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Old 04-10-2007, 07:11 AM
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Best location for ballast

In another forum, a poster wanted to know whether some ballast weight would do more good (dragracing application) in the rear bumper or above the axle. I thought this was a good question and worked out a graphical solution for him. I figure this question either has been...or will be...brought up in other forums, so, rather than send someone on a search for my post, I thought I'd just repeat it here:

There is a graphical means for evaluating different positions for ballast.

Draw a horizontal line and locate a point on that line. That will be the rear tire patch in side view. The front of the car will be to the right. Construct a line which extends upward and to the left from the point. This line will have an angle, from the horizontal, having a tangent equal to the acceleration, in G's, that you expect during launch. With good tires, that could be, say, 2.5 G's, meaning that the angle would be 68.2 degrees from the horizontal.

Now, locate, to an appropriate scale, the two location points you're considering for ballast. Suppose point "A" is the bumper location and point "B" is the location above the axle. Construct a line that passes through point "A" and is perpendicular to the first line. Construct a similar line through point "B." Each of these last two lines will intersect the first line. The intersection which is furthest from the tire patch...along that 68.2 degree line...passes through the point ("A" or "B") at which you want to locate your ballast.

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Old 04-11-2007, 09:16 AM
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this is a good read but a graphic would get the idea across in a much better fashion, can not find my protractor and don't have a cad app loaded.
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Old 04-11-2007, 11:40 AM
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Here's a picture:

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Old 04-11-2007, 12:04 PM
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Quote:
Originally Posted by BillyShope
Here's a picture:

now that I can understand .. thanks
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Old 04-11-2007, 01:10 PM
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Does this mean that there is no relationship between the distance from the ballast to that "G force line", whether the ballast is in front or behind the "line".
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Old 04-11-2007, 03:41 PM
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Quote:
Originally Posted by home brew
Does this mean that there is no relationship between the distance from the ballast to that "G force line", whether the ballast is in front or behind the "line".
That's correct. This is because point "B" is taking advantage of the moment created by its inertial force and the vertical distance from the tire patch. Since point "A" is so low, it must achieve the same effect by being further to the rear.

Here's my response, in another forum, to a similar question. Perhaps it will make it clearer:

If a car weighed 3000 pounds, had a 50/50 weight distribution, and a 100 inch wheelbase, the addition of a ballast of one pound, placed 150 thousand inches behind the rear tire and at a height of less than one inch would result in a load, on the rear tires, of 3001 pounds. On the other hand, if that one pound of ballast were placed 60 thousand inches directly above the rear tire patch, the rear tires would be loaded to 1501 pounds while the car was at rest, but, when accelerating forward at 2.5 G's, the rear tires would be carrying 3001 pounds.

In other words, the same effect can be achieved be placing ballast either to the rear or up in the air. Ideally, you'd want to place your ballast on that angled line (which takes advantage of both effects), but that usually isn't practical.
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Old 04-11-2007, 03:58 PM
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Thanks for the reply.
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Old 04-11-2007, 06:25 PM
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Hey Billy,
I'm not trying to be a smart ***** or anything, but wouldn't
placing the ballast above the axle raise the vehicles roll center,
making is less stable to side loads at high speeds??

K
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Old 04-11-2007, 07:20 PM
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Quote:
Originally Posted by bates_k
Hey Billy,
I'm not trying to be a smart ***** or anything, but wouldn't
placing the ballast above the axle raise the vehicles roll center,
making is less stable to side loads at high speeds??

K
The roll axis is determined by suspension geometry, not by weight distribution. But, certainly, anything which raises the center of gravity has a harmful effect on handling. With a dragstrip car, however, some compromises on handling can be tolerated.
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