Nobody has to tell you adjustable coilovers are just about the most versatile suspension upgrade you can make. Here, a measly set of dampers and springs can yield dramatically different ride heights and change a car's handling characteristics all at the hand of a few twists and some clicks of a knob. They can also flub up your suspension worse than you'd ever imagined. But we're not here to talk about Civics that can hardly turn or 240s that eat tires for breakfast. There's a bigger problem, and it starts before those coilovers of yours have even been bolted on.
ABOUT THAT PRELOAD
We're talking about preload, which is really just another way of describing the pressure that's been applied to a spring when compressing it and that everyone from coilover manufacturers to forum kings have all wrong.
The myth: Preload affects spring rates and handling characteristics.
Why it's wrong: The short answer is that preload won't make a spring any stiffer, and it won't make it any softer, but all you want to know is why you've been told the opposite.
First, what kind of spring you're talking about matters. According to Eibach Springs' Tony Jackson, just about every aftermarket coilover uses linear-rate springs, including Eibach's own Multi-Pro R2s, which means unlike progressive-rate springs how stiff they are won't change, no matter how far they've been compressed.
Imagine any old coilover spring free-floating around its shock without any preload applied and with a rate of, say, 200 lb/in, and now imagine your 200-pound self standing on top of it, squashing it down a whole inch. (Remember how spring rates work and that a 200-lb/in spring will compress 1 inch for every 200 pounds of pressure.) Now get your butt off of it and move that spring perch up an inch, adding enough preload to simulate that same amount of compression. Hop back on and notice that, this time, that spring won't compress since that 200 pounds of force (preload) is working against you and your 200-pound self. Notice that in both cases spring height remains the same.
Now imagine that coilover's on your Civic and your 200-pound self is the car's corner weight (it'll be more, but let's keep things simple). Plow over a bump and that linear-rate spring of yours will react just like it would without any preload, compressing the same amount, and absorbing that bump just as it normally would.
The exceptions: Go buck wild with introducing preload and it's possible to muck up suspension travel, though. Add another inch of preload to the example above, for instance (400 pounds now), hit that bump again, and you're in for trouble since now the car's corner weighs less than the force of the spring. Give it just the right amount, though, and you'll have a little more grip on the side of the car that's been unloaded. The key is moderation, and is why in most cases a slammed ride height only makes sense to car show judges.
BOTTOMING OUT HAPPENS
Compress a shock far enough and at some point it'll reach the end of its stroke where all sorts of solid parts come into contact with one another. All you know is that something underneath your car just made an expensive-sounding thud.
The myth: Bottoming out should never happen and can damage the shock.
Why it's wrong: The Accord you drive to pick up Pizza Hut should never bottom out. When it comes to race cars, though, the same isn't always true. "The bumpstop's job is to act as a secondary spring to control suspension travel down at the limit and to prevent the spring and shock from becoming a solid unit," Jackson says, "[which can cause] permanent damage to the chassis or suspension, [like] an upper control arm smashing into a shock tower."
That much you already know, but here's why, according to Jackson, reaching full compression and landing on those bumpstops isn't always so bad. "In some forms of racing, the bumpstop is used as a tuning tool to spike up the suspension rate once the car [has been] fully loaded," he says. "Eibach even offers a full range of bumpstops and bump springs for racers looking to get the right balance [out of] of their setup when the car's fully loaded in a corner."
The exceptions: But all of this assumes those bumpstops are soft enough to properly absorb everything in the first place. It also assumes all shocks are created equal. Stick a soft bumpstop onto the wrong shock and, in some cases, its shaft nut can smack into its floating piston, which will never end well. In many cases, shocks that aren't able to withstand the effects of bottoming out have to use a stiff enough bumpstop to prevent that sort of contact.
SETTING THE RIDE HEIGHT RIGHT
Chances are you picked up that set of adjustable coilovers for no other reason than to establish a specific ride height. Many coilovers allow you to do this in two ways: by compressing or uncompressing the spring by way of a pair of jam nuts at the spring perch—which will alter preload—or by threading the shock's base up or down—which will alter shock travel.
The myth: Ride height should be adjusted by the coilover's base and not its spring perch.
Why it's wrong: All this time you've been afraid of addressing ride height by way of compressing those springs in fear of all that preload, and all this time you've been wrong. By now you ought to understand preload won't change your spring rate, and when applied conservatively, it won't affect handling. Every spring should be subjected to at least some preload. It's what keeps them from flopping around when fully extended. Give it too much, though, and you can introduce all sorts of other problems, like reduced suspension travel.
Adjust ride height by way of those shock bases, though, and you could be headed for trouble. Jackson explains why he believes it's never a good idea: "You want to have droop travel and suspension travel; this is what keeps the tires planted on the road and gives you maximum grip," he says. "When you shorten the shock body, you're taking away precious suspension travel."
And it doesn't end there. "When you shorten shock body length, you're also increasing bump travel, which means the bumpstop now can't do its job properly," Jackson says. "This also means that the suspension can potentially bottom out on shock towers [and] wheelwells." According to Jackson, shorten that shock body enough and you'll also prematurely wear things out like ball joints, bushings, even CV joints.
"Eibach sets up [its] shock body lengths the same as [the] factory so we can retain the car's proper suspension travel and geometry," he explains. "Doing so allows us to pick the proper length and rate bumpstop to properly pair with each application."
The exceptions: None of this necessarily means you should stay away from coilovers with adjustable bases, though. Many high-end systems use them, however, most of the time their lengths are preset from the factory or specified in whatever instructions they're sold with. Trouble happens when you go ahead and adjust them beyond what the manufacturer says you should, resulting in a Civic that hardly turns and a 240 that eats tires for breakfast.