Street bicycle tires have a smooth surface, in contrast to the knobby thick tread of most mountain bike tires. The two types are optimized for different types of riding. However, there is a wide range of tread thicknesses and patterns among street tires and mountain bike tires, allowing you to pick a precise design of tire to meet your needs.
Purpose
Knobby mountain bike tires are designed for off-road riding, including terrain such as gravel, mud, dirt and rock. Not all mountain bike tires are knobby, however; smooth-surface tires are useful on slick rock. Street tires are designed for paved roads and may also, depending on the tire, work on hardpacked unpaved trails.
Traction
A knobby texture allows the tire to grip complex terrain. When climbing over tree roots, for example, the tire gains traction on jutting parts of the root. On muddy or squishy surfaces, the tire tread knobs dig in to gain purchase. A smooth street tire is ideal for getting traction on paved surfaces because it has more surface to contact the road on each rotation. A knobby bicycle tire would contact the road only on its knobby tread.
Width
Knobby tires are typically wider than street tires, with the exception of balloon tires used on beach cruisers. Knobby tires are 2 to 3 inches wide to give the rider more control and traction on complex terrain; street tires are about 3/4 to 1 1/2 inches wide to give the rider more maneuverability and speed on roads. Balloon tires on casual riding beach cruisers are wide for shock absorption and smooth for easy rolling and traction on roads.
Pressure
The recommended inflation for knobby tires is significantly less than that for street tires; knobby tires often have a maximum recommendation of 50 pounds per square inch. Street tires, particularly the very narrowest ones used for racing, may function optimally at 100 psi or more.
Speed
The terrain-optimizing tread on each type of tire makes it the faster choice on its intended terrain. Knobby tires are slower on paved roads because of the low tire pressure and the rolling resistance generated by its tread. Street tires are slower on unpaved roads because they can't grip complex terrain and are therefore more prone to skidding.
Sizes
Manufacturers produce knobby tires of various styles for mountain and cyclocross bikes, which generally take 26-inch wheels and 700c wheels, respectively. Street tires are available for mountain, cruiser and road bikes; the first two types use 26-inch wheels and the last may take wheels of various sizes, including 700c, 650c and 27-inch, depending on the size, style, and age of the bike.
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