Tungsten weights
Once again, not sure if this is old news, but....
I've been exploring some pinecar accessories in the the interest of finding some items that might cross over to smaller scale racing(besides graphite) and this one seems perfect. Tungsten is more dense than both lead or steel which means more mass in a smaller 'package'. Its available in a variety of preformed sizes, many small enough to considered for our applications. The apparent downside to tungsten is its lack of workability...that is, its too dense to be easily cut or drilled. But "tungsten putty" is available which can be easily formed & shaped to fit into tiny areas.
Here's an example of what I'm referring to: http://www.maximum-velocity.com/pinewood_derby_weight.htm
It seems like a viable option, IMO. Besides, I think depleted uranium is out of the question in this current global climate.
GspeedR
I usually go with Kyrptonite.