Mountain bikes and racing bikes have very different frames, though there also is considerable variation within each type of frame. Generally, racing bike frames use thinner tubes and are considerably lighter than mountain bike frames. Mountain bike frames are frequently outfitted with rear and/or front suspension.
Function
Mountain bike frames are designed for off-road riding across a variety of terrain, while racing bikes are designed to be used on paved roads or cycle tracks. The frames are not interchangeable. Cyclocross bikes are an exception: Using a particular style of racing frame, they are outfitted with knobby tires similar to mountain bike tires. They can be ridden on or off the road.
Materials
Mountain bike and racing bike frames are usually made of cromoly, aluminum, titanium or carbon fiber. Their older counterparts, particularly those dating from before the 1990s, were made almost exclusively of steel, cromoly and sometimes aluminum. Steel frames are the cheapest and heaviest of these and are seen on old racing and old and new mountain bikes. Carbon fiber and titanium are the most expensive and the most lightweight, and are common on high-quality racing bikes.
Frame Sizing
Both racing bikes and mountain bikes come in different sizes. Some bicycles also come in men’s and women’s size; women typically have shorter torsos in proportion to their legs than men, but a woman’s bike might be the perfect solution for a man with a shorter torso and longer legs. Sizing designations can vary by manufacturer, so it’s important to pick a bike based on its fit to you rather than the size number. Whereas a racing bike should have about an inch of clearance between the top of your inside leg and the top tube, a mountain bike should have 2 inches or more.
Component Sizing
REI notes that mountain bike frames typically take 26-inch wheels, though some very new models of mountain bikes use 29-inch wheels for greater stability and decreased rolling resistance. Racing bike frames use 700c wheels almost exclusively. Another major difference between racing and mountain bike frames is the sizing of the head tube, the semi-vertical front tube of the frame into which the handlebar stem and front fork are inserted. Racing bikes usually have smaller-diameter head tubes--and smaller tubes in general--than mountain bikes, meaning that the stems, forks and handlebars are not interchangeable between frame types.
Limitations
Bicycle expert Jim Langley cautions that racing bike frames might not have the appropriate eyelets to attach fenders or luggage racks, while mountain bikes frames can require special fenders that accommodate rear and/or front suspension without cracking.
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