Progressively wound springs
These are the things to go for when you upgrade your springs. In actual fact, it's difficult not to get progressive springs when you upgrade - most of the aftermarket manufacturers make them like this. Most factory-fit car springs are normally wound. That is to say that their coil pitch stays the same all the way up the spring. If you get progressively wound springs, the coil pitch gets tighter the closer to the top of the spring you get. This has the effect of giving the spring increasing resistance, the more it is compressed.
The spring constant (stiffness) of a coil spring equals:
k = compression / force = D^4 * G / (64*N*R^3)
where D is the wire diameter, G an elastic material property, N the number of coils in the spring, and R the radius of the spring.
So increasing the number of coils decreases the stiffness of the spring. Thus, a progressive spring is progressive because the two parts are compressed equally until the tightly wound part locks up, effectively shortening the spring and reducing its compliance.
So for normal driving, you'll be using mostly the upper 3 or 4 'tight' winds to soak up the average bumps and potholes. When you get into harder driving, like cornering at speed for example, because the springs are being compressed more, they resist more. The effect is to reduce the suspension travel at the top end resulting in less body roll, and better road-holding. Invariably, the fact that the springs are progressively wound is what accounts for the lowering factor. The springs aren't made shorter - they're just wound differently. Of course the material that aftermarket springs are made of is usually a higher grade than factory spec simply because it's going to be expected to handle more loads.
Note:Make sure you get powder-coated springs! This means they've been treated with a good anti-corrosion system and then covered in powdered paint. The whole lot is then baked to make the paint seal and stick and bring out it's polyurethane elastic properties. It's the best type. If you just get normally painted springs, the paint will start to flake on the first bump, and surface rust will appear within days of the first sign of dampness. Not good. Besides - powder coated springs look cool too!
Electronic damping force controllers.
edfc Remember way back at the top of the page I mentioned that some dampers allowed you to change the damping rate by altering the size of the constriction hole? That's all very well and good but you have to stop your car, get out and twiddle a knob or screw on the top or side of the strut each time you want to make a change. In 2005 the aftermarket saw the first appearance of an EDFC - electronic damping force controller.
The premise is really simple. Four servo motors (the four smaller boxes in the picture here), one for each strut, each one designed to replace the manual screw adjuster. A control unit mounts inside the car and allows you to change the damping force of the shocks front and rear without leaving the drivers seat. The way it works is dead simple. When you first install the system and power it up, all the servos spin clockwise for a few seconds. This ensures the adjusters are screwed all the way in on all four struts. From that point, you can dial in any number from 0 to 20 on the control unit. When you do, the servo motors spin a certain amount - the same as you getting out of the car and spinning the adjuster with your finely calibrated fingers. The units currently have three memory settings so you can store motorway, city and track-day settings (for example), and recall them at the push of a button.
Installing the current-generation EDFCs is pretty simple - about the most difficult thing you'll face is running the wires from each servo back to the control unit inside the car.
There's a few different companies selling EDFCs right now.
Torsion bars
Torsion bars deserve their own section because they are a type of spring which can be used in place of coil- or leaf-springs. It's one of the topics I get the most e-mail on, so instead of continually sending the same answer, I thought I'd cover it on this page.
A torsion bar is a solid bar of steel which is connected to the car chassis at one end, and free to move at the other end. They are almost always mounted across the car, one for each side of the suspension. The springing motion is provided by the metal bar's resistance to twisting. To over-simplify, stick your arm out straight and get someone to twist your wrist. Presuming that your mate doesn't snap your wrist, at a certain point, resistance in your arm (and pain) will cause you to twist your wrist back the other way. That is the principle of a torsion bar.
Torsion bars typically have splines on one end so that they can be removed, twisted round one spline and re-inserted. This can be used to raise or lower a car, or to compensate for the natural 'sag' of a suspension system over time.
Lift Kits
Because of the mechanical nature of suspension, all sorts of mods are available. Lifting suspension is a popular mod used to try to increase ground clearance. This is often a source of misunderstanding. A lift kit doesn't really give you more ground clearance. What it does is increase the height between the axle and the underside of the body. Whilst this does give more ground clearance for the bodywork, the lowest point on the vehicle is still the axles - or on a 4-wheel-drive, the bottom of the transfer case. For this reason, you'll often see trucks and SUVs with lift kits and larger wheels and tyres. The lift kit boosts the clearance under the bodywork whilst the larger wheels and tyres result in the axles being lifted higher off the ground. Technically of course, in a 4-wheel-drive, you don't really need a lift kit - bigger wheels and tyres would do it. BUT lift kits typically end up being required because adding on the larger wheels and tyres can often mean they will no longer fit in the wheel arches. The lift kit will help solve that problem.
Lift kits come in literally hundreds of shapes and sizes, all dependent on the final application as well as the design of the vehicle the kit is going to be used on. For street cars, typically with independent suspension, the kit will basically be longer struts, longer springs and remounted shocks. For off-roaders with beam axles and transfer cases, the suspension system is typically leaf-spring, so the kit will be a set of blocks that fit between the beam axle and the bottom of the leaf spring. Alternatively, some kits have blocks which lower the spring mounts themselves so that the spring-to-axle joint isn't changed. The image below shows an example of a typical leaf-spring beam-axle suspension system along with two examples of how it can be raised.
Fitting a lift kit is pretty basic engineering but it's really difficult to do without access to a hydraulic lift, so its best to either get a garage to do it, or to find a mechanic friend who has a decent sized hydraulic lift. Trying to mess with the suspension whilst a vehicle is on the ground is just asking for trouble.
