subject for a different discussion. 

The point of this thread was to enlighten us on what we can do to maintain some semblance of car control after traction is loss by paying attention to our car's CG in relation to its CP,

Sumner

[panic]

Re: "it is a force and not a weight"

Part of the confusion is that identifying down-force (or any non-gravity based effect) as weight is that it conflates weight (gravity pull, proportionate to the relative masses of the object in question and the astronomical body on which the event occurs) with mass (directly proportionate to the number of atoms present in an object).
This is why chute selection is not down-force compensated - it only controls mass, not weight.
On Earth (and at sea level, to be specific) weight (in lbs.) = mass (in slugs) × 32.172 (acceleration rate by Earth gravity). On the Moon weight (in lbs.) = mass (in slugs) × 5.322 (acceleration rate by Moon gravity).

[tortoise]

. . .mass [is] (directly proportionate to the number of atoms present in an object).

Oh, really?

[jl222]

I guess I don't understand how an external force of 7000 lbs (downforce) that will flatten tires if not for centrifugal force is not to be considered as weight. In my other post if you could have a 320 mph wind in the tunnel, the car would weigh 8200 lbs on the scales. How is that not weight?

Ron

  Ron...You not drinkin either?
 
   And all that 7000 LBS of downforce [ or what ever that's flattening the tires] doesn't change the CG dynamically.

   It'S like a car doing a wheel stand and these guys say the center of gravity hasn't changed.

                             JL222

[Stan Back]

I finally figured this out . . .

Convinced People vs. Convinced Guessers

[tortoise]

   It'S like a car doing a wheel stand and these guys say the center of gravity hasn't changed.

I think everyone here would agree that  wheelstanding  raises the center of gravity.

[jl222]

I think we have lost sight about the intent of this thread which was...

....then he said I'm only going to tell one thing because you're probably going to kill yourself. Make sure the CG is ahead of the CP! And bang the phone slammed down. Pretty sage advice. Anyway, that was one the driving factors that led us to build the Danny Boy as a front motored, front wheel drive car and eventually with a huge vertical stabilizer....

The title is "CP vs CG" .  The thread is/was suppose to get that message out.  Maybe someone should start a new thread on all the different forces that can effect a car but that don't effect the car's CG,

Sumner

  Sumner... CG ahead of CP is a great design for Streamliners and other classes of cars that allow changes to body styles.
 But I would warn about moving CG  forward to improve CP on some cars, loosing traction and causing a spin.

  I was  trying to point out that even if CG was ahead of CP, aero down forces as from a wing would cause the CG to move dynamically. You don't think so. Explain how Top Fuel tires are flattened without added weight on chassis and how the CG hasn't changed dynamically.

  Some one reading the original warning and thinking its gospel, then moving weight forward for a better CP could make things worse.

  Some of the things we have done on the 222 Camaro are completely wrong according to a lot of advice from seasoned
racers.

               JL222