I didn't made this! I take this from GTA Finland mod, all respect goes to it!!!

Add this to carcols.dat
bf400, 54,1, 58,1, 66,1, 72,1, 75,1, 87,1, 101,1, 36,1

And this to handling.cfg
! BF400 0.25 0.08 0.25 0.08 40.0 30.0 0.93 0.65 0.5 0.1 35.0 -40.0 -0.009 0.5 0.5

and this too:
BF400 200.0 70.0 4.5 0.0 0.05 -0.09 103 1.4 0.9 0.48 4 180.0 50.0 5.0 R P 15.0 0.50 0 35.0 0.85 0.15 0.0 0.15 -0.20 0.5 0.0 0.0 0.15 10000 1000000 0 1 1 4

This to vehicles.ide
581, bf400, bf400, bike, DIRTBIKE, BF400, biked, motorbike, 10, 0, 3f341210, 16, 0.44, 0.44, -1 3f341210, 	16, 	0.68, 	0.68, 	-1

HAVE FUN!!