CP vs CG

[tortoise]

. .  don't be moving weight forward to improve CP and make thinks worse by increasing chances of wheel spin.

So don't move weight forward, add weight forward. You may have to live with some aero instability, though, if this added weight slows acceleration too much to get to speed in the distance available.

[Sumner]

If the traction is good enough the CP can be at the push bar . Salt and dirt traction is poor so the CP location matters to us .

John did you mean CG at the push bar.  Having CP back there would be good,

Sum

[Richard Thomason]

Of course CG vs CP is only one factor in going consistently fast and straight at B-Ville. Traction, aero and Polar Moments of Inertia are all also critical components. Part of the design criteria should hopefully be to make the vehicle as inherently stable as possible. All things have to work together and a lot of principals can be overcome by other factors. Witness the Thrust SSC, which was essentially a fork lift. They used that particularily unstable design because of other necessary design criteria, but had the technology to make it work. Anyone else want to give that a try? Not me.
What we tried to do, was to incorporate any and every stability positive factor that we could. Different classes and build types have their own unique circumstances that can and will require compromise, adjustments and just plain experience and know how.
That's what certainly makes it fun.

[jl222]

 I would like more answers on the static vs dynamic down force on CP that no one is talking about.

You're mixing 2 questions. The CP/CG relationship concerns whether aerodynamic forces  act to turn the vehicle. Downforce concerns whether the wheels can produce steering and tractive force. If the wheels never break loose, a bad CG/CP relationship may never show up as a problem.

   The question is, does the CP get worse from the down force of wings-spoilers-ECT?

  One question 2 forces. Seems to me as down force is added at rear at speed, the CP is going to get worse.

  jl222

[John Burk]

"did you mean CG at the push bar.  Having CP back there would be good"

Thanks Sum , meant the nose .

[tortoise]

. Seems to me as down force is added at rear at speed, the CP is going to get worse.

For The CP, "getting worse" means moving forward. I can't see how rear down force would do that.

[kiwi belly tank]

The percentage difference between them will decrease as speed increases due to the added weight on the wing at the rear of the vehicle. The CG will move rearward.
  Sid.

[tortoise]

The CG will move rearward.

CGs don't move, unless you move mass in the vehicle. Aero force is not gravity.

[kiwi belly tank]

The wing down force is applying weight to the vehicle. It gets heavier at that location, that changes the CG unless the change is on the axle centerline.
  Sid.

[tortoise]

From Wikipedia:
"Center of gravity is the point in a body around which the resultant torque due to gravity forces vanishes. Near the surface of the earth, where the gravity acts downward as a parallel force field, the center of gravity and the center of mass of an arbitrary body are the same."

What you are referring to might be called the "center of down force".   It does affect what forces the wheels can produce, but that's a different question.  When the CG is ahead of the CP, the vehicle yawing produces a self-correcting aerodynamic torque. 

[Interested Observer]

The wing down force is applying weight to the vehicle. It gets heavier at that location, that changes the CG unless the change is on the axle centerline.
  Sid.

This is nonsense.  Tortoise’s reply #22 is correct.

Rear wing drag and downforce aft of the rear axle may, and likely will, reduce front axle contact with the salt and produce reduced steering capability and effectiveness.  Also a possible change in pitch attitude.

[maj]

Then is centre of gravity the right force to be using for calculating stability ?

maybe it should be cg +other mechanical or aerodynamic downforces at any given speed 
   

[manta22]

Maj;

CG and CP are the basis for aerodynamic stability but vehicle "stability" also depends on lots of other factors-- bump steer F & R can cause instability: front toe- out can, too. Roll centers, roll stiffness, tire sidewall lateral stiffness, height of the CG, weight transfer, etc., etc., are all factors affecting vehicle stability.

One is tempted to think that in straight- line LSR, these things don't matter since the car is not going around a corner as in circle track or road racing. As long as the car is going in a straight line these things matter little but once the vehicle deviates from a perfectly straight line, these factors begin to come into play. If one or more of these factors cause the deviation to increase, it gets out of hand quickly-- i.e., instability.

Regards, Neil  Tucson, AZ

[wobblywalrus]

My partially streamlined bike, when analysed as a solid shape, has a CP farther forward then the CG.  It is very stable and does not behave like it should given this.  My figuring is my toes, the rear wheel, my and helmet top all catch the wind and this moves the CP back behind the CG.

[tortoise]

My partially streamlined bike, when analysed as a solid shape, has a CP farther forward then the CG.  It is very stable and does not behave like it should given this.  My figuring is my toes, the rear wheel, my and helmet top all catch the wind and this moves the CP back behind the CG.

I'm getting into tricky, controversial territory here, but bikes are different than cars; they lean over. When a bike gets turned left a little bit, the wind pushes on the right side, leaning it left. If the CP is way back, the wind has a long lever arm to push the bike to turn back right. Now you've got a bike leaned left and turning right: maybe not a good thing.