Tirxer

whats the difference between sway bars and underbraces aka lower arm bars?

is it better to have both or one of the above?

Sputnik

An underbrace is similar to a strut bar, in that it is bolted to points on the bottom of the car to stiffen the chassis. Like everything else in the automotive aftermarket, some cars benefit from these more than others, and some braces are more effective than others. And I've seen some useless braces in this category (in other cars, I haven't seen any offered for the RX8 yet, not that they don't exist). But one thing you need to remember, the stiffer suspension items you add (springs, shocks, lower profile tires, poly bushings, sway bars, etc.), the more effective strut tower bars, roll bars, and other valid bracing items will be.

When you are talking about stiffening a chassis (not the suspension), stiffer is better.

A sway bar is more accurately known as an anti-sway bar, or anti-roll bar, and sometimes simply called a roll bar (not to be confused with a roll bar which protects occupants in the event of a roll-over). Most sports cars will have one on the rear suspension, and one on the front suspension. This is basically a U-shaped rod of steel (sometimes hollow, sometimes solid). The tops of the "U" are connected to the hub carrier on each side, and move up and down with the suspension. The bottom of the "U" connects to the chassis itself, and swivels freely in those connections.

When both sides of the suspension compress (like when you go over a speed bump), both ends of the sway bar simply move up and down with the suspension, and have no effect on compression, ride quality, or performance. When one side of the suspension compresses, and the other side decompresses (like when you are cornering), this causes the anti-sway bar to twist. The stiffer the sway bar, the harder it is to twist, and the more it will keep the body of the car from rolling.

The adjustable versions of sway bars simply have two or more holes on the ends of the sway bars. To adjust the stiffness, you simply mount it in a different hole. Further towards the end is stiffer, closer in is less stiff (it's all about leverage, just like if you use a short or long wrench on a bolt).

Like everything else in a suspension, stiffer is not always better, you have to find the happy medium. Stiffer allows the car to corner flatter, and keeps it from having to shift it's weight as much when changing direction. But you need to have some roll in the car to get the most grip out of the tires. If you put too stiff of a swaybar in a car, the tires won't be able to grip as well, especially with street tires. It might feel better, but your best speed around a corner will be less, and the car will be less capable of keeping optimum grip and recovering from a slide.

You can change a car's tendency for oversteer or understeer by only adjusting the front or rear. Basically, if you have a car that "pushes" or has understeer, you can either soften the front swaybar, or stiffen the rear swaybar. This basically shifts a little more weight from the rear tires to the front tires, decreasing grip in the rear and adding grip in the front at the same time. There is much more to tuning a suspension than that, but that's the basic idea.

---jps

Tirxer

thanks for your input, by the way, how about the rear shock tower brace? will it help body roll?

epitrochoid

good post sputnik!

a rear shock tower brace does the same thing as a front, stiffens the chassis, and has no noticeable effect if any on body roll.

Sputnik

Thanks guys.

A rear strut tower brace will help a little with body roll, and it's effect will be greater the stiffer the suspension is. But like Epitrochoid said, it probably isn't noticeable as far as body roll, especially on the street. It will be noticeable in other ways.

Now, I don't think that a basic rear strut tower brace will be effective in an RX8. under the rear parcel shelf is a very big structural piece that runs from side to side, and connects to the chassis very close to where the top of the struts connect to the chassis. Adding another bar to that area will have little effect, if any.

---jps