Here's a one with crossover appeal. Two not straight tire tracks with mud splatter around them, insert phrase about getting dirty, bashing, etc.

Some sort of design with a pistol grip transmitter and the phrase "keep it pinned" "no lift" "don't lift" "brakes? We don't need no stinkin brakes" "bash it like you stole it". You can do a silhouette style transmitter design, something more realistic, with or without a hand.

I kind of like the shirts that show X + Y = Z. Maybe a picture of a RC + nitro engine = musical notes, glorious noise or face palm to make fun of the tuning issues some have. Picture of electric motor + RC car + a really big lipo with lots of layers/cells = a picture of a RC doing a burnout or a broken gear. Picture of a RC + a pic of a jump, mud, anything = smiley face. Lots of possibilities for the X+y=z shirts.

Might I suggest coming up with a cool simple name for the company. Print the name small but easily legible up near the collar on the back and maybe put a small pistol grip transmitter in front of the name.