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Good Handling Is Fun: The Truth about Coil-overs

Topic #6 from the "Good Handling Is Fun" series of MM newsletters.

Welcome to the sixth installment of our weekly Good Handling Is Fun series. This week's topic is all about the power of coil-overs. (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 springs (Put a Spring in Your Step!), you'll be prepared for our expose on the good, the bad, and the ugly about coil-overs. We love them, and...there are some insider tricks for using coil-overs right to get the biggest bang for your buck. Read on for more expert advice on transforming your Mustang into a high-performance handling machine!

What's a Coil-over?
"Coil-over" simply means the coil spring is located on the damper (strut or shock), encircling the damper shaft and housing. Converting to coil-overs is one of those rare win-win situations: you can improve performance without paying the usual penalty of worsened ride quality, which happens when higher rate springs get installed in the stock location on the front control arms. With coil-overs, the wheel rate can go up without increasing friction in the suspension, which as we learned in last week's topic, "Put a Spring in Your Step!", is not fun. (See Wheel Rates & Motion Ratios below for more info.)

Coil-overs for Fox and SN95 Mustangs
These Mustangs have front struts like hundreds of other car models, yet are unique because the spring is located on the control arm rather than the typical location on the strut. In the Fox and SN95 Mustang world, "coil-over" always refers to aftermarket parts that convert the suspension to a coil-over design, which invariably include provisions to adjust ride height, usually via an adjustable lower spring perch. Coil-overs can be installed on the front and/or rear of these cars.

Coil-overs for S197 and S550 Mustangs
Ford outfitted these models with front coil-overs at the factory, but without providing any means to adjust ride height.

So I should convert my Fox/SN95 Mustang to front coil-overs?
Yes! First of all, as we mentioned, coil-overs can get you a higher wheel rate without ruining ride quality. They also make it really easy to adjust ride height and corner weights. Coil-overs are usually available in 25 lb/in rate increments, which makes it simple to get exactly the right spring rate for your application, and in 2" length increments so you can find the correct spring length for your Mustang's weight. Plus, if you ever swap out your springs for a new rate, you can sell coil-over springs outside the Mustang world because they're not vehicle specific. Bonus.

What about converting to rear coil-overs?
Rear coil-overs offer the same benefits as front coil-overs, but to a lesser degree because the motion ratio of a stock location rear spring is less. It's important to note here that you can't use standard coil-overs in the rear of a Fox or SN95 Mustang when you need a low wheel rate, since there isn't enough room for the size of the required spring without making significant modifications to the unibody. In other words, it becomes a big, expensive deal involving a plasma cutter and a welder.

Important factors when choosing your new coil-overs
  • Coil-over springs are available in several standard inside diameters. While the most widely used is 2.5", Maximum Motorsports' rear coil-over kits for Fox and SN95 Mustangs use 2.25" springs. The smaller diameter increases clearance between the spring and the chassis, reducing the possibility of interference.
  • Coil-over springs are available in several standard free lengths. The free length typically used on Mustangs ranges from 6" to 14".
  • Coil-over springs are available in many standard spring rates. A range of 150 lb/in to 750 lb/in covers the Mustang front, rear solid axle, and Cobra IRS.
  • When selecting a coil-over spring, choose the rate first, then the free length.
  • For any given Mustang installation (front or rear), there are usually only one or two suitable free lengths that will work well, so that simplifies your choice.

Don't let coil bind happen to you! Here's how.
Coil bind is when the adjacent coils of the spring stack up, touching each other. That turns the spring into a solid steel block, causing disastrous consequences for handling and component life. The spring length you choose has to provide enough travel to prevent coil bind at any point in suspension travel-even when the spring compresses so much the suspension crushes the bumpstops. Softer springs (i.e., a lower spring rate) compress more for any given load than do stiffer springs. Therefore, the lower the spring rate the longer the spring must be to avoid coil bind.

Another factor to consider is that all springs bow when compressed. We want to keep this effect to a minimum, though, because when a spring bows it can rub on other suspension and chassis bits. The longer the spring, the more it bows. To avoid excessive bowing, always use a high-quality spring, never use a spring longer than necessary, and use a barrel-shaped spring if clearance allows.

And now, for something completely technical: wheel rates & motion ratios
OK, so we admit we're car nerds. So are you, if you're reading this! We appreciate that, because implementing these concepts can yield truly amazing Mustangs. Here goes: the "motion ratio" is a bit of math we use to calculate the wheel rate for any given spring rate, which lets us easily compare a coil-over spring rate to a conventional spring in the stock location. There is a motion ratio because each spring, whether stock location or coil-over, doesn't act directly on the all-important tire contact patch. The "wheel rate" is the spring rate as measured at the wheel, i.e., at the tire contact patch. It's what affects handling by controlling body roll, dive, and squat. This is a little complicated, so let's look at some practical examples.

Fox and SN95 front
  • For a stock Fox or SN95 Mustang front suspension, the front spring is located halfway between the ball joint and the inner control arm pivots, yielding a motion ratio of 2:1. We'll square that to find the conversion factor to the wheel rate, so in this example, the wheel rate is 25% of the rate of the stock location spring. That means an 800 lb/in stock location front Fox spring provides a wheel rate of 200 lb/in.
  • For a coil-over suspension, the wheel rate is approximately 85% of the coil-over spring's rate. It's not 100% because the strut isn't vertical; the angle from vertical causes a reduction in the spring's effectiveness at the wheel. That angle is the result of the camber and caster settings and the Steering Axis Inclination (SAI). Thus, a front Fox or SN95 coil-over spring rated at 350 lb/in would result in a wheel rate of about 300 lb/in.
Fox and SN95 rear with solid axle
  • For a stock Fox or SN95 rear suspension, the rear spring is located on the rear lower control arm. Unlike the front spring, which is located midway on the control arm, the rear spring is a bit toward the rearward arm pivot.
  • By measuring the location of the spring relative to the control arm pivots, we find that the rear wheel rate for a Fox or SN95 Mustang with a solid axle turns out to be approximately 50% of the spring rate. That means a 200 lb/in stock location rear spring provides a wheel rate of 100 lb/in.

