Portland Bike Hire is a mobile bike hire company. they bring the bikes to you and pick them up after your tour, full details here portlandbikehire.com. They provide routes, guides and trails around Portland, Dorset. You can download free maps and guides from their website. You can download the Portland Cycle Map here.
With Dorset Mountain Biking you can share Routes, Guides and Trails around Dorset. Share your Mountain Biking Routes and Experiences with DorsetMTB
Mountain biking on Portland has some spectacular views and brilliant trails. On a weekend the trails can be a bit busy with walkers and dog walkers, especially if the suns out. However its well worth a ride around the coastal path which consists of around 10 miles. Some light to moderate riding.