CP vs CG

[thundersalt]

Good info in this article Rick Yacoucci shared on FB the other day
http://scienceenvy.com/race-car-engineering-400-mph-how-to-stay-straight-and-on-track/

[jacksoni]

Killacycle beat you to it, but bit buried perhaps:

http://www.landracing.com/forum/index.php/topic,14924.msg272851.html#msg272851

[jl222]

Good info in this article Rick Yacoucci shared on FB the other day
http://scienceenvy.com/race-car-engineering-400-mph-how-to-stay-straight-and-on-track/

  I don't know where the CP is on the street roadster and coupe is but the center of gravity should be pretty far forward
on those front mounted engine cars.
 Kind of contradicts what they think their talking about.
 Of the 2 types least likely to spin ''according to the article'' funny that they shows these
 
  2OO mph like a pig on ice. What a bunch of crap,  if the 222 Camaro ever felt like that its shut off time.

   JL222

[thundersalt]

I just thought it was a good tutorial for us idiotos the do a quick CP equation 

[jl222]

I just thought it was a good tutorial for us idiotos the do a quick CP equation 

  It is a good as figuring the CP-CG I just don't agree with adding weigh to front with a car having traction problems making traction worse.

  Example-Street Roadsters they have a lot of front end weight so their CG should be ahead
of CP but still spin same as gas coupes why if CG and CP are correct?

  I know that above 160 mph '' Chaparral Jim Hall'' discovered enough lift on coupes to unload
the suspension calling for a spoiler for downforce. Roadsters and gas coupes are not allowed
spoilers, or body mods, so weight keeps the lift down with less wheel spin.

  How about you roadster and gas coupe guys posting some results.

  I'M just saying that wheel spin causes the instability and solving that should be looked at 
just as much.

 I guess a pig on ice could ''and have'' been built, just have a wheel rate of infinity, skinny tires
with 90 lbs in them and not much weight on rear tires. Hey spin city.

             JL222

 

[Stan Back]

Our Street Roadster is 40% front and 60% back.

Stable as hell until it spins the tire(s).  At El Mirage with a cross wind in the "brown stuff" . . .

[Stan Back]

Here are the top secret weights . . .

[hotrod]

<quote> Example-Street Roadsters they have a lot of front end weight so their CG should be ahead
of CP but still spin same as gas coupes why if CG and CP are correct?</quote>

Part of the answer to that, is that the approximation in that article is only an approximation. It just gives you a ball park idea of where the cp is under ideal conditions.
In the real world the center of pressure moves forward as you go faster.
The center of pressure at 100 mph will be behind the center of pressure at 200 mph.
The center of pressure also moves around depending on the angle of the air flow over the car.
Likewise down force can change drastically due to small changes in the wind direction.

A good portion of your down force and drag force (which contributes to center of pressure) is due to air pressure build up on the windshield and cowl area at speed. This is negligible at speeds below about 60-70 mph but becomes more and more significant as you get to 200+ speeds.

For example on the 1960's corvette body, if it was placed in a side wind so the effective airflow was coming from about 15 degrees instead of straight ahead (this as I recall from a very old book) the diagonal wind would cause that high pressure bubble of air at the base of the windshield to basically disappear as it dumped off the down wind side of the windshield.

Result, mild cross wind caused huge increase in front end lift, higher front end stance due to unloaded springs increased exposure of the suspension and undercarriage and increased frontal area and aero drag shot up (moving the center of pressure forward of where it was at lower speeds).

Same effect is the result of flow separation toward the rear of the body. As that transition point moves forward at higher speeds so does the center of pressure as only the front portion of the car still has more or less attached flow and once the air flow separates the aerodynamics of the rear of the body become meaningless.

[wobblywalrus]

Stan, we both use the same performance tuning method, a #2 pencil and some not previously written on paper.

[racergeo]

  8888 views and 188 replies. Not anymore.

[7800ebs]

What's New is Old... From Bonneville Racing News early 90s  Harry Hoffman Jr.... 25 years ago....

maybe we should look toward Galileo .... He might have known ... 

Center of Gravity of A Horse drawn Carriage is...?

Wait.... Roadster or Coupe? one or two horsepower?

 

[ronnieroadster]

Doing some calculations on my race car I find the CG is 2 1/8 inches forward of the CP this is without moving any weight around by relocating stuff in the car. Is there a prefered distance forward of the CP for the CG?
 Ronnieroadster

[hotrod]

It depends on what problem you are trying to cure.

For aerodynamic stability only, move the weight as far forward as possible

The bad news is that as you do that, you will reduce rear wheel traction and the ability to put power down by the rear wheels.
Not as big a problem if you are front wheel drive, then forward weight distribution would be a benefit to both.

As they say moderation in all things. Move it as far forward as you can without creating traction problems.

[ronnieroadster]

 The problem I experienced as the speeds increased the front of the car would move around more and more. Checking the CP and CG last year I discovered my CG was behind the CP. At 150 MPH no issues at 160 a little movement at 170 lots more movement and over 180 a whole lot of movement. As the speeds increased I thought it was front end issues tried many different things nothing would help. Then I learned a bit about this CP and CG after checking the cause was finally figured out.
 Now with a longer wheel base and a few other changes the CG is finally ahead of the CP so now the next test is to see how this all effects the traction. Interesting stuff for sure.

[manta22]

That could also be due to front end lift.

Regards, Neil  Tucson, AZ