Here we go again. I get a lot of flak for this, but:

There are tons of cheap ($500-$1000) used dirt bikes out there that can help you.
The lessons you learn on a 150lb mini-bike
do translate to bigger bikes.
And, if you do something stupid, you fall down in grass and dirt at 20 mph.
You can try braking in a corner without getting run over by a Buick.
You don't have to try and be a Supercross rider, just push the limits of a bike whose limits are within reach.
Plus, goofing around in the mud on a mini-bike is
fun.So there.
