Originally Posted by BESRK
It's the additional weight of the tread pattern.
Thanks BESRK! BTW, also thank you for your bumper build thread and many contributions to this forum...that thread single-handedly inspired me to go buy a welder and attempt to learn. I am still horrible, but learning everyday and having a blast doing it.
Only thing that caught me off-guard was the price of steel...wow is it expensive for fresh stock. My 3'x4' .half inch was going to cost me $250!
Lucky I found a scrap yard around here that sells stuff by the pound for a bit cheaper.