Just adding a bit of info for some suspension newbies since this is a sticky anyways.
Spring rates are the key factor in balance of the complete chassis. This leads to the debate between "linear" and "progressive" spring rates. There's no mystery about progressive springs: A progressive spring has a variable rate increase throughout its compression stroke. For example a progressive spring with a starting rate of 200 pounds per inch for the first inch of compression and an end rate of 400 pounds per inch for the next two inches of compression would then equal a load of 1000 pounds.
A linear spring rate has one rate throughout its deflection. This means, if you have 300 pounds per inch spring rate, it takes 300 pounds to compress that spring one inch. A 300 pounds per inch linear spring, compressed three inches, would equal a load of 900 pounds. As you can see, one progressive spring can do the work of two or more linear springs. This is a big advantage in modern automotive chassis design, fulfilling the needs of today's discerning customers.
So why are linear springs still popular? Linear springs are readily available and inexpensive, allowing most race teams to use several different sets depending on track conditions. Linear springs are also easy to work with because the spring rate never changes, allowing for quick chassis set-up. This user friendly appeal is why so many chassis tuners are critical of progressive rate springs. These chassis tuners do not have the know-how to use progressive rate springs, or if they do have the knowledge, the manufacturer that they use is not capable of producing the design specified. Springs with a high linear rate would be used on a smooth racetrack, while on a rough or bumpy road course; you would use a softer spring rate. Since many racetracks have different road surfaces a suspension that is adaptive to changing road surfaces is desired. Progressive rate springs can offer a chassis tuner the means to achieve a compliant suspension in the rough and a tight suspension for high-speed turns.
Another issue that adds to the debate between "Linear" and "Progressive" rate springs, is that when most spring manufacturers say that their springs are progressive they are not! Springs may be wound progressively, but that does not mean that they function progressively. Some suspension springs are wound progressively but function as a linear spring. These springs can be called "dual-stage" coils, but are generally referred to as springs with "dead" or "inactive" coils. Dead or inactive coils are coils that are in contact with adjacent coils at loaded height. Inactive coils do nothing but give the spring enough free-length to stay tight in the spring perches at full rebound (when the tires and wheels are hanging in the air like when the car is on a lift). A spring that is wound with inactive coils and no progressive coils that are active, is actually working as a linear-rate spring. This is why when you call a spring manufacturer for spring rates for your application you must ask, "What is the actual working spring rate?" This ensures that you do not just get numbers quoted from a design sheet. For example: A design sheet may have rates of 69lbs. per inch, to 160lbs. per inch, to 220lbs. per inch. When the actual rate is 170lbs. per inch to 220lbs. per inch. As you can see, getting the correct information is important in making a true comparison.
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