CP vs CG

[maguromic]

I finally figured this out . . .

Convinced People vs. Convinced Guessers

I am still trying.  Tony

[kustombrad]

I'm pretty sure it's come to this... If you've figured it out, you'll go fast. If you haven't, you'll always be chasing your tail. If you don't have a car it doesn't matter anyways...

[Sumner]

....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....

This is my last response to a question such as this.  Aero downforce flattens the tires and the CG of the car itself is in the same location relative to the car.  If the car does a wheelstand then the CG goes to a new location in the universe along with the car.

You just keep coming back to aero forces on the car that for sure are there and I'm not disputing that but they don't change the CG.  A NASCAR car should have what you are talking about with all the downforce built into the car and if you are correct it should keep the car from spinning once it loses traction but it doesn't.

I'm not saying you need to give up trying to gain traction, that would be foolish.  We now have the CG ahead of the CP on Hooley's Stude and I"m convinced that was what kept it from going around in 2013.  Now we can more or less leave the car alone and stop adding more weight since we can gain traction via the rear wing, using its size, profile and angle of attack.  But while experimenting with that if we loose traction with the CG ahead of the CP that relationship will go a long ways towards keeping the car from spinning.  It isn't just the spin that is dangerous, some cars once sideways tend to start flying.  Long cars like streamlines also don't tend to spin very well as witnessed by the number that receive a lot of damage once sideways.

My remark about the blown gas/ blown fuel referenced that now with the development of turbos and in your case centrifugal blowers it isn't that hard to build any motor larger than an F motor that will put out over 1000 hp and a lot more.  So with that kind of HP available a lot of the cars that can't have a wing and the vertical stabilizers that can really, really help with the CP part they can add both weight ahead of the rear axle for traction and weight further forward to move the CG forward.

I've already repeated myself to the point I'm even tired of hearing myself on this so people will just have to decide what they want to take from this thread.  I'm not trying to convince you to change your car.  If it is working for you stick with it.

Sumner

[Ron Gibson]

Because the "cg" is up in the center of the mass, maybe for practical purposes we should call what we're discussing the balance point. As I understand it, the static "cg" would be somewhere along a vertical line at the balance point. As long as the tires are on the ground this is what we're talking about. The balance point would change as force is applied to either end. The cg can be low and the cp much higher, think Nascrap cars blowing over backwards after sliding sideways while going forward.


Ron

[tortoise]

So with that kind of HP available a lot of the cars that can't have a wing and the vertical stabilizers that can really, really help with the CP part they can add both weight ahead of the rear axle for traction and weight further forward to move the CG forward.

You might be better off adding the weight you're talking about in one lump rather than two, at what would be the CG of the two lumps. This gives you the same CG relocation but a lower polar moment of inertia.  
But what some of the others are saying, I think rightly, is you want the least possible or even no ballast if you are drag racing, even a 1.3 mile drag race on dirt.

[tortoise]

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

Don't ask me why, but I'll try once again to explain how weight distribution can change without the CG moving. Think of your car moving at a constant speed up a gradually steepening hill. If the hill gets steep enough, and you maintain traction, the car is going to tip over backwards. The CG is at the same point in the car, but the direction of the gravitational force relative to the road surface changes. Accelerating is like going up a hill: the vector sum of inertial and gravitational forces is no longer straight down but slanting toward the rear.

By the way, according to Einstein, gravity and inertia are really the same thing, but let's not go there.

[Sumner]

.....You might be better off adding the weight you're talking about in one lump rather than two, at what would be the CG of the two lumps. This gives you the same CG relocation but a lower polar moment of inertia.

I agree with that and probably 80% of the weight we have added to the car is in weight boxes ahead of the rear tires and back of the driver with the CG about where the steering wheel is.  Running out of room there we also have weight under the transmission area and a small amount just ahead of the engine.  Cars are still compromises and we have had to make some.

I once built a 'pump trailer' that was used to do water testing and pump-down tests on wells for an anticipated strip mine.  You could pump or bail wells up to 400 feet deep with it.  To keep the cost down I used the drive-train from a 4 wheel drive Bronco to supply power for the winches and generator and such.  The drive train was pretty much intact except for swapping the front axle and rear axle locations and was mounted on a trailer.  So a lot of weight at both ends of the trailer.  The trailer was a real pain to tow since if it started to wag its tail it only got worst.  The next one I made had a different design and all the weight central to the trailer.  Lesson learned.  That is why I really try and tell people to not put the weight they need for traction back behind the rear axle.  We have nothing back there except for 1 normal size battery and another for data-logging only.

Edit:  Added what is below this at a later time.
=======================================



I added the photo above to show the 'approximate' locations of the CG and CP on the Stude.  Actually the CP would be a little further back as Tom B. pointed out to me that I should use the side view area of the vertical stablizers twice in predicting the 'approximate' location of the CP since both of them see the air.

