Where was I - oh yes. Frames. The main thing about the frame, besides having the manufacturer's name on it, is that the frame also determines the kind of bike you are looking at. Welcome to the world of bike design - geometry and specifications. The most important part about buying a bike is buying a bike that fits - the fit begins with the frame size.
It helps a bit to think that there are are two main types of bikes - road bikes and mountain bikes. Think of these two types as a opposite ends of a spectrum - there are, of course, many different types of road bikes and mountain bikes and there are lots of bikes which are in between. However, think of the various kinds of bikes as variations the basic theme ...
Mountain bikes are like Land Rovers - strong (often heavy) intended for rough, muddy and steep terrain. Some mountain bikes are designed for cross country, others for downhill racing and then others for designed for jumping... but most are distinguished from road bikes in that they have:
- straight handle bars and a more up right position (so you can see all around and up the road...)
- 26 inch diameter wheels (although some with 29 inches are now available)
- fat nobby tyres so you can get through mud etc. The width of the tyre vary from 23 mm for road to 2 1/2 inches for mountain...
- triple chain rings on the front and 7 to 9 in the cassette at the back with low gear ratios (ie small chain ring in the front - on the crank - and big in the cassette on the back wheel)
- adjustable seat (one moves the seat up or down depending on the terrain - going up the seat may be higher etc; going down the seat may be lower so that the rider can get right off the seat and over the rear wheel etc)
- suspension - some mtn bikes have front suspension. Those with only front suspension are called hardtails) and some have suspension under the seat as well (called dual suspension). Of course, it is not necessary to have any suspension but it can make the rise more comfortable. The problem with suspension is that suspension makes the bike heavier and less efficient to ride (ie some of the rider's energy is spent bouncing up and down rather than being directed along the road, up the hill etc). As a rule of thumb a dual suspension bike is expensive - cheaper dual suspension bikes are a horror to maintain (and basically a waste of money)...
- caliper brakes or most recently disc brakes (either mechanical or hydraulic). For more information check out http://en.wikipedia.org/wiki/Bicycle_brake_systems
There are four main types of mountain bikes (cross country, enduro / all mountain; freeride; downhill, etc.) For more info check Wikipedia on mountain bikes.
Now the second type of bikes are road bikes. If a mountain bike is like a Land Cruiser, then the road bike is a Miata... (or any other kind of sports cars) Road bikes are designed for long fast rides (on relatively smooth roads)... A road bike usually has:
- drop handle bars for a lower (stretched out) to allow the rider more aero dynamic positions...
- high gear ratios (so you can go really fast!) (often only two rings in the front and 9 or 10 in cassette)
- light wheels with a diameter of 700mm (yes the mountain wheel is measured in inches reflecting its origin in the US while the road wheel is measured in centimeters)
- high pressure skinny tyres (less road resistance so you can go really fast)
- light - ie no suspension or anything else unnecessary (less weight so you can go really fast)
Like mountain bikes there are also a number of different kinds of road bikes. The main kinds of road bikes - racing (or traditional) road bike, triathalon or time trial (TT bikes are the same as triathalon bikes);cycle cross bikes, and touring bikes.
In sum, the main difference between mountain and road bikes reflect their intended uses. The frames reflect this difference as the geometry of the design is difference the materials used (ie steel, aluminum or carbon) and the running gear (gears and brakes) are different. There are also many bikes which are designed taking a bit from the mountain and a bit from the road. For example, a touring bike may look like a road bike but have a heavier steel frame and disc brakes... The bikes in the middle are often referred to as "hybrids". Many commuting bikes began as hybrids (although some might argue that they are in a category of their own.)
In general a hybrid bike is a straight handle bar road bike (ie a road bike - road frame, wheels and gears but with mountain bike handle bars...
Another up and coming bike category is the comfort bike. This kind of bike is usually a heavier bike with a mountain type frame and wheels (fat tyres). Beach bikes or cruisers may also fit into this category. The bike is intended for casual usually slow short "beach" rides.
The commuting bike can either be a road bike with some mountain features or a mountain bike with road features... the gear ratios are usually higher than for a mountain bike... the tyres fatter (28 or 35 mm wide) and you can put fenders on ...
And here I stop. There are bikes I haven't mentioned - BMX bikes, bikes for trialing and fixies (bikes without the freewheel). Maybe later...