Coil Springs

Lowering e21's has become a fad within the tuner community...this is not without unanticipated effects...while lowering the mass will certainly have the aftereffect of less body roll, lowering a mcpherson strut can actually have the opposite affect. this occurs because the geometry of the strut when lowered will also lower the 'Roll Center'. think of it as a lever. if the pivot point (roll center) of the lever was about the top of the trans tunnel, the the lever to the roof is short, but if you lower the car and by doing so lower the roll center to the road surface the the lever becomes quite a bit longer and has a greater mechanical advantage. the added mechanical advantage will cause the car to roll more not less. this is an exaggeration of course to make the point.

Cutting off coils can only lower the car. Progressive coil - cutting some off will have less effect on height but more on stiffening. Non progressive - cutting some off will have more effect on height and less or none on stiffening.

Road & Track said that the E21’s ultimate cornering speed was limited by its predilection for lifting the inside rear wheel, and that aggressive drivers would be well served by specifying the optional limited slip differential. The press was not alone in experiencing this characteristic, which was blamed for the car’s tendency to suddenly oversteer and quickly became a known quirk of the 1977 320i. BMW’s chassis engineers responded by stiffening the front springs on all E21s for 1978, while U.S.-market cars also received a smaller front anti-roll bar and the total deletion of the rear anti-roll bar. The fix seemed to work, as Road & Track’s re-test of the 320i in 1978 found it to be “a very balanced and forgiving car to drive.”

 Coil Springs

Springs are wound with the same wire diameter wire from start to finish. Spring rate is defined as follows:

k= (Gd^4)/(8nD^3)

G=modulus of rigidity of spring material
d=spring wire diameter
n=number of active coils
D=mean coil diameter

As you can see, as you reduce the number of active coils, spring rate will go up. Progressive rate springs act on this principle. A certain number of the total coils are wound closer together. As the spring compresses, all coils will compress equally but the closely spaced coils will bottom out and become inactive. In the case of the e21, the rear springs have about 8 active coils, 4 are wound very close. Once those 4 bottom out, the active coils are cut in half and spring goes from somewhere around 90 to about 180 lbs-in.

Changes in coil diameter also will affect spring rate (such as a bee hive shape), but this does not make a spring progressive as there is no change in rate as a spring compresses. It just means the active coils with a smaller coil diameter will be stiffer and compress less than the larger diameter coils, having the same affect on mean coil diameter through out the entire compression range.

To put it simply: cutting coils in a manner to reduce the number of active coils will both shorten the free length of the spring (which will lower the car) and increase the spring rate. To fully reduce the number of active coils, you have to re-close the end of the spring.

Removing one of the coils on the tight end of a progressive rate spring will reduce free length, and change the total number of active coils (changing both spring rates), and change the point at which the spring changes from one rate to the other.

 

Spring rate: refers to the amount of weight needed to compress a spring an inch (Example: 500 lb. per inch) To understand and properly check a spring for rate you need to know the factors that determine the rate of the spring. Fortunately, there are only three things that affect spring rate. 

 

1. Wire diameter - effects rate, the greater the wire diameter the stronger the spring, the lesser the diameter wire the softer the spring. Therefore, as wire diameter is increased, spring rate increases.

 

2. Mean diameter of spring - Mean spring diameter is the overall diameter of the outside of a spring, then subtract the wire diameter. As the mean diameter of a spring increases, the spring rate decreases.

 

3. Active coils - Count the total coils, then subtract 2 for springs with both ends closed. Count the total coils then subtract one for springs with one end closed and one end open. Bottom Line: as the number of active coils increases, the spring rate decreases.

If a spring's rate is linear (i.e. racing springs have linear rates) then its rate is not affected by the load put onto the spring. For example, a linear rate spring rated at 500 lb. per inch will compress 1" when a 500 lb. weight is placed onto the spring. If another 500 lb. weight is put onto the spring the spring will compress another inch. At this point the load on the spring has increased to 1000 pounds. The rate of the spring, however, remains constant at 500 lb. per inch.

If the load put onto a spring increases the rate of the spring, the spring is said to have a progressive rate. Progressive rate springs are sometimes used on torque arms to absorb engine torque. Keep in mind that the load (or preload) put onto a progressive rate spring can greatly increase the rate of the spring.

Progressive rate springs are made by varying the spacing between the springs' active coils. During compression the close coils bottom out and deaden. This reduces the amount of active coils and spring rate increases as a result.

Springs that are designed to include coils of different diameter or are wound using a tapered wire will also produce a progressive rate.