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How To Pick The Right Spring Rate - Rated A For FunWith The Help Of Chassisworks, We Show You How To Pick the Right Spring Rate For The Perfect Balance Of Ride And Handling From the February, 2010 issue of Chevy High Performance By Stephen Kim Photography by Courtesy of the Manufacturers
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Spring terminology can certainly... Spring terminology can certainly get confusing. A spring's free length, as pictured here, is its length when detached from a car with no load placed on it at all. Its solid height is its length when compressed all the way, and a spring's travel is the difference between its free length and solid height. "Everything" may be a rather vague term, but when trying to figure out what factors determine the ideal spring rate for a car, it's pretty darn accurate nonetheless. To put it succinctly, anything that adds or removes weight from a car affects the optimal stiffness of a spring. Aluminum flywheels, A/C systems, fiberglass hoods, power steering boxes, aluminum heads, and a driver's midsection girth all swing the weight total in different directions, but each and every component that has mass plays a role in spring rate selection. With the countless variables that can alter a vehicle's weight, it's not surprising that GM had to use more than 100 different rear spring rates on the first-gen Camaro and its Nova platform-mate at the factory. The OE suspension designers back in Detroit-equipped with mechanical engineering degrees, chassis schematics, CAD/CAM software, and really big brains-can precisely calculate the perfect spring rate by feeding data such as control arm lengths and suspension pickup points into exotic math formulas. Those of us that can't are stuck with test-fitting boatloads of springs onto our project cars and hoping for the best, or settling for a setup that compromises ride quality, handling, or both. Obviously, neither option is particularly appealing. So when Chris Alston of Chassisworks informed us that he's devised a method of selecting the ideal spring that's both 100 percent accurate and easy to execute, we were all ears. "People call us up all the time and want us to recommend the perfect springs for their car. If I were that brilliant, I'd be in the business of buying lottery tickets, not building race cars," Chris quips. "We can get you in the ballpark by comparing your car's setup to other cars we've supplied springs for in the past. However, no two cars are the exact same, so you can't get it perfect that way. For people who have already made the plunge for coilovers, our method involves starting out with a baseline spring to get you in the ballpark, making a few simple measurements once they're installed, then plugging those numbers into a simple formula to determine the ideal spring rate for your car. That still requires buying a second set of springs, but if you think it's worth the money we'll sell them at a discount. That's still better than trying out dozens of different springs to no avail." It's All About Support
Unless the role of a spring in relation to overall suspension dynamics is made clear, the procedure required to select the right rate-and why it's necessary-won't make any sense at all. Consequently, at the risk of being redundant, we'll briefly recap some info covered in previous installments of CHP's ongoing suspension tech series. According to the vast majority of chassis tuners we've consulted, a spring's job is to simply support the weight of the car. Although a higher spring rate reduces body roll, it comes at the expense of a harsher ride. "Not only will a softer spring improve ride quality, it will also enable the tires to follow uneven road surfaces more precisely for improved grip. A tire that momentarily loses contact with the road due to having too stiff of a spring won't produce any grip at all," Chris explains. "Upgrading to bigger sway bars is a far more effective method of limiting body roll and managing weight transfer. Once the spring rate is dialed in, the bulk of track tuning is accomplished by adjusting the shocks and trying out different sway bars. If you prescribe to this school of thought, where the purpose of the springs is to merely support the weight of the car, then picking out the right rate isn't hard."  If you tried to determine...  If you tried to determine the ideal spring rate for your car using the same formulas and data they use in Detroit, it would take you a month to figure out the math.
