center of pressure determination

[Carl Johansson]

OK - I know it's been cobvered here before -  but apparently I am inept at the search feature.  If I recall someone here said to determine center of pressure you just use a cutout of a photo of the car?  is that correct?  and if so how would i do it?

also for CG -  can you use 4 wheel scales and some formula -  or do you just go about jacking the car up  along the side rails till you get a balance?

Carl "dummber than ever" Johansson

[1212FBGS]

you can do the cutout thing but what your really getting is the side load, weather cocking, guestimate of about close to cp. scales will give you corner weight but you need to find cg with a floor jack and some blocks...
kent

[JackD]

Take a photo enlargement of the side view as seen by the wind.
Glue it to a piece of poster board and cut it out exactly to the side view.
Pinch it lightly between 2 fingers and hold it up along the top edge.
The point that it balances tells you that the surface area in front and to the rear are close to being the same.
The shape of the body as the air passes over it will make a slight difference but it is close enough for your purpose.
When doing the weight balance, be sure the operator is in place with all the other stuff ready to run also like fluids, and do both sides.

[Carl Johansson]

thanks Jack,  You too 1212

anyone have a good side view of a 1955 2door studebaker?

Carl J

[Sumner]

thanks Jack,  You too 1212

anyone have a good side view of a 1955 2door studebaker?

Carl J

 

Blow it up and cut the people out and the scoop and decide if you want to leave the spill plates on.  It is not exactly a "true" side view, but probably good enough.  I don't know what class you are considering, but this one has been chopped.  There are more pictures of the car here:

 http://purplesagetradingpost.com/sumner/hooley/hooley-index.html

There is more on CP/CG here:

 http://purplesagetradingpost.com/sumner/bvillecar/bville%20-%20LSR%20Thoughts-Index.html

I'll try and find you next week,

Sum

[tortoise]

scales will give you corner weight but you need to find cg with a floor jack and some blocks...

Scales will work. If 40% total is on rear wheels then cg is 40% back from front to rear wheel centers. If you think this is wrong, give an example. (Or were you joking?)

[JackD]

Keeping it simple with tools at hand will allow you to check it without anything you don't already have, and recheck it as you add or subtract stuff.
It is not near as scientific as a car set up to corner.
 

[hotrod]

There are lots of different ways to do it depending on what degree of precision you want.
As JackD says mostly you want a ball park location. If you cut a printout of the side view on a stiff piece of cardboard and cut the perimeter away with an exacto knife and then poke it with a pin you  can find the center of area for the sideways car. It won't be exactly the center of pressure because form is important too. A smoothly rounded front fender will not have as much side drag as a flat spill plate, but it will be in the ball park to locate your center of pressure if the car gets completely sideways. That will give you an idea what kind of turning moment the car will have in a strong crosswind or if it gets really out of shape.

The same goes for the front - rear location of the center of mass (CG). On a typical sedan it is usually about the height of the cam bore on a V-8 engine or a bit lower. Front to rear it usually falls (on an unballasted car) some where in front of the drivers hips and behind the drivers knees. As mentioned above all you need are the front to rear weight split to locate that point. If the car is drivable, and you have a truck scale nearby (or the drag strip scale) just weigh the front and rear axles and then figure the split. If you come up with 58% of the weight on the front wheels and 42% of the weight on the rear wheels you know that the CG is 42% of the wheel base back from the front axle. The 4 corner weight scales will help you determine if you have a strong weight bias left to right as well.

As crude as it sounds yes you can also locate the front to rear position of the CG by simply jacking the car up on the two frame rails and finding out which direction it wants to tip, forward or rearward. Then move the jacks toward the heavy end a few inches and try again. It will take about as long as setting up 4 corner scales so 6 of one and half a dozen of the other.

Larry

[interested bystander]

Not wanting to dispute JackD's legendary knowledge  but methinks the cardboard side view is a little bit simplistic.

I believe its a three dimensional thing. REX?? BETTES? RAILTON? KELLY JOHNSON?

 Scaling the racecar one needs to remember that all corners have to be guaranteed LEVEL with the planet (or the weigh points 90 degrees to the center of the earth) or yer data is flawed. Carrol Smith? Bill Millikan? Steve Smith Autosports? Dave Morgan?

Have Trojan problems (on the internet- too old to worry about the other Trojans)), Jack. I'll get back to U.

[hotrod]

anyone have a good side view of a 1955 2door studebaker?

Try these!




Larry

[hotrod]

and this


Larry

[hotrod]

but methinks the cardboard side view is a little bit simplistic.

You are correct, discussed this in another thread some time ago.
http://www.landracing.com/forum/index.php/topic,1594.0.html

 It is only reasonably accurate when the car gets nearly broadside to the airflow. At low yaw angles the center or pressure will be some place else, and is best located with simple models and a low tech wind tunnel setup. Some of the fast streamliners used that simple setup to get ballpark locations for their design by blowing air on a model and seeing how it "windvane'd" with various pivot points.


The side view is sort of a worst case condition if the CP is infront of the CG in that attitude you will have a backwards day.

Larry

[panic]

As we say in Court: "asked and answered".
Read the previous post.

[John Burk]

A test I did of a 3 dimensional model of my streamliner with no tail fin showed that the center of pressure was 33% from the nose . The side area method mentioned would have put it at about 58% . The CP of wings is at 22% to 29% .