CoG vs down force

[hawkwind]

Hello all,

Does the down force created by an inverted wing change the Cog ? and if so is there a method or formulae to calculate the change.
thanks
Gary 

[hotrod]

No the CG (center of gravity) is a physical property of the structure, like its height width and weight, you cannot change it without moving components around in side the car.

The down force of a wing, does however change weight distribution on the tires. A rear wing for example will add "apparent" weight to the rear wheels and in most cars will lift some weight off the front wheels due to its lever action. This apparent weight only exists as long as the wing is producing down force, so as soon as the car spins and the down force disappears weight distribution on the wheels will change, and the car will want to spin around the center of gravity.

Larry

[hawkwind]

Thanks for that Larry  .

[1212FBGS]

it depends where the wing is mounted.... if it is mounted at the COG then no.... but a wing will absolutely change the dynamic COG.... why wouldn't it?..... if you had the vehicle on 4 wheel scales and someone jumped on the back of the car wouldn't the scales on the rear tires show more weight? that obviously would move the COG rearwards

[Interested Observer]

To reinforce Hotrod's correct statement, the CG is purely a function of the mass distribution of the vehicle.  Any external forces applied, such as tractive or aero, have no bearing whatsoever on the location of the CG.

[Old Scrambler]

The effect of a rear wing at speed is to counter any body lift and maintain the CG.  If you were a contortionist and could crouch on your feet whle driving the car, your CG would be slightly lower..... assumming the usual seat is somewhat higher than the floor panel.   

[Bootleggerjim]

Cool stuff !

[Dr Goggles]

The effect of a rear wing at speed is to counter any body lift and maintain the CG.   

I dunno about maintainbut I can see your point about countering the lift characteristics of a given body shape , it's a compromise.

So. Landspeed is a trading game, valuable assetts include power, reliability, traction, weight distribution and streamlining.Liabilities include drag, low power poor weight distribution and lift amongst other things.

A wing uses airflow to create downforce, thus increasing the "apparent" weight on the rear of the car/bike.

Using the arrow analogy we need the Centre of Gravity to be ahead of the Centre of Pressure or the tail will want to pass the front.

As the speed increases the wing begins to produce downforce , shifting the dynamicCoG rearwards....If the wing maintains the relationship between CoG and CoP by significantly adding to the drag at the very rear of the vehicle it will remain stable , but at the added expense of the additional drag......this can be compensated for by increasing the power............this may introduce it's own problems with traction.....

However if the drag created by the wing does not maintain the CoP/CoG relationship then the limiting factor may well be the aero-stability.....

I'm being careful not to tread on any sacred ground here but the bottom line is drag , the less you have , the faster you'll go............

....................... brains and brawn, David and Goliath................



It's the SOS, but the same questions keep getting asked.......

The real questions are who's gone  100/100.......200/200.....300/300

[Rick Byrnes]

Just DON'T mention frontal area!

[Gwillard]

Just DON'T mention frontal area!

The large frontal area of some women shifts their CG forward substantially. 

[hotrod]

I understand what you guys are saying, but by mislabeling things, (and misusing the term CG) you are setting yourself up for misunderstandings down the road as you try to figure out what your car is or should be doing.

The CG (center of Gravity) is the same as the Center of Mass (CM). It is a physical property of the car, and will not change not matter how slow or fast is is moving (right up until it impacts the salt after taking flight and it starts to shed parts and change shape).

It is fixed -- it does not change -- the spoiler has absolutely no effect on the location of the center of mass/center of gravity. It is where it is when the car is parked when the car is at speed.

The spoiler does change the so called "weight distribution" on the suspension, as it adds (or subtracts) load from the wheels depending on if it is creating down force or lift (yes the aerodynamic experts see both as lift but we will stay with conventional usage here).

It creates an additional load that is a function of aerodynamic forces and as such changes with speed and wind direction over the car. It also usually tends to move the center of pressure to the rear (not the CG).

In a car with no spoiler, as the car goes faster, the center of pressure tends to move forward.

If the center of pressure ever gets ahead of the CG (center of mass) then the car will want to turn around and put the center of mass ahead of the center of pressure. This is why roadsters like to spin. When the CP and CM get close to each other, the car becomes directionally unstable and any slight upset will start it spinning.

The presence of a rear spoiler that produces only down force does not change this much.

In some cases, a simple flat plate spoiler will actually reduce drag and cause the center of pressure to move farther forward that it was without the spoiler. It will also if it produces too much down force tend to lift the front of the car, so that it catches more air, causing the center of pressure to move even farther forward.

It will however increase down force on the rear wheels, so you now have (hopefully) the traction to maintain directional control, right up to the moment you spin the tires then "poof" all your stability is gone, and since traction is also gone, your rear wheels no longer have the lateral traction to keep the car straight.

If the spoiler is designed to not only create down force but to include side plates so it increases the "fin area" of the rear of the car compared to the front of the car when the airflow is at a slight angle, then you get the best of both worlds.

