I am currently looking to design and build my own ATV/quad frame. However I don't know what material to use. Could someone please advise me what materials of steel are best suited to this type of application.
IMHO... the AISI 41xx series of alloy steels is commonly used in tubular form for different applications requiring high strength, one thing to keep in mind is that after welding the frame will need to be stress relieved.
The bigger Diameter of tubing will give higher strength, adn if designed properly you should be able to optimize for both strength and weight by adjusting diameter and wall thickness.