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What is downhill?
A downhill bike (also known as a downhill mountain bike) is a full suspension bicycle designed for particularly steep, rocky trails. Unlike a typical mountain bike, durability and stability are the most important design features, compared to lighter, more versatile cross-country bikes. Downhill bikes are primarily intended for high speed descent, and downhill riders will usually push, or shuttle via chairlifts or motorized vehicles, to the trailhead.
Downhill bikes are typically made of aluminum or steel, will have 7-10 inches (180-250mm) of rear suspension travel, and be designed around a 7-8 inch (180-203mm) suspension fork. The suspension sag is also much higher than cross-country bikes (25%-50% of total travel vs. 10%-20%) for a more supple ride at higher speeds.
These bikes will also have very slack head tube angles (66 degrees or less), long wheelbases (over 45 inches), and will accommodate the use of up to 3 inch width knobbed tires. Downhill frames are also overbuilt to handle the stress of riding over rocky terrain, drops, and jumps. Bike weights have been typically over 40lbs (18kg), but modern downhill bikes have broken the 45lb weight barrier (such as the Trek Session 10, weighing in at almost 50lbs). Some newer (2007/8) downhill bikes can be built to weigh under 38lbs, such as the Santa Cruz V10 or the GT DHi.
Other features include 8 inch (203mm) disc brakes and a chain guide to prevent accidental chain deraillment. Other innovations include the use of OnePointFive headtube standard, which uses a 1.5" wide headtube (see headset), instead of the more conventional 1.125" diameter, for added stiffness and strength. Adjustable headtube angles are also available to adapt the bike to the owner's preferred style of riding.
There has been a recent trend in downhill for lighter, shorter, better modulated suspension travel in bikes, such as the new Sunn downhill bike, which only has 7 inch rear wheel travel, compared to most modern downhill bikes with 10-12 inch rear wheel travel.
