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Author Topic:   Pendulum Effect is a Myth
gahainsey
Member
posted August 01, 2003 08:25 AM
Just wanted to post some thoughts on the quote/unquote Pendulum Effect. Weight placed far back in the car increases rear percentage by moving the center of gravity for all of unsprung weight in the car rearward relative to the center of the wheelbase. This allows the rear percentage to be higher per given total weight. As the car goes around the turn the force due to centripital acceleration acts on the center of gravity of the car. NOT just on the weight behind the axle!! I have heard a million stories about people putting weight in the back then having loose problems. Did they upset their setup (bite, cross etc.)by just throwing the weight in?? My point is that putting weight behind the rear axle does not act independantly of the rest of the mass in the car. THE PENDULUM EFFECT DOES NOT EXIST.


rocket36
Member
posted August 01, 2003 10:51 AM
when i first started messing with dirt cars i couldn't understand why you would want the rear to weigh more than the front, i assumed that if you wanted to over steer (loose) that you would require the rear to weigh less.

the best explaination that i read (in a chassis tech book) was "a light (weight) object will/can change direction faster than an equivalent heavy object", when you think about this it makes perfect sense. it is called "polar moment of inertia".
there is however, a point (like everything) where it becomes too much.


ratracer10
Member
posted August 01, 2003 10:55 AM
The polar moment of inertia argument does not hold in this case. Spinning with a barbell close versus far from the center of rotation proves that the moment of inertia increases as a function of the diameter of the circle of rotation. You got me thinking about the car rotating about it's own center, though(sliding) I still say that it is a function of the overall CG of the car. Let me take it to an extreme to illustrate. Say I have a 3800 pound car with 40 percent rear weight. If I add 200 pounds to the bumper is the 200 pounds going to pull the car around, or is the 2300 pounds in front of the rear tires going to pull the car other way?


ratracer10
Member
posted August 01, 2003 12:55 PM
DECIDE FOR YOURSELF...

TAKE A CARTON OF EGGS PUT 4 IN THE MIDDLE ONLY, AND HOLD IT IN ONE HAND AND TURN IT BACK AND FORTH.

NOW PUT THE EGGS AT THE ENDS AND TURN IT BACK AND FORTH.

YOU DECIDE WHICH ONE IS EASIER TO CONTROL IN YOUR HAND AND OFFERS THE LEAST RESISTANCE TO CHANGING DIRECTION!!!!

JEFF JOLDERSMA

Rychtr
Member
posted August 04, 2003 05:05 PM
Great example!


sixwillwin
Member
posted August 04, 2003 05:11 PM
yea but..... the eggs (ballast) weighs more than the carton (racecar)!! Of course that would be the case, but to compare apples to apples in your egg scenario.....you would have a 3000# stock car with 9000# pounds of ballast!!


66jj
Member
posted August 04, 2003 06:55 PM
doesnt some ballast in the trunk weigh more than none in the trunk??

I guess im wrong I quit, I wont enter these discussions on wt transfer, placement etc again.



outlawstock17
Member
posted August 04, 2003 08:27 PM
i think the added forward bite of the weight placed farther back more than offsets the disadvantages of the so called "pendulum effect".


tilley88
Member
posted August 05, 2003 01:30 AM
OK, car is way nose heavy. If I add all my weight on the bumper, do I still get the "pendulum effect"? Does this apply to assfault or dirt, or both? How high do I need to place this weight, depending on how far I place it towards the rear bumper? I can understand how the bumper weight would make it "shear" on assfault, but why does'nt it do the same on dirt? Come on smart guys, I need closure on this subject!


popwillis
Member
posted August 05, 2003 06:47 AM
the so-called pendilum effect is not in the car, unless you consider gas sloshing in the tank, it IS the car.
using the egg carton example..........
face the end of the carton, looking long ways, [or back end of a car]. tie a string around the center of the carton and lay it to the left. [a sideways T]. place 2 eggs in positions 3 & 4 representing the motor & front suspension weight. take the loose end of the string and swing the carton around. the heavy end with the eggs go to the outside equating to a severe push in your racecar. since the objective is to maintain a neutural axis at the center of the car, [or string], thru the corner, add 2 eggs at positions 9 & 10 [rear axle] or 1 egg at position 11 or 12 [rear bumper][lighter is faster]. swing the carton again. this tells us a 50/50 split, front to rear, maintains a neutural axis thru the corner. [as the 'T' relation , string to egg carton remains]
adding braking, acceleration, track banking, traction, springs, etc., modifies this slightly.
i hope this explanation helps.


merlinmech2
Member
posted August 05, 2003 08:37 AM
Outlawstock17 has got it. Get your percentage where you want it with the least amount of weight.


widebody
Member
posted August 05, 2003 08:48 AM
Were forgeting about slip factors that come into play. And what about that third dimension of height. Add that into all this, that changes alot of things.

EXAMPLE: 400# of weight.
A. in front of axle centered,36" off ground.
B. in front of axle extreme left 12" off ground.
C. behind axle centered 36" off of ground
D. behind axle centered 12" off of ground.

Im not sure about your cars. But mine will react differently with each placement.
Widebody


gahainsey
Member
posted August 06, 2003 08:14 AM
So like I said originally, It is overall CG of the car, not a force acting independantly on the ballst and only the ballast. Stock car racing is just a big loud dusty Physics experiment.


Big Bear
Member
posted August 06, 2003 01:18 PM
With a shot of adrenaline to keep it interesting. I always fell asleep in Physics class. Every thing I've read has said to keep the weight low and close to the CG

On the gas and kickin a$$
Big Bear


sixwillwin
Member
posted August 06, 2003 03:20 PM
Dont believe everything you read, on asphalt that may be true, but on dirt, esp. dry slick with hard tires, sidebite needs to be created.


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