Mountain Biking - why so many gears needed on a downhill bike?

I see these huge downhill mountain bikes, w/ 18, 21, 27 gears? Why?
Why not make them singal speed? Why on earth would a cyclist need 21 gears, if they're doing nothing but flying downhill? I would imagine the only purpose they serve is the rear derailleur acts as a chain tenioner.

Have you ever tried pedalling one of those downhill bike up something? Try it, you'll find out soon enough.

Yes, you're going downhill but you don't start out with some residual velocity. You still need to build up speed. Your legs will work most efficiently at a certain spin rate and so you still need to cover that range with multiple gears. Granted most DHers don't run a full spread of front chainrings and some of them might not even have a very wide rear cluster either. The big front chainrings they do run are usually pretty big... as much as 60T in some cases.

umm, most downhill bikes have single gear up front and like 9 on the rear, and those gears are there for finding a gear fit for the trail/track...and come to think of it if a full suspension had one gear wouldnt the chain snap as the rear end compresses? or does the chain keep the same length all the time?

well just sayin that DH bikes usually have 9 gears :)

I run a 42t front chainring with MRP chain device and a 9 speed rear cassette (I forget the range) but have the bottom gear locked out. First you need to have 3 or more gears in a proper Norba, UCI race and nationals. Also do you actually know how much pedaling we do on a DH race run. If you aren't in the air, you are pedaling or turning. And on some tracks there are flkattish sections or you come out of a hard corner so you need to change the gear to keep at optimal pedaling range.

You do however see some people race single speed DH bikes and in the UK there is an even they holdevery year where chains are not allowed. I think that would be a really good race, teaching you how to brake as little as possible. Apparently they are only about 12 sec slower than usual.

If you aren't in the air, you are pedaling or turning.

Not really. if WE (ie, us here in Adelaide) arent in the air we are pedalling or turning but not everyone. Its just that the Adelaide tracks arent exactly 'super steep'. On some tracks, even the pro's coast because they don't have the gearing or they simply dont need to.

But in a answer to the question, you need the gears for different steepnesses of tracks and for the odd uphill pedal. And like Hopper said, Downhillers pedal a lot more than you think.

[QUOTE=Hopper]If you aren't in the air, you are pedaling or turning.[QUOTE]

Not really. if WE (ie, us here in Adelaide) arent in the air we are pedalling or turngin not everyone. Its just that the Adelaide tracks arent exactly 'super steep'. On some tracks, the pro's coast because they don't have the gearing or they simply dont need to.

But in a answer to the question, you need the gears for different steepnesses of tracks and for the odd uphill pedal. And like Hopper said, Downhillers pedal a lot more than you think.
How do you ride upside down?

I'm kidding, really.

How do you ride upside down?

I'm kidding, really.

You really got me confused now :S

I thought of another reason last night- there are sometimes uphill sections in Downhill tracks believe it or not. So DH bikes need the lower gears for uphill sections!

And for the most obvious, most dhillers do multiple things with one bike. It is not uncommon for me to see someone trail riding with an Orange 223.

I would also assume as mentioned, standing, you have one rpm basically, fast. At different degrees of steepness you still pedal close to the same speed but need to keep up to the spin of the cassette. Gearing allows that.

Thats said, I know guys who race the bikercross course chainless...now thats beyond hardcore...not even fixed :)