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Make Driving Fun! Let's Put a Spring in Your Step.

Topic #5 from our series.

Read on for more expert advice on transforming your Mustang into a high-performance handling machine. This week's topic is all about springs. (If you missed previous episodes, want to read them again, or send them to a friend, they're posted on our website in the FAQs & Tech Tips section.)

If you read our newsletter last week about how to lower your car without ruining its handling, you'll remember that installing the right springs is critical to enjoying the big benefits of lowering. Springs are kind of a complicated topic, though, so to make it easier to understand, we're adopting a bullet point approach this week. If you have any questions for your 1979-2004 Mustang, you can also check out our popular and very thorough Guide to Choosing Spring Rates & Dampers.

Why are springs so important to make your Mustang awesome?
  • They hold your car off the bumpstops.
  • Springs are the link between the unsprung part of the suspension (the tires, wheels, and some attached bits) and the sprung part of the car (the chassis and everything else attached to it).
  • The springs' compressibility and extension is what allows the chassis to move relative to the unsprung part of the suspension.
  • The spring design sets the chassis' ride height and the spring "rate" affects ride quality and handling.
  • Get used to this fun fact: spring design is always a compromise. Ford designed your Mustang's springs to meet several goals: achieve a certain ride height, level of ride comfort, and degree of handling ability. Unfortunately for us enthusiasts, Ford prioritizes the first two goals, yielding ride height suitable for curb hopping and ride quality with our grandmothers in mind.
  • While changing springs is most often done to lower the car for improved appearance, even that simple task can have unintended and negative consequences; see our newsletter Good Handling Is Fun: How Low Can You Go?
Spring Design Specifics: Nitty Gritty Part 1
  • Mustangs are supported by coil springs, front and rear.
  • Springs are made from spring wire, which consists of very specific steel alloy designed just for this application. Different spring manufacturers have small variations in their own particular wire alloy.
  • Pro note: luckily for us consumers, the top spring manufacturers are in a race to produce the highest-quality product possible; they are not engaged in a price war, which would inevitably drive quality down.
  • Aftermarket spring manufacturers design their springs to provide a different set of tradeoffs than the OEM springs.
  • Since aftermarket springs have to fit into the same physical space as the OEM springs, certain elements are always fixed and unchangeable.
  • The coil diameter and the end design are rarely altered from OEM design.
  • That leaves wire diameter, the length of the wire (before being wound into a coil) and the spacing between coils as parameters that can be changed.
Spring Design Specifics: Nitty Gritty Part 2
  • The combination of wire diameter, wire length (for the active coils), and the modulus of steel elasticity used establishes the spring rate (in other word, how "stiff" the spring is).
  • Spring rate is usually measured in lb/in. As in, it takes a load of X pounds to compress the spring 1 inch. The more pounds required to compress the spring 1 inch, the higher the spring rate and the stiffer the spring.
  • The free length (the length of the uncompressed coil spring) has nothing to do with spring rate.
  • The spring rate coupled with the spring's free length determines the vehicle ride height.
  • Further complicating the design process are variances in car weights and the prevalence of variable rate springs, which reduce compromise in the eternal ride-quality-vs.-performance battle.
  • And then…there's a whole other level of complication caused by the mounting location of the spring on the vehicle. 1979-2004 Mustangs locate the springs on the front and rear control arms. 2005-2014 Mustangs are equipped with front coil-overs (the coil spring is wrapped around the strut), while the rear springs are on top of the rear axle. 2015 and newer Mustangs have front coil-overs and rear springs located on the IRS lower control arms.
Spring Design Specifics: It's All Relative
  • The spring's location affects how stiff or soft the suspension is relative to the spring rate. That's because the spring location determines the motion ratio of the suspension. That's the ratio between how much the spring compresses compared to how much the wheel actually moves.
  • The motion ratio and spring rate are used to calculate the wheel rate. Wheel rate is the effective spring rate as seen at the wheel.
  • At any ratio other than 1:1 the spring will have a greater or lesser effect than the spring rate indicates.
  • The wheel rate (also measured as lb/in) is a measure of what limits body roll and brake dive.
Fox & SN95 Front Springs: Fun Friction Facts
  • Each front spring is located on the front control arm at the midpoint between the inner pivots and the ball joint.
  • That location creates a motion ratio that lessens the spring's effect on the sprung mass (i.e., the chassis), resulting in a wheel rate that's about one-quarter of the spring rate.
