Wound up Tight – Coil Springs Part 1

Wayne Scraba

22 Mar 2018

Click Here to Begin Slideshow Aside from shock absorbers, springs are one of the most important and often most misunderstood pieces on a car (race car or otherwise). All too often, you might use a spring that's identical to what your pal uses on their car - even if both cars have a different setup (and more importantly, a different corner weight). More than that, springs tend settle with age. How old are the springs on the car? Far too many cars run down the road (and track) with springs that have been improperly modified - coils cut by way of a torch, springs that are coil bound, springs that are sagged and so on. Improper modifications to springs can do more harm than good. According to chassis builders we polled, in a car with a stock style front suspension system, it's common for people to go out and buy a set of “trick” front coils. They install them in the car, but to their surprise, the ride height is too tall. The "normal" solution is to cut the springs. A month or two later, the car sags. Instead of buying correct springs, most people will simply use a set of black plastic spacer donuts on the spring to restore the ride height. This practice eventually stiffens the spring rate to the point where the car is seriously over-sprung. You've messed up the spring rate by shortening it. Sooner or later (likely sooner), the car will develop strange handling or performance characteristics. The real solution is to use a quality spring that isn't cut, even if you have to have a set of springs custom wound. The same can be said for coil over springs. With a coil over arrangement, it’s easy to install springs with the wrong rate on a car. Coil over springs are selected by way of mathematical formulas based upon the geometry of the car (short-long control arms, MacPherson strut, etc.), the corner weight of the car, the dimensions of the spring and a series of component measurements. Simply stated, there are no generic coil over springs. Each car will require a different spring and spring rate. You'll also find some springs are far more costly than others. Ask yourself "Why?" The better springs are made from materials similar to those used in good valve springs (chrome silicon steel wire). That's why some valve springs last longer than others (although they still won't last forever). In the end, you usually get what you pay for with springs. According to the folks at AFCO Racing Products, there are basically three types of coil springs commonly used today: • Type 1: Closed and ground on both ends (usually a coil over spring or a conventional spring used on the rear of a car). • Type 2: Closed on both ends, but ground on one end only (conventional front springs are usually of this configuration). • Type 3: Closed and ground on one end and open on the other end (similar to a conventional spring that has been cut). AFCO goes on to explain: "The three spring types are used in different situations and provide different effects to rate. Since the designs are so varied, it only follows that the dynamics of each design are also varied. You must remember, however, that the only factors that affect spring rate are wire diameter, mean diameter and the number of active coils. No active coil should touch the seat." Spring rate refers to the amount of weight needed to compress a spring one inch. (For example, a 250 pound per inch spring with 250 pounds resting upon it will compress 1 inch). As mentioned above, three factors influence spring rate: Wire Diameter: This affects rate since a greater wire diameter is stronger than a smaller diameter wire. When the wire diameter is increased, spring rate increases. Mean Spring Diameter: This is the overall outside diameter of the spring, less one wire diameter. When mean diameter increases, the spring rate decreases. Active Coils: Figuring out the number of active coils varies with the design of the spring. Count the total coils minus two for springs with both ends closed (Type 1 & 2 above). For springs with one end closed and one end open (Type 3 above), count the total coils minus one. As the number of active coils increases, the spring rate decreases. So far so good. But how do you figure out which spring rate to use in which car? The first thing you should do is to determine what the exact corner weights of your car will be (left front, right front, left rear, right rear) with the car at final weight, including the driver weight, fuel, oil, etc. Spring rates are determined by the amount the spring deflects versus the weight placed upon it. As an example, a 250 pound spring rate is a spring which will deflect 1 inch for every 250 pounds placed upon it. This sounds simple, but there's more: The actual suspension geometry can influence how much the spring deflects. You have to compensate for the geometry when determining the correct spring rate for your car. According to racing chassis construction expert Jerry Bickel: "When you buy front springs, the listed spring rate (lb./in.) will not be the same as the rate at the wheel. Front control arms are lever systems that alter the motion and applied forces between the coil springs and the wheels. The front wheels gain a mechanical advantage against the coil springs, so the wheel rate is always less than the spring rate.” Bickel goes on to provide an easy-to-use formula for approximating front wheel rate (WR) versus spring rate (SR) – see the following illustrations, courtesy of Jerry Bickel Race Cars. "If the car has MacPherson struts, the top line should be drawn from the top strut mount at 90° to the strut. Actual wheel rate is extremely difficult to calculate with accuracy. This is because the angles of suspension members change continuously though the normal range of travel and the front coil springs are not perpendicular with the pavement. "Remember this rule when selecting spring rates: "The farther the spring is from the tire, the lower the wheel rate will be. The closer the spring is to the tire, the higher the wheel rate will be. "All of our Pro race cars are equipped with MacPherson struts in front, with the springs very close to the tires. This is why the front spring rates we use are low compared to those used on cars with unequal length control arms. "Another problem with unequal length control arm suspensions is the lack of spring height adjustability. Some people have been known to cut a coil or two off their front springs or heat them until the front ride height sags an inch or two. I do not consider this to be a safe and reliable practice. You may have to try several different spring rates and heights to find those that provide the correct chassis ride height. "Rear suspension members may also create leverage against the rear coil springs and affect wheel rate. This depends on the type of rear suspension system and the location of the coil-over-shocks. "Depending on the builder, the rear coil springs may be located in front, on, or behind the rear axle centerline. These are the formulas to determine rear wheel rate for a modified car: "The same formula can be used for ladder bar suspension systems. Simply use the front pivot as the IC reference point. I prefer to mount the coil over shock behind the axle centerline. This keeps it from interfering with the 4-Link bars and lowers the top mounting location." "Here is a handy front spring selection chart for cars equipped with MacPherson struts. Remember that is only a rough guide for spring rates. The correct rate for your car may be somewhat higher or lower than those listed below." Gross Vehicle Front Spring Rate Rear Spring Rate Weight (lbs) (lb./in. approx.) (lb./in. approx.) _____________________________________________________________ 2000 185 85 2350 200 95 2500 215 120-140 3000-3500 250-350 150-200 Coincidentally, the approximate spring rates supplied by Jerry Bickel are very close to those required in many cars without MacPherson strut front suspension systems. That’s a wrap for this segment. Next time around, we’ll dig deeper into the world of coil springs. There’s more here than meets the eye. Watch for it.

