To unlock the Pro bikes you have to beat the Pro Bosses. Each Pro Boss you beat will unlock a different Pro bike. You unlock the Pro bikes in the following order.
Pro Bike 6 - beat Super Shrimp
Pro Bike 5 - beat Up Chuck
Pro Bike 4 - beat Wonder Wind
Pro Bike 3 - beat Limitless Liz
Pro Bike 2 - beat Champion Caine
Pro Bike 1 - beat Ludicrous Loco (good luck! 😜)
Pro bikes allow you to fully upgrade the bike to the max in speed, jump and pump.
