hello dynamike if you have the same 21st 1/9/ scale abrams m1a2 purchased at wal-mart? words of caution as for i added more weight to bottom hull i have already done this and yes it will give you better traction and will go just about anywhere [&o] but the gear boxes are not able to withstand the strain so i have found out the hard hard way i made the mistake of tring to run it thru rough terrain as in backyard with sticks rocks ect. after the breakdown stripped gears i look into the gearboxes and found that they are only made of what i call pot-metal cast aluminum and are only 3/4" dia. and have very soft shafts for gears to run on the rear sprocket s are 3" dia. and gears are 3/4" do the math? and yes i tried to find replacment gears the same dia. and no. of teeth but not luck[:@] any way i still like the tank short of shortcommings i have already replaced gearboxs with bigger metal gears from linear accuators waiting for new radio controller to finish it up and get it back running again as i said be very carful where you run it and not to bind up tracks,sprockets at all or it will strip gear boxes tankmandan