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Wound for Glory Springs? What's so important about coil springs? Plenty. Aside from shock absorbers, springs are one of the most important and often the most misunderstood pieces on a racecar. All too often, a racer will use a spring that's identical to what a buddy uses on his racecar -- even if both cars have a different setup (and more important, a different corner weight). There's more too: Springs tend to settle with age. How old are the springs on your car? In addition, far too many drag cars run down the track with springs which have been improperly modified (coils cut by way of a torch, springs which are coil bound, springs which are sagged and so on).
Improper modifications to springs can do more harm than good. According to Super Stock chassis builder Mike Pustelny (MPR, Almont, MI): "In a car with a stock-style front suspension system, it's common for racers to go out and buy a set of race 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 racers 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. 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 drag race 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 definitely last longer than others (although they too won't last forever). In the end, you usually get what you pay for. a
d v e r t i s e m e n t 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 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 effect 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# per inch spring with 250 pounds resting upon it will compress 1 inch). As mentioned above, three factors influence spring rate: Wire Diameter: This effects 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 race 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 race car. According to Jerry Bickel's "Complete Guide To Chassis Performance" book, "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. "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. a
d v e r t i s e m e n t "Fortunately for us, front spring rates are not quite as critical in drag cars as they are for road race or stock cars. If your spring and wheel weights are within the correct range, little or no performance gain should be expected from changing them. 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 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 racers 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. "Here is a handy front spring selection chart for drag race 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."
Coincidentally, the approximate spring rates supplied by Jerry Bickel are very close to those required in many drag race cars without MacPherson strut front suspension systems. Moroso has long offered "trick" front coils for drag cars. These springs are designed with a small wire diameter, and a long length which definitely promotes weight transfer. Moroso points out that many factors can influence front end height of a race car: "Wheel offset is a major consideration. A front wheel offset to the outside will increase leverage of the lower A-arm against the coil spring and the nose of the car will be lower. Disc brake spacers will further effect the height. Adding or removing as little as 50 pounds can also make a big difference in front end height. Take this into consideration when adding a fiberglass hood, aluminum heads, or when putting the battery in the trunk." a
d v e r t i s e m e n t The Moroso catalog lists a number of springs for common drag cars along with the rates of their springs. You'll note that the front end weight is the ultimate deciding factor when selecting springs (this is something you'll need to deal with in all cases -- that's why sharp racers check exact corner weights on their cars during setup). You'll note too that many of these cars have very different front suspension arrangements (modified MacPherson strut, short-long control arm, etc.) with considerable differences in geometry. Because of this, the spring rates can differ dramatically, even when overall front end weights are similar.
As mentioned earlier in this article, the rate of the spring is determined by the diameter of the spring, the number of active coils in the spring and the diameter of the wire used in the spring. In the case of coil over springs, most are built to the same overall diameter. This means the spring manufacturer must decide upon the diameter of the wire along with the number of active coils needed to produced the desired rate. If the designer chooses a smaller than normal diameter of wire (this has a tendency to "soften" the rate), they will have to compensate by using fewer active coils (which has the effect of stiffening the rate) to achieve the desired spring rate. AFCO points out here are two possible reasons for a spring designer to use a smaller than normal wire diameter for a specific rate spring: (1) The ideal diameter wire may not be made and using the next larger wire (which requires more active coils) would produce a spring with insufficient spacing between the coils. This could cause the spring to bind during normal operations. (2) Cost could be the prime consideration and by using a smaller diameter wire and fewer coils (shortening the length of the wire used) material cost is reduced. Unfortunately, this "cost is an object" manufacturing perspective is more common than you might think, and the end result can be a number of different problems which can plague the chassis tuner. According to AFCO, many racers mistakenly believe extra spacing