You must complete these things to get the corresponding bikes.
Bubble Bike (small): Win the Mirror Leaf Cup.
Dolphin Dasher (medium): Win the Mirror Star Cup.
Kamek (small): Defeat all the Nintendo ghosts in the Star Cup.
Magikruiser (small): Play on 8 different courses in Time Trials Mode.
Phantom (large): Win the Mirror Special Cup.
Quacker (small): Win the 150cc Star Cup.
Shooting Star (large): Win the 100cc Star Cup.
Sneakster (medium): Get a "One Star" ranking or better in all Wii Grand Prix Cups.
Spear (large): Unlock 12 expert staff ghosts in Time Trials Mode.
Zip Zip (medium): Win the 100cc Lightning Cup.
Aero Glider (large): Get a "One Star" ranking or better in all 150cc Retro Cups.
Blue Falcon (small): Win the Mirror Lightning Cup.
Honeycoupe (large): Win the 150cc Lightning Cup.
Piranha Prowler (large): Win the 50cc Special Cup.
Royal Racer (medium): Win the 150cc Leaf Cup.
Sprinter, B Dasher MK 2 (medium): Unlock 24 expert staff ghosts in Time Trials Mode.
Super Blooper (medium): Get a "One Star" ranking or better in the 50cc Leaf Cup.
Tiny Titan (small): Get a "One Star" ranking or better in all 50cc Retro Cups. Alternately, get 50 Wi-Fi Wins.
Subject: Bonus Bikes! 
Wed Jun 25, 2008 3:14 pm