A Rigid Mountain Bike is a frame with a rigid fork and fixed rear, no suspension. A mountain bike or mountain bicycle (abbreviated MTB or ATB (all-terrain bicycle) is a bicycle created for off-road cycling. This activity includes traversing of rocks and washouts, and steep declines, on dirt trails, logging roads, and other unpaved environments—activities usually called mountain biking. These bicycles need to be able to withstand the stresses of off-road use with obstacles such as logs and rocks. Most mountain bikes use wheels with rims which are 559mm in ISO sizing, one of the sizes colloquially referred to as "26-inch". Wheels used on mountain bikes usually use wide, knobby tires for good traction on uneven terrain and shock absorption