Can I ride my fat bike on regular mountain bike trails in summer?

Yes, but it won't be as fun or fast as a standard mountain bike. Many fat bike owners swap to 27.5" plus-size wheels and tires (2.8–3.0") for summer riding, which transforms the bike into a capable trail hardtail. If you keep the fat tires on, expect slower climbing, higher effort on flats, and less precise handling. The traction is excellent though, making fat bikes surprisingly capable on loose, rocky summer trails.