on my trials bike i run 22t on front and 17t on back dunno wat psi softer front than back bmx u want 36t front spocket or higher for dirt, hard as you can get psi, so u can get alot of speed but for park u want 32t or 34t front spocket, hard as you can psi again, this is so u can accelerate quick from stall or sumthin probably use 17t on the back for both park and dirt