Until recently, mountain bicycles had road bicycle style frames and geometry. Mountain biking has since become more mainstream with riding styles becoming more aggressive. Newer frames are better-designed, lighter, and stronger, with a geometry that allows for much more spirited riding over obstacles like logs, rocks, wooden bridges, and man-made ramps.

Newer mountain bikes have either 21, 24, or 27 speeds, with 3 gears in the front and 7, 8, or 9 gears at the rear wheel. 30 speed mountain bikes are currently unworkable, as a slimmer 10-speed chain is not strong enough.

Geometry
The critical angles in bicycle geometry are the head angle (the angle of the head tube), and the seat tube angle (the angle of the seat tube). These angles are measured from the horizontal, and drastically affect the rider position and performance characteristics of the bicycle. In general, steeper angles (closer to 90 degrees from the horizontal) are more efficient for pedaling up hills and make for sharper handling. Slacker angles (leaning farther from the vertical) are preferred for high speeds and downhill stability.

Suspension
In the past mountain bikes had a rigid frame and fork. In the early 1990s, the first mountain bikes with suspension forks were introduced. This made riding on rough terrain easier and less physically stressful. The first suspension forks had about 1½ to 2 inches (38 to 50 mm) of suspension travel. Forks are now available with 6 inches (150mm) of travel or more (see above under "Design.") Bikes with front suspension and rigid, non-suspended rear wheels, or hardtails became popular nearly overnight. While the hardtail design has the benefits of lower cost, less maintenance, and better pedaling efficiency, it is slowly losing popularity due to the increases in full suspension designs.

Many new mountain bikes have a "full suspension" design, meaning that both front suspension forks and some form of rear suspension are used, as opposed to front suspension only ("hard tail"). The advantages of dual suspension are increased comfort on rough terrain, and improved handling over obstacles. Disadvantages of rear suspension are increased weight, increased price, and with some designs, decreased pedaling efficiency. At first, early rear suspension designs were overly heavy, and susceptible either to pedaling-induced bobbing or lockout at certain points of the suspension arc or travel. One of the most popular rear suspension designs to solve these issues has been the 'Horst Link' which first appeared with the AMP series of bikes, and was later adopted by Specialized and many other mountain bike manufacturers.

Disc brakes
Most mountain bikes now feature disc brakes. These offer improved stopping power over rim brakes under adverse conditions, because they are located at the center of the wheel (on the wheel hub) and therefore remain drier and cleaner than wheel rims, which are more readily soiled or damaged. While the traditional cantilever and V-brake style braking system provided ample braking for fully rigid bikes and the earlier, less sophisticated suspension fork-equipped bicycles, as suspension has evolved bicycle speeds have increased. Disc brakes offer the capacity for sustained heavy braking with fewer problems of brake fade than are encountered with rim brakes, allowing greater safety margins with less rider fatigue, greater modulation and therefore control.

The disadvantage of disc brakes is their increased cost and often greater weight. Hydraulic disc brakes, which work by moving brake fluid through a hose or line to squeeze the pads together, require much more technical maintenance but enjoy much longer service intervals than their mechanical counterparts. Mechanical disc brakes, which are simpler and somewhat less expensive, work in a similar fashion to rim brakes by pulling one pad towards the disc with a cable.

The braking power of a disc brake also depends on the size of the rotor. For example, an 8-inch rotor has more stopping power than an 6 inch rotor of the same design (about 33% more). This is because the brake caliper can apply more torque with the same amount of force because the larger disc provides a longer moment arm.