Best location for ballast

[BillyShope]

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.

[ST7317]

Interesting. Some questions:

1) Quote "This line will have an angle, from the horizontal, having a tangent equal to the acceleration, in G's, that you expect during launch."

Is this because the vector representing the force has its tail at the contact patch and extends to the right with a magnitude equal to the force?

2) Can I find the "G force" by just figuring the acceleration (velocity divided by time)?

3) Has this been tested against scaling a car and moving weight around?

[mudracer]

is the application process the same for mud racing as it would be for pavement drags? I would like to be able to figure out ballast placement to enhance traction in that environment.

[BillyShope]

Interesting. Some questions:

1) Quote "This line will have an angle, from the horizontal, having a tangent equal to the acceleration, in G's, that you expect during launch."

Is this because the vector representing the force has its tail at the contact patch and extends to the right with a magnitude equal to the force?

The vector is simply a pictorial representation of a force and can be placed anywhere along the force's line of action. In this case, that which you're describing is the tractive force which propels the car forward. An inertial force, equal in magnitude but opposite in sense, acts through the center of gravity.

2) Can I find the "G force" by just figuring the acceleration (velocity divided by time)?

If you have the acceleration in feet per second squared, you divide by 32.2 to find the acceleration in G's.

3) Has this been tested against scaling a car and moving weight around?

Since inertial forces are involved, it cannot be checked by wheel scales alone. You can, however, use the traction dyno, as described in my blog, to verify the results:

http://home.earthlink.net/~whshope

[ST7317]

Wow, that's quite a "blog". Gonna take some time to go through. Thank you for posting that information and calculators online.

[BillyShope]

is the application process the same for mud racing as it would be for pavement drags? I would like to be able to figure out ballast placement to enhance traction in that environment.

I've seen a bit of mud racing on TV, but I'm not all that familiar with it. I am, however, certain that the principle would apply, though I'm equally certain that your launch acceleration is quite a bit less than a pavement dragracer. This would reduce the angle of that line. I'm curious. Do you clock the 60 foot times? Those mud cars ARE surprisingly quick!

[mudracer]

Depending on venue, the track can range from 120 feet to 300 feet. Generally, the accepted measurement is 200 feet.

2 and 3 second times are not unheard of for a 200 foot pit.

most intermediate classes *(which I race in) run 4 to 9 seconds, depending on track depth and moisture.

[BillyShope]

200 feet in 6 seconds would be an average acceleration of a little more than a third of a G. This is about what I'd expect. 200 feet in 3, however, would be a little over one and a third G, which is very quick! Sounds like fun!