Wound up Tight - Coil Springs Part 1 — Click Here to Begin Slideshow Aside from shock absorbers, springs are one of the most important and often most misunderstood pieces on a car (race car or otherwise). All too often, you might use a spring that's identical to what your pal uses on their car - even if both cars have a different setup (and more importantly, a different corner weight). More than that, springs tend settle with age. How old are the springs on the car? Far too many cars run down the road (and track) with springs that have been improperly modified - coils cut by way of a torch, springs that are coil bound, springs that are sagged and so on. Improper modifications to springs can do more harm than good. According to chassis builders we polled, in a car with a stock style front suspension system, it's common for people to go out and buy a set of “trick” front coils. They install them in the car, but to their surprise, the ride height is too tall. The "normal" solution is to cut the springs. A month or two later, the car sags. Instead of buying correct springs, most people will simply use a set of black plastic spacer donuts on the spring to restore the ride height. This practice eventually stiffens the spring rate to the point where the car is seriously over-sprung. You've messed up the spring rate by shortening it. Sooner or later (likely sooner), the car will develop strange handling or performance characteristics. The real solution is to use a quality spring that isn't cut, even if you have to have a set of springs custom wound. The same can be said for coil over springs. With a coil over arrangement, it’s easy to install springs with the wrong rate on a car. Coil over springs are selected by way of mathematical formulas based upon the geometry of the car (short-long control arms, MacPherson strut, etc.), the corner weight of the car, the dimensions of the spring and a series of component measurements. Simply stated, there are no generic coil over springs. Each car will require a different spring and spring rate. You'll also find some springs are far more costly than others. Ask yourself "Why?" The better springs are made from materials similar to those used in good valve springs (chrome silicon steel wire). That's why some valve springs last longer than others (although they still won't last forever). In the end, you usually get what you pay for with springs. According to the folks at AFCO Racing Products, there are basically three types of coil springs commonly used today: • Type 1: Closed and ground on both ends (usually a coil over spring or a conventional spring used on the rear of a car). • Type 2: Closed on both ends, but ground on one end only (conventional front springs are usually of this configuration). • Type 3: Closed and ground on one end and open on the other end (similar to a conventional spring that has been cut). AFCO goes on to explain: "The three spring types are used in different situations and provide different effects to rate. Since the designs are so varied, it only follows that the dynamics of each design are also varied. You must remember, however, that the only factors that affect spring rate are wire diameter, mean diameter and the number of active coils. No active coil should touch the seat." Spring rate refers to the amount of weight needed to compress a spring one inch. (For example, a 250 pound per inch spring with 250 pounds resting upon it will compress 1 inch). As mentioned above, three factors influence spring rate: Wire Diameter: This affects rate since a greater wire diameter is stronger than a smaller diameter wire. When the wire diameter is increased, spring rate increases. Mean Spring Diameter: This is the overall outside diameter of the spring, less one wire diameter. When mean diameter increases, the spring rate decreases. Active Coils: Figuring out the number of active coils varies with the design of the spring. Count the total coils minus two for springs with both ends closed (Type 1 & 2 above). For springs with one end closed and one end open (Type 3 above), count the total coils minus one. As the number of active coils increases, the spring rate decreases. So far so good. But how do you figure out which spring rate to use in which car? The first thing you should do is to determine what the exact corner weights of your car will be (left front, right front, left rear, right rear) with the car at final weight, including the driver weight, fuel, oil, etc. Spring rates are determined by the amount the spring deflects versus the weight placed upon it. As an example, a 250 pound spring rate is a spring which will deflect 1 inch for every 250 pounds placed upon it. This sounds simple, but there's