Automotive Coil spring is a torsion bar with an extremely accurate outside diameter which gets wound in spiral by tempering process of heating and cooling metal under controlled conditions  which increases the resilience of metal. Resilience is the ability of metal to return to its original shape after being compresses or stretched. Most factory springs today are made out of steel alloy, aftermarket springs however are made out of a more superior material called chrome vanadium alloy, a more expensive but lighter and more durable. Because of their superior ability to provide a comfortable ride for the passengers, and support a significant amount of weight, coil springs are adopted by most vehicles you see on the road today.The strength and handling characteristic of a spring generally depend on the following:

Coil diameter -The smaller the diameter, the softer the spring.

Number of coils-The more coils, the softer the spring.

Height of spring -The longer height, the softer the spring.

The suspension spring of a vehicle serves several vital purposes when the vehicle is in motion. First, it acts as a buffer or protection between suspension and the frame of the car. When the wheels rise and fall over road surface irregularities, the springs act as energy storage devices that absorb the vertical wheel movement and greatly reducing the magnitude of impact transmitted to the car chassis, in other words the springs make the driving experience adequate enough to keep the passenger comfortable and the cargo intact. Second: each spring transfers part of the vehicle weight which rests upon it to the suspension components which transfer it to the wheel contact patch with the road. The ideal spring should keep the wheels stuck to the road over variety of road imperfections and driving conditions for maximum traction.

In actual practice, these two basic requirement of a spring tend to conflict since softer springs which permit generous deflection will reduce to the minimum most unexpected surface impacts, on the other hand will allow a considerable amount of body roll when cornering which deteriorates the handling of the vehicle. However an acceptable compromise can be achieved between the two with progressive springs.

Generally spring modification is performed for two reasons, to lower the ride height of the vehicle and to increase the stiffness of the suspension ( increase spring rate). Both of these factors should improve the handling characteristics of your vehicle in comparison to what they were when factory springs were employed.

 
Fist lets attend to the matter of lowering the vehicle. Imagine a truck and a passenger vehicle going at the same speed of 65 mph, they come up to the point where the highway starts to curve abruptly,  due to its low center of gravity the passenger car may continue at the speed it was going and will only experience a minimal body lean to the outside. The truck however, with its considerably higher center of gravity is forced to reduce the speed or it will end up on its side. To explain why the lateral (side) force acting on a vehicle with higher center of gravity is considerably stronger as opposed to that of a vehicle with lower center of gravity we need to embark on some physics, I will address the issue later in the article. For now one thing should be clear is that vehicles with higher CG will have greater amount of body roll due to a larger centrifugal force acting the CG during cornering.  Evidently by lowering the vehicle a respectable amount you can reduce the amount of lateral force exerted upon you vehicle during cornering and reduce the weight shift to the outside wheels.

N9ow let us talk about the stiffness of the springs and how it alters you car’s handling characteristics.  Installing aftermarket springs with higher spring rates will increase the stiffness of you suspension which will reduce excess body roll during cornering, body squat and body dive while accelerating and braking. Reducing the roll of the un-sprang weight of the vehicle is an essential step in improving the handling of you vehicle during all race conditions. Body roll slows chassis response to steering, braking and accelerating by altering the suspension geometry and contributes to the lack of confidence of the driver. I would like to stress the point that body roll does not cause weight transfer, total weight transfer to the outside wheels during cornering is determined by the centrifugal force acting on the CG of the car, however there is a small amount of lateral motion during body roll which adds to the weight transfer.
Progressive Springs vs. Linear Springs.

Linear Springs exhibit a constant spring rate when under pressure. This is achieved by wounding the spring to have the same amount of spacing between all coils. Constant or fixed spring rate means  that if the rate of a certain spring is 100lbs per inch, it will take exactly 100 lbs to compress the spring the first inch, and every other inch after that throughout the complete range of compression until the suspension bottoms out. Because these springs exhibit constant spring rate they are ideal for a vehicles with a specifically set task where compromise between performance and comfort is not required. Most racing vehicle use linear springs because they offer that consistency which allow the race teams to anticipate the handling and calculate the movement of the suspension for a certain track.. Most aftermarket coilover set-ups including all TEIN coilovers utilize linear springs.

Progressive Springs offer a spring rate that varies depending on amount of vertical pressure exerted on the spring. The actual spring rate is not fixed, this is achieved by special design where the coil is wound conically or the spacing between the coils is decreased from the bottom to the top of the spring which makes it exhibit a variable spring rate. For example; if it takes 100-lb to compress a variable spring 1 inch it will take 120-lb to compress the spring an additional inch, the weight required for the spring to compress every additional inch is increased. These kind of springs offer a considerable amount of compromise between performance and ride quality making them ideal OEM parts for most car manufacturers. Aftermarket lowering springs including all TEIN products utilize the variable design because it can be manufactured in a certain way to improve the performance characteristics over the OEM parts and at the same time retain a respectable amount of comfort.