I'll grant that it is going to be very hard to get a 'precise' location for the CP, but John Burk has pointed to one method to get close and I've used a different method that comes from people with far more knowledge than I have to figure ours.  I outline that method here for anyone interested....

http://1fatgmc.com/car/14-Hooley/14%20-%20hooley-construction-2014-2.html

I'm not saying you are going to be within inches of the CP with John's method or what I presented but it sure beats doing nothing about something that you can do to build a safer car,

Sumner

[Eddieschopshop]

I think looking at the car as if it were traveling down track at a 15 deg angle helps a lot for center of pressure.  Lets face it if the car is straight all is good,  but at 15 deg sh&*ts about to get real.  At that angle the cars center of pressure can change dramatically and aero downforce is typically nearly gone.  The shape the air sees is totally different,   this shape/cp and the center of mass is what will make the difference on a car going around or not.  Some car with a taller front cockpit/body will see a significant change in the cp at an angle and suddenly the tail is not enough to overcome the pressure trying to push the front around.  In the video it appears as though speed demon's cockpit is ripped off from the air prior to the roll.  Imagine the cp while the car is sliding sideways.  With a large/tall cross section forward of the cg it seems the car would be harder to recover.

The cg and or mass of the car is what is continuing to push the car down track so the pressure will rotate the body around the cg point depending on which end of the car is easier to push sideways (considering air and scrub/tires).  So a car with less than ideal cp/cg relationship can get away with it up untill it sees a certain degree of slip/yaw. 

[Sumner]

So that we are clear on CP.  It is the point that if you turned the car 90 degrees to the air the car wouldn't rotate one way or the other if a pivot point was put vertically through the car at the CP (this is if we discounted the CG for a moment).  The pressure behind and in front of the CP would be equal. 

Now bringing CG into the equation the car should weather-vane around with the nose still down track if the CG is ahead of the CP just like a weather-vane keeps pointing into the wind.  If the CG is behind the CP it is going to weather-vane around trying to put the tail in the lead.  Likely it won't stay there because now the car is spinning and inertia is involved but if we can keep it from going around in the first place that will be a good idea.

It is too bad that all that are reading this don't get BRN as George's article is a good one and explains the canopy coming off and other items of interest.  That article and others like it are a good reason for anyone involved in this to subscribe.  What you will learn from the interviews and articles that Bob Hellmuth and Bill Hoddinott write and others could save you thousands in building a better car and not having to repeat and relearn what many have learned before us.  You can also order a lot of those articles from back issues.  I go back and re-read them from time to time and always pick up something new,

Sum

[panic]

Correcting my earlier typo (duh):

"mass (directly proportionate to the number of atoms present in an object)."

Yes, it's the atomic weight, not the atomic number.

[panic]

Re: "if you turned the car 90 degrees to the air the car wouldn't rotate one way or the other if a pivot point was put vertically through the car at the CP"

True with the car in the intended vector and alignment.
However, with the car in yaw (unless it's radially symmetrical!), the CP almost always moves - the questions are:
1. how much? and
2. forward or backward? and
2. acting at increased or decreased height (will it induce a roll moment)?

I'm sure we all agree that yaw resulting in an aero change that moves the CP backward (at least, in the range of predictable steering corrections, not 90°!) is a useful safety feature.
Has anyone heard of spill plates (e.g., end & end "capture" fins on a rear wing) linked to the steering (toward motion parallel with steering corrections) so that they tend to sustain down-force by controlling spillover?
Another "wing" thought: if yaw begins to become a slide, isn't increasing wing down-force also helpful (since drag is no longer a concern)? How about a dive brake, which cranks the leading edge's nose down (ref.: Stirling Moss's 1955 300SLR, linked to brakes)?

[manta22]

OK, I think this is about exhausted............

[Seldom Seen Slim]

I like that icon, Neil,  I have been reading along on this discussion but haven't tried very hard to absorb the information.  Not that it isn't interesting or perhaps not important, but there's so much of it.

[Elmo Rodge]

Relax Jon. Not all of it is information.  Wayno

[Richard Thomason]

Sorry, I didn't really mean to start a treatise on aero, design etc., I was just reflecting on my own experience and thinking about George's comments. As I posted before, there are so many differant classes and design constraints that there is no one size fits all. Yes it is more simple in a streamliner to achieve ideal configurations, but there can be other design constraints that violate the ideal rule, such as the fork lift Thrust SSC. They made it work, as many of you have in other situations. I applaud you all for your ingenuity and cleverness to make things work the way you have. I'm certainly not as smart as many of you in "tricking the laws of physic" and therefore that is why we built the way we did. It eventually (after two relatively high speed endos) worked for us and hopefully our experience will help us in our new build.
Richard