Main Methods of Bite Detection when Ledgering.
There are a number of ways of detecting bites when ledgering. The 2 main ones are watching part of the rod for movement (such as a quivertip), or attaching something to the line (such as a bobbin), and watch it go up or down.
It is also worth mentioning "touch ledgering", with this method you hold the line near the reel and feel for a bite. Please don't try wrapping the line around your finger if carp fishing, the speed of the takes can be such that the line could cause injury.
If conditions allow, you can also watch the line where it enters the water. Both these methods can be very sensitive, but I don't want to get involved with them at the moment.
Using the Rod as a Bite Detector - Quivertipping.

Quivertip Rods.
A quiver tip rod is a adapted to be very fine at the tip which is used for bite detection. Most quiver tip rods come with interchangeable tips of different stiffness, so they can be changed for different fishing methods or conditions. The last few inches of the tips are normally painted a bright colour, so they stand out and are easier to watch.
Bear in mind that rods do not always need ultra sensitive tips, depending on the sort of fish you are after. I remember a good few years ago, when someone took me barble fishing for the first time. I was told to leave my quiver tip rod at home and bring my lite carp rod. The bites where so savage, some nearly pulled my rod into the river!
Bite Detection.
As the illustration above shows, the ledger or feeder is cast out and is then tightened to so that there is a bend in the rod tip. It is possible to get rods with really thin tips, which allow you to put a bend in it, when tightening up to small ledgers.
The rod tip is then watched for bites. Some can be very positive where the rod tip slams around, others can be very delicate little vibrations or tiny single movements. Be aware that you can also get "drop backs" where instead of bending around, the tip and rod will go straight, if a fish picks the ledger up and comes towards you. An animation of "drop backs" is shown below where the bobbin drops down towards the floor. The principle is exactly the same.
When quivertipping it is normal to sit at an angle to the water. This is so that the bait can be cast out in front of you and the rod set at an angle to the main line. This allows you to put a bend in the rod tip to register bites.
I also like to set the rod on rests so that the tip is very near the waters surface. This helps to stop the wind blowing the line and quiver tip about, making bites hard to see. If it is very windy then I will set the rod so that the tip is just below the surface.
When setting the rod up bear in mind that you will need to strike backwards, so make sure there are no obstructions in the way before you start.
Swing Tip.
The animation above shows how a "swing tip" works. As with a quivertip, the tip of the rod provides the bite detection. This time however the swing tip is attached to the tip of the rod by rubber tubing. The tip points down towards the water and when a bite occures the swing tip is either pulled upwards. In the case of a drop back it falls downward. This method seems to have fallen out of favour, people prefering to quiver tip or use a bobbin. It can be a bit of a fiddley method with the swing tip getting tangles.
Bobbin - Attached to the Line.
The Bobbin.
The picture on the right shows a "bobbin" which is clipped onto the line. They work on a simple principle, if a fish picks your bait up and takes line, the bobbin goes up towards the rod. If the bobbin falls downward, it means a fish has moved your ledger towards you and is causing slack line. The bobbin moves downward to take up this slack.
They are used in various shapes and sizes by carp anglers, but I see an increasing number of pleasure anglers using them for other species such as bream and tench. I think it's the fact you can use them with a bite alarm, which makes it more relaxing than watching a quivertip all day.
I know this sounds crazy, but someone asked me once if you had to use a bite alarm. I've also heard someone say after a blank when packing up "no wonder I didn't catch anything, my bite alarm wasn't turned on". Needless to say that if I'm getting a lot of runs I very often turn off the bite alarms as they can get on peoples nerves.
Setting Up.
The picture right shows a typical setup. The rod is supported by 2 rests. If your using a bite alarm then the bobbin goes behind the bite alarm, not in front. I know I'm stating the obvious, but I have seen it done the over way - with the bobbin in front of the bite alarm. If you do this the bite alarm will not register drop backs or sound if the bobbin moves upwards until line is taken from the reel.
If fishing for carp make sure that you have set the "bait runner" on the reel so it can give line. If you haven't got a bait runner reel then the best thing you can do is set the clutch so a fish can pull line off the reel if you get a fast take. The way to strike then is to put your finger on the spool to stop it spinning, tighten the clutch and then play the fish. Don't have the clutch set to loose when waiting for a bite. If you get a very fast take, the spool could over run and you end up with a massive birds nest tangle with the line. I've seen it happen many times and to be honest I've had it happen to me as well - more than once.
Bite Detection.
Someone once asked me why you need a bobbin when carp fishing, as I was using bite alarms. Well bite alarms may be a great at showing screaming takes, but that's all without a bobbin. The bobbin is able to show small movements of the line and "dropbacks". For an explanation of how a bobbin shows bites, look at the animations below
The animation above shows what happens when a fish picks your bait up and moves in a direction away from the angler. The bobbin is raised to indicate a bite or take.
Above shows what happens when a fish moves towards the angler and the ledger stays in position on the pool bed. Even though the fish is moving towards the angler, the bobbin still goes upwards.
Above we see what happens when using a fixed lead, or if the hook length line becomes tackled around the ledger. Sometimes a fish will pick your bait up and bolt off towards you with such speed, that it drags the ledger with it anyway. In these cases the line becomes slack, so the bobbin drops down towards the floor. This is called a "drop back".
Not only do "drop backs" apply to bobbins. A similar thing happens when you are quivertipping and instead of bending further around to show a bite, the quivertip straightens up, this is a drop back. So what ever method you choose to show bites when ledgering, make sure you can detect "drop backs".
Balancing the Bobbin to the Ledger or Swimfeeder.
Just a word about what type of bobbin to use. Don't use a bobbin that's so heavy it's going to drag your ledger along the lake bottom. Also if you are doing delicate presentation for roach etc., attach a bobbin that's very light weight. It's common sense, but it's amazing what you see people doing on the bank side.
Some bobbins are made so you can screw different size weights into the bottom of the bobbin body. I find these excellent and will adapt to most fishing situations.