Coilover sleeves resemble lowering spring. The main difference is that they are height adjustable. You can choose from a wide range of rates and lengths. They do not have a shock or strut. They are installed on your existing shocks, and they come with top mounts. These are entry level suspension upgrades – they have a low price and do not come with a Camber Kit.

The key feature of the non-shock adjustable coilover is height adjustability. This is a full-bodied coilover that comes with shocks. However, the shock is non-adjustable. That is to say; you cannot adjust the rebound, compression, and stiffness of the shock.
Different manufacturers have different designs. Most provide them pre-assembled with top mounts. These coilovers are a good option if you want to lower your car and have the choice in the amount of lowering you want. With this type of coilover, you have to purchase a Camber Kit separately.

These coilovers sport the same features of the non-shock adjustable coilovers. The difference between the two is the shocks. In the shock adjustable coilover, the shock is adjustable and perfectly dials into the suspensions.
The shocks allow smooth rebound and compression, and you can adjust the stiffness. Shock adjustable coilovers are the most popular type of coilovers. They provide a smooth ride in your daily driving. Racers like them as they can adjust the stiffness – a feature that is crucial for track use. The price is mid-range, and they usually do not come with a Camber Kit.

A shock adjustable coilover with a camber kit is one of the best suspensions upgrade you can get for your car. These coilovers come with height adjustment and adjustable shocks. Normally, they feature a front camber kit, with some advanced versions sporting the rear camber kit.

Most coilovers on the market will use a pillow-ball upper mount. This means a spherical bearing is used as the joint between the strut assembly and the top-hat, where it mounts to the car. A pill0w-ball will produce sharper, more precise handling, but it will also transmit vibration and harshness through the suspension more easily, producing a harsher ride on the street.
Many newer brands are offering rubber upper mounts. These provide a slight amount of deflection, compared to pillow-ball. While virtually of them will be stiffer than a stock setup, a coilover using rubber mounts will be much more streetable than one using pillow-ball mounts. Here are some popular coilovers and which mounts they use:
BC Racing BR Coilovers – Optional Rubber or Pillow-ball Mounts.
Powertrix Sport Street (SS) – Rubber Mounts.
Powertrix Road/Track (RT) – Pillow-Ball Mounts.
Additionally, some coilovers forgot the inclusion of upper-mounts altogether, and instead you just re-use the upper mounts your stock suspension uses. Because the stock suspension uses a large, rubber spring seat, this will almost always offer the smoothest ride, but is not always the best choice for performance.

Coilovers are generally all adjusted the same way. You loosen the lock collars at either end, and twist the main threaded shock body (sometimes called the “cartridge”). Twisting the shock body effectively screws it into the lower mount (called the “cup”). Screwing it further into the cup will lower the car, and unscrewing it will raise the car. If you're looking to go extremely low, you may find that the car isn't as low as you like once you screw the shock body into the cup. The only way to go lower from here is to unload the spring. When you unload the spring, by spinning the spring's lower mount around the threaded shock body, you create a condition called “droop”. This means the spring is basically loose; you can physically jiggle it around. But when you set the car on the ground, the shock compresses and sits on the spring like normal, and the result is that the car is now lower. Droop can be a dangerous condition, however. If you hit a large bump, the spring can free-float between the upper and lower mounts while the coilover is unloaded, and there's nothing forcing it to land correctly. So once the coilover compresses, the spring may land incorrectly, causing serious damage. Helper springs are thin, light springs that are installed between the main spring and the spring mount. They are easily compressible, and exist purely to let the car go lower. Keeping helper springs in place means that even if the main spring is unloaded during driving, the helper spring will stretch out to maintain constant tension on it, keeping it in place to ensure it will land correctly. This gives you the benefit of lowering the car more, but negates the dangerous side-effects of you would otherwise get from the “droop”.

The spring sits in a cup on a control arm, and the shock mounts to spindle and body of the car. Many coilovers are available in both styles. Neither is truly superior to the other, though many feel a “true coilover” setup is a better choice for a car where performance is a higher priority. This is partially because switching to a true coilover setup allows you to replace the rear control arm which otherwise houses the bulky, heavy, spring seat.

If you're wondering whether your vehicle needs to have the shocks or struts replaced, look for the following 6 signs:

The most obvious sign of a problem with your shocks or struts is that your car is giving you a much more uncomfortable ride than normal. You'll feel bumps more, so the ride will feel rough. If you feel like you're about to lose control of your car when you drive over a bump or pothole, it's likely your shocks or struts need to be checked out.

Another telltale sign of a shock or strut problem is difficulty steering. Your steering wheel may feel stiff or hard to turn. And you may feel like your car is swaying or leaning when you take a turn or switch lanes.

Because struts are a structural part of your vehicle, faulty struts can result in other problems beyond just suspension issues. You may notice that your car feels unstable, takes a nose dive or lurches forward when you brake.

You may notice visible wear and tear to struts if you know where to look. But the most common visual clue that your shocks or struts may need repair is a fluid leak.

When your suspension or wheel alignment is off, your tires will experience uneven tread wear, and it will usually look different than when your tires are just aging. Suspension damage can cause scalloped dips (“cups”) to develop around the edge of the tread. And this problem is very dangerous. If you notice any such signs on your tires, see a mechanic right away.

Cars need regular replacement of shocks or struts every 50,000 miles or so, depending on your car's make and model. Have your mechanic make a routine check of the shocks or struts every 50,000 miles, or however often your operating manual recommends it.

It is recommended that you get your coilovers installed at an auto shop. Still, a D-I-Y installation is not out of the question. Depending on the manufacturer and model, coilovers are shipped in different ways. You can either get them fully extended, collapsed, with the springs mounted, or with separate springs. The first thing you should do when you buy them is to inspect the contents of the package. You should determine that they are the right size for your car. Also, check that you have received sets for the rear wheels and the front wheels.

Installing the springs should be the first step if the coilover came with the springs separate. Most coliovers have an instruction manual detailing how to install the springs. Typically, you will have to first extend the shock. Then, slide the spring over the shock until it comes in contact with the spring retainer, also called the preload nut. If the coilover is dual rate, install the slider first before mounting the lower spring from the lower end of the shock. After installing the lower spring and the upper spring, there should be zero preload. So, you should make sure that both springs snug together. Also, at zero preload, there are no more than 3 inches of threads above the coil adjustment nut. Zero preload guarantees that there is no play between the coil nut adjusters, the springs, the slider, and the spring plate.

To install the fully assembled coilover, suspend the chassis using a jack and make sure the suspension is in full droop. Then, bolt the coilover using the right bolts (usually grade 8) with the lock washers and nuts. Lastly, lower the vehicle slowly. If you did everything correctly, after the vehicle weight rests on the coilovers, it will settle at your desired ride height. If it is slightly below the ride height, you simply need to add the amount of preload. You add preload by rotating the upper coil adjustment nut down. However, if it settled way below or way above the desired ride height, you need to change the springs because the problem could be that the spring rate.

Now that you have installed the coilovers and set the correct preload, you can go for a test run. The test run will let you know if the coilovers need shock tuning or shock valving. Your main concerns on the test run should be the feel of the suspension. You want the coilover to achieve light handling, and also need the suspension to be strong enough to adequately support the weight of the vehicle.