more: The actual suspension geometry can influence how much the spring deflects. You have to compensate for the geometry when determining the correct spring rate for your car. According to racing chassis construction expert Jerry Bickel: "When you buy front springs, the listed spring rate (lb./in.) will not be the same as the rate at the wheel. Front control arms are lever systems that alter the motion and applied forces between the coil springs and the wheels. The front wheels gain a mechanical advantage against the coil springs, so the wheel rate is always less than the spring rate.” Bickel goes on to provide an easy-to-use formula for approximating front wheel rate (WR) versus spring rate (SR) – see the following illustrations, courtesy of Jerry Bickel Race Cars. "If the car has MacPherson struts, the top line should be drawn from the top strut mount at 90° to the strut. Actual wheel rate is extremely difficult to calculate with accuracy. This is because the angles of suspension members change continuously though the normal range of travel and the front coil springs are not perpendicular with the pavement. "Remember this rule when selecting spring rates: "The farther the spring is from the tire, the lower the wheel rate will be. The closer the spring is to the tire, the higher the wheel rate will be. "All of our Pro race cars are equipped with MacPherson struts in front, with the springs very close to the tires. This is why the front spring rates we use are low compared to those used on cars with unequal length control arms. "Another problem with unequal length control arm suspensions is the lack of spring height adjustability. Some people have been known to cut a coil or two off their front springs or heat them until the front ride height sags an inch or two. I do not consider this to be a safe and reliable practice. You may have to try several different spring rates and heights to find those that provide the correct chassis ride height. "Rear suspension members may also create leverage against the rear coil springs and affect wheel rate. This depends on the type of rear suspension system and the location of the coil-over-shocks. "Depending on the builder, the rear coil springs may be located in front, on, or behind the rear axle centerline. These are the formulas to determine rear wheel rate for a modified car: "The same formula can be used for ladder bar suspension systems. Simply use the front pivot as the IC reference point. I prefer to mount the coil over shock behind the axle centerline. This keeps it from interfering with the 4-Link bars and lowers the top mounting location." "Here is a handy front spring selection chart for cars equipped with MacPherson struts. Remember that is only a rough guide for spring rates. The correct rate for your car may be somewhat higher or lower than those listed below." Gross Vehicle Front Spring Rate Rear Spring Rate Weight (lbs) (lb./in. approx.) (lb./in. approx.) _____________________________________________________________ 2000 185 85 2350 200 95 2500 215 120-140 3000-3500 250-350 150-200 Coincidentally, the approximate spring rates supplied by Jerry Bickel are very close to those required in many cars without MacPherson strut front suspension systems. That’s a wrap for this segment. Next time around, we’ll dig deeper into the world of coil springs. There’s more here than meets the eye. Watch for it.

The differences between coil over springs and conventional springs are obvious. What may not be so crystal clear is the difference in spring types. The coil over spring in the center is what AFCO considers to be a "Type 1" spring. it's closed and ground on both ends.

Springs which are closed and ground on one end but have the other end "open" are classified as a "Type 3" spring by the folks at AFCO.

The overall diameter (or "mean diameter") of a spring plays an important role in determining spring rate. When mean spring diameter increases, the rate of the spring decreases.

The diameter of the wire also has an effect on the spring (keep in mind that a spring wire which is larger is stronger). As the wire diameter increases (gets stronger), the spring rate increases.

The number of "active" coils in a spring has an effect upon the spring rate. Unfortunately, this isn't as simple as counting coils. When springs are closed on both ends, count the total coils minus two to determine the active coils. For springs that have one open end and one closed end, count the total coils minus one. As the number of active coils in the spring increases, the rate of the spring decreases.

Image courtesy Jerry Bickel Race Cars.

Image courtesy Jerry Bickel Race Cars.

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