-Chris Alston  To account for varying loads...  To account for varying loads and road conditions, Chassisworks says that a street car must have between 5 to 7 inches of wheel travel to maintain smooth ride quality. Since the springs and shocks mount farther inward and travels in a shorter arc than the wheels, the typical shock used on an A-arm-style suspension has just 3 to 4 inches of total travel. That leaves very little room for error when attempting to set the suspension to operate in a shock's ideal operating range.  When pairing springs and shocks,...  When pairing springs and shocks, it's extremely important to make sure that the spring has at least as much travel as the shocks. Otherwise shock travel will be reduced. The consequences are that the spring will coil-bind and become overstressed, and shock performance will be severely compromised. Any time a car is lowered with lowering springs, they should be matched with shorter shocks. Unlike the front suspension,... Unlike the front suspension, the shocks on a solid-axle rearend are usually mounted directly to the housing. This virtually eliminates the effects of the motion ratios imparted on an A-arm front suspension, and yields wheel travel that's very similar to shock travel. Consequently, most rear shocks have between 5 and 6 inches of travel.
If the role of the springs is so simple, then why the big stink about picking the perfect set for your car? It all comes down to making sure that the shocks can operate in their ideal range of motion, and the importance of the shocks in relation to overall handling performance. "In my opinion, the only thing more important than the shocks in regards to handling is the tires. By dampening the motion of the springs, shocks control the rate of weight transfer in a corner," Chris explains. "Optimizing the rate of weight transfer is the essence of good handling, and the shocks control how the entire suspension operates. That said, shocks have a very limited range of motion, and their performance degrades dramatically as they reach the extremes of their travel. It's up to the springs to make sure the shocks operate in their sweet spot, which is why proper spring selection is so important. If your car sits too high or too low due to having the wrong springs, it will take the shocks out of their sweet spot and they'll never perform well. People often think there's something wrong with their shocks, but 80 percent of the time perceived shock problems are a result of not having the right springs on their car." Shock Travel
A typical A-arm-style front suspension has between 5 to 7 inches of wheel travel. However, the wheels are mounted farther outward than the springs and shocks, and travel in a longer arc. That means that the 5 to 7 inches of wheel travel necessary for a smooth ride and secure handling equates to just 3 to 4 inches of total shock travel. When attempting to determine the correct spring rate for a car, the trick is to set the ride height so the chassis rests in the shocks' sweet spot within this small 3- to 4-inch window. A solid-axle rear suspension, on the other hand, is a bit more forgiving. Since the shocks are usually bolted directly to the rearend housing, the motion ratio of the wheels is more proportional to the total travel of the shocks. As a result, the shocks mounted to a solid-axle rear suspension typically have 5 to 6 inches of travel. How much of a shock's range of travel is allocated to compression and rebound depends on a car's intended use. Street cars require more compression than rebound, while drag cars are the exact opposite. Cars built for the road course or autocross, on the other hand, can benefit from an even split of compression and rebound. "A street car needs about 60 percent of its travel in reserve for compression, and the other 40 percent for rebound (60/40). The bias toward compression improves ride quality and has a built-in safety guard for unexpected road hazards," Chris explains. "Street/strip cars need roughly 40 percent of their shock travel for compression and 60 percent for rebound, as the extra extension assists in front-to-rear weight transfer. Since road course and autocross cars usually run on smooth surfaces which require less compression, they can benefit from a 50/50 split. However, variations in suspension geometry or track conditions may necessitate altering the travel percentages to prevent the shocks from bottoming- or topping-out." Whether you bias the shock travel toward compression, rebound, or keep it neutral, the first step in accomplishing this is determining the total travel of your shocks. Chassisworks publishes the shock travel specs of all its shocks and struts, as do many other manufacturers. It just takes some research. Next, figuring out how much the springs and shocks should be compressed at ride height to get the shocks in their sweet spot is merely an exercise in simple math. A street-oriented setup (60/40) requires that the shocks and springs collapse 40 percent from their free length at ride height. Consequently, a shock with 4 inches of travel should compress 40 percent, or 1.6 inches, at ride height. That results in 40 percent of travel reserved for rebound and 60 percent, or 2.4 inches, reserved for compression.  