You may be thinking, “It’s about time he got to sway bars!” They’re normally one of the first things you think about when it comes to parts designed to improve handling performance. They certainly are important, but sway bars are tuning aids and you can’t tune what you haven’t built yet. Plan out your whole suspension package first, and then consider which sway bars are most appropriate. That is, unless you relish the idea of leaving a lot of performance on the table or buying several sway bars until you get it right.
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Basics of Function
They’re referred to by many names: sway bars, anti-sway bars, anti-roll bars, stabilizer bars, and so on. Whatever you call them, they play a critical role in your suspension package. Put simply, sway bars contribute roll rate without contributing any vertical spring rate. No other mechanism on a car can do this. What does that mean? It means no other item on the car can contribute as much toward cornering performance with as small a compromise in ride quality.
A sway bar is simply a torsion bar with an arm at each end, typically integral, which is linked to the suspension on each side of the vehicle. Any time only one side of the suspension moves, it twists the torsion bar and contributes rate to that corner of the car. If one wheel moves a bit more than the other, that wheel gets more rate. If the suspension moves the same amount, at the same time on both sides, the torsion bar simply pivots in its mounting bushings and contributes no rate. This is why sway bars have less effect on ride quality than springs, which always contribute to rate. Still, be prudent about your sway bar choices. You’ll still get a spike in wheel rate from a sway bar when a single wheel hits a bump, and that still contributes to some degradation in the ride quality of the vehicle.
As a tuning aid, you also have to be conscientious about selecting bars that help balance the car. Modified muscle cars are a widely varied breed, and it concerns me that some sway bar manufacturers offer only one set of bars for every car of a given model. Whether it has an aluminum small-block or iron big-block, stock or modified geometry, lowered or stock height, stock 14-inch wheels or 19s and R-compound tires, do they all take the same sway bars, and can they be just right on each car? I’d rather you think about what your car’s needs are, and then select your sway bars based on the results rather than on convenient merchandising.
Learn What You Need
How do you know which sway bars your car needs? The bottom line is that you won’t know for certain until you try them. And if you only try one set, you won’t have anything to compare them to. This is one reason I’m a big fan of adjustable-rate bars. It’s like purchasing several bars for the price of one and being able to swap them out in minutes to determine which actually works best on your car, relative to your driving style. Still, you need a starting point, and if you apply some logic, you can be pretty sure to get it right the first time. You generally start with the front bar, and then match the rear one to it. Wait, do you even need a rear bar? Some folks say you don’t. I’ve read that if you need a rear sway bar, you haven’t selected your springs properly. Let’s keep in mind that virtually all road race cars (including Formula 1) have rear sway bars. Do you suppose there’s anyone on the Ferrari or McLaren Grand Prix teams who knows anything about spring rates and sway bars? I bet there is.
Of course, muscle cars are not Formula 1 cars, so let’s reason through it. We are starting off with cars that typically have a lot of understeer. Adding rear roll rate reduces understeer, and sway bars add roll rate. Modifications have probably been made to the front end, so higher-rate springs and larger front sway bars both help the car corner flatter but increase understeer. Modifying the geometry for a higher roll center reduces body roll and makes whatever bar is up front have less to fight against, which can further induce understeer if you don’t properly select the front bar.
In the rear, you’ve probably added some spring rate, and that’s probably all you’ve done. Does it seem unbalanced to do several things up front that add understeer and only one small thing to the rear to reduce it? Sure, the car may corner flat and it may have good mechanical grip, but even a cursory look at this package tells you some additional rear roll rate could really improve the balance of the car. This improves turn-in and responsiveness while giving the car a much lighter, more sports-car feel.
Couldn’t you just add more spring rate in the rear? Sure you could. A reasonably effective rear sway bar may contribute about 150 lbs/in of rate. A typical performance rear spring runs from 135 to 175 lbs/in, so to get the same amount of total roll rate, all else being equal, you’d need to run a spring roughly twice as stiff. You’d also need very firm shocks to match. Now your roll couple is fairly good but your natural spring frequency ratio is all out of whack, front to rear, and you have a car with very rough ride quality. The imbalance in spring frequencies will likely cause the car to pitch fore/aft at certain road speeds. If you increase the rate of the front springs to correct the balance, the whole car then has a very high ride frequency and a very rough and nervous ride all around. So the short answer is yes, you probably do want a rear sway bar.