  • Locating the spring on the control arm increases friction in the front suspension because the spring forces pass through several suspension pivot points.
  • Friction in the suspension, as you may guess, affects ride quality and performance.
  • Thus, higher spring rates cause more friction, which worsens ride quality. This is why you don't automatically run out and buy the stiffest springs you can find! There's definitely an art to this science.
  • Think of it being two curves (like supply and demand). As spring rate goes up along the X-axis, the handling curve goes up (Y-axis) and is better. But the ride quality curve goes down (Y-axis) as the spring rate goes up. The art is finding the intersection point of the two curves. The handling curve is determined by science, and the ride quality curve is the art. Ride quality degrades from the increased friction caused by a higher spring rate much faster than handling degrades from increased friction.
Swapping Springs
Disregarding changing springs for the sole purpose of lowering the car, which we cover in our newsletter Good Handling Is Fun: How Low Can You Go?, swapping to a spring with a higher rate (stiffer spring) will, in general, improve handlingby allowing:
  • Less body roll when cornering, keeping the tire footprint flatter on the ground for best grip (traction).
  • Less brake dive, reducing upset to both car and driver.
  • Less acceleration squat, making it easier for the driver to tell what the rear tires are doing.
  • Quicker vehicle response to the driver turning the steering wheel, helping the car make directional transitions much more quickly.
Finding Your Ride Quality Sweet Spot
  • Ride quality is a subjective evaluation. Put two people in the same car and one might say the ride feels fine while the other calls it jarring. So, when we discuss ride quality we're talking in generalities that apply to most people.
  • When looking at a spring rate, you have to consider the overall weight of the car. The same front spring will fit in a 1979 Mustang 2.3L and a 2004 Cobra convertible, yet there's a 55 percent weight difference between the two! The difference in vehicle weight means the same front spring will provide vastly different ride quality and handling abilities in each car.
  • As we mentioned, there's a trade-off in ride comfort as the spring rate increases. For example, Ford equipped most 1979-2004 V8 Mustangs with front spring rates of around 440 lb per inch.
  • That rate means the spring compresses 1 inch for every 440 lb of load applied. Each front corner of these V8s weighs in the neighborhood of 850 to 1,000 lb. The motion ratio translates that weight into a spring load of between 1,700 and 2,000 lb, so the spring should actually compress 4-4.5 inches.
  • A higher front spring rate will make the ride feel firmer.
  • Go even higher and firmness gives way to harshness.
  • Increased harshness comes only partly from the spring rate; much of it comes from the increased friction the higher spring rate causes in the front suspension pivots. That increased friction requires a greater input force to overcome and move the suspension. At a lesser force input (a smaller bump), there isn't enough force to overcome the friction, so the suspension doesn't move; the car harshly jolts up over the bump instead of the suspension compressing to absorb it. For those of us who aren't professional race car drivers, and above such paltry considerations, we feel that as poor ride quality.
  • To maintain acceptable ride quality, we consider the upper limit for a street-driven Fox/SN95 spring rate to be a variable rate spring of 650 to 750 lb/in.
  • Note that special-edition Mustangs such as the Bullitt, Mach One, and 2003-04 Cobra were equipped with springs with a rate of about 600 lb/in.
Variable Rate Springs
  • A variable rate spring is a spring whose rate increases as the spring compresses.
  • The purpose is to have a relatively softer rate at and near normal ride height, but have the spring stiffen when it's compressed as the car rolls, dives, or squats.
  • As a variable rate spring is compressed, some adjacent coils begin to touch, one by one, in a controlled manner. That increases the spring rate because it effectively shortens the spring wire.
  • For a given wire diameter, the shorter the wire length the higher the spring rate.
  • The length of spring wire is what affects the rate, not the free length of the spring.
  • Free length is the length of the coil spring under zero load. Two springs could have the same wire diameter and same free length, yet very different wire lengths and, therefore, rates. The difference comes from how the spring was wound to create the spaces between individual coils.
  • The increase in rate as a variable rate spring compresses is especially beneficial during body roll. As the car body rolls from cornering forces, the spring rate goes up, limiting how far the body rolls compared to a linear spring with the same starting rate, all while providing better ride quality most of the time.

Phew! OK, those are the basic what, where, how, and why on springs, a complicated but critical topic. If you can wrap your head around this info, you'll be well on your way to designing your dream Mustang. If not, MM's always here to help.
Thanks for reading! Stay tuned for next week's closely related topic: The Truth about Coil-overs.