Double A-arm front suspension cars are also referred to as double-wish-bone suspensions. Either name is an accurate description of the parts being discussed in this chapter. I refer to them throughout this book as A-arms. Depending upon the vehicle, they can be rather flat and triangular shaped. Looking at the physical part itself, you can imagine either a wishbone from last Thanksgiving’s turkey or the capitol letter “A,” thus their name.
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Since they are not part of the frame itself but attach to the main frame they are considered a subframe. The end of the “A” with two legs attaches to the frame and pivots, while the pointed end of the “A” attaches to the steering knuckle and pivots upon a ball joint. Two A-arms per wheel make up a double A-arm suspension. If your car has only one A-arm per wheel, usually the lower, then your car has a single A-arm, or MacPherson strut suspension (see Chapter 7). If the front wheels are connected by a rigid axle, the car has a dependent front suspension (see Chapter 5). If the front wheels move independently from each other, the car has an independent front suspension.
While there are several different possible configurations, a double A-arm independent suspension typically uses two A-shaped arms to locate the wheel. Each side utilizes a shock absorber and a coil spring to absorb vibrations and bumps. Double A-arm suspensions allow for more control over the camber angle of the wheel, which is the degree to which the wheels tilt in and out. They also help minimize roll or sway and provide a more consistent steering feel. Because of these characteristics, the double A-arm suspension is common on the front wheels of larger cars as well as most GM and Ford cars from the muscle car days.
Three areas that need attention to align the wheels of a double A-arm independent suspension-equipped car are: caster, camber, and toe. The following is an explanation of these three settings as they pertain to a drag race car. Settings for street-driven cars are much closer to factory specifications.
Camber changes drastically as the suspension moves up or down when hitting bumps and potholes. This also occurs in a stock double A-arm independent suspension car doing a wheel stand, as the tires tilt in toward each other dramatically at the top. Typical body roll in such a car launching hard at the starting line lifts the driver-side tire 6 to 12 inches off the ground with the passenger side still on the ground. This can also dramatically influence camber.
For maximum straight-line acceleration, the camber angle should be near zero so that the tread is flat on the road. Positive or negative camber has a scrubbing effect (due to the tire being at an angle) and increases the rolling resistance. Significant suspension modifications like raising, lowering, or using dropped spindles may require camber adjustments. Near 0 camber helps the car accelerate quicker, as do narrow tires, which have less rolling resistance. In this case, the narrower the tire, the better. However, don’t get yourself into a situation where safety is an issue because your tires are not rated in a high enough capacity for your car’s weight (see “Front Tires” on page 47 for more information about front tires specifically designed for drag racing).
If your front end is extremely worn or well used, the toe-in might be set as high as 1/4 inch. Since most race cars are pushed beyond the limits of normal street cars this may help you see the importance of maintaining your car’s front-end components.
Now you can see that toe settings have a great impact on directional stability as well as tire wear. Besides continually checking the front steering components for wear and replacing them at the first sign of wear you should install polyurethane bushings in the A-arms where they attach to the frame. For a quieter and softer ride, manufacturers installed soft rubber bushings into A-arms. This allowed for movement when pushed to the limit. Race cars need to get rid of this mushy, comfortable, suspension flexing ride by installing spherical bushings (heim joints), polyurethane, plastic, or metal bushings to provide optimum control of suspension links. That is why a street car requires more toe-in than a race car and a worn street car requires more toe-in than a better condition street car.
Over the years with many pot-holes or speed bumps the frame and the crossmember below the engine can sag or become bent without the car ever being in an accident. Some vehicles therefore require occasional realignment.
On double A-arm front suspensions, a car’s camber or caster settings are changed by the placement of shims between the cross shaft (where the upper A-arm is mounted) and the frame. There are two studs pressed into the frame on each side. The cross shafts are loosened and horseshoe-shaped shims are slid over each stud and then retightened to make adjustments.
Camber is affected by adding or removing an equal number of shims from both studs. If you add shims to the shim pack, the top of the tire moves inward, creating more negative camber. Removing shims from the shim pack tips the tire out at the top creating more positive camber.
