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The spring's roll resistance characteristics helps to resist the forces during dynamic changes, and make the car more stable during the transition. This implies a stiffer spring is needed to minimize the compression and elongation, and therefore minimize the change to the suspension geometry.
However, even purpose-built race cars cannot simply use the stiffest spring available. If we return to the case of having no springs at all (the ultimate in stiffness), even a "smooth" race track would be violently bumpy without some suspension dampening. At some point the spring becomes too stiff for the road surface, and the vehicle will lose traction as it bounces over surface imperfections. The race technician and driver have to find the most effective balance between being soft enough to allow the tire to stay in contact with the road surface over bumps, and being firm enough to control suspension geometry and keep the tire as flat as possible on the road surface.
A driving enthusiast's car which does double duty on the street and the track has a larger window to find compromise in than does a race car. Putting full race springs on your street car may seem the macho thing to do, and though your car should be faster on the track, it will make your life miserable on the street. In fact, it is quite likely to cause damage to other suspension components when you come across that surprise pothole.
Today, after-market springs offer features that not too long ago would have been found only on race cars. The research done in sports car class racing has resulted in several manufacturers producing high performance progressive rate springs for virtually all sport enthusiast cars that allow an acceptable comfort level on the street yet significantly increase handling performance over the stock springs.
Most factory stock springs have a constant or nearly constant factor of stiffness called the spring rate. As the spring is compressed or elongated, the force required to change the spring's length stays the same. The spring rate is linear as the spring goes from full elongation to full compression. This provides greater comfort across minor and major bumps, but does little to minimize body roll under hard cornering.
Progressive rate springs have a softer spring rate during some initial portion of compression or elongation, but then get progressively stiffer as continued force is applied. This is typically accomplished by changing the shape of the spring. This ability to start soft and get firmer with higher compression allows the spring to accommodate typical street bumps with satisfactory comfort. On the track under high braking or cornering forces, the spring's stiffer region comes into effect to reduce the body roll compared to the stock spring. Compared to a full race spring, there is a little more body roll before the spring takes a firm set, but that's the compromise of a dual purpose car.
Most after-market progressive-rate springs start out about 15% firmer than the stock part, and get stiffer from there. Though they offer acceptable bump absorption, they do give the vehicle a noticeably rougher ride, especially with larger bumps where the spring becomes stiffer. However, given the success of these springs, the comfort for performance trade off is considered well worth it by sports car enthusiasts.
In selecting an after-market spring set, you should know how much stiffer than stock it is, whether it is progressive or linear, and how much it will change the car's ride height. If you're concerned about losing too much ride comfort, you should ride in another car as closely prepared to yours as possible. Some people stiffen their suspensions for periodic racing only to discover they really don't care for it the remaining 97% of their driving time.
You should also know what other suspension changes you're going to make to the car including wheel and tire sizes, and talk with a technician experienced with your car type. Certain combinations of springs, shocks and tire sidewall sizes will function better than others. A mechanic from a specialist shop or race team may offer some advice learned from experimentation and testing.
One other thing related to spring selection is that of vehicle ride height. On the street, the variety of road surfaces, speed bumps, drainage channels, and steep driveways requires the car's lowest point have a certain practical height above the road to avoid damaging the car.
In racing, ride height has significant impact on the vehicle's center of gravity ("CG") which is one of the major influences in a car's weight transfer characteristics. Ideally, the CG should be as close to the ground as possible, and race cars will be lowered as much as allowed by the rules. Open-wheel formula cars are lowered as much as possible without bottoming out while racing which often ends up being 1/2" or less on very smooth tracks.
The most straightforward way to lower the CG is to lower the car, and the most direct way to do that in a street car is with shorter springs. Most street cars can be lowered somewhat from their factory setting, but there are several practical limitations in the design of the suspension system. A realistic compromise needs to be made that considers the clearance needs for the street, and the suspension system of the car.
Extremely low cars ("slammed" in today's vernacular) may look good (or at least look like the racing sedans they seek to imitate), and if done right will handle better on the track, but there are some limitations on the street. Springs which are too short may cause interference problems with other suspension components such as the shocks. Additionally, the suspension geometry (the connection points, and lengths of it parts) are designed with a certain spring length in mind to keep the wheels in proper alignment. A severely lowered car that does not also alter the suspension will cause the wheels to have excessive camber for sure, and will likely also adversely affect the castor and toe. You might think it looks great, but this will severely reduce the handling performance of the car.
