Your car's suspension is subjective. It's also a whole lot of sorcery. Combine those two things and you've got something that nobody can agree upon and that hardly anybody understands.
At the heart of all of that confusion is just how firm it all ought to be. Different roads, driving conditions, driving styles, curb weights, and a whole lot of other inconsistencies all mean that what you think is too soft, your mother will think is way too stiff.
Millennials, go find your safe spaces now because going faster doesn't care about your opinion and will almost always hurt your feelings. It'll tell you that things like too much negative camber, stretched tires, and slammed ride heights will always make your car handle like shit. It'll also say that blowing air through a stock intake and then past a four-into-one header will never make sense. And it'll want you to know that, as far as your suspension goes, there is such a thing as too stiff.
Think of how stiff your suspension ought to be and the first thing that comes to mind is its springs. That's good because your springs are at the core of all of this. They're what keep your chassis from bottoming out and control your tires when going over bumps. They're what limit body roll when cornering, squatting when stepping on the gas, and nose-diving when pressing the brakes. They also establish your car's ride height. As it turns out, your car's springs, more than any other suspension bit, determine its basic handling tendencies. It should be obvious then that you arbitrarily slapping on the stiffest ones that you can find can foul up a whole lot of things.
You can't talk about how stiff a spring is without talking about spring rates. In simple terms, a spring's rate is the amount of weight required to compress itself a single inch. It's a universal measurement, it applies to everything from lowering springs to valve springs, and it'll look something like this: 500 lbs/in. The bigger the number, the stiffer the spring.
Linear vs. progressive: To complicate things, know that there are two types of spring rates that you need to know about. Linear-rate springs remain constant, no matter how far they've been compressed or how much load's been placed on them and feature coils spaced evenly apart. Their predictable nature makes them ideal for smooth surfaces, like well-prepared race tracks, where you aren't likely to encounter bumps, dips, or rough terrain. Progressive-rate springs have varying rates that increase alongside load and are made up of unevenly spaced coils. Their dynamic rates make them ideal for the street where rough surfaces are more common. Here, rates can vary between stiff and soft depending on how far the spring's been compressed.
When stiff is too stiff: Whatever springs you've just stuck onto your S13 are going to make it a whole lot lower, which means they've got to be stiffer than what Nissan thought you needed in order to keep your shocks from bottoming out. If they're too stiff, though, ride quality will suffer and, more importantly, you'll sacrifice how well your tires are able to do their job on bumpy or uneven surfaces. Excessively stiff springs can also lead to premature understeer or oversteer. In other words, go too stiff and you've just made your car handle a whole lot worse than before you started.
Striking the balance: There are two disappointing realities you ought to know right now. First, whatever spring rates your friend's got on his Miata are pretty much meaningless to you unless you're both driving the same thing, with the same wheel rates, and with the same suspension frequencies. But you're not. Second, you can't have a soft ride your mother will approve of yet still be stiff enough to do all the things you want it to on the track. Progressive-rate springs are the closest you'll get to accomplishing this, but it's still just another fantasy of yours that'll never come true.
What spring rate's right for you: There's no magical spring rate that the Internet or any magazine article can prescribe to you. As it turns out, settling on the right spring rate for you, your car, and what you plan on doing with it isn't easy. First, you've got to use all sorts of complicated math to figure out what suspension frequency you want to arrive at, which is really just another way of expressing how heavily sprung you want your car to be. You'll need to know your car's wheel rate and sprung weight to get the answer you want, and that's before you've even figured out your suspension's motion ratio. And after all that, you still haven't come up with a potential spring rate.
Which is exactly why you ought to just start with whoever it is that made your springs and, presumably, tested them out on a car exactly like yours. You know that experimenting with something marginally stiffer can reduce suspension travel, improve lateral grip, and make your chassis more responsive, but unless you're spending the majority of your time on the track, you'll only be making things worse going too much stiffer. On the flip-side, if the roads you typically drive on are excessively bumpy, you may do well with springs that are slightly softer or that feature a progressive rate. Just be mindful that low ride heights and soft springs typically don't play well together.
You already know that a spring's rate is expressed by how many pounds of force it'll take to compress it a single inch. But not all spring rates are expressed in inches. It turns out that the rest of the world uses something called the metric system and, chances are, whatever springs you've got are advertised that way. The next time you see something like this: 8kg/mm, and you want to know how it compares to something like this: 500 lbs/in., know that 1kg/mm is equal to about 56 lbs/in. In other words: kg/mm x 56 = lbs/in. Or, lbs/in ÷ 56 = kg/mm.
ANTIROLL BAR RATES
Antiroll bars (or sway bars) do exactly what they sound like they do: control body roll. They also affect handling balance and, when applied the right way, minimize camber changes during suspension travel, which means your tires just became a whole lot more effective and your springs are now able to do what they should.
Corner hard and physics says that some of your car's weight'll be thrown in the opposite direction, causing a rolling effect between the chassis and the suspension. It's the anti-roll bar's job to counteract some of this. Anti-roll bars do all of this by bolting directly to the chassis through a series of bushings and to the suspension by means of a couple of links. Take that corner and the anti-roll bar acts as a giant spring, twisting and, ultimately, resisting body roll better than your springs are able to. The stiffer the bar, the better it'll be able to do all of this. It doesn't happen immediately, though, and that's because of its bushings. And the degree to which it does all of this is determined by the bar's diameter and, geometrically, how it links up to the rest of the suspension.
