Cascade Crew

[blue89]

I took the camaro out to a parking lot finally and setup the old G-tech meter. The car pulled a 0.90g cw and a 0.96 ccw! I'm still suffering from understeer badly. The parking lot is a year old and still freshish, surface was about 60*. Street tires, 245/40r18's, Trac A temp A tread 240. GO ME! Woot woot!

Now I want to get the sway bars balanced more (front smaller or rear bigger) , subframe connectors in, and a strut tower brace. If i had wider tires, I'd be catching up to mike!

[Dewey316]

g-tech is a little in accurate, because the rolling motion will effect it.

The best way to do it, is put out a 200ft circle, and measure the time it tacks to make 1 full lap. Then you can calculate the average g-forces over that lap. That is how it is done when any magazine or manufacture publishes skid-pad numbers.

[blue89]

Where am I going to find a lot big enough for a 200ft circle.... Do you happen to have the equation handy?

[Dewey316]

(1.226 x R) / T^2

R = Radius of the circle (in feet) (this should be 100)
T = time (in seconds)

To make it work like the mags do, you NEED a 200ft diam. circle. That is the standard (speed and arc effect the g's, since you are changing the slip angle.

[blue89]

check this out;

http://www.welltall.com/ymc/discovery/car/road_resist.html

Pretty interesting. I got this out of it, if the cars suspension is setup correctly with a low CG and 0 yaw, the cornering g's is the cf of the tires.

I'm going to revisit that site I used with the gtech and measure it out. I'm guessing that its about 100ft diameter. I'll use the same method you described to calculate the g's and see how they compare. I'm hoping that the numbers are within 1-5% of eachother. Since my car experiences minimun "yaw" I'm hoping for the best. A would imagine a larger circle would give a better result because the car is tweaked less on a larger circle.

[Dewey316]

Generaly steady state corning is completely a limit of the tires. Transistions and bumps are where you suspension really works.

[Dewey316]

Also, if you want a really good read with a TON of information on the physics of what is happening, between suspension and tires and dirver, and such.

Brian Beckmans -- Physics Of Racing

http://phors.locost7.info/contents.htm

[QwkTrip]

Yeah, I wouldn't get too excited about the G-tech numbers. Odds are you are somewhere in the mid 0.8G range. Dewey is right on. But I might add that the key is to make sure you are traveling at a constant speed. So go around the corner a lap or two before you begin measuring.

And if you're interested in the reasoning... When you have uniform circular motion at constant speed, then the acceleration is constant and directed towards the center of the circle. You can calculate g-force using either your speedometer or a stop watch. A stop watch is going to be more accurate because your speed will vary slightly and your tires will likely slip.

If speed is constant, then the radial acceleration is

a = 4*(pi^2)*(R) / T^2

and

g-force = a / g

Where pi=3.14159, R is the radius of the circle, T is the period of revolution (time to go around the circle), and g is the acceleration due to gravity at sea level.

With R measured in feet and T measure in seconds,

g-force = (1.23)*R / T^2

Same as what Dewey posted.

[QwkTrip]

Ah heck, let's look at the velocity formula too. The radial acceleration can also be represented as,

a = V^2 / R

where V is the velocity and R is the radius of circle. My guess is that you will end up pulling 0.86g. So put me in the betting pool for a speed of 36 mph.

V = sqrt (a * R)
V = sqrt [ (0.86)*(32) * 100]
V = 52.46 feet per second
V = 36 mph

[CheezX]

The accelerometer in the g-tech itself seems pretty accurate to me. Hold it vertical and you get 1.00g. But Dewey is right about error due to roll. This however, doesn't render the g-tech useless. You can you can account for the error due to roll:
Assume a roll rate of 4.2 deg/s (typical for stock F-body's with performance suspension option)

so pretend actual lateral g's is 0.90
roll = 0.90 x 4.2 = 3.78 deg
component of lateral acceleration = a x cos(angle)
component of gravity = a x sin(angle)
measured acceleration = 0.964g

So taking roll into account, you've got approx. 0.85g cw and 0.90g ccw.

But the steady state will also be affected by steering angle. You will measure a lower value on a 50ft circle vs a 200ft circle. So you can say the numbers above are conservative.