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Guideline: Every single suspension modification is subjective. This means that there is no "better" or "best". It ENTIRELY depends on the surfaces you drive on, the conditions you see, or the ride comfort you want. The only real rating that can be given to parts relates to their quality of manufacture. Moton shocks are "better" than Tokicos due to their quality. That doesn't mean that one is "better" than the other for the conditions that YOU drive in, and the goals YOU seek.
Guideline: Suspension is a key part of what makes the stock RX-8 so good compared to other unmodified cars. Our suspension really is that good. It's probably a large part of why you bought an RX-8 to begin with. Please don't screw it up! The comparison with other cars is also stock to stock. We start with a fantastic baseline and don't have to "overcome" inherent penalties, but other cars can be made to have just as "good" handling with modifications. Focus on your goals, not on the labels.
Shocks are a critical component of the suspension feel. It's the shocks that provide the limiting factors to the spring's velocity (not it's position). Just randomly throwing shocks on the car is more likely to hurt your performance than it is to benefit it. Really understanding shocks takes much more than a short blurb here, but understanding them can reap significant benefits. If you change your springs for any reason, make sure that your shocks will have the appropriate damping for the spring rate you will be running! Stiff springs with shocks meant for soft springs aren't going to be able to control the springs, which is what you commonly see with many "lowering springs" where the car seems to bounce and quiver along at every sliver of pavement change. The reverse, soft springs with shocks meant for stiff springs, means that you are placing too much load on your shocks, and it's going to wear out the shock internals really fast, as well as hurting the ability of the springs to manage the load changes of the car. The ride quality also become very harsh, as the springs aren't doing anything to check the speed of the suspension, and everything is fine right up until the shock is flowing it's maximum flow rate, at which point there is an abrupt change in behavior. The MazdaSpeedd Miata's factory shocks are a good example of this, being valved too stiff compared to the springs, and the ride becomes much harsher than a stiffer spring setup that is properly matched.
The primary contributing factor to the quality of shocks is the valving. It's what makes a shock absorber what it is! Low quality shocks typically have poor valving, which you have no real way of solving. Some poor quality shocks won't hold their valving rates, or the adjustments on them are not consistent, or when you buy a pair or a set of 4, you end up with shocks that are valved differently. This can easily cause more handling problems than you are attempting to solve. It takes a shock dyno to match shocks properly, so if a shock dyno doesn't exist, it will be sheer luck if you actually have matched sets. An easy rule of thumb is that if the vendor or manufacturer isn't able to give you shock dynos, they are probably not actually equal. Don't just blindly accept any dyno though. The low-speed damping of the shocks determines the car's balance during transitions (i.e. turn-in), where high-speed shock damping controls ride quality and keeps the tire in contact with the road over bumps. (High vs low speed refers to the speed of the shock itself compressing and decompressing, NOT the vehicle speed) If you are looking to upgrade for better performance handling, low speed damping is the critical part. If you are looking for pure ride comfort, high speed dampingis your focus.
If you want to do some much more in-depth reading about shocks, and their impact on vehicle performance, this Grassroots Motorsports article is good: Grassroots Motorsports.com: Shocking Behavior
This site also has quite a bit of information (Farnorthracing.com: Shocks) including lots of diagrams, examples, charts, etc... though I recommend that you do not take it all as gospel, but only as more information to consider. There are many things in that site that purists will disagree with or take objection to, but that doesn't mean that the rest of the information should be rejected. It's a good baseline to start your self education.
Springs are often misused and heavily abused by people looking to move away from an OEM suspension. A common preference is to just "drop it" or "slam it", and is usually done by just grabbing cheap "lowering springs" that lower the ride height with little concern or attention to the impact that the springs have on the shocks and ride performance. There have even been cases where unfortunately minded individuals have cut their springs, which is all sorts of wrong.
Springs are critical to controlling the motion of the car's weight, directly contributing to body roll, squat, and dive characteristics, and for a given corner, the ability of the suspension to absorb surface changes. The springs work directly with the shock valving rates, and the spring rates should be selected that directly complement the shock rates (remember, shocks then springs).
Also, before you randomly lower the car, know that the suspension geometry was designed for the OEM ride height. Yes, there are benefits to lower, but there are also penalties. What you goals are will depend on how low is too low, and how high is too high.