You have aero-down force to increase rear wheel traction for both acceleration and lateral control, but if the car gets a bit sideways, the added fin area, creates more drag  in the back of the car from the off axis air flow than the drag created by the front of the car, and it naturally wants to windvane back into the wind and get back pointed in the direction it is traveling.

A station wagon body behaves like this in cross wind. Its large side area in the rear makes this body style want to turn into a cross wind rather than turn away from it.

It is the center of pressure that moves, and the downward force on the tires that changes due to rear spoilers but the CG/center of mass never changes until you start tossing parts off the car.

Larry

[hawkwind]

I understand what you guys are saying, but by mislabeling things, (and misusing the term CG) you are setting yourself up for misunderstandings down the road as you try to figure out what your car is or should be doing.

The CG (center of Gravity) is the same as the Center of Mass (CM). It is a physical property of the car, and will not change not matter how slow or fast is is moving (right up until it impacts the salt after taking flight and it starts to shed parts and change shape).

It is fixed -- it does not change -- the spoiler has absolutely no effect on the location of the center of mass/center of gravity. It is where it is when the car is parked when the car is at speed.

The spoiler does change the so called "weight distribution" on the suspension, as it adds (or subtracts) load from the wheels depending on if it is creating down force or lift (yes the aerodynamic experts see both as lift but we will stay with conventional usage here).

It creates an additional load that is a function of aerodynamic forces and as such changes with speed and wind direction over the car. It also usually tends to move the center of pressure to the rear (not the CG).

In a car with no spoiler, as the car goes faster, the center of pressure tends to move forward.

If the center of pressure ever gets ahead of the CG (center of mass) then the car will want to turn around and put the center of mass ahead of the center of pressure. This is why roadsters like to spin. When the CP and CM get close to each other, the car becomes directionally unstable and any slight upset will start it spinning.

The presence of a rear spoiler that produces only down force does not change this much.

In some cases, a simple flat plate spoiler will actually reduce drag and cause the center of pressure to move farther forward that it was without the spoiler. It will also if it produces too much down force tend to lift the front of the car, so that it catches more air, causing the center of pressure to move even farther forward.

It will however increase down force on the rear wheels, so you now have (hopefully) the traction to maintain directional control, right up to the moment you spin the tires then "poof" all your stability is gone, and since traction is also gone, your rear wheels no longer have the lateral traction to keep the car straight.

If the spoiler is designed to not only create down force but to include side plates so it increases the "fin area" of the rear of the car compared to the front of the car when the airflow is at a slight angle, then you get the best of both worlds.

You have aero-down force to increase rear wheel traction for both acceleration and lateral control, but if the car gets a bit sideways, the added fin area, creates more drag  in the back of the car from the off axis air flow than the drag created by the front of the car, and it naturally wants to windvane back into the wind and get back pointed in the direction it is traveling.

A station wagon body behaves like this in cross wind. Its large side area in the rear makes this body style want to turn into a cross wind rather than turn away from it.

It is the center of pressure that moves, and the downward force on the tires that changes due to rear spoilers but the CG/center of mass never changes until you start tossing parts off the car.

Larry

Thanks for the info Larry .
Im assuming that the above holds true for pitch ,does it also apply to roll and yaw? conditions

[hotrod]

Yes especially yaw, as the fin area of the side plates on a rear spoiler have a major impact on how the car behaves in yaw.

Roll is a bit more complicated because suspensions sometimes have roll steer where as the chassis rolls the  rear axle rotates slightly as the effective length of the suspension links change with roll. This can if properly engineered cancel out some of the tendency of a car to turn away from a cross wind, and as the chassis rolls due the wind gust it steers slightly toward the force that caused the roll. In poorly designed (or just unlucky) design, roll steer can make the car turn with the force that is causing the roll making it a real handful in cross winds.

The car is a system and all elements aero and chassis characteristics must work together to get stable handling at speed, you cannot fix a chassis problem with aero, and you cannot fix aero problems with chassis adjustments (including ballast), all you can do is try to cover up the problems. Sometimes successfully sometimes not so successfully.


It would really take a thick text book to cover this stuff properly.

Larry

[Dean Los Angeles]

Thanks Larry. I was hoping someone would get it right.

The Center of Mass (not Gravity) is the sum of all of the different materials in the car and their molecular weight. Very difficult to calculate.
Weighing the vehicle gives you an idea, but doesn't tell you how high the mass is in the vehicle. Center of Mass never changes.

A wing changes the apparent weight distribution on the tires. A wing creates a lever action based on location. Between the wheels creates down force proportional to the location. If the wing is behind the rear wheels the lever action is multiplied on the rear at the same time it is lifting the front wheels.

[johnneilson]

Thanks Larry. I was hoping someone would get it right.

The Center of Mass (not Gravity) is the sum of all of the different materials in the car and their molecular weight. Very difficult to calculate.
Weighing the vehicle gives you an idea, but doesn't tell you how high the mass is in the vehicle. Center of Mass never changes.

Dean,

Are you saying that measuring the CG (inclined weight calculation) is not the CM?
Please elaborate.

John