CP vs CG

[7800ebs]

This is what I was referring to .. "Sitting on scales in the shop looking for CG is pretty much irrelevant from my perspective."

[Sumner]

I so love this "techy" stuff and having to twist what little brain function I have around it! When referring to CG though, my question would be are we looking at it while "static" or while at speed? I'm building my streamliner to go fast and yes, chase records at "speed". If I have it on scales with me in it and CG is is in one point while sitting " static", but at 300mph (using my magic "speed" scales) it's now in a different location because of airflow, isn't that what we're actually looking for? In this sport/hobby it's actually ALL about airflow. Sitting on scales in the shop looking for CG is pretty much irrelevant from my perspective. Am I wrong?

As mentioned above the CG is the same place except for a small difference with the fuel burning off and changing the car's weight and where it is.   Center of Gravity is center of gravity, it is static.  Other forces can change loadings but they are forces not changes in the car's weight and where the center of that weight is located.  The other forces such as a wing or spoiler are going to play into things and need to be considered and studied and respected but do not change the CG of the car,

Sumner

[Sumner]

This is what I was referring to .. "Sitting on scales in the shop looking for CG is pretty much irrelevant from my perspective."

Not really since once you loose that traction that the wing helped with the CG as found in the shop along with the CP will now come into play and will influence which end of your car will be facing down track.  Your 2 tails are going to go a long ways towards ensuring the car goes straight but it is very simple to get an approximate location for the CG and CP so why not do it.  60 minutes or less and you would at least know the approximate locations of both,

Sumner

[NathanStewart]

Sum, would you not concede to the idea of a "dynamic" center of gravity?  You and Tortoise and a few others are batting around semantics.  Yes, CG is static but what Sid and JL are talking about is DYNAMIC.  Look at the larger picture and don't put CG in a box.  If CG is found by establishing a fore-to-after "balance" point when the vehicle is not moving, then it's entirely conceivable to me that the EFFECTIVE CG would move rearward as aero devices add down force to the back of a vehicle.  Just like in a wind tunnel - car is setup on scales.  With no air moving we see static CG.  Then with the air moving either in scaled or real speed, we can now measure the dynamic forces applied to the car which will indicate a shift in weight and effective "make the car heavier" on one end or the other.  Or conversely it could just as well make the car effectively lighter but the scales under the car will indicate a change - this is what they're talking about.  It is a real change and it is measurable.  Yes, we all know and understand that the mass of the car obviously didn't actually change therefore the static CG is fixed.  Without regard to the chance of having zero tractive force, I think it makes perfect sense to say that aero forces will affect the effective weight and balance of the car.

This discussion isn't far from an argument about calling motors engines and vice versa. 

[kustombrad]

So how how is "air" force different than "static weight (fuel loss)" force? It's weight regardless (gravity) of how it's there isn't it? Our deal is always at speed and with airflow causing lift or down force. If my car is jacked up in the shop and sitting balanced (like a teeter totter) but at speed  (our main goal) and that balance point is now a foot farther forward or back, isn't that the center of gravity we're actually looking for?

[kiwi belly tank]

Next time you guys head off down the road with your race car in tow, make sure to leave the tie downs lose so it can roll further back on the trailer & you have a nice day!
  Sid.

[7800ebs]

Tires connected to salt... go very very fast... with HEMI  

tires not connected to salt...  crash very very fast...  

unless CG is in front of CP

aero planes always have CG in front of CP.. well since the Wright Flyer..  

[7800ebs]

For your entertainment..  one run two views   NO WING  .. Most Steering provided by throttle.. aka sprint car ..

note to self... focus harder to keep it on the track...  


https://www.youtube.com/watch?v=PBqKu1zbR-Q      front view

https://www.youtube.com/watch?v=DCBLSkynrv8      rear view

[Ron Gibson]

I'm with Brad on the little brain part.  To begin with, we're not talking spinning, yawing, losing traction, side wind, tractive force, torque, sidewalls or any thing other than cg /cp in a straight line. All that other stuff just muddies the water.

If I have enough wing at the rear and far enough back so that it lifts the front wheels off the ground at speed, (as some dragsters have done). It sounds like I'm supposed to believe that the cg of the moving car is still the same at speed with all the weight of the car plus all the downforce on the rear tires. You could theoretically could build a front motored rear wheel drive car that had 80/20  static front bias and with enough wing far enough back and enough speed, lift the front wheel off the ground.

. If you have a bar with a weight on one end and an equal weight in the middle and both weights sitting on two scales. If you apply down pressure to the free end of bar til the end weight is off the scales, the second scale will show the weight of both weights plus the weight of the bar, plus the weight of whatever downforce it took to lift the first weight.

With the logic that if a wing doesn't add or remove weight, an airplane while flying would have no weight whatsoever and therefore would not have or need a center of pressure. (good luck on that one)

YMMV
Ron

[tortoise]

Much of the confusion in this discussion would be removed if we stopped calling the center of mass the center of gravity.

