Downhill mountain biking

Downhill mountain biking (DH) is a genre of mountain biking practiced on steep, rough terrain that often features jumps, drops, rock gardens and other obstacles.

Downhill bikes are heavier and stronger than other mountain bikes and feature front and rear suspension with over 8 inches (200mm) or (20 cm) of travel, to glide quickly over rocks and tree roots. In competitive races, a continuous course is defined on each side by a strip of tape. Depending on the format, riders have a single or double attempt to reach the finish line as fast as possible, while remaining between the two tapes designating the course. Riders must choose their line by compromising between the shortest possible line and the line that can be traveled at the highest speed. If a rider leaves the course by crossing or breaking the tape they must return to the course at the point of exit, unless they do not gain a time advantage from crossing the tape, in which case they can continue with their run.

Riders start at intervals, often seeded from slowest to fastest, and courses typically take two to five minutes to complete with winning margins being often less than a second. Riders are timed with equipment similar to that used in downhill skiing.

The 1st downhill time-trial race took place in Fairfax, California on October 21, 1976 on a fireroad now referred to as Repack Road, due to the need to repack the hub brake(s) after a descent. The bikes used were based on cruiser bicycles that had Drum brake(s) or a single rear coaster brake that worked by pedalling backwards. A mechanism came into operation causing a conical metal (bronze?) brake shoe to be wound on a thread into a conical metal hub. To prevent a metal to metal brake from snatching it was always filled with grease. Heavy use of the brake during the descent would cause the brake to over heat, melting the grease till it drained from the hub and required repacking. Ten riders descended 1,300 feet (400 m) of Repack in about 5 minutes.

The first bikes used for descending were known as "klunkers" or "paperboy bikes": cruisers using balloon tires and coaster brakes, a sturdy bicycle, designed by Ignatz Schwinn in the depths of the Great Depression, that could endure abuse that could damage other bicycles, by changing features from the Henderson and Excelsior motorcycles his company had built during the 1920s, including a heavy "cantilevered" frame with two top tubes and 2.125-inch-wide (54.0 mm) "balloon" tires from Germany. Innovations like the fat-tire Schwinn with derailleur gears by Russ Mahon of The Morrow Dirt Club in Cupertino at the 1974 Marin County cyclo-cross and Gary Fisher's 1975 use of a tandem rear hub (from a flea market) with internal steel drum brake and threaded for a freewheel derailleur cluster developed the sport and by 1979, two organizers and competitors of the Repack downhill, Charlie Kelly and Gary Fisher founded the company which named the sport, MountainBikes. As mountain biking grew enormously during the 80s, downhill riders continued to use either rigid or limited suspension travel (under 2 inches (5 cm)) bicycles, and purpose made downhill bikes were not made until the 90s. Some of these innovations included dual crown suspension forks and disc brakes, as well as very elaborate frame suspension designs.

...
Wikipedia