well if you want a mountain bike, buy a NEXT bike, they are
extremely resistant, and relatively cheap,but they come with shock absorbers, so if you ride your racing bike afterwords, the ride may appear very bumpy, assuming that you are just as handy with bikes as you are with robots, you could easily replace the shocks with some extrusion.
if you want a mid range bike, i like to use a
FSR XC mountain bike, these are sweet bikes, and quite comfortable on road and off road. The "
stumpjumper" version weighs only around 25 pounds, and quite fun to ride because of its maneuverability.
here is a quick list of bikes in low range, mid range, and high quality bikes
Low-End $100-$399
Specialized Hardrock ($169 at Performance Bike Shop)
Cignol Ranger ($189 at The Clean Machine)
Specialized Hard Rock Classic ($299 at Franklin Street Cycles)
Trek 820 ($299 at Franklin Street Cycles)
Trek 850($399 at Franklin Street Cycles)
Mid-Range $400-$999
Specialized Rockhopper ($424 at Franklin Street Cycles)
Gary Fisher Hoo Koo E Doo ($569 at The Clean Machine)
Specialized Rockhopper FS ($649-749 at Franklin Street Cycles)
Specialized Ground Control FS ($749 at Franklin Street Cycles)
GT Tequestra ($999 at The Clean Machine)
High-End $1,000-Unlimited
Proflex 756 Aluminum full suspension ($1399 at Franklin Street Cycles)
Trek 8500ZX SHX ($1399 at Franklin Street Cycles)
Sprecialized Stumpjumper M2 Comp ($1499 at Performance Bike Shop)
Cannondale Super V 900 ($1599 at The Clean Machine)
Trek 9800 SHX ($1899 at Franklin Street Cycles)
Trek Y33 ($3199 at Franklin Street Cycles)
Have fun biking, and Congratulations on your 5000th post