Ok. I'm really not familiar on the particular rules on bikes at each trail area.
Hashawa, or more commonly known as Union Mills, has trails slightly north of Westminster. They have horse friendly and bike friendly trails. I'm not sure which trails bikes are allowed on. They have maps at the parking lots. It might be clarified there.
Liberty Reservior also allows bikes I do believe. Parking can be difficult for horse trailers though. That's kind of south and west of westminster. Not sure how long it would take to get there from Frederick.
Little Bennett is an obvious choice. Lots of trails. Big parking area. South of Frederick.
Sugarloaf Mountain. It's also south of Frederick. According the website, bikers are allowed on the horse trail Memorial Day to Labor Day. Has an area for horse trailers to park.
Black Hill Regional Park- Also south of Frederick. You are supposed to park in the boat overflow parking area -which is a big area. Allows bikers and riders on most of the natural suface trails.
Here's a list of activities and then which state park offers them. Maryland Department of Natural Resources - Maryland Park Service
Out of the parks that offer cycling and horse back riding on those lists, I've been to Morgan Run and Patuxent River. I parked at an aquaintance's house on private property the one and only time I've been to Morgan Run. So, I don't know the parking area at all. And I don't know if there's specific trails for either activity. I was at Patuxent River State Park last year. Loved the trails. Huge parking area. I don't know anything about biking there either though.
There's always the option of going to a Rails to Trails trail (I'm not familiar with any near Frederick) or the C and O Canal (allows bikes right? Lol). The downside is the the ride will be straight out and back. Posted via Mobile Device