This wheel clamps on to the footrest of most manual chairs. It changes the dynamic of your chair by going from two small front wheels to one larger one to allow movement on tough terrain.
Retailing at around 600euro from MMS Medical, http://www.mmsmedical.ie/wheelchair-accessories/freewheel/, this wheel enables your chair to move easier over small gravel and stones eg. over grounds that would normally prove impossible for wheelchair users.
As a quad with good physical strength in my shoulders, I found the wheel to work best on flat ground but on uneven surfaces the wheel proved difficult to keep in a straight forward-moving motion, wasting a lot of unnecessary energy.