OK, let's pause for breath while we recap our comparison of front stock location wheel rate (200 lb/in) to front coil-over wheel rate (300 lb/in). The wheel rate is also a measurement of the resistance to body roll when cornering. That 300 lb/in coil-over wheel rate allows much less body roll than does the 200 lb/in wheel rate of the stock location spring. Yet ride quality will be better with the coil-overs! Keep reading to learn why.

Still with us? Awesome!
Hang in there, because when it comes to rear coil-overs, it gets a little more complicated. But remember, MM is here to help. When you buy our parts direct, our experienced Tech Associates will help you figure this stuff out so you can build the Mustang of your dreams.

A matter of suspension
Now, for rear coil-overs, the wheel rate formula is based on the type of rear suspension: 4-link or torque-arm. Why does the formula change with suspension type? Because the motion ratio formula includes the Side View Swing Axle (SVSA) length. The SVSA length of a torque-arm is radically different from the SVSA length of the stock 4-link suspension. Therefore, we need two equations, one for each suspension type.

Note: Unlike the front, the wheel rate of a coil-over spring is higher than the spring rate. This is because the coil-over spring is located behind the centerline of the Mustang's solid axle. Let's look at these example situations.

Stock Fox and SN95 Mustang 4-link with coil-overs
  • For a rear coil-over suspension, the wheel rate is approximately 110% of the coil-over spring rate.
  • This means a 200 lb/in coil-over spring provides a wheel rate of 220 lb/in.
Fox and SN95 Mustang with an MM Torque-arm and coil-overs
  • For a rear coil-over suspension, the wheel rate is approximately 130% of the coil-over spring rate.
  • This means a 200 lb/in coil-over spring provides a wheel rate of 260 lb/in.
SN95 Cobra IRS
  • The wheel rate for the rear of a Cobra Mustang with IRS is approximately 28% of the spring rate, for a spring in the stock location.
  • Thus, a 600 lb/in stock location spring (that's the stock rear spring rate of a 2003-04 Cobra hardtop) has a wheel rate of 168 lb/in.
  • For a coil-over conversion, the wheel rate is approximately 50% of the coil-over spring rate, so a 600 lb/in coil-over spring has a wheel rate of 300 lb/in.

There's one last small complication in setting up the rear suspension with coil-overs: the roll stiffness (resistance to body roll) of the suspension increases at a slightly greater rate than one would expect from wheel rate increase. This happens because the shocks are mounted a little further outboard than the stock spring location. Fortunately, the increase in roll stiffness helps reduce understeer! So, really not much of a complication. In fact, even a benefit.

What's the deal with coil-overs and ride quality?
Plain and simple: from a ride quality standpoint, you can have a much higher wheel rate with a coil-over conversion than a stock location spring. As we mentioned earlier, that's the win-win of coil-overs—you get the handling advantage of a relatively high wheel rate without the significant decline in ride quality associated with stiffer stock location springs.

Let's throw in a few numbers here to play around with. Say we do our math and discover that a stock Fox front spring rate of 440 lb/in gives a wheel rate of 110 lb/in. If you go by the MM-recommended maximum stock location front spring rate of 750-850 lb/in, you get a wheel rate of 188-213 lb/in. Or take 325 lb/in, a favorite coil-over front spring rate for street performance, which gives a wheel rate of 277 lb/in. Yet most people describe the ride quality of those coil-over rates as similar to their 110 lb/in stock springs. So with coil-overs, you get a huge increase in wheel rate with the associated improvement in handling, yet with the ride quality of a stock Mustang spring.

How is that possible? Although a front coil-over spring may increase the wheel rate significantly, often by well over 50%, the ride quality won't get worse, since a stock-location spring on the Mustang's front control arm contributes to ride harshness. Harshness comes from the friction in the control arm bushings and ball joints, which increases as the spring rate increases because the spring is pushing the control arm down, away from the chassis.

Friction prevents the suspension from easily absorbing small bumps. An input load into the suspension (aka hitting a bump) has to be large enough to overcome the friction before the suspension will move. Small bumps mean small input loads, and therefore no suspension movement. The result is the whole car bouncing up and down on the tire sidewalls instead of the suspension moving and absorbing the bump. And there goes your ride quality!

A Mustang front coil-over kit eliminates this friction by applying the spring force directly to the spindle and upper strut mount instead of the control arm. An added benefit here is that coil-over springs are much lighter than conventional Mustang springs. The resulting reduction in unsprung weight allows the suspension to absorb bumps more easily. Rear coil-over conversions provide a similar improvement in ride quality, although it's not as dramatic of a difference as the front kits because the motion ratio is significantly different.

Ready for coil-overs?
We have:

Well, that's about it. Hopefully you understand the complex topic of coil-overs a little better now, along with why they're so beneficial for your Mustang.
Thanks for reading! Next week we'll discuss the mysterious and wonderful Panhard bar, another MM part that will substantially improve your Mustang's fun factor.