1.5mm frame thickness

[Longerr]

I know that 700kg is too much, but we did everything according to the book, but we used quite heavy 16" wheels and bmw 1.8L engine, but I dont know the weight of it, that's why we want our second build to be as light as possible, we will be using duratec 2.0 with type9, light 15" wheels and 1.5mm steel, lets see what will happen, but still 500-580kg is great with 2mm steel, I really don't know were is the problem with our car