Due to all of the conversations we have regarding suspension setups, springs, etc. I thought I would consolidate them into one thread. The tC responds very well to handling mods and can be made to handle very well for a relatively small amount of money. I am not getting into the rates and drops for specific setups, as that is in another thread. This is to explain the basics and general topics. I am sure I will forget some things, so feel free to add if it is good, pertinent info. While I have spent time learning this and playing around with it, there are others on here that know much more than I from sheer experience. I would ask that the moderators watch this so it is not filled with too much opinion and remains on topic.
First, lets discuss the main components and what they are. To many this will be common knowledge, but will help explain to those not familiar.
Obviously, the springy part that car sits on. On the tC they are all 4 coil springs. It is sometimes useful to understand that a spring is nothing but a torsion bar (like a sway) that is wound into a coil shape. The behavior of the unit still abides by the same basic principles. Springs are specified in spring rate. This can vary in units, but if a spring is rated at 500lbs, that means that it requires 500 lbs of weight to compress the spring 1 inch. The number of coils, diameter of the spring and the thickness of the wire it is made of all affect rate. A spring with coils that are closer together at one end or with decreasing spring diameter towards one end is a progressive rate spring. A progressive rate spring changes rate as it is compressed. A linear rate spring will keep the same rate (so for a 500lb spring, every 500lbs added will compress it 1 more inch).
The damper. Cars with the shock absorber separate from the spring perches has shocks, a setup (like ours) with springs mounted onto the damper has struts.
Coilovers are technically any spring mounted over the damper.. so technically the tC has them stock. However, most refer to them as adjustable aftermarket coilovers. Adjustable coilovers are great if you need the adjustment.
These help to control body roll by applying some of the force from one side of the suspension towards compressing the other. If the outside spring is compressed in a turn, the sway will transfer some of this to the suspension on the inside wheel. This will compress the inside spring to some degree, helping to keep the car flat in the corner. A stiffer sway will transfer more compression to the inside, thus helping to keep the car even more flat. Keep in mind that you are taking suspension independence away from the vehicle by using larger sways. So while body roll is decreased, the car will be more skittish on bump surfaces and will not ride as smooth over uneven bumps.
Now, for the other terms mentioned in the alignment portion of the setup (handling is highly dependent on alignment).
This is the amount your wheels are turned in or out. Toe out means that they are pointing outward (away from each other), toe in means they are pointing in towards each other. This has the greatest effect on tire wear, but also has a big effect on how the car responds to a corner.
This is the way a wheel leans in or out top to bottom. A negative cambered wheel leans in towards the top, for example.
This is the angle (vs a vertical line) of the vertical line between the upper and lower pivot points in the steering. For example, the strut tower mount and ball joint on a setup like ours. This effects steering effort as well as how well the car tracks in a straight line. Positive caster means the line slopes back towards the top, negative is the opposite.
There are other measurements, but for the purposes of this discussion we will leave it at that for now.
The below terms discuss the geometry of the suspension. These are VERY important when setting up the car, but are much harder to measure.
Center of gravity:
You may recognize this from physics class
This is the point at which an object can be balanced in all directions and is considered the point at which gravity acts upon an object for calculations.
A car does not sit in free space. Its body rotates about some point defined by the angles and geometry of the suspension components. This point is the roll center. This center will be different for front and rear. To be more precise, the instant centers are the roll centers for front and rear, the roll center is the line between the two.
This represents the distance between the CG and the roll center. If you think of the CG of a car trying to move outward in a turn, this momentum is applied to the roll couple via an imaginary lever arm (The roll couple). Think of your hand on a wrench. If the roll couple is longer, the body will roll more.
A cars mass is distributed unevenly. The engine is heavy, parts of the car are not. Between the front and rear of the car somewhere (based upon weight distribution) is the center of rotation. A car with the heaviest portion distributed a long ways from the center of rotation, it is considered to have a high polar moment. This means it will be harder to rotate. A car with low polar moments (like a mid engine car) will be more responsive. This is a general statement, and I will leave it at that
Now, lets apply this to our car and some of the various conversations that pop up.
How much should I lower it?