If the vehicle for any reason (loss of traction, side wind),the vehicle finds itself not moving directly into the wind, there will be a side force. The inertial resistance to this side force acts at the center of mass.
 
If the car is yawed to the left, the wind force will be to the left, centered at the CP. If the resistance to the side wind force is behind the CP, The wind will push to yaw the vehicle further left.

If the inertial resistance to the side force is ahead of the CP, the wind will push to yaw the vehicle right, straightening the vehicle relative to the wind.

Yes, of course it's important to have the appropriate down forces on the wheels, and this can be done with wings; nobody's disputing that; but it does not move the center of mass.

[kustombrad]

I love this! Airplanes and our deal are TOTALLY different from my perspective though. Is their CG/CP figured in the shop floor or calculated at cruising speed? Most planes I've seen have the nose pointed in the middle as to shove air over AND underneath. In our world this is a bad thing. The basic physics theories here ARE the same old thoughts that said it was IMPOSSIBLE to accelerate past 150mph in the 1/4 mile aren't they? If the rules hadn't changed, fuel cars would be in the low to mid 340mph range now. Theories and ACTUALLY shoving bricks through the air are two totally different things. If Speed Demons CG/CP were wrong like George said AND he still went 460+mph with a small motor, doesn't that kind of kill the theories here?  What we do with horsepower and aero out there is still very much always changing uncharted territory!

[tortoise]

The basic physics theories here ARE the same old thoughts that said it was IMPOSSIBLE to accelerate past 150mph in the 1/4 mile aren't they?

That theory was presented in an article in HOT ROD by an engineer named Roger Huntington. He was a real smart guy, but he made two mistakes; he didn't consider using downforce and he didn't realize that a very low inflation pressure slick tire, heated to where it got sticky, could have a friction coefficient greater than one.  NOBODY ELSE HAD THOUGHT OF THIS YET EITHER.  Give the guy a break. Predicting the future is hard.

[Rex Schimmer]

Tortoise said:

Much of the confusion in this discussion would be removed if we stopped calling the center of mass the center of gravity.

If the vehicle for any reason (loss of traction, side wind),the vehicle finds itself not moving directly into the wind, there will be a side force. The inertial resistance to this side force acts at the center of mass.
 
If the car is yawed to the left, the wind force will be to the left, centered at the CP. If the resistance to the side wind force is behind the CP, The wind will push to yaw the vehicle further left.

If the inertial resistance to the side force is ahead of the CP, the wind will push to yaw the vehicle right, straightening the vehicle relative to the wind.

Yes, of course it's important to have the appropriate down forces on the wheels, and this can be done with wings; nobody's disputing that; but it does not move the center of mass.

He is completely correct! Aero down force has NOTHING to do with the center of gravity (mass). You are confusing lbs of down force with lbs of mass. Actually the mass of an object is a property of that object and does not change even if you would take it to a place that did not have gravity. The mass that we are concerned with is called inertial mass, which is the property of a body that resist acceleration, linear or rotational. The basic physics formula F=MA (Force=Mass x Acceleration), which came from Newton, can also be written as A = F/M which shows that acceleration increases proportionally to force and inversely proportionally to mass, i.e. more force faster acceleration, less mass faster acceleration. To use Kustombrads example of a top fuel car if the down force of the wing(s) added mass then they would actually would accelerate slower!!

Rex

[kustombrad]

I'm not trying to argue here, but now I'm kind of confused. Center of Mass vs. Center of Gravity. Say for arguments sake we'll use a neutral (even on all sides but airflow is even) shape like a boat tail bullet. We do the see saw/teeter totter thing again and have it balanced evenly from the bottom. Is this mass or gravity? Now we slide our pivot point up (any distance up to the top edge) and it'll still balance evenly. Where in there is it the CG if it's two different things? Now I fire said bullet and stop it at 500mph, will it balance on the same spot? If it does, is that considered mass or gravity? Is the bullet designed to balance sitting on the floor or at 500mph? On a fuel car last time I checked it was around 7000lbs down force in the lights. When you talked about adding mass I'm assuming you referring to just adding that weight to the back. Yes a static 7000lbs will move slower since it's not progressively being added like it would be during a pass. If your talking about mass (as in size) moving slower, wind resistance would again come into play if we're talking about 7000 lb mass of wood or the same weight in tungsten (much smaller mass) being moved. I guess I should've gone to to college instead of becoming a custom painter...

[tortoise]

Where in there is it the CG if it's two different things?

The CG and the CM are at the same point (almost, see my post #46, but that's an insignificantly small difference for our purposes). I proposed referring to it as the center of mass to focus attention on the inertial property of mass relevant to the discussion of aerodynamic stability, rather than just how hard the wheels are pushing down on the ground.

Yes a static 7000lbs will move slower since it's not progressively being added like it would be during a pass.

That downforce itself does not slow the vehicle; that wing has a lift/drag ratio, and it's the resultant aero drag. The 7000 lbs. of downforce  makes much less than 7000 lb. induced aero drag at top speed, and no inertial resistance to acceleration.