You should consult someone experienced with your car type before just buying the spring which seems to lower your car the most. Such a spring may also require a specific matching shock or other suspension changes to actually improve the handling performance.
Speaking of shocks, it is generally necessary to buy stiffer shocks at the same time you change the springs. Springs alone will lower the CG, and will reduce body roll, but neither is the primary function of the spring. For road imperfections, shocks work in conjunction with the spring, and are designed with each other's ratings in mind. Going over bumps, a stiff spring may resist the first compression well, but without sufficient shock capacity, the car will bounce more than it should afterwards which ultimately reduces the car's handling performance. Also, stiffer springs will prematurely fatigue stock shocks. They'll last a while, but will eventually get weaker and decrease the handling performance.
If you can only afford shocks or springs, either keep saving to get both, or start with the shocks. Performance shocks alone which provide firmer bump and rebound control, and greater control over weight transfer rate, will improve handing performance more than stiffer springs alone will.
There is increasing availability of coil-over suspension setups for street cars. The coil-over is a combined assembly of the shock and spring where the spring rests in a height-adjustable base.
Coil-over setups were designed to optimize racing performance, and so several characteristics of coil-overs seem to make them the ultimate choice in suspension modifications.
First, coil-overs allow the car to be lowered more than conventional shock and progressive rate spring upgrades. This of course is a good thing for weight transfer control, but as we mentioned earlier, can cause real comfort and damage-potential problems for a street car.
Secondly, the spring rates are usually much stiffer than even progressive rate springs, and they're usually linear. On the race track, there's no need for a progressive spring rate, and coil-overs with their racing purpose, aren't intended to use them. This also allows the spring to be shorter which is where the greater lowering potential come from.
The main purpose and advantage of the coil-over design is to allow independent ride height adjustability of each corner of the car. This ride height adjustability allows manipulation of the center of gravity of the car. In particular, by manipulating the front to back or side side to side location of the center of gravity, weight transfer can be controlled to influence specific tires on the car. If you can pre-dispose the car to distribute weight transfer a certain way, you can optimize the grip of certain tires and improve the overall speed of the car through corners.
Raising the back of the car puts the CG more forward. Raising the front of the car pushes the CG more rearward. The left or right side could also be independently raised or lowered. With these adjustments, you can reduce the amount of grip lost to specific tires during weight transfer.
For example on a race track with a high percentage of high-speed right turns, you could set the car up to reduce the weight transfer to outside left tire, and increase the weight that remains on the right rear. Compared to the same car with fixed ride heights, the one with the adjustable coil-overs will be able to increase the overall grip through more even wieght distribution during the dynamic load of the right turns.
So, this sounds like an awesome modification to run out and get, right? Well, yes it is -- for racing. Like we say, for the street this is an extreme modification. You may be die hard, and live with the harshness of the ride, but you'll likely have very grumpy passengers.
It is possible to minimize the harshness by using softer springs, or even progressive rate springs for that matter. However, those springs will tend to be longer, will not allow the car to be lowered as much negating some of the advantage. Some people will use stiff springs for the track, then swap to softer springs on the street between events.
Springs allow the wheel to remain in contact with the road surface over bumps. The bumpier the road surface, the softer the spring needs to be. In a street car, it's not likely you'll be set up to change springs easily, but in selecting an after market spring, you should understand the balance of ride comfort and track performance you want, and identify the spring best suited to that compromise.
Lowering the car with shorter springs will also reduce weight transfer, but excessive lowering (beyond the design range of the other suspension components) will cause other problems that will diminish handling performance, and potential create very unsafe conditions if the suspension bottoms out easily.
Progressive-rate after-market springs are designed to reduce body roll to maintain better tire contact patch during cornering, and reduce weight transfer while sacrificing only a little ride comfort.
A coil-over spring and shock assembly is generally the ultimate set up for racing. The car is lowered significantly, the springs are usually much stiffer for body roll control, and the ride height of each corner of the car is adjustable allowing tuning control over the center of gravity and weight transfer. Coil-overs are an extreme modification for street use, and renders the ride comfort intolerable on the street for most people.
For all changes to the spring stiffness, especially coil-overs, be sure to test drive a car that has a setup similar to what you're looking at. Ride stiffness is a definate try before you buy situation to make sure you can live with it the 97% of the time you're not racing.
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