What are the differences between Single speed bicycles and multi-speed bicycles?

A single-speed bike has one gear, whereas a multi-speed has considerably more, though not usually more than 21. Lower gears trade speed for power, which is useful for negotiating hills, grass, and rough terrain. Higher gears do the opposite, making them suited for speed over level terrain.
A multi-speed bike will let you do more, but the single-speed, by virtue of simplicity, is more reliable and in many cases, less expensive. BMX bikes are single-speed, road bikes and mountain bikes are multi-speed..

Everything @Nullo said plus it’s fun to go ripping through the gears, just like it is in a 4 speed manual car! Definitely opt for a bike with gears.

@Nullo Good description!

I’d add that the brakes tend to be different as well.
Multi-speed bikes have hand brakes one for the front and one for the rear, operated with levers on the handlebars.
A single-speed bike often has a pedal operated rear drum brake, that engages when you pedal n reverse. Typically a single-speed bike will have only that rear brake, although some supplement with a front hand brake.

@Nullo covered everything, except for the generalization in the final, whispered sentence – road bikes and mountain bikes can both be single speed. The road bike type is quite common where I live in NYC, and they’re called fixies. Single-speed mountain bikes are less common, but certainly do exist. I nearly bought one recently off craigslist – a beautiful Gary Fisher Big Sur hardtail. Really should have grabbed it, actually, but I didn’t want to go to the trouble of swapping out the derailleurs; ended up with an excellent 21-speed Hoo Koo e Koo instead. I personally am not a fan of the stand up and hammer your way up the hill method, and think that single-speed mountain biking sounds like the stupidest thing I ever heard of, but others think otherwise.

Fixies are only suited for “trick” riding or track competition where weight matters enough to where you piss before the race just to be a few ounces lighter. For those that see many hills, and/or actually need more braking power than human legs can manage, they are outright dangerous. Single-speed bikes that have a freewheel and coaster brake are far better on that second count, but only the fittest cyclists can handle the sort of hills I am accustomed to.

I vastly prefer multi-speed as I often go places where single-speed bikes would either not be able to make it, or would fatigue anybody short of an Olympic athlete. They are a little harder to maintain, but well worth the hassle the first time you try to climb a mountain.

I have only ridden a fixed gear road bike once. It was weird and I didn’t really like going fast because you have to somehow slow your legs down to slow the bike down. I ride a single speed and geared mountain bike.

Gears: Able to find the right one for the trail or road you are on to either spin up a hill or fly down it; more expensive to maintain; more things to go wrong on the trail

Singlespeed: no worries about switching gears to find the right one; fly up the hills (if you are fit enough); sometimes run out of gear on the flats or downhills; not much can go wrong on the trail and you can bring what you need to fix a broken chain to get back to the trail head; cheap

A single speed mountain bike can go anywhere one with gears can. I have ridden my single speed in Maryland, Delaware, New Jersey, Pennsylvania, Vermont, New York. There are places that I prefer the bike with gears, but that is a function of my fitness, not because of the bike.

One of my riding buddies, 53 years young, only rides his single speed. He kicks the rest of our asses, no matter what or where we are riding.

We met riders at a bike festival in the Shenandoah Valley that were riding single speeds. And those were the hardest trails that I have ever ridden.

I’ve ridden in Waterbury, Vermont (the best cross country trails I have ever ridden) with guys on single speeds.

I don’t think I would recommend starting with a single speed, but man, are they fun to ride.

Single speed bikes are for dorks. The cool people have shifters, because they know they are better bikes. If you want to struggle with just one speed then go ahead and get a shifter and leave it in the same gear ratio until it frustrates. It will be obvious at that time. I state this… having had both.

@captainsmooth Thanks for your thorough reply; single speed mountain biking makes more sense now.

A single-speed bike only makes sense in perfectly flat terrain, or if you (especially your knees) are in outstanding shape and can handle the strains of cranking very hard at low speeds.

Some of us have to be a bit kinder to our knees so we can live to ride another day.

@dabbler A single speed isn’t much fun cranking very hard at low speeds. It is more fun to keep your momentum going by pedaling as fast as you can. You would be surprised how easy it is to get up what you thought would be a killer hill when you pedal as hard as you can to avoid that grind. My fitness usually stinks, so my buddies know to get in front of me on the uphills otherwise they get to grind like me. Trails that have a lot of up and down are more fun than flats; I usually spin too fast on the flats. To change to a better gear on flat terrain would make it really difficult to get up a hill.

@woodcutter Getting the shifters, derailers, extra cogs, extra chain rings and cables off your bike saves a significant amount of weight.

“A single speed isn’t much fun cranking very hard at low speeds.” Indeed, my intended point precisely.

That’s why I’d advocate multi-speed for most people in most conditions.
If you don’t like all the usual multi-speed derailer/multi-gear-hub rig, how about a multi-speed hub?

Unless you have one of those single speed track bikes (ala Olympics), a regular single speed cruiser is a dog of a machine. They ‘re pretty heavy. Thick steel frame big balloon tires, they’re all there. You can now go to wally world and get a 21 speed mtn bike with disc brakes that weigh less.

And not look dorky

It’s like saying I love driving a car with one gear???

@woodcutter Have you tried the disc brakes yet? Are they any good as brakes, and do they work when wet?

Haven’t experienced disk type. Mine has “v” brakes and are pretty stout. Couldn’t imagine needing brakes stronger. Personally I think they are a bit of hype just to keep things competitive among the bicycle manufacturers but they do look cool.

First time I tried disc I sent myself flying out over the handlebars. But the bike guy said that shouldn’t have happened, they were not adjusted properly. That was great consolation for the road rash! I don’t go fast enough that they are necessary, overkill for me and you do have to be a bit tentative applying them even if they are adjusted properly.

For mountain biking, disc brakes blow away “v” brakes. Get the Avid BB7’s. They are cable actuated, not brake fluid. You can use them with the levers you already have and they cost about $50/wheel.

@captainsmooth Do they work when wet, though? I would have trouble with my regular brakes in the rain.

@captainsmooth 50 bucks a wheel? That includes the forks and whatever it takes to convert the back? Plus labor?

@Nullo I had my bike changed from cantis to V -brakes and I can lock up the wheels using one finger on the levers, even though it’s a bad idea to do that. I suppose the advantage of a disk is if you warp a wheel somewhat you won’t have that annoying pad drag on the rim while you’re out.

Most bikes come with disc ready forks as well as frames. Heck, most mountain bikes come with discs.

Discs blow away V-brakes, wet or dry. They stop you with less effort.

And returning to the original question, I just got back from a weekend in North Jersey, camping and riding some ridiculously difficult trails. Two of our crew rode single speeds. The rest rode geared bikes. All of us had a blast.

I don’t think anyone looked too dorky.