Height of center of mass

[tigerdad]

Gentlemen,

I've attached an image of a car we are building. It's a dragster shape which tapers from 1/8 at the nose to to about 13/16 in the rear. The wheel base is just under 5-5/8 inches and there are 3, 3/8 holes drilled into the car (above the rear axle) from the rear to a depth of 1-1/4 inches. The center of the 3/8 holes is 5/15 above the axle on center, and 7/16 above the bottom of the car. These holes are obviously for weight and it is our intention to have the weights extend to be flush with the rear of the car and painted black to look like "jet engines". It also allows the weight to be as far to the rear as possible. I've read that one should keep the COM as low as possible to insure stability. Well in this case COM will not be as low as I could possibly make it, but I don't think it is too high. This is because additional weight will be added underneath the three holes on the beveled area on the rear of the car and perhaps just in front of the axle. Therefore, it's not like all the weight will be packed into above the center line of the car. Anyway, I would greatly appreciate any and all feedback/opinions about the weighting. Thanks!

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[dkgracing]

I think you will be fine as long as the balance point is about an 1" in front of the rear axle. One of my fastest cars has the weight above the rear axles like that.

If you can keep the the car from popping a wheelie, go for it.

[the woodbutcher]

You must already have this car done, but I thought I'd chime in anyway.

DK is right. Be cautious of how much of your weight is behind the axle. You may acheive a terrific COM on the workbench, but on the track it can change. I had a truck last year with too much behind the axle. When it went through the transition, the front would lighten and the wobbles set in. Not enough RR steer was also a part of the problem, but it actually jumped off the track in the Final and cost me the win.
To say it further, I would rather have the COM higher than scrifice stability due to too much weight behind the axle giving a teeter totter effect. That's another reason tungsten weights are so popular. You can pack a lot into a small space.

Your car design looks good and I wouldn't expect your in danger like I was.

By the way, how did it turn out? How did it go down the track?