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.
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.