Coilovers are basically a specific shock and spring combination that is designed to work together as one unit. They typically have the advantage of being able to adjust ride height for each corner. This is often used as an excuse to "lower in the summer and raise in the winter", but you should be aware that this rarely happens. Changing the ride height on a set of coilovers is not a small task, once you have everything set at a different ride height. Most owners will set it and never touch it again. The intended use for this ride height adjustability is so you can set the ride height to exactly where you want the first time. People looking for an actual improvement in handling take this a step further and use the ride height adjustment to corner balance their car, which can lead to significant gains over just blindly setting the height. All coilovers are not created equal however, and usually the differences in performance and quality lies in the shock valving. The same quality rules apply to coilover shocks as do non-coilover shocks.
Adjustability is not a solve-all however, as a complete stock non-adjustable RX-8 will beat the most adjustable Ford Pinto silly if the owner of the Pinto doesn't know what they are doing. And most owners REALLY DON'T know what they are doing when it comes to suspension.
Sway bars are what helps to control the difference in suspension action from side to side. If both front corners are reacting the same (acceleration or braking in a straight line, a bump or dip that hits them equally) then the sway bar has no impact. It's when one corner is compressing or extending greater distances or more quickly than the other that a sway bar impacts the handling. Cornering (when the weight compresses the outside corners and uncompresses the inside corners), bumps or dips that only hit one side (like track curbing) are the key points here. The sway bar then acts to try to keep the corners as similar as possible. Put another way, a stiffer sway bar allows you to run a softer spring to account for bumps and desired weight transfer on braking and acceleration, without losing stiffness on cornering.
In general, a thicker sway bar with shorter arms will act against the difference more greatly than a thinner bar with longer arms. The length of the sway bar arms is largely fixed for a given vehicle, though sway bars like Progress are adjustable vs end-link mounting holes, so you can effectively shorten the arm length by using holes closer to the bar, making the car act stiffer. Tubular bars have a slightly stiffer torsional rigidity than a solid bar of the same thickness. However, due to the weight saves, it will be rare that you will find both types of the same thickness for the same car. Vendors will make the tubular bar larger because the weight reduction allows them to manage it easier. And thickness is by far the biggest king in sway bar stiffness. An increase from 1" diameter to 1.25" diameter is about a 60% increase in stiffness. 1" to 1.5" is an increase of nearly 3 times stiffness.
Don't automatically assume that stiffer is better however, as too stiff of a sway bar can lead to unpleasant vehicle behavior, such as lifting the inside front wheel on accelerating out of a corner (making you horrifically understeer) or lifting the inside rear wheel on braking into the corner (leading to snap oversteer) or on acceleration (more oversteer plus making the LSD work overtime trying to put the power down). Many raced RX-8s end up removing the rear sway bar completely... assuming they have their shocks, spring rates, and front sway bar all working together to make the rear sway bar unwanted. The key critical point is that the sway bars must be selected along with the springs and shocks to work together. The sways and the springs are what work together to control body roll, and the shocks work to control the rate of change of the springs (and thus the sways as well). Adjusting front and rear roll-stiffness will determine the car's steady-state balance (understeer / oversteer / neutral). Which swaybar is appropriate for your application will depend on spring rates, ride height, static camber, suspension geometery, grip level, aerodynamics, and desired handling traits.
Most professionally set up suspensions will have stiff sway bars and soft springs, or soft sway bars and stiff springs. Soft on both and the car is just floppy, and stiff on both doesn't let the weight transfer enough, which reduces overall grip significantly
The end links used to attach the sway bar to the lower control arms are largely a matter of preference. Breaking an endlink is certainly possible, however there are no real brands more or less prone to braking than any other. Many people go for adjustable end links. Ideally you want them to be relaxed with the car sitting on level ground (relaxed but still secured on each end, shouldn't be under tension). Adjustable length end links can assist with making this happen. Don't "pre-load" your sway bar with your adjustable end links, as this can lead to unpredictable behavior. If you can do the last tightening of the end link bolts with the car sitting on the ground and the suspension settled from rocking it forward and back a bit, it is certainly best.
For a great read from Eric Meyer, read this post: RX8Club.com: Anyone Broke Rear Sway Bar Endlink Before?
