So I decided to make a new base from my benchtop, this base would allow me to place vises where I want them.

The previous base was rock stable, not a hint of movement, it had legs at the far corners of the bench. the new is base pictured here:
unnamed.jpg

the base is only 40" long, and the bottom bars about 23", which is the same as the benchtop. My issue is that the bench is highly unstable, it rattles, and the far right corner bounces when I use the face vise.

-The legs are flat and all for edges are co-planer.

I found that I would need about 30kg of mass on each side on the legs to keep the bench stable, and while that is kinda doable, I use thinking maybe there is a better solution.

also I'd like to know, did I make this with bad mechanics, or is it just not long enough at 40" to counter balance the cantilever effect from the top hanging over the edges?

I'm open to all Ideas besides building a new base.... I have no wood, or time for that anymore.