between the spring indicates a better spring. While a spring must have sufficient stroke capacity, it must also have sufficient material to absorb the load placed upon it. If the spring material is not sufficient for the load placed upon it, the material will become over stressed and the spring will take a set. This usually results in a loss of height which in turn, can have a profound effect upon the way your race car works. Worse yet, this is an extremely difficult problem to detect, unless, of course, you pay very close attention to the springs in your race car (which is seldom the case for most drag racers). Given the long stroke requirements for many types of drag race springs, material strength must sometimes be sacrificed to achieve significant stroke. Couple this with the fact that the ideal wire diameter is not always made and you can see why some springs have a real potential to take a set. AFCO claims they have seen some springs lose as much as 15/16" of free height during normal operation. The AFCO solution is to answer the stress problem by design. They've found that, if done correctly, the springs can be pre-set during manufacture. Done correctly, this pre-setting manufacturing process will usually eliminate any potential for added spring set, even when springs are designed with smaller-than-ideal wire. AFCO points out that shot-peening will further enhance the durability of a spring. a
d v e r t i s e m e n t What if a spring does in fact take a "set"? When a spring "sets", it will normally stabilize at the new height. AFCO reminds us that the rate effectively remains the same since no appreciable change have been made to any of the three factors which determine the rate of the spring. Other than creating a need to readjust the chassis (to restore the original set up and ride heights), the spring would provide satisfactory performance. AFCO claims it is not uncommon for springs (even those which are well designed and properly manufactured) to settle up to 1% of their free height. There is a catch though: If the spring is a poorly designed or manufactured example, and it is subject to extreme over-stressing, the height many never stabilize. The spring could continue to change height (both shortening and lengthening) as long as the spring is worked. Because of this, the set up on your car physically changes each time the spring height changes. Obviously, this can lead to some pretty severe chassis tuning headaches. The only easy solution is to carefully monitor ride height in your race car. If it changes regularly, you definitely have a spring "set" problem. Coil bind is another factor which can plague race cars. Coil bind occurs whenever a spring is compressed and one or more of the active coils in the springs contacts another coil. The rate of the spring increases whenever a coil binds since the bound coil (or coils) are no longer active (if you think back to the parameters of spring design, this changes one of the three rate-determining factors). In simple terms if the spring is compressed to solid height (all coils touching) during suspension movement, the suspension will cease to work. AFCO offers a simple inspection process: "If any coils have bound, the finish between them will show contact marks that appear as though they were drawn with a lead pencil. Normally, any spring that is binding should be replaced with a taller spring. Be aware there are racing springs on the market which are built with wire that is heavier than required. These springs will boil bind before others constructed with the proper wire size."
Under very extreme conditions, coil binding can cause a spring to unwind slightly. This can cause the mean diameter of the spring to increase and reduce the rate of the spring. AFCO points out that the potential for coil bind is increased whenever short springs are used. Bottom line? Always match the spring to the job. The final thing you have to consider in regard to coil springs is "bowing". Springs that have lengths greater than 4 times their diameter have a natural tendency to "bow" when loaded. Because of this, tall springs tend to bow more than short springs, and small diameter springs tend to bow more than larger diameter springs. Generally speaking, the more a spring is compressed, the more it will "bow". AFCO reminds us of something which should be obvious: "The rate of a spring will increase if an active coil rubs another part of the race car." AFCO offers the following tips to minimize spring bowing: • Use correctly fitting coil over hardware or install weight jack assemblies so that the spring mounting surfaces are kept as parallel as possible during suspension travel. • Use springs that do not lean excessively (when positioned on a flat surface). This indicates that the ends are ground parallel to each other. This reduces the tendency for a spring to bow. You should check both ends. • If a coil over spring is rubbing the shock, try reversing the spring so the bowed part of the spring is around the shaft where there's more clearance. • Use coil over springs that have straight sides rather than an hour glass shape. This maximizes the clearance between the shock and the spring. • Use springs that are wound straight. Your can roll the spring on a flat surface to check for straightness. As you can see, there's much more to coil springs than first meets the
eye. Select your springs carefully, and inspect them often. You won't be
sorry, and neither will your ET slip.
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