CP vs CG

[Ron Gibson]

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

[jl222]

It the center of gravity doesn't change from acceleration, why do drag cars have wheely bars?

                          JL222

Because the external accelerating force applied in the forward direction to the tires at ground level induces an opposite acting (rearward) inertial force (F = ma) at the CG which is above ground level, thereby creating an overturning moment tending to raise the front end--if the overturning moment is greater than the moment acting in the opposite direction due to the weight acting downward at the CG forward of the rear axle.

The CG may move, rotating about the rear axle with the chassis, but it does not move relative to the chassis.

  Yea, that's the point CG does move in dynamic situations.

  Gas coups, not allowed engine setbacks have a better CP vs CG but are more prone to spinning than a comp coup or
altered class. See videos on U-tube.

 So whats better on cars not able to make body changes, a good CP or more rear CG?

  I always liked Joe Laws mid mounted wing on his lakester that still holds the C/BFL AT 349+mph and pretty sure on gas

                      JL222

  

[Sumner]

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...

Real simple it is a force and not a weight.  Weight can push down on the tires and also a wing can supply downforce on the tires.  They both push down on the tires but only the weight of the car and where it is located is going to determine where the CG of the car is while at rest or underway and it is not going to change.

Once the car is out of control we are no longer interested in the weight of the car to maintain its stability but where the CG of that weight is in respect to the CP. 

The relationship of CG to CP is of importance to us not to provide traction for the car but to maintain car stability so the back doesn't become the front.

Sum

P.S. For another time and thread but I don't think just because roadsters have spun in the past is a reason to believe you can't do better.  There are options for them related to this discussion, especially if they are blown gas or blown fuel.

[jl222]

  Sum..the options on blown fuel  roadsters are not a better CP vs CG.

                 jl222

[tortoise]

 Yea, that's the point CG does move in dynamic situations.

The whole car is moving, so of course the CG is moving, but it remains in the same position relative to the car.

  Gas coups, not allowed engine setbacks have a better CP vs CG but are more prone to spinning than a comp coup or
altered class. See videos on U-tube.

 So whats better on cars not able to make body changes, a good CP or more rear CG?

You can, you know, put more weight on the rear axle without moving the CG. Just add weight centered at the existing CG. The only problem with that is if your existing weight distribution is, say, 70/30, you need to add 1000 lb. to increase the weight on the rear axle by 300 lb.  Maybe not a problem on the long course on the salt for most cars. If you can't make it to speed in the distance available with that much weight,  than you have no choice but to add ballast at the rear, let the CG move back, and live with the aero instability.  Just don't get off the gas until the chute hits. You've probably figured this out from practical experience.
  

[Sumner]

  Sum..the options on blown fuel  roadsters are not a better CP vs CG.

                 jl222

If you don't want to spin they are, but just my opinion,

Sum

[wobblywalrus]

One way to look at it is to draw an outline of the side view of the vehicle on a piece of graph paper and to make five copies.  The first one will be the forces on the vehicle at 0 mph.  It is the weight on the wheels.  The force arrows from the gravitational attraction are straight down through the axle centers.  Weigh the thing to figure that out.

Measure the frontal area and pick the centroid of that area as best as you can.  The first analysis will be at 50 mph.  Figure out the force from the air pushing on the front at that speed.  This force, acting on the centroid, tends to roll the vehicle up and over itself.  That does not happen.  Force on the rear tire contact patch increases and it is counteracted by the salt being there.  Downward force on the front contact patch decreases.  There is also the tractive force vector at the bottom of the rear tire pushing the vehicle forwards.  It equals the rolling friction and aero forces pushing against it.  All of the forces should equalize each other.  In other words, the upward forces should be the same as the downward, the forces on the front equalizing those on the back, etc.

Do the analysis at 100 mph, 150 mph, etc.  There will be a speed at which there is no downward force on the front tire.  Try the exercise with front wheel drive.  See how fast the vehicle can go without the front tire lifting.  Do everything with a smaller frontal or lower area.

This is a simplistic way of modeling things.  It is a big help to figure out the critical relationships.  A free body diagram it is.

This has been a big help.  It told me to lower the center of pressure on the front, to lengthen the swingarm a bit, to add weight up front, not too far forward, but just enough.  What I end up with is a bike that is front heavy while sitting still that has a decent weight distribution at speed.  The target speed for me was 150 mph and I wanted no more than a 45% front and 55% rear distribution at that speed.  I do not get that, the front end is lighter, but is handles much better than it would if I had not done the changes based on math.     

