CP vs CG

[entropy]

...
A few years ago there were some horrific bike crashes on runways.  I do not know the exact causes of them.  They got me to thinking.  Suppose the bike hit a bump, the front end lifted, the wind got under the bike, and it lifted the front end up and spun the bike onto its side?
This is something I think about.  Is there any evidence of it happening?     

WW,
I was at the events where most of those horrific asphalt crashes ocurred, even had "the best seat in the house" at one at Texas in Mar 2012.  Thus i've given this issue a bunch of thought. 
Unfortunately I have no profound conclusions. 
I do know that most of the bad crashes happened in shut down with Bill Warner being a notable exception.  Personally, i believe Bill's crash at something pushing 300mph was somehow aero-related, but no one knows.
karl

[jl222]

The vehicle with the most weight in the front will be less likely to spin [duh]

According to you, aero downforce and mass are both "weight". Both vehicles have (at speed) the same "weight" (as you define it) on each wheel.

              Now please explain how down force is not a weight that puts more load on the tire and does not change the CG.

I cannot speak for Sumner or anyone else, but I have no problem calling downforce "weight". After all, we speak of weight jacking by adjusting the suspension without moving any mass around. Forget about "CG" when talking about aero stability; think of center of mass. When you've lost traction, inertia is resisting the slowing of the vehicle, and this forward inertial force is centered at the center of mass. The rearward aero force (drag) is centered at the CP. If the force pulling forward is in front, the vehicle "wants" to face forward.

And while your at it explain how weight is not a force as Sumner says.

I think he would say that weight is a force, but not all force is weight, even if it's pointing down. I agree, but don't think it's important.

The 222 Camaro 3000 lbs on rear tires 1680 lbs on front. tilting up spoiler and adding a 2'' piece of metal
at 45 deg angle for more down force. lets just guess 500 lbs. Now rear weight is 3500 lbs at speed and you guys say that's not moving the CG back at that dynamic point and I don't have to worry about it?

The center of mass has not moved. Aero stability is unaffected. Fear not; go fast.                

          

          

  ON THE black Ice, both vehicles would loose traction and probably spin. the #1 wouldn't spin as many times. [maybe]

  OK...Why hasn't the center of MASS moved dynamically from more aero weight on rear wheels?
  Don't say because it doesn't change. And I know after aero down force is lost MASS CG goes back to static state.
  You keep talking about what happens when you loose traction I'm talking about before you loose traction and aero
  Down force.

                          JL222
  
              

[tortoise]

    OK...Why hasn't the center of MASS moved dynamically from more aero weight on rear wheels?  Don't say because it doesn't change.

Because it doesn't change.  What you call moving weight (by aero force) doesn't move the CG. Aerodynamic directional stability is about side forces, not down forces. The mass of the vehicle resists a side force. This resistance to the side force is not transmitted through the wheels, it's inertial. It's acts at the center of mass.  If the center of mass is behind the center of gravity, the vehicle yawing to the left creates a side force on the vehicle pushing it to the left, and the inertial resistance creates a moment yawing it further left. If the downforces at the wheels, and their coefficient of friction don't allow them to produce enough force to resist this yaw, around you go. No matter how much downforce you have, it's better to have aero producing a self-correcting force when you yaw.

And I know after aero down force is lost MASS CG goes back to static state.

"MASS CG" is not a term I'm familiar with. CG and CM are the same point, they never moved in the vehicle.

  I'm talking about before you loose traction and aero  Down force.

Me too. 

[Interested Observer]

Why hasn't the center of MASS moved dynamically from more aero weight on rear wheels?

Because aero downforce does not, in any way, redistribute the center of MASS.  And until you understand that, you will continue to have an incorrect and confused concept of vehicle dynamics.  In reality, the conventional practice is a much simpler and more useful in that the mass and external forces are considered separately and combined as appropriate to predict a particular situation.

most of the bad crashes happened in shut down

A motorcycle does  not have the advantage of having the center of mass between the front wheels while under braking as a car would, which for a car provides a somewhat stable situation under deceleration. 

Assuming that M/C shutdown on a relatively short, paved course would tend to invoke fairly heavy braking, weight transfer to the front relieves the rear wheel of vertical load and its stabilizing effect, leaving one with the large rearward acting braking force of the front wheel in opposition to the inertial force of the bike mass which is well behind the front wheel--an unstable set of opposing forces.  This could easily lead to putting the bike down.  How many crashes occurred in the absence of front braking?

Aero drag would tend to stabilize the situation.  Bikes with dive brakes or parachutes?

[wobblywalrus]

The events, including Bill's, caused me to take a hard and long look at vehicle stability.  A challenging aspect of airport racing is pushing the bounds of speed in both directions.  Accel and decel. 

[entropy]

IO & WW,
I'd love to have a beer or 2 with you guys, kick around MC asphalt LSR handling issues.
As you point out, 2 wheels can make things different than 4 wheels
Now back to topic trying to get consensus that Cg is a static quality of the car with implications on dynamic handling  

With my recent switch from 2 wheels to 4 wheels and my lakester's perceived handling issues at 2014 WOS, i am keenly interested in this topic.

