Well we certainly have mountains!! - but riding any kind of trail is unlikely, the bush is just too thick - perhaps some of the newly graded bulldozed dirt roads or - some of the longer "improved' roads like the ones to Bordeaux beach, Stumpy Bay, Neltjberg, etc. would be worthy rides but fairly steep. Road riding on the main roads is doable - on Sun. mornings [early] I often pass some 10-20 riders on the roads before the traffic gets too bad. I see the occasional rider on the roads during the day sometimes especially in the west end and Peterborg areas.
I would not bother to bring your own rig unless you are seriously training for a triathalon type event or are an all out serious biking fanatic. There used to be a cruise ship type excursion that operated out by Magens Bay but I haven't seen that for a few years - there is occasional mention of a bike tour of Water Is. - maybe someone knows about that?
There may be some kind of biking operation on STJ ?? Of course STX has a huge triathalon event yearly - you could check into what goes on there during the non racing season for bikers.

Don't know of anything on St. Thomas for bike tours or renting bikes; there aren't any bike trails. There was a company called Endurance Sports that rented bikes, but I don't think they are around anymore.

The Water Island Bike Tour is run by Water Island Adventures.

Arawak Expeditions, on St. John, they have mountain bike tours: see their website at http://www.arawakexp.com/biketrips.htm.

Information on St. Croix Ironman at http://ironman.com/events/ironman70.3/stcroix70.3.

--Islander

You could bring your own bike and ride on your own on Water Island. It's a short ferry ride from STT, and has very little traffic which is why somebody has a business providing mountain biking excursions there instead of STT. You could probably do biking on Hassel Island if you can get the ferry to take you there. There are no roads on Hassel Island! But there are a few trails.

You could probably also ride along the roads on the West End of STT near the University, and maybe even further along that road. There's less traffic.

There used to be mountain biking near Magens Bay Beach, and the trail may still be there. You may want to contact the Magens Bay Authority to see if they still allow biking on the property.

I too am an avid cyclist but after seeing the drivers (and roads) decided to stay in the rental car. One of times I tried walking I got hit by a car. Nothing major just a nice bruise in the shape of his side mirror. Others have suggested Water or Hassle Island and I humbly defer to their knowledge. If you want a good and safe workout walk up from Fungi's to Agave Terrace a few times or from the road on the east end up to Agave Terrace. 🙂

Might be difficult to get approval to put your bike on the Water Island ferry. You might want to call ahead to make sure they will permit you and you'll likely have to take a not-as-busy ferry time to/ STT/from WI.