There are four things that'll determine an antiroll bar's rate, or how stiff it'll be: its diameter, its length, its arms' lengths, and the metal's strength. Need to impress some friends? Let 'em know that an antiroll bar's stiffness increases at a rate four times the bar's diameter, so double the bar's size and you've just made it eight times stiffer.
The consequences of stiff: A lot of times, what you thought swapping in stiffer springs would accomplish the right anti-sway bar could've done better. Get it right and you'll experience better traction when turning into and out of corners, but go too stiff and you'll defeat your independent suspension almost entirely. Bumps, dips, or ruts where one wheel experiences more travel than another will lead to a worse tire patch and less overall stability than had you no bars at all. Like with spring rates, experiment with what the aftermarket's already figured out and, if you've got an adjustable bar, start with its softest setting, and be sure to not introduce any pre-load when bolting it in place.
It's your springs that determine how far your suspension will travel and where weight'll be transferred to, but it's the shocks that'll decide how fast all of that happens. The stiffer the shock (or strut)—or its damping rate—the slower the spring will oscillate, or move up and down; soften it up and the opposite will happen. Shocks are your suspension's most complex component, and they can be broken down in three ways:
Under-damped: Shocks that are too soft will allow your springs to oscillate for several cycles before stopping, resulting in your chassis bouncing around and your tires losing with the pavement long after you've passed over that bump. It'll also make whatever it is you're driving look pretty ridiculous from behind.
Over-damped: Shocks that are too stiff won't even let your springs compress all the way and will cause your tires to skip or bounce across the road. You'll look equally ridiculous here only with a whole lot less traction.
Critically damped: Here, the shocks are able to complete a single spring oscillation before stopping, resulting in the sort of traction that you want.
Generally, you want to target the range in between over-damped and critically damped, which'll provide the best response so long as you've got a smooth surface to drive on. If you've got fancy coilovers with some sort of adjustable damping knobs on them, now's the time to use them. As with anti-roll bar rates, start soft and work your way up.
Your car's filled with all sorts of bushings. Right now the only ones we care about are those that mount any suspension component directly to the chassis. Here, as far as performance goes, stiffer's better. But you've got to do realistic things like drive over potholes and take your grandmother to the podiatrist, which means stiffer won't always be best.
But stiffer's almost always best: In terms of anti-roll bar bushings, for example, stiffer means a more immediate response from the bar when cornering. Go too stiff, though, and you'll experience a harsh ride and with a whole lot of noise. Polyurethane is the best compromise between the rubber bushings Honda said you need and the aluminum ones that you think you want. Stiffer bushings elsewhere also help resist alignment variations while driving hard since they're less compliant.
The wonkier and more flexible your chassis is, the more it'll act like one big, fat, uncontrollable spring. Contrary to everything we've talked about so far, you can never stiffen your chassis up too much.
Bars and braces: You won't be stitch-welding the seams along your Celica's unibody in an effort to increase its rigidity, but you just might throw a strut bar onto it. These sort of braces, as well as under-body tie-bars, roll bars, and roll cages all increase chassis rigidity, which means your springs, shocks, and tires just became a whole lot more effective.
FIRMING THINGS UP
- Stiffer springs limit suspension travel (good for lower ride heights)
- Stiffer springs and shocks increase tire temperatures for better traction
- Stiffer springs and shocks increase driver sensitivity
- Stiffer springs, shocks, and bushings improve handling response
- Stiffer anti-sway bars reduce body roll
- Stiffer shocks and anti-sway bars improve the tire patch
- Stiffer, polyurethane bushings last longer than rubber
- Stiffer braces and bars increase chassis longevity
- Stiffer braces and bars allow the suspension to work more optimally
- Stiffer springs can decrease ride quality
- Stiffer springs can decrease handling performance and grip on bumpy or uneven surfaces
- Stiffer anti-sway bars can decrease the inside tire's patch
- Stiffer anti-sway bars can reduce grip on turn-in and exit
- Stiffer bushings can yield a harsh and noisy ride
SETTING IT ALL UP
You know you want to improve your suspension. You even know what needs to be done. But you aren't sure where to start. As it turns out, there's an order that you ought to follow, which'll give you the best results.
Step 1: Select a spring rate and whatever dampers you plan on using.
Step 2: Establish your ride height.
Step 3: Align it, test it, and go back to Steps 1 or 2 if the tires aren't doing their job.
Step 4: Select the right anti-sway bars based upon what happened during Step 3.
Step 5: Align it, test it, and go back to Step 4 if you made things worse.
Step 6: Fine-tune the dampers (if adjustable).
Step 7: Align it, test it, and go back to Step 6 if you made things worse.
KNOW THIS BEFORE YOU LOWER
You're smart enough to know that there's more than one kind of coilover. Higher-end versions feature adjustable damping, and ride-height changes can be made by adjusting the shock body's length independent of its spring. Here, all you've got to do is preload the spring to keep it from moving around and adjust the shock's length as needed. Not all coilovers work this way, though. Less-expensive versions often rely on compressing the spring to make ride-height changes. Since, most of the time, a linear spring's used here, its rate won't change, but squashing it down can reduce suspension travel more than you'd like, bringing you one step closer to bottoming out.