[wobblywalrus]

The moments that try to spin the vehicle around its center of mass should be cancel each other out, too, as well as the forces.  This I forgot to say.

[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...

Real simple it is a force and not a weight.  Weight can push down on the tires and also a wing can supply downforce on the tires.  They both push down on the tires but only the weight of the car and where it is located is going to determine where the CG of the car is while at rest or underway and it is not going to change.

Once the car is out of control we are no longer interested in the weight of the car to maintain its stability but where the CG of that weight is in respect to the CP. 

The relationship of CG to CP is of importance to us not to provide traction for the car but to maintain car stability so the back doesn't become the front.

Sum

P.S. For another time and thread but I don't think just because roadsters have spun in the past is a reason to believe you can't do better.  There are options for them related to this discussion, especially if they are blown gas or blown fuel.

  IF down force is not a weight [of course it is and measured in lbs] what unit of measurement do You give it.
  Of course CG changes dynamically. Would you agree that a CG taken at rest at a certain rear level and another
2 inches lower at rest would be different? But you and others would say a lower rear height from speed or acceleration doesn't change the CG.

            JL222
 

[tortoise]

. . .you and others would say a lower rear height from speed or acceleration doesn't change the CG.  
 

Not me.

[Eddieschopshop]

I have been eating popcorn while following this discussion

I think most involved aren't going to change their opinions....  however food for thought.  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.  Your parachute needs to bisect the center of mass too function properly and not upset the car.  This is calculated with the car static not moving and no aero "weight" factored in  (at least that's how I have done it ).  Mount and size your chute with a calculation using a lot of aero "weight" factored in and your chute could really upset the car when it deploys.  Have you ever heard of DJ asking how much aero "weight" your car has at speed so they could properly size your chute.....

Just wanting to be involved in the conversation 

[Sumner]

I have been eating popcorn while following this discussion

I think most involved aren't going to change their opinions....  however food for thought.  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.  Your parachute needs to bisect the center of mass too function properly and not upset the car.  This is calculated with the car static not moving and no aero "weight" factored in  (at least that's how I have done it ).  Mount and size your chute with a calculation using a lot of aero "weight" factored in and your chute could really upset the car when it deploys.  Have you ever heard of DJ asking how much aero "weight" your car has at speed so they could properly size your chute.....

Just wanting to be involved in the conversation 

 

I think I've exited the building on this so glad someone else entered the room  ,

Sumner

[desotoman]

I don't think I have read anything about the word "GRIP" in this thread. There are two types of grip on a race car, aerodynamic grip and mechanical grip.

So here is a video that will explain it in such a way that everyone should understand.

Tom G.

http://science360.gov/obj/video/64358df1-5f21-430e-b66e-cadaed8f3951/science-speed-grip

PS. Sumner, don't leave yet.

[jl222]

I don't think I have read anything about the word "GRIP" in this thread. There are two types of grip on a race car, aerodynamic grip and mechanical grip.

So here is a video that will explain it in such a way that everyone should understand.

Tom G.

http://science360.gov/obj/video/64358df1-5f21-430e-b66e-cadaed8f3951/science-speed-grip

PS. Sumner, don't leave yet.

  Thanks Tom, explains how aero makes weight.

            JL222

[Sumner]

I don't think I have read anything about the word "GRIP" in this thread. There are two types of grip on a race car, aerodynamic grip and mechanical grip.

So here is a video that will explain it in such a way that everyone should understand.

Tom G.

http://science360.gov/obj/video/64358df1-5f21-430e-b66e-cadaed8f3951/science-speed-grip

PS. Sumner, don't leave yet.

Good video and it explains downforce over and over and some of us are not disputing downforce (force, force not weight, weight)  I didn't see anything in the video about moving the cars center of gravity with downforce. 
They mentioned that to the tires the downforce looks like added weight to the car but they then explained that the good thing was that it didn't actually add weight to the car.  No weight change, no change in the center of gravity and that is what this whole discussion has been about.  About the relationship of CG to CP . to the stability of the car once it looses traction.

I've watch about every NASCAR race since 1990 and it seems like every time one of those cars looses traction it spins.  A second before it had all that downforce on the car but once it started to spin that downforce did nothing to halt the spin and that is what this is all about.

Can I leave now  ,

Sumner