From MangaHelpers:

Natsu was grandpa’s girl and she loved every minute they spent together. He was the one who taught her to love motorcycles and showed her how to ride them. It devastated her when he passed away, but she knows he will always be somewhere in her heart. As she goes through his possessions, Natsu finds out that in his younger days, grandpa and his group of 7 motorcycle friends made the dream bike, the CB72, that was the fastest hog at the time. When the group eventually disbanded, they broke the bike up into 7 pieces as a reminder to their younger years. As a tribute to her late grandpa, Natsu decides to go on a trip around the country to find his friends, retrieve the parts, and recreate the CB72. [vyc]