1. Multiple gears vs. one gear - While the full suspension bike has three rings in front and eight cogs in back, a singlespeed has -- you guessed it -- only one gear.
2. Bigger wheels - Standard mountain bikes have 26" wheels; the new wave of singlespeed bikes have 29" wheels, like hybrid/touring bikes.
3. Rigid fork - My bike is very bouncy with front and rear suspension. Elden's bike has no suspension.
4. Different pedals - Elden's Time pedals are a little more difficult to clip into and out of than my Speedplays. Within fifteen minutes of the first ride, I stalled while going up a ledge, couldn't clip out, and fell 8 feet onto my back. I had a headache the rest of the weekend, but it's always fun to claim you're "concussed."
Advantages of the Singlespeed
One of the reasons I didn't want to ride a singlespeed bike was because of an experiment I did a few years ago. While on my geared bike, I rode side-by-side with Dug and shifted until we pedaled the same revolutions and speed. I ended up in the middle ring and fifth or sixth gear, and there's no way I wanted to push that gear all day. What I didn't realize is that I was comparing apples and oranges. Having multiple gears comes with a price -- the drivetrain absorbs some of your energy and the bike is heavier. When you're riding a singlespeed, the gear you're pushing is much easier than its standard mountain bike equivalent. This means you have more pop.
When you ride a singlespeed, you have to get used to going fast over obstacles and going easy on the brakes. The bigger wheels and higher speeds allow you to roll over obstacles that snag bikes with smaller wheels. This is perfect for riding in Southern Utah. There are some moves -- especially drops -- that I can do on a singlespeed but not on a standard mountain bike.
There's also something to the feel of riding a singlespeed bike, but I don't know how to describe it. It just feels good.
Disadvantages of Singlespeeds
When you want to ride fast down a long hill, you spin out. Also, certain moves can't be done on a singlespeed, and that's frustrating. Any difficult move in which you lose all your speed and have to pedal up a steep incline has to go unconquered (this can also be a benefit in that certain moves that we've mastered on a geared bike have now come back into play on a singlespeed). Also, the larger wheels make tight turns more difficult -- a definite negative for riding in the Northwest.
Another drawback is that you get more beat up on a singlespeed. By the third day, I no longer enjoyed just riding along the trail because my hands and forearms were so sore.
I am going to purchase Elden's extra singlespeed bike for all Southern Utah riding, and I'm going to convert my old backup Stumpjumper into a singlespeed for riding in Seattle. It'll have the same small wheels, but I'll replace the old suspension fork with a rigid fork.
I'll have more details on the bike trip later.