Types of Compound Bow
Compound bows comprise a riser and limbs combined with a system of cams, strings and cables that enable the bow to use the energy stored in the limbs as efficiently as possible. Compound bows come in various axle lengths ranging anywhere from about 30" to about 42"; typically the shorter axle bows are used for hunting whereas the longer axle bows are used for target shooting. They come in varying draw weights as well, anywhere from 20lb up to 70lb. Note however, that the maximum draw weight for target shooting is 60lb. Most bows today provide a range of draw weights and are adjustable without dismantling the bow. When drawn the compound bow typically has a reduction of 65 - 85% in the draw weight at full draw; so for example, a 60lb bow with a 65% let-off cam will have a holding draw weight of 21lb.
Though there have been many different configurations of compound bows over the years the main 2 types are the single cam bow and the twin cam bow:
These bows have a single cam at one end of a limb and an idler wheel at the other end. They typically have a long string and one cable. The string attaches to the cam, goes around the idler wheel and attaches to the cam again. The potential problem with single cam bows is that the nock travel is not level throughout the shot sequence. Also changes to the draw length can change the draw weight slightly.
These bows have 2 cams, one at the end of each limb. Typically, they have a string and 2 cables that slave the cams together. The issue with these bows is cam timing; both cams must rotate to the same degree at full draw otherwise the bow exerts uneven force through the arrow.
Note that if set up correctly either type of bow will be fast and accurate; ordinarily the bow is capable of out-performing the archer.
Choosing a Compound Bow
The prime concern when choosing a compound bow is to get one that fits you; the bow must fit you, you cannot fit yourself to a bow that has too short a draw length etc. Consider the following:
The draw length is probably the most important factor. Too short and you will be moving your head forward to see through the peep sight and too long you will struggle to reach full draw. Neither situation is good for your form. To get an idea of your draw length, stand as if your are holding a bow at full draw, make a fist with the outstretched arm and measure from the front edge of the fist to the corner of your mouth; this is a decent starting place. Ideally, you go to the archery shop and try the bow you are interested in before buying it. You can also buy compound bows that have adjustable draw lengths; this allows you to change the draw length without having to buy a new bow. Note that your draw length can change over time as your form improves.
Draw weight is the poundage you will have pull to shoot the bow. Don't listen to the recurve archers who would tell you you only hold 20lb so it must be really easy! You still have to pull the peak weight for every arrow. Choose a weight that will be comfortable for you to shoot over the course of the round and not the maximum weight you can pull for a few minutes. If the draw weight is too heavy your form will suffer when you get tired. Most bows have adjustable draw weight so choose one that has a range comfortable for you.
Axle to Axle Length
Shorter bows are typically used for hunting and longer bows are used for target shooting. Archery is about repeatability, being able to execute that perfect shot again and again. In order to do this we want a number of reference points that we can ensure are in place for every shot. One of those is the string touching the end of your nose. One of the differences between the shorter and longer bows is the string angle, the longer the bow the steeper the string angle and the more likely you are to get the string to your nose. You need to choose a bow that fits you.
Bracing height is measured from the throat of the grip to the string. Typically a short bracing height indicates a fast bow but an unforgiving bow whereas a higher bracing height indicates a slower bow but a more forgiving one. If you are just starting out you don't need the fastest bow in the world, you need the bow that supports your efforts to improve your shooting. You only need a bow that can shoot the distances you need to shoot.
Is more expensive better? Well, the answer is probably yes, if you are a world class archer. For the likes of you and me the bow we shoot will almost always be able to out-perform us. There will be more improvement to be had by perfecting our form than by buying a new or more expensive bow.
Hopefully the above will help you make a better informed choice when choosing a compound bow. Basically, choose a compound bow that fits you! The best way to do this is to visit an archery shop where the staff have the knowledge to help you do that and you can try various different bows.