Snot Wheels is a company owned by Nelson, and invested in by Lisa, that does up bikes and makes them look better. It becomes very successful and Nelson also hires Seymour Skinner. It eventually shut down after Nelson discovered that he was using a water-based glue, which meant that the bikes weren't weather-resistant and fell apart.
Behind the laughter
Snot Wheels is a parody of Hot Wheels both in name and logo.