Suspension Physics: Sway Bars and Springs

In a recent blog we talked about springs and shocks.  But there is another player in this game – sway bars, also referred to as anti-roll bars.  In this article, we will look at sway bars and try to understand how they work.

Image result for sway bar adjustment porsche 944"
Rear adjustable M030 968 sway bar

In its simplest form, a sway bar is a chunk of metal that is attached at either end to the wheels on the same axle – front sway bar, rear sway bar.  But what does it do and how does it do it?

Metal has several properties, one of which is the resistance to twisting.  If you take a paper straw, hold each end and twist, it will kink, bend and maybe even break.  Try the same thing with a plastic straw and it will take a lot more force to make it fail.  Use a metal straw and, well, good luck kinking and bending it.  That is why we don’t make sway bars out of paper.

Sway bars some in different sizes – the bigger, the stiffer.  Some are adjustable, some are not, but we will get into that later with something called “moment arm.”  First let’s understand how they work by looking at the front sway bar.  The sway bar is attached to the lower control arm at each end with two hangers in bushings at either end of the “straight part” of the bar.  These hangers do not grip the bar; they simply hold it in position relative to the ends to keep everything in place.

As the car is driven straight, The sway bar does nothing.  It rides along without a care in the world.  When you turn, the sway bar dives into action.  As the car leans in the turn, the two ends are twisted against each other – one up and one down, twisting the metal.  Since metal doesn’t like to twist, there is resistance.  The amount of resistance is determined by two factors: the size of the bar (thickness) and the length of the moment arm (coming up in a bit – be patient.)  The thicker the bar and the shorter the moment arm, the more resistance there will be and the harder it will be to twist it.  Of course, the opposite works too.

Image result for roll center 944"So what does all that mean in an overly-simple way?  When you go into a turn, the sway bar twists a bit to help keep the car from rolling over on the suspension (anti-roll bar?). In that way, it helps the springs by adding resistance to both compression and rebound (one on each side), effectively adding spring rate to the springs.  So setting up the sway bar and spring combination to assist in the turns is part science and part art – and the science part can be a bit of black magic.  A little spring rate here, a little sway bar there, and whoosh!  Great handling in the turns.

With that said, also understand that the spring rate that you choose applies only in a straight line – braking and accelerating.   When you turn, you also involve the sway bar.  Some will choose “hard” front springs coupled with a thick sway bar, and then find that the car understeers – a little body roll helps the car transfer weight to the outside and therefore “hook up” better, and too much stiffness in the front end can defeat that weight transfer.

Moment Arm

There is a physics principle that involves this thing called “moment arm” or “moment of inertia.”  This principle involves a lot of complex math that we don’t really need to discuss.  A few simple examples will suffice.

  1. If you have a wheelbarrow full of dirt, grabbing the handles out at the ends allows you to pick it up and move it, as opposed to grabbing the handles near the tub.  It takes less force (strength) to lift the load when you lengthen the distance between the lifting point and the front wheel.
  2. Image result for breaker bar"You are prying a box open with a crow bar.  You grab the bar at the end, away from the point where you are trying pry instead of close to that point.  If you are not having any luck, you get a longer pry bar and grab it at the end to get more leverage.
  3. A nut will not come loose with a wrench.  You go to your 1/4″ drive ratchet that is about four inches long, and that doesn’t work.  Next you go the 3/8″ ratchet that is about seven inches long with no luck, so out comes the 1/2″ drive twenty-four inch breaker bar – and that does the trick.

In each of these cases, we needed to move farther and farther away from the fulcrum (pivot) point to be able to use the lever to exert the force needed to actually do the work.  The distance is between the force and the pivot is the moment arm.

Image result for sway bar adjustment porsche 944"On our adjustable sway bar, we first look at thickness, then adjustability.  The thicker it is, the more force will be needed to apply the needed torque to twist it.  However, increasing or decreasing the stiffness of the bar works opposite of our examples above.  To stiffen the bar, decrease the distance between the pivot (the straight part of the bar) and the connector to the control arm.  To soften the bar, increase the distance.  That means that on an adjustable bar, if the adjustment is on the end of the bar, the bar is set at its softest setting.  By using the longer length, you are using less force to twist the bar.

Some say that they want the stiffest suspension possible, so stiff springs and torsion bars are the start with thick sway bars added, along with stiff shocks.  But again, some body lean is needed to help the tires grip in the corners.

We bought a 1987 924S PCA SP1 race car in South Carolina many years back.  The seller was proud of the “professionally fabricated” front sway bar setup – built by a NASCAR mechanic.  It was almost an inch and a half thick and the entire assembly weighed about thirty pounds.  With 400 pound front springs, the car would NOT turn because there was literally NO body lean and therefore no weight transfer.

Spring, shock and sway combinations are the “dark magic” part of this equation.

To oversimplify this equation, consider this.  The spring allows some movement between the tire/wheel and the rest of the car.  The shock is there to control the motion of the spring by adding some resistance to that movement.  The spring is adjustable by choosing the spring rate – the amount of force that it takes to compress it one inch.  (There are progressive springs that provide a changing spring rate depending on how much you compress it, but suffice to say that here we are talking about constant spring rates.)

In a straight line, the springs and shocks work together in acceleration and braking.  In a turn, the body leans toward the outside of the turn, compressing the spring on the outside and extending the spring on the inside.  The faster you turn, the more the body will lean.  The sway bar is connected to each wheel, and as the car leans it twists the sway bar to effectively add spring rate to the wheel and keep the car from leaning as far.  It also helps the shocks by helping restrict the movement in the spring and help slow body lean.

So for some the hot ticket is to decrease spring rate and add sway bar stiffness, if possible.

Again, Spring, Shock and Sway Bar combinations are a bit of a dark magic.  Understanding the what the three elements do and how they do it then allows you to make better sense of adjustments.  Overly stiff springs and huge sway bars may not be the answer…depends on the driver and driving style; the car; the track; and of course, personal preference.

This is the exit to the east horseshoe at Daytona.  Note the inside front wheel is off the ground, causing the car to understeer on exit – because there was no traction on the right front.  In this case, we reduced both spring rate and sway bar (adjustment) to allow the body to lean without lifting the wheel off the ground.  

Author: Kevin Duffy, LLC, DeLand, FL

After retiring from a career in Law Enforcement, Kevin Duffy turned his attention to one of his passions, Porsche 944's and 924S's. He owns LLC in DeLand, FL, rescuing and restoring forgotten Porsches, bringing them back to a useful life. He is especially interested in the rare-but-beautiful 924S Special Edition. He can be found at Porsche Club events, including track days, tours and shows, as well as other car-focused events around the southeastern United States.

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