In order for Chassisworks'...  In order for Chassisworks' procedure to work properly, a car must be 100 percent complete with all interior components, glass, and fluids in place, in addition to ballast to simulate the weight of the driver. "A lot of times you don't even know what specific parts are going to be on your car until you're done building it, so fine-tuning the spring package should be the last thing you do," Chris explains.  With the coilover assembly...  With the coilover assembly removed from the car, all preload must be removed from the spring before recording any measurements. To accomplish this, the lower spring seat should be tightened just enough to remove freeplay from the spring. Accurately measuring the free length of the spring involves recording the distance from the upper spring seat slot to the bottom edge of the spring within 1/16-inch. This length may differ slightly from the spring's published free length. For instance, the 9-inch spring pictured here actually measures 815/16 inches. This will be used as the "F" value in the spring rate equation. Chassisworks recommends taking measurements for one front coilover and one rear coilover, as most production cars will have very little variation from right to left.  After reinstalling the coilover...  After reinstalling the coilover assemblies, wheels and tires, the next step is measuring the height of the springs once again with the car resting on level ground. This will be used as the "L" value in the spring rate equation. Anything that may interfere with the movement of the control arms and rearend, such as bump stops, should temporarily be removed. On most cars, there will be very little front-to-rear variation in terms of how much the springs collapse at ride height. To establish the numerator... To establish the numerator in Chassisworks' spring rate formula, the difference between the spring's free length (F) and loaded length (L) must be multiplied by the baseline spring rate (R). Most manufacturers print the rate of a spring right onto the coils.
With all the foreplay out of the way, it's time to finally get into some number crunching. Calculating the optimal spring rate for any car can be expressed in the following equation, where "F" is the free length of the spring, "L" is the loaded length of the spring, "R" is the baseline spring rate, and "T" is how much the shock collapses at ride height.
| (F - L) R |
= |
Spring Rate |
| T |
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|
For the sake of illustration, let's presume that a 500 lb/in spring (R) measuring 8.94 inches (F) compresses down to 6.50 inches (L) at ride height. The difference between the two spring height figures is 2.44 inches, which yields a product of 1,220 when multiplied by the spring rate of 500 lb/in. When matched with a shock featuring 4.25 inches of travel, setting it up for a 50/50 cornering application requires the shock to collapse 2.13 inches (T) at ride height. Finally, dividing 1,220 by 2.13 results in a quotient of 572.77, which is the ideal spring rate for this application. The ideal amount a shock should... The ideal amount a shock should collapse at ride height is contingent upon the total travel of the shock used and its intended application. For example, let's presume you're running shocks with 4 inches of travel in a street/strip setup (40/60) where more rebound is desirable. In this case, biasing 60 percent of the total shock travel involves collapsing the shock 2.4 inches at ride height. This results in 60 percent of travel (2.4 inches) reserved for rebound and 40 percent (1.6 inches) of travel reserved for compression. Consequently, 2.4 should be used as the "T" value in the spring rate formula. In this example, the baseline spring rate of 500 lb/in was too soft, which resulted in a shock that collapsed more than the desired amount at ride height (2.44 inches). Not only would this spring move the shock outside its sweet spot, it would also bias its travel too far toward rebound, not leaving enough travel for optimum compression valving and potentially degrading ride quality. Although it may seem trivial, the extra 73 lb/in as calculated in Chassisworks' formula would effectively decrease the amount the shock collapses at ride height to hit the target sweet spot of 2.13 inches. That yields an equal amount of shock travel in each direction for compression and rebound (50/50), which is perfect for many road course or autocross setups.
