Here's how to unlock new kart parts. All you need to do is collect enough coins while racing. If you do, you'll unlock a new part. The exact part you unlock is random. The gold kart parts are unlocked by doing one of 2 requirements. You can unlock the Gold Standard Kart by scoring 10,000 VR (don't know what it stands for), or collecting 20,000 coins. You can unlock the Gold Tires by finishing all engine classes with gold trophies and star rankings on all cups, or collecting 15,000 coins. Finally, you can unlock the Gold Glider by meeting 100 people through StreetPass, or collecting 10,000 coins.
