THE GOODS : Chain Reaction : A little worried about protecting your bicycle? <i> Very</i> worried? There are enough locks to match your fears.

A bicycle is really the perfect object to steal: It’s valuable, is often left out in public and provides transportation from the scene of the crime.

Although the marketplace offers many security products (and some you can put together yourself), there are myriad ways to foil any lock. But the incidence of theft in which a professional thief uses extreme methods to break through a high-quality locking device is relatively rare.

A more alluring target is the cycle that’s either lightly locked, or not locked at all. It’s up to bikers to match the level of protection their situation requires with product cost and ease of use.

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Cheapest and easiest is the minimalist, or “faith in your fellow man,” approach.

Product: The thinnest, lightest chain or cable available. Prices go from $5 on up.

Advantages: Easy to carry. Quick to put on and take off. Some have built-in combination locks so a key doesn’t have to be carried. Deters the thief who just grabs and rides.

Disadvantages: Thieves don’t need bolt cutters to go through these. Any sharp tool will do. They could almost bite through some of them.

It will cost you only a little more to use the mid-level, or “this and prayer should work,” approach.

Product: The curled cable (about $10) with at least a 1/4-inch thickness secured by a lock.

Advantages: The typical six-foot length is enough to lock both wheels and frame. Flexibility makes for easy use in many situations. The cable is usually covered in vinyl to protect the bike’s finish and is slippery for bolt cutters to get a grip on. Some manufacturers now make cables that can be attached to the bike’s frame for easy transport.

Disadvantages: Cables can be a bit unwieldy for carrying on the bike. Don’t wrap a six-foot cable around a parking meter: the whole bike can be lifted off. A determined thief won’t have much trouble cutting a cable.

Note: Some of the more expensive cables are made from braided stainless or brass-plated steel, which offers more resistance to bolt cutters. The combination of the weave and metal tends to make the cable flatten rather than split when attacked with cutters.

If you have an expensive bike or a mistrustful attitude about your fellow man, go for the maximum.

The Product: U-locks

Advantages: These are the toughest locks for a crook to crack. They’re made especially to fit bicycles. They come in a number of sizes. Most experts believe that these are the only real protection in a theft-prone urban area.

Disadvantages: They won’t fit around most objects you’d encounter on the street, such as lampposts or trees, though they do they fit around parking meters, bike racks or thin street signs. They’re relatively heavy. They take up room on the frame. It takes a little more time to lock the bike. The front wheel must be removed for completely effective locking.

Note: U-locks come in several sizes; the rule of thumb is the smallest is most effective against the bike thief. And not all U-locks are created equal. A basic type is about $20. Top of the line models include Avenir’s Fail Safe ($40), Kryptonite’s Evolution 2000 ($50) and New York Lock ($80). A U-lock’s protective quality can be enhanced through use of a BAD Bones ($10), a device that reduces the space inside the U, making it difficult to insert a tool to pry it open. Finally, some experts recommend using both a U-lock and a cable, forcing a thief to have multiple types of tools.

The do-it-yourselfer who believes almost everyone is a bike thief will want to try the “It’s what we do in New York” approach.

The Product: A six-foot length of case-hardened steel chain with a high-quality case-hardened lock. With some effort, you can encase the chain in a two-inch diameter bicycle inner tube to protect your bike from nicks.

The Advantages: The six-foot length is enough to lock both wheels and frame. It’s flexible and can fit around a variety of stationary objects. In terms of security, it approaches the level of a U-lock, but is more adaptable.

Disadvantages: The thing weighs a ton. The idea is to leave the chain at home or work--wherever the bike is left unattended for long periods--using other protection for short stops.

Note: Be absolutely positive the chain is case-hardened. This type of steel is so difficult to cut, it’s typically sold in pre-cut lengths. (A 5/16th thickness is $6 per foot at Koontz Hardware in West Hollywood.) If the hardware store’s bolt cutters can cut the chain for you, that usually means it’s not case-hardened.

Other tricks and options:

* Write down your bike’s serial number and engrave a driver’s license number on the frame. If the bike is stolen and the police find it, at least there’s a chance of its return. Another trick is to take a picture of yourself with the bike, write your name and address on the back and drop it down the seat post. If the bicycle’s found, you have immediate proof it’s yours.

* Away from home, lock the bike where passersby see it. Don’t give the thief a hideaway where he can work on the lock for hours.

* If there’s a whole row of bikes, try to park yours toward the end of the rack where it’s exposed to passersby.

* If you forget your lock, disengage the bike’s chain. Or wrap the toe straps around the frame. Or remove the front wheel and the seat post. One bike mechanic says he loosens the skewers on the front and rear wheels. “If someone rides on it, they crash,” he said.

* If you have enough cable or chain to go through the frame and only one tire, choose the front. It’s easier to remove than the rear. The downside of this advice is that the back wheel, with its hub of gears, is more expensive to replace.

* Be sure to chain to something solid. One especially bad habit is used by bike messengers in a hurry: “free locking,” or slipping a U-lock through one tire so the bike can’t be ridden away--but it can still be picked up and hauled off.

* Even if you’re storing the bike in your home garage, lock it to something. One trick is to drill a hole through the exposed wall studs and run a cable through the wood.