The Fonz's Motorcycle -- Hitting the Auction Blockamundo

If you've ever wanted to sit on it ... now's your chance -- because The Fonz's iconic motorcycle from "Happy Days" is hitting the auction block next month.

The 1949 Triumph Trophy TR5 hasn't been ridden since the show ended in 1984 -- and for the last 15 years, it's been gathering dust in some motorcycle shop.

Auctioneers expect the bike to fetch in the $90,000 range -- which is shocking ... because the damn thing doesn't run anymore. And it's speedometer is broken.