Ruby Campground Singletrack is a 5 mile loop trail located near Avoca, MI and is rated as moderate. The trail is primarily used for mountain biking and is accessible from April until October.
This is the best trail in the Thumb and Blue Water Area. They cram a lot of trail into a relatively small area. There are a lot of steep and quick climbs, and some fun little descents. I prefer it for trail running, but it's primarily a mtb trail. It costs 2 bucks to use, and it's based on the honor system as they have a little cash box. The trail crosses over itself a lot, so it can be somewhat difficult to stay on trail at first, but usually if you just trust your instincts, you'll stay on course. I measured it to be about 4.5 miles on Strava.
In an area starved for trails, Ruby stands out as the best. We don't have real mountains in the thumb, but Ruby does a lot with the hills that they have. It's a fun course, and you can give yourself a helluva workout. Highly recommended if you're in the area, and looking for a trail. You won't find anything better, and if you do, let me know.
Nice little trails with a stream crossing. Maintained by local mountain bikers.