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0344021005.jpgWhen discussing which modifications to start with when working with a blank slate, aftermarket sway bars are often near the top of the list for the simple fact that a nicely tuned suspension will transform how a car drives immensely. Often times the actual purpose of these parts can be lost on the user, or they don't know why an oddly bent piece of metal pipe can cost so much.

Over and understeer

If you’re reading this you likely are aware what oversteer and understeer are, but maybe you don’t know what causes these situations. Oversteer and understeer events are caused directly by the difference in traction between the front and rear axles of a car. The amount of traction the rear has in relation to the front can fluctuate greatly from several different causes, braking, accelerating, weight distribution, tire compound, contact patch and a host of others.

Now let’s talk about a sway bar (a.k.a. anti-sway bar, stabilizer bar, anti-roll bar) and how it works. A sway bars sole purpose is to minimize the difference in the amount a specific wheel (and its partner it shares an axle with) are suspended. A bigger/stiffer sway bar reduces the delta (difference) more than a small sway bar, or no sway bar.

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In practice

Let's use a car simply coasting around a corner at speed without accelerating and without braking. As I'm sure you will all agree, the car will lean towards the outside of the curve, this is caused by centrifugal force. As the car leans there is weight transferred to the outside wheel, where the car is rolling towards the outside of the curve. But if you only pay attention to that you're missing the bigger effect of weight being transferred off of the inside wheel. This might be hard for some to wrap their head around, but stay with me.

When you're driving around this corner, the outside wheel suspension is being compressed by the centrifugal force of the car rolling outward, the sway bar is taking some of this compression energy and transferring it to the inside wheel in the same direction (compressing) and reducing the amount of coil spring weight being pressed down on this inside wheel, in addition to that the centrifugal force is also reducing the amount of weight being placed on this inside wheel. Thus, the outside wheel is getting slightly more weight (traction) and the inside wheel is getting far less than before the car entered the turn.

By installing only a larger rear sway bar, you will reduce the amount of cumulative traction to the rear axle, and induce more oversteer when at the limit. For spirited driving this is generally a good thing. If we need to discuss why oversteer is more fun than understeer, you might need to watch some more Top Gear. If you want the car to both be more fun to drive and hug the road better, a matching pair of sway bars is advised. The car will handle at its best when the suspension is set up to be neutral at the limits, balancing the traction of the front with the traction of the back. This takes a good deal of engineering and/or trial and error to get right and that’s why a set of sway bars cost more than the material used to create them.

A good sway bar (or set of sway bars) tunes the car in question so that the traction differences front-to-rear are minimal, neutrally balanced. Now lets take our imaginary factory fresh car to a skid pad and measure the lateral G-forces it can withstand, and lets say that it’ll reach .75G’s before losing traction at the front wheels and understeering while the rear axle still has a lot of traction to spare. By installing a rear sway bar that is matched to the front, or a pair of sway bars that were engineered together for this car, we drop the rear traction slightly but bring the front traction up even more, and we even up the traction limits front-to-rear. So instead of losing traction to the front at .75G’s we now have more cumulative traction to work with and lose traction to 3 or 4 wheels around the same time at .95G’s.

The beauty of a car that is set up neutrally is that there are so many other things that effect traction differences front-to-rear that you can essentially add or remove traction from an axle with slight throttle inputs or counter steering. a.k.a. driving bliss.

The act of reducing body roll has very little to do with extending the abilities of a car to hold more of a lateral G-force, and a LOT to do with increasing drivers confidence and limiting any inverted pendulum effect. The only real effect the body roll has on a car and it’s cornering abilities is the slight changes in caster and camber you get when the suspension is loaded and unloaded, and how the car behaves as it exits the turn.

So when you buy a set of sway bars, the evening out of traction differences stabilizes the car, the body roll reduction stabilizes the drivers mentality.

Different suspension styles react in vastly different ways as well. For instance, FWD Volkswagens used a trailing/torsion arm rear suspension design for many years which in and of itself acts as a sway bar of sorts. Cars with solid rear axles, weather driven or not, have the detail that the traction patch of the rear tires can remain relatively unchanged as the car leans. Independent rear axles behave differently but will also benefit from a tuned set of sway bars.

Regardless of what you drive, an aftermarket sway bar or two can improve your spirited driving experience, but as always stay safe and happy motoring.

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Written by :
Glen Cordle

Glen is a mechanical designer from Minneapolis Minnesota. An old-school motorhead at heart, he respects anything that’s had passion poured into it. A jack of all trades, master of some.


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