This is a huge item that comes up all the time on the forums. Some want that "slammed" look, while some care more about performance. Lets explain now why that is a different answer. If the CG is lower, that has to be better, right? Sort of, except that the roll center and the CG do not tend to move the same amount as you lower (I wont get into the math.. you can look that up on various websites if you want to do it). This means that you may lower the CG, but lower the roll center more.. meaning you have increased the roll couple. You can also get to a point where the angles of the suspension components are at non-optimal values, further degrading handling. A few people and groups have worked this out and found that a drop of approximately 1.4" tends to be the sweet spot on the tC. This means that big 2 inch drop you are looking for is most likely handling worse than your stock tC did. While it may feel stiff and good on the street to you, if you really took it out and pushed it compared to a stock tC, you would see a degradation in handling.
What about spring rates?
If you lower your car, your spring rate should increase. This is because you decrease overall travel in general. If the springs are too soft, under hard driving you will be hitting the bump stops in the corners. You can think of the bump stops as very high rate springs. So this is the same as your springs going from too soft to too hard all at once. If your car feels like it is bouncing during a hard corner, this is probably what you are feeling. Many springs tend to be designed with a large drop, but only a relatively small increase in rate to please those looking for that big drop and a smooth ride. The truth is that this is not a good setup at all for handling. Spring rates also affect body roll. A very stiff spring will allow less body roll than a very soft one. However, there are limits to how stiff you want to make the springs if you are driving it every day on the street.
Should I buy coilovers?
The good answer here is why do you need them? If you simply want to lower the car, just by a set of springs you like. Coilvers are significantly more expensive due to their adjust-ability. If you are not going to tune the setup and use the adjust-ablity, then you may be wasting your hard earned money purchasing them.
How about sways and body roll?
Sways work along with springs to control body roll. The best way to control body roll is to run very stiff springs and keep as much independence in the suspension as possible. However, for a street driven car and cars driven on most typical tracks, you only want to go so stiff on the springs. So, a set of sways bars are used (some cars only have a front bar... ours has both). Obviously per the definitions above, stiffer sways will reduce roll more than soft ones. The trade off is independence, which means you need to balance body roll control with the ability to handle rough surfaces.
The other effects of sway bars is control of oversteer and understeer. In general, if you run stiffer sways on the front of the car in relation to the rear, it will have more tendency to understeer or push. If you run stiffer sways in the back in relation to the front, it will have more tendency towards oversteer. Playing around with these two will help you tune the amount of over/under steer you want into the car. Be careful, as you can make the car almost dangerous at an extent. This is why for super stiff bars, like the hotchkis bars, they sell them in sets. Running that much more stiffness in rear without increasing front stiffness will make the car very prone to being backwards in a corner
One of the important settings for handling is toe. I have seen autox'rs toe the car out so much you can see it from 50 ft away. They may play with front,rear or both toe settings. This can make the car rotate very nicely on an autox course, but those same people will dial that back out if they drive it home after, since at highway speed the car can become downright scary set up that way. Toe will also eat up your tires if set too far one way or another. If you daily drive your car, it may be a good idea to set toe out as far as the OEM spec allows but no further. This will help to add some responsiveness, but not make it too twitchy or eat up your tires. If you see the inside edge wearing faster and in a feathering or cupping pattern, your toe is most likely set too far out.
Camber is another big setting for handling. Since the outside tire tends to lean towards its edge in a hard corner, having it set with some negative camber can provide more contact patch on the ground in a turn. Contrary to popular belief, you can get away with a lot of negative camber as far as tire wear is concerned. For example, a lot of us have run -2 degrees with no negative tire life effect.
Another item to consider is corner balancing. Corner balancing is one of the reasons for adjustable coilovers. It involves placing the car on a set of 4 wheel scales and then setting preload on the individual springs in order to achieve the best balance of cross weights. This helps to ensure you have the optimum amount of weight on each corner of the car for the bestgrip. It also helps to ensure the weight transfer in corners will be the same in both directions, so it corners the same way in both right and left hand corners.
Handling in general
With all of this, and with all of our discussions around autox and track days, keep in mid that a car handles differently in lower speed, sharper corners (Autox) than it will at high speed sweeping corners. A car will generally tend to understeer much more in an autox environment than it will on a road course type setup. So while you may want to set it up to be very prone to oversteer on an autox course, you want to be careful if you take it to the track, or even on the street, as it may become very hard to drive.
I wont go too much further into tuning for handling, since there are a million ways to discuss it and do it. I wanted to focus this mainly on the basics so people understand how to make the right decisions. We tend to repeat a lot of this info here and there, so hopefully this can just be referenced in the future.
Lowering your car too much