A mountain bike is a bicycle designed for off-road cycling. Mountain bikes are designed to enhance durability and performance in rough terrain, which makes them heavier, more complex and less efficient on smooth surfaces. These typically include a suspension fork, large knobby tyres more durable wheels more powerful brakes, straight, extra-wide handlebars to improve balance and comfort over rough terrain, and wide-ratio gearing optimised for topography and application. The rear suspension is ubiquitous in heavier-duty bikes and is now common even in lighter bikes. Dropper posts can be installed to allow the rider to quickly adjust the seat height.