depends what you are going to use the steel for ali is good as a replacement aswell????????

you can use thicker foam aswell to shock absorb and its lighter

Kevlar as far as im aware has never been used in modern robots and i believe its Pricey++++++++ stick to well know materials, polyc, hdpe, nylon, steel, ali, ti and not forgetting the ultimate WOOD!!!!!!!!!!!! (block of wood = champion)

alex