Next, how do you go about selecting the sway bars? This is what David Wheeler of Hellwig Products had to say about sway bar selection:
“Many customers fall into the ‘bigger is better’ trap and shop only for the largest sway bar they can find. Sometimes this works, but many times they are compromising their vehicle’s performance. If other modifications have been made—increased spring rate, lowered height, change in roll center, etc.—the best sway bar may be smaller. The vehicle is a combination of its parts and it doesn’t care how massive and cool the bar may look, it only cares how the sway bar interacts with the rest of its components. This is why Hellwig produces adjustable sway bars for muscle-car applications. The adjustable sway bar allows you to tune the vehicle to your driving style and suspension components. We also manufacture more than one diameter for an application to allow the customer to match the bar to the vehicle.
“Sway bar rate is getting more attention from enthusiasts these days as they are getting more technical in their suspension tuning. With the availability of affordable suspension analysis programs, the sway bar rate can get the tuner into the ballpark of what size of sway bar is needed, but sometimes the enthusiast gets caught up in too much detail. There really is no substitute to actually driving and tuning the car. The calculated rate of the bar is great to look at as a predictor of performance but the installed rate of the bar can lose up to 25 percent of its theoretical rate due to compliance in the bushings and suspension components. Unless you can predict all of the losses in your program, you will not get accurate results. It’s best not to get caught up in the numbers the program is giving you, but to look at the trends. Drive the vehicle, evaluate what it needs, and look at your options.
“What affects sway bar rate? The most important influence on sway bar rate is its diameter. The influence of diameter is intuitive, as increasing diameter will provide more stiffness. The next most important influence is arm length. The arm of the sway bar is the portion that extends from the frame bushings to the end links. The length of the arm influences the rate of the bar as the longer the arm is, the less effective the sway bar becomes. The adjustable sway bar exploits this relationship by providing multiple holes at the end of the sway bar. By moving the end link attachment to a hole that provides a shorter arm length, you increase the effective rate of the sway bar. Moving the end link attachment to a hole that provides a longer arm length decreases the effective rate of the sway bar.”
You also need to keep in mind the effects of modifications you’ve done to the car. If your car suffers from poor front end geometry, a very large, high-rate bar is often used as a bandage to inhibit individual wheel movement and body roll. If your car is lucky enough to have fairly good geometry, or if you’ve taken meaningful steps to correct poor geometry, then you are likely much better off with a more-moderate-rate bar. The huge, high-rate bar only causes what could be a nice balanced car to push and feel less responsive. If your car is an elbow-out-the-window cruiser, you’ll probably enjoy the slightly smoother ride of a moderate-rate bar.
On the other hand, if you’re into serious handling, autocross, or track days, you may want a high-rate front bar, which you then match with a comparable high-rate rear bar to balance the car. If you’re in doubt, call a few manufacturers or well-informed retailers, tell them your combination, and get their recommendations.
Once you select the size and relative rate of your front sway bar, you need to match up a rear bar to complete the package. Again, look at your combination and select accordingly. If your spring rates are well matched, front and rear, and you selected a moderate-rate front bar, then select either a comparable rear bar or an adjustable-rate rear bar. Note that you want to select the bar by rate, not just diameter. Some bars (such as those on GM A-body and G-body, Ford Fox body, etc.) have unusual factory mounting that makes the rear bars very inefficient, so a large bar still contributes little useable rate. On some cars, the rate can be as little as one-quarter the size of the same-diameter bar mounted with conventional links. (See Chapters 8, 9, and 10 for more details.)
If you’re lucky enough to have adjustable-rate sway bars available for your application, always start out with them in the softest position. Typically, these bars use a change in end link position to alter the motion ratio of the bars in order to effect rate change. The softest position is the one farthest from the transverse (or torsion bar) segment of the bar. Once set, put some seat time in to get a feel for how the car behaves and let this be your baseline for continued tuning. I cover this more completely in the Appendix, but for now just start easy and work your way up to the higher rates.
Sway bars have various options for types of end links. For many years, most factory links used rubber bushings and washers sandwiching the lower A-arms or frame mounts and the flattened ends of the sway bars. The link portion was formed by a long bolt with a separate tubular sleeve in the center. These work okay in stock applications, allowing some deflection to prevent binding or breakage, but they allow much lost motion due to bushing deflection.