For a compiled list of all options as of January 2013, see this great list put together by HiFlite: RX8Club.com: New Sway Bar Options - Sway Bar Summary
Suspension bushings are something that many people overlook or ignore. The bushings are there specifically to reduce vibration from the road that travels through the A-arms. This means that the bushings are inherently flexible, and can degrade over time. This also means that you can change the compound of the bushings for more or less flex. Less flex will generally improve your feel of the road, which is a good thing for feeling what the car is doing on the track, not so good for feeling the less-than-stellar paving job that Mr. Joe Contractor did to pocket a share of the funding. So it will depend on what you want, and what you are willing to sacrifice.
Some of the compounds possible:
The lubrication you use on your bushings is critical as well, as the lubrication is what keeps the bushings from binding, and binding is what makes them split and break. There are many options, from lithium grease to copper grease etc... Ideally, you want to pick something that will continue greasing for a VERY long time, and should be a 'dry' grease so it doesn't pick up dirt. Dirt + wet grease = grinding compound.
An alignment is the cheapest handling performance mod you can do. At ~$80 for a 4 wheel alignment at most shops in the US, you can make dramatic changes to your car's handling away from the OEM alignment specs.
Getting an alignment done:
The 3 dimensions are:
The OEM brakes are actually very very good. You will need different pads for track use, but otherwise the OEM stuff is some of the best street quality you can find. Most upgrades are done for appearances, or because the car is going to get tracked more. The "best" upgrades are usually to 2-piece rotors, or big brake kits with superior calipers. Lower upgrades are usually just pads, maybe rotors. If you upgrade the rotors, avoid cross drilled if you are going to track the car. Cross drilling, no matter how well done, will make the brake rotors more prone to cracking. The grooved method of slotting is far less compromising on the rotor's integrity. Slots and drills are mainly used to promote cooling of the rotor and cleaning of the pads when contaminates such as water or dust are present.
The most important thing you can do for your brakes however, is to bed the brakes in on first use. There are many subscribed methods, so do some searching for one that appeals to you, or that you can do in your local area without getting arrested (many cops would take exception to the acceleration->hard braking cycle repeated over and over)
There is an excellent brake writeup by justjim for more in-depth reading about our brakes and what options we have, here: RX8Club.com: Brake FAQ
The RX-8's braking system is DOT 3/4 based. DOT 3, like DOT 4 and DOT 5.1, is a polyethylene glycol-based fluid (contrasted with DOT 5, which is silicone-based). Fluids such as DOT 3 are hygroscopic and will absorb water from the atmosphere. This degrades the fluid's performance, and if allowed to accumulate over a period of time, can drastically reduce its boiling point. Water contains oxygen which is released as it is boiled and oxygen is compressible, meaning your brake pedal moves a portion without the pads getting that pressure. The more water contamination you have, the lower the boiling point and the worse your braking will match your pedal.
Wheels are more subjective in nature than most upgrades, as they tend to be selected more for appearance than for performance. However, there are three factors for performance to keep in mind:
Tires can be the single biggest contributing factor to handling ability that you can make. Within the Summer Tire category, you can largely base the general life and grip on the treadwear rating. A lower rating means more grip, lower life. Generally. Some tires, such as the R888s, have significantly more rubber contacting the road in the contact patch than other more road friendly summer tires, so even if they might have close treadwear ratings, the grip levels will be different. The other factors typically don't come into play with dry grip unless you are tracking your car, stuff like heat cycling.
Caution: A common mistake made is to buy tires for the best conditions you will see, rather than the worst. If you daily drive your RX-8, don't go for high dry grip tires and sacrifice rain grip and have to dangerously limp around in the rain. If you can swap off to a different car or avoid such weather completely, this advice goes out the window. All too often however, a rain storm through the sunny south will catch an owner off-guard that has had summer tires on for a while, ignoring their fading rain grip because it doesn't feel any worse in the dry. Rears loose traction suddenly and into the guardrail they go. If we see a thread of a wrecked 8, and the ground looks wet in the pictures, the story is inevitably "suddenly snapped loose and couldn't react in time". And a closer look at the pictures shows nearly bald rear tires. Don't lose your 8 because you ignored the more dangerous conditions you will see.