Caster is changed by adding or removing different quantities of shims from both studs. Removing a shim from the front stud moves the upper ball joint out and back, creating more positive caster. Adding a shim to the front stud moves the upper ball joint in and forward, creating more negative caster. However, notice the words “in” and “out.” Your camber setting also changes and may need to be reset. If you need a dramatic change you can take a shim off the front stud and add it to the rear stud. This adds more positive caster without changing camber.
Toe is changed by turning the threaded sleeves connecting the tie rod ends. Threading the tie rod longer creates toe-out. Threading the tie-rod ends shorter creates toe-in.
Each chassis needs different camber, caster, and toe settings. These settings are programmed into modern wheel-alignment machines by year, make, and model, and are updated periodically. Normally for drag racing cars, you want the tires as straight as possible. This translates to 0 camber and nearly 0 toe. With stock A-arms, go for as much positive caster as possible. Usually, you are limited to about 2 degrees. Offset upper control arm shafts are available through most distributors to provide an additional 1 to 2 degrees of positive caster. Most aftermarket upper control arms are set for 5 degrees of positive caster. Some chrome-moly upper control arms are adjustable and allow for as many degrees of positive caster as needed (such as those from Dick Miller Racing). In this case you should shoot for between 5 and 7 degrees of positive caster.
The first improvement for cars with a front sway bar is to completely remove it for drag racing purposes. Most performance-oriented double A-arm independent suspended cars have sway bars on the front to give the car additional handling stability. A sway bar is a round piece of steel that fastens to each frame rail and then to each lower A-arm which connects the driver’s side of the suspension to the passenger’s side. When the suspension on one side moves up or down the sway bar transfers movement to the other side. This keeps the car more level with less roll and better handling while reducing sway during hard cornering. Knowing how a sway bar works allows you to better understand why you don’t want one on your drag race car. Be careful if your car is a dual purpose street/strip car. Depending on your driving style, you may need to re-install the sway bar when returning to the street.
By design, sway bars limit the travel of the front suspension and therefore don’t allow enough front-end rise or weight transfer (pitch rotation), which is necessary to add down force to the rear tires and plant them as hard as possible. The more weight transfer, the better.
Sway Bars versus Anti-Roll Bars
Most muscle cars came from the factory with front and/or rear sway bars. They are available direct fit with many aftermarket suppliers, in different diameters (bigger being stronger or lighter being weaker), and some are more universal. They are normally made from a solid round bar of steel shaped as needed for a particular application.
While road course cars and street driven only cars can benefit with the use of sway bars, they should not be used on drag cars. They will get rid of a lot of the body movement or dipping in the corner in a hard turn. On a low-horsepower drag car they could be used to help with wheel hop. Notice I did not say get rid of wheel hop. Because of its design a sway bar never gets rid of a problem. It simply takes half of that problem and transfers it to the other side, such as with wheel hop. Taking half of the wheel hop problem and transferring it to the other side on a low horsepower car can make it seem like it has gone away. Going into a corner with a less than adequate spring and shock with a sway bar can add stability to the car.
If you are turning in the same direction all the time a stronger spring would be a better answer. If you are turning the car in both directions then a sway bar is mandatory. Muscle cars with front sway bars usually have the center section bolted to the frame and the ends are fastened to the steering arms. At the frame they are mounted inside rubber bushings to allow them to twist and flex, but in a minimal amount as compared with a car trying to do a wheelstand. In the 1970s, NHRA required Stock class cars that came from the factory with sway bar’s to run them while racing in a Stock class. To get rid of the sway bar’s tendency of limiting front suspension travel, some racers would ream out the rubber bushings to make them almost useless. Since the rear sway bars were bolted directly to the lower control arms there wasn’t much that could be done without leaving it off. Just as helping with wheel hop a rear sway bar hinders in trying to get individual corner tuning such as preload. It will fight the preload.
Anti-roll bars are not the same as sway bars. Anti-roll bars are used on drag cars to get rid of the typical body roll upon launch having the driver’s side front tire about a foot higher off the ground than the passenger side tire.
Antiroll bars, unlike sway bars, are made from round tubing instead of being solid steel. All of my anti-roll bars are chrome moly because of the demand needed of the anti-roll bar to twist and return to its original shape.
Using different methods, the anti-roll bar is fastened to the frame and each side of the rear end housing. By being welded to the frame rails, the anti-roll bar, upon launch, will push on the housing mounts and absorb the twisting from the car which is trying to raise the driver’s side front wheel and then the antiroll bar will use the passenger side of the frame as leverage to prevent this from happening, thus creating a launch which is level at the front of the car.
