Anti-Sway Bars

Do you yearn to turn? Driving a production vehicle illustrates the fact that its design was a compromise between the average driver’s capabilities, the average road surface, and the average turn; do you see a pattern here? Let’s see a show of how many of you will enjoy driving a mediocre vehicle. When we purchase a vehicle it is up to us to improve handling performance to somewhere higher than above average, to match our expectations and abilities. The sway or anti-roll bar is a way to greatly improve how the car feels. Upgrading your vehicle will remove its nasty habits like understeer and/or body roll.

Understeer is a condition where the car seems to “plough” or “push” through the corners. This is done by design. The auto manufacturers determine the skills of the “mediocre” driver and set the vehicle up to understeer on purpose.
This is because the auto manufacturer assumes that you are incompetent at handling your vehicle up to and beyond the limit of traction. If the vehicle understeers and pushes around a corner the rear end will still have traction so the mediocre driver will not get in trouble spinning out. In the world of paranoia concerning lawsuits, this makes some sense.

Race cars and autocross vehicles are set up to be neutral steer or slight oversteer. Neutral steer is self explanatory. Oversteer is where the front end retains traction, but the rear tires loose traction first and the car has a tendency to swing the rear end out in a hard corner. You have better directional control with neutral steer or slight oversteer because the front tires still have traction. The tires are not sliding like an understeer condition. With a neutral or over steering vehicle now the driver is responsible for keeping the car headed through the turn in the road track, or course. It is now easier for the driver to control up to the limits of traction and negotiate the turn as fast as possible under the prevailing conditions, and your driving skill. This is better than accepting a compromised vehicle that an engineer and corporate lawyer decided was good enough for you.

“My car has a sway bar.” That’s true, most modern vehicles have cute little paper clip sway bars that increase spring rate, just not quite enough, and most have no rear stabilizer bars at all. Bigger is better is good up to a point, most factory suspensions are wimped out to meet the lawsuit requirements of corporate management.

Here are some guidelines for achieving improved handling:

  1. Understeer: Install a larger rear stabilizer bar to restore balance
  2. Oversteer: Increase the diameter of the front bar
  3. Match Bars: with spring rates

Selection of a sway bar should be done in conjunction with your springs. If you are planning an upgrade to stiffer springs, then you probably wont need to increase the stabilizer bar diameter since the added spring rate will handle body roll. A larger diameter bar is required if you have soft springs. Some prefer a softer ride, but if you want to limit body roll and opt for soft springs. Here are some helpful suggestions:

  1. Stiff springs small stabilizer bars.
  2. Soft springs large stabilizer bars.

With all generalities there are tradeoffs. The stiff spring / small bar arrangement is generally better for overall handling because it reduces front to back weight transfer. This is because the spring rate is higher and helps resist the tendency for a vehicle to “squat” during acceleration or “dive” in breaking. Both of these conditions will upset the vehicle by changing suspension geometry and can make handling difficult.

The Rock & Roll Motions

When driving a vehicle that is a “leaner” is no fun. You may wonder if the door handles may scrape off negotiating a tight corner. If your vehicle has stiff suspension and properly sized stabilizer bars, it will improve its attitude in a corner. That is because the stabilizer bar adds spring rate as the vehicle tries to lean. On the straight away or when not maneuvering the spring rate increase is nonexistence. To add the additional spring rate needed to stay relatively flat in a corner would mean a stiff, uncomfortable ride.

Possible to just selectively add spring rate? The stabilizer bar is a torsion spring which is attached to the suspension and body through the stabilizer bars bushing and the vehicles moveable suspension parts like the lower a-arm in front or the differential housing (non-independent cars) in the rear. It is affective only when the vehicle tries to roll. You can consider it a spring on demand. The more the vehicle tries to roll the more it resists. When you go over a bump that is across the entire road the stabilizer bar rotates in its bushing with no increase in spring rate. This directional bias makes it perfect for reducing vehicular body roll.

Body roll is also connected to whether a vehicle has undesirable characteristics like oversteer previously discussed. That’s because the more the body rolls the more is upsets the steering geometry (mostly camber) causing the vehicle to do unexpected things when you corner. The rear suspension can use the same components only with a much smaller stabilizer bar diameter since front engine vehicles have so much more mass to affect the front suspension system.

How Much Is Too Much?

You would think you would want to bolt on the biggest, baddest stabilizer bar you could find. Bad idea, the suspension needs to be able to react to the irregular surface of the road or track in a straightaway and in a corner. Installing a much larger sway bar on when you start to corner the spring rate (stiffness) of the suspension will go way up disproportionately to the weight of the vehicle. The suspension system becomes so rigid that it can not properly follow the road surface to keep the maximum contact patch (tire footprint) against the surface. You then loose traction, lower the maximum speed you can take the corner, and it will ride just like a go-kart when you hit a bump in a corner – like it had no suspension at all. Now you can see that properly sizing the stabilizer bar is paramount if you expect to improve the handling, reduce body roll and retain the ride qualities.

This article is the intellectual property of Simmons BOSS CREATIONS. Any reuse of the contents must include the following attribution:

Marcus Simmons, ASE Certified
Phone: (248) 461-6977