[javajoe79]

JL222 in regards to your car it seems inevitable to me that you will have traction problems no matter what, considering the HP that you have. Your data has shown impressive acceleration, that is for sure. I have also noticed that you guys have had quite a few spins including one that I personally witnessed that ripped the hood off the car. Wouldn't you rather have a car that didn't WANT to rotate due to its weight distribution? Maybe dial back the power and correct your CP vs CG? Seems like a number of door slammers have gone faster with far less power.

[tortoise]

Wouldn't you rather have a car that didn't WANT to rotate due to its weight distribution?

First, I would think, does the car take maximum advantage of the rules to move the CP as far aft as possible? Strakes and spill plates don't need to add much drag.

[jl222]

Wouldn't you rather have a car that didn't WANT to rotate due to its weight distribution?

First, I would think, does the car take maximum advantage of the rules to move the CP as far aft as possible? Strakes and spill plates don't need to add much drag.

   We have added spill plates close to as large as allowed. My rule change request for additional plates have been approved and we are adding 2 or 3 more. When we added the original spill plates we made the mistake of not raising the spoiler rods to where we usually have them so were losing down force. Strakes are not allowed in our class.
 We also have the 2% body stretch in cowl area.

  javajoe.. Yeah, it would be nice and SAFE if we changed the CG ahead of CP But it wouldn't perform as well.
                I also believe that 3000 lbs of weight on rear helps to keep the 222 car from lifting in a spin.

               A lot of other cars have much better CP vs CG than we do but still spin. Gas coupes and street roadsters
               are some.

                              JL222
 

[javajoe79]

This interests me because as you know I am building a similar car. It will be real heavy for starters but we will have lots of options for placing extra weight.

[jl222]

JL222 in regards to your car it seems inevitable to me that you will have traction problems no matter what, considering the HP that you have. Your data has shown impressive acceleration, that is for sure. I have also noticed that you guys have had quite a few spins including one that I personally witnessed that ripped the hood off the car. Wouldn't you rather have a car that didn't WANT to rotate due to its weight distribution? Maybe dial back the power and correct your CP vs CG? Seems like a number of door slammers have gone faster with far less power.

  Yeah, I saw that hood up in the air and thought of all the time we had in it and knew it was going to be thrashed   Tacking 280 mph at that time and spun going through the traps at 260+ with the chute out.

  Where were you when I spun? It was sprinkling on the windshield at the start, just a few drops, after I spun in the 1st
timed mile, my chute was completely soaked when I picked it up. So course was wetter down there. And our wide tires
don't like water. Did you notice any rain if you were near the 3 mile?
  Other spins have been because of 2 much throttle even though I never give it to much until hi gear.

  Other cars have gone faster, but 294 mph in the 1st mile with 2 more to go keeps us coming back.

                  JL222

[Richard Thomason]

Not to hijack my own thread, but just a quick thought. You cannot apply more forward force than the down force (weight plus aero and any other force that acts as weight [corealis? Ha] times the coefficient of friction. On the salt, the CF can and does vary greatly. In the 60's, no one expected a tire to exceed a 1.0 CF.  Dragsters exceed the then supposed theoretical limits. Guess that's why a cog wheel train works.

[Richard Thomason]

Meant to say "weight" on the driving wheels.

[javajoe79]

JL222 in regards to your car it seems inevitable to me that you will have traction problems no matter what, considering the HP that you have. Your data has shown impressive acceleration, that is for sure. I have also noticed that you guys have had quite a few spins including one that I personally witnessed that ripped the hood off the car. Wouldn't you rather have a car that didn't WANT to rotate due to its weight distribution? Maybe dial back the power and correct your CP vs CG? Seems like a number of door slammers have gone faster with far less power.

  Yeah, I saw that hood up in the air and thought of all the time we had in it and knew it was going to be thrashed   Tacking 280 mph at that time and spun going through the traps at 260+ with the chute out.

  Where were you when I spun? It was sprinkling on the windshield at the start, just a few drops, after I spun in the 1st
timed mile, my chute was completely soaked when I picked it up. So course was wetter down there. And our wide tires
don't like water. Did you notice any rain if you were near the 3 mile?
  Other spins have been because of 2 much throttle even though I never give it to much until hi gear.

  Other cars have gone faster, but 294 mph in the 1st mile with 2 more to go keeps us coming back.

                  JL222

Keep it up!  We were in line and ran behind you in the black sonoma. Would have bumped our own record up a couple mph but we also spun. There was alot of wind and we didn't want to run but they yelled at us in the staging lane to go

[jl222]

Not to hijack my own thread, but just a quick thought. You cannot apply more forward force than the down force (weight plus aero and any other force that acts as weight [corealis? Ha] times the coefficient of friction. On the salt, the CF can and does vary greatly. In the 60's, no one expected a tire to exceed a 1.0 CF.  Dragsters exceed the then supposed theoretical limits. Guess that's why a cog wheel train works.

  Guess that's why 4 wheel drive is better, more contact patch.

 Wider tires? Naw everyone knows they don't work, you know, traps air at speed, blaw,blaw,blaw, like Indy cars,formula 1 Sports cars

            JL222