Downhill biking is a sport where you start the top of a rough rocky hill on a specially built well-sprung bicycle, and then you race downhill mostly under gravity. Each contestant is timed; the one who does the run the fastest, wins. Downhill mountain bikes really do have to be specially built – with a proper suspension that gives the shock absorbers at least 10 inches of travel.
Since downhill mountain bikes need to negotiate extremely rough terrain and take flying leaps through terrifying drops, the suspension really needs to be pretty serious. It may not readily be evident how the different design features of downhill mountain bikes are put in place. But every one of them is quite essential.
For instance, if you were to take a regular bicycle through a 4-foot leap, it wouldn't just be bone jarring, it would be chain jarring too. In other words, the impact would lift the chain off the gear wheels. So a downhill bike needs a special chain guide that will help keep the chain on the front chain ring. The rear does already have its regular multispeed gear system, so that's kind of okay.
You need some pretty serious road grip (except that there are no roads where you are going) on a downhill bike. The tires need to be made from very sticky rubber that can really grip any terrain, and they need to be wide. For stopping power, you always need disc brakes. That's why these bicycles are quite expensive. They weigh at least 40 pounds, and the begin at $1000. It wouldn’t be difficult to find $10,000 bikes, either.
No matter how small your budget, you need to be sure to look for tires that are at least 2.5 inches wide, and superstrong rims that don’t bend under all the pressure. The wheels on downhill bikes at least need 36 spokes to not fold into a pile of twisted metal the first time you take a leap.
Take a close look at the chain retention mechanism, too. With all the serious bumps and jolts your bike will experience, you need the best retention mechanism possible. It just won't be possible to ride a bike downhill without something on the front that keeps the chain in place.
It can be a bit difficult deciding how much to spend, seeing that these bikes can be very expensive. If you plan to be riding regularly, count on spending at least $2500.