river, mud or snow. These types of terrain can sometimes only be travelled on with vehicles designed for off-road driving (such as SUVs, ATVs, snowmobiles and mountain bikes in recent decades, and minibikes even earlier) or vehicles are designed to better handle off-road conditions. Compared to road-going motorcycles, off-road machines are lighter and more flexible, typically having long suspension travel, high ground clearance, and are geared higher to provide more torque in off-road situations. Wheels (usually 21" front, 18" rear) have knobby tires, often clamped to the rim with a rim lock. Many competitive events have emerged and developed into a variety of off-road motorcycle sports, for which a number of specialized motorcycles have been built: Motocross—Such bikes are raced on short, closed off-road tracks with a variety of obstacles. The motorcycles have a small fuel tank for lightness and compactness. Long-travel suspension allows riders to take jumps at high speed. Motocross engines are single-cylinder two-stroke or four-stroke units, which vary in size from 50cc up to about 500cc. At the professional level, bikes are split up into two levels based on their displacements: MX and MX Lite. The MX Lite class contains 125cc two-stroke engines and 250cc four-stroke engines, while the MX class pits 250cc two-stroke engines against 450cc four-stroke engines. The actual cc's for both four-strokes and two-strokes can be under what is listed for the different bikes. The differences in power, displacement, torque, and weight are all variables that balance the competition between two-stroke and four-stroke engines. Motocross sidecar outfits have bigger engines, usually four-stroke and often twin-cylinder. Motocross bikes are also used in freestyle motocross. Trials—Trials riding is a specialized form of off-road competition testing balancing skills and precision rather than speed. For a trials bike, low weight and crisp throttle response power are prioritized, so trials bikes tend to have a small (125 cc to 300 cc) engine, with two-strokes being common. During the trial, the rider stands on the foot-pegs, so a trials bike will have only a vestigial seat, or no seat at all. Fuel tanks are very small, giving a very limited range. Trial usually consider the best for start riding off-road because of its low sit and light weight. Trial motorcycles are design to pass the tracks which is impossible for another kind of motorcycles to pass. Enduro—A modified and road-legal motocross bike which is something between a Trial motorcycle and