| SHOCK TRAVEL USED AT RIDE HEIGHT (COILOVERS) |
| Coilover |
Street |
Handling |
Drag |
Spring |
Street |
Handling |
Drag |
| Shock |
60/40 |
50/50 |
40/60 |
Free |
Max. |
Center |
Min. |
| Travel |
(T) Spring Travel Used At Ride Height |
Length |
Installed Spring Length At Ride Height |
| 2.80 |
1.12 |
1.40 |
1.68 |
7 |
5.88 |
5.60 |
5.32 |
| 3.50 |
1.40 |
1.75 |
2.10 |
7 |
5.60 |
5.25 |
4.90 |
| 4.25 |
1.70 |
2.13 |
2.55 |
9 |
7.30 |
6.88 |
6.45 |
| 5.15 |
2.06 |
2.58 |
3.09 |
12 |
9.94 |
9.43 |
8.91 |
| 6.15 |
2.46 |
3.08 |
3.69 |
12 |
9.54 |
8.93 |
8.31 |
| 7.15 |
2.86 |
3.58 |
4.29 |
14 |
11.14 |
10.43 |
9.71 |
| Use above values for "T" in calculation table. |
Compressed spring length will be close to above values when using the correct baseline spring rate. |
 Many hot rodders use adjustable...  Many hot rodders use adjustable coilovers as a means of changing ride height, but Chassisworks advises against it. "The threads on coilover shocks are for positioning the springs correctly in relation to the sweet spot of the shock, not for changing ride height. You only have about 10 percent leeway in either direction for ride height changes. For instance, if your shocks have 3 inches of travel, you can only change ride height 0.300-inch in either direction before you move the shocks out of their ideal operating range. Unless you have progressive-rate springs, the spring rate won't change even if you crank in preload on the coilovers. If you do that, the reason the car will ride stiffer is because you've moved the shock travel out of their sweet spot, not because you stiffened up the spring rate," says Chris.  Since ride height can only...  Since ride height can only be adjusted marginally without ruining ride quality, the preferred method is changing the mounting points of the shocks. This rear coilover conversion kit from Chassisworks will install on virtually any solid-axle rear suspension, and makes adjustments easy.  Unlike a solid-axle rear suspension,...  Unlike a solid-axle rear suspension, changing the shock mounting points on an A-arm front suspension is nearly impossible unless it's completely re-engineered. Fortunately, drop spindles can easily lower most cars up to 2 inches. If a car needs to be raised for any reason, Chassisworks offers upper shock eye extensions that screw onto the company's extensive catalog of shocks.
| SHOCK TRAVEL USED AT RIDE HEIGHT (OEM STRUTS) |
| Strut |
Street |
Handling |
Drag |
Spring |
Street |
Handling |
Drag |
| Travel |
60/40 |
50/50 |
40/60 |
Free |
Max. |
Center |
Min. |
| (T) Spring Travel Used At Ride Height |
Length |
Installed Spring Length At Ride Height |
| 6.50 |
2.60 |
3.25 |
3.90 |
12 |
9.40 |
8.75 |
8.10 |
| 7.00 |
2.80 |
3.50 |
4.20 |
12 |
9.20 |
8.50 |
7.80 |
| Use above values for "T" in calculation table. |
Compressed spring length will be close to above values when using the correct baseline spring rate. |
 Without the right tools, assembling...  Without the right tools, assembling and disassembling coilovers can be a treacherous affair. Spring compressors come in several different configurations, but Chassisworks' design is remarkably simple and easy to use. It cradles both the upper and lower spring seats simultaneously, and allows the spring to be compressed with simple handtools.  Turning a spanner wrench to...  Turning a spanner wrench to adjust spring preload isn't easy when hundreds of pounds of force are pushing down on the spring seat. Chassisworks offers thrust bearings that install between the spring and lower seat to allow the spring to twist freely for easy adjustments. They also keep dirt and road debris out of the spring pocket.  Cars that require a soft spring...  Cars that require a soft spring rate will exhibit lots of wheel travel, in which case bump stops become a wise investment. These progressive bump stops from Chassisworks attach directly to the shock piston shaft to prevent abrupt changes in direction before the shock bottoms out. The shorter the shock, the more important they become. Additionally, bump stops can also be attached to the lower control arms and rearend housing.
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