Another option was to use the same end link, but polyurethane bushings. This format causes less lost motion and (if properly used) not much more binding. Factory cars eventually went to a ball socket or tie-rod-end-style configuration. This design has very little deflection and articulates smoothly. Several major auto manufacturers have had durability issues with them, though. Just ask a dealership mechanic how many he’s had to replace. This is not due to a configuration issue. Rather, it’s a build quality issue; the lowest bidder tends to get the job. This can make choosing a bar with this type of end link a bit of a gamble.
On the other hand, race cars have mostly gone to a Heim-joint spherical-bearing end configuration. This offers high strength and excellent articulation. The normal caveats about running exposed high-precision bearings on the street do apply here, but at least in this application they’re under much lower stress than on a suspension arm. At least be sure to run Teflon-lined Heim joints and expect to replace them eventually, when they start to loosen up and make noise. On an occasionally used, fair-weather-only car, they may last as long as you own it. Some manufacturers have also developed their own end link configurations, like Hellwig’s hourglass bushing links. These use poly bushings that can be greased during installation. Their unique shape allows for 10 degrees of free lateral articulation without binding and smooth, quiet operation. These seem to hold up very well even on desert racing trucks.
Splined bars are a big recent trend. Originally used in racing, the idea is to have a straight torsion bar with the ends splined like an axle and separate arms that fit on each end to mount the end links. This lets a fabricator build the bar in any width, with arms of different lengths and offsets, and makes for a large assortment of torsion bars to choose from. Often these bars are tubular, which allows a race team to quickly swap out one rate/wall thickness torsion bar for another bar of a different rate/wall thickness very quickly and easily. That’s very handy. Otherwise, splined-end bars work exactly the same as coined, bent, solid, or tubular bars. The arms of the splined bar can sometimes free up a little extra tire clearance, sometimes not.
They do look pretty cool, but they’re also hard to see once they’re mounted on the car. If a splined-end bar happens to be the proper rate for your combination and it fits your budget, it may be a good choice for you. But don’t expect any increase in performance or clearance simply because it’s splined.
Say you’ve selected the size and style of bars you want to run. Now you’re faced with a couple different options and some cost two or three times as much as the others. What makes a $99 sway bar different from a $150 bar or a $350 bar? One big difference can be the steel they’re made of and whether they have had any heat treatment. These factors affect both the performance and durability of the bars. Again, a few words from David Wheeler of Hellwig Products:
“Many enthusiasts don’t realize that many aftermarket sway bars are made from mild steel. That is OK for a street cruiser, but if you’re going to push the limit, you need to have the best materials. Many muscle cars can benefit from a quality tubular sway bar, as the geometry of the sway bar and limited suspension travel allow tubular steel to be used, providing up to a 50-percent weight reduction over a solid sway bar. Pay attention to the quality of the bends and the end treatment. Are there any sharp corners in the ends that can act as a stress riser? Is the end finished with a radius? The little details can indicate if the manufacturer places quality first.
“For the ultimate strength, a heat-treated solid chrome-moly sway bar is the answer. Chrome-moly steel is a great material but its benefits are not realized unless it is heat treated. Some companies advertise the use of chrome-moly steel, but in its annealed condition, as required for cold forming, its tensile and yield properties are not much better than 1026 steel. If you heat treat [i.e., quench and temper] chrome-moly steel, its tensile and yield strength will increase 50 percent over its annealed condition. What this means is a heat-treated sway bar will survive in an application where a non-heat-treated sway bar will fail. It’s important to select a sway bar made from quality materials and processes, as inferior components can lead to failure.
“Another big difference is fit. Every time a client brings a set of low-budget bars to our shop, you’ll hear a collective sigh. We already know what we’re in for. Fitment issues commonly include end links that don’t line up, bars that hit the oil pan, idler arm, or differential, flimsy mounting brackets that bend when you install them, and ‘grade nothing’ hardware you can twist off with a half-inch wrench. Some brands have been selling these things for years, and I don’t know how. The bottom line is: Buying the cheapest bars you can find just ensures you’ll also get the cheapest construction.”
Like the rest of the suspension system, there’s nothing especially complicated about selecting sway bars, just a few fundamentals and taking a little time to consider the variables that impact your choices. If in doubt, go adjustable. That’ll let you easily tweak the car’s handling characteristics without having to buy a truckload of additional sway bars.
Written by Mark Savitske and Posted with Permission of CarTechBooks