As mentioned earlier, each side of a car utilizes a shock absorber and a coil spring to absorb vibrations and bumps. Good quality shocks are very important for today’s higher horse-power cars. An inexpensive set of shocks like 90/10 or three way adjust-able shocks should work on cars with as much as 300 to 350 hp.
If you have a car that does very high wheel stands, you can control how hard or soft the car comes back down on the ground. Too hard can cause the car to bounce back into the air repeatedly. That causes the rear tires to un-plant and then re-plant with each undulation. Not only is this hard on traction and ETs, it is also hard on driveline parts. Where you are in the torque band determines just how bad the effect is. As the tire unloads, the converter with less load stalls to a lower RPM as the engine spins to a higher RPM. This action makes achieving an accurate dial-in impossible.
With most front-adjustable shocks, the best setting to start with is near the middle of however many adjustments the shock has. If your car needs more front end lift (pitch rotation), a softer setting is needed. If your car’s front end is raising too far, a firmer setting is needed.
As mentioned in other chapters, adequate shock travel is essential. Measure the total amount of suspension travel between fully compressed and fully extended without a shock or spring in place to determine the amount of suspension travel you have. Then specify a shock absorber capable of extending and compressing within those limits. A shock that bottoms out or extends fully before the suspension travel arc is reached inevitably bounces back when it reaches its limit. This generates unpredictable results—dangerous in a racing situation. Don’t let this happen to you. Measure carefully, consult with your shock manufacturer, and get the proper components in place.
Not only does your car go faster, but your life may depend on it!
Measuring For Shocks
Companies such as QA1 have shocks for most muscle car applications. Just as it is up to an engine builder to be sure all machine work and all parts measure to the spec necessary for the job, it is your responsibility to be sure your shocks have the right measurements to suit your car’s needs. Therefore, you must take three measurements for your car and compare them to the measurement of the shocks you are about to use. Shocks are necessary on your car to control the car’s suspension (see side-bar “Proper Shock Length is Critical” on page 13). However, if the shock runs out of travel in either direction your suspension movement will stop abruptly and control will be lost.
To dimensionally check your car’s shock needs, you must get the measurement for the car’s shock compressed, extended, and ride height lengths. In order to verify the correct shock by length for your vehicle, the following procedure can be used. This way you can be sure your shock won’t bottom out or top out by comparing the measurements you get either with the shock listed for your car or go to the shock companies tables and pick the one to suit your car’s needs.
The shock’s measurement at ride height is usually all that is needed. However, some shock companies only publish compressed and extended measurements. If you are using this method then you need your car’s actual ride height to be somewhere in the middle or so that there is minimally more travel on the extension side as opposed to the compressed side. Then call your favorite shock supplier (like me) and buy a shock with the same recommended ride height. However, my belief of not taking anyone else’s work for granted or as being right prevents me from stopping there. To get these measurements for the car’s shock compressed, extended, and ride height length you need to take the following measurements from the car’s chassis.
First raise the vehicle off the ground and let the suspension and wheels hang freely. Remove the shocks and carefully allow the suspension to drop as far as it can, being sure your are not damaging any connected brake lines. Measure from the center of the upper shock mount to the center of lower shock mount. This measurement is the car’s maximum necessary extended shock length. Place the car completely back on the ground, resting on all four tires. Now remeasure from the center of the upper shock mount to the center of lower shock mount. This is your ride height.
Next measure from the bump stop to where the bump stop contacts. The bump stops may be different from the front of the car to the rear of the car. Subtracting the bump stop travel measurement from the ride height measurement you now have the compressed measurement. Shocks are measured from the center of loops and/or shock shaft/stud shoulders. The measurements are taken from mounting surface to mounting surface.
Now determine the upper and lower mounting type and size. For loop mounts, the inside bolt hole diameter is needed. For cross pin mounts, the bolt hole size is needed, as well as the distance between bolt holes. Quite often the ends you need may not be offered in the size you need, but normally I can change the shock from one type end to another style.
Front-spring selection is very important yet very simple. Your car needs the lightest spring available that will hold the car at the desired ride height. That usually requires a lighter-weight and taller front spring. Using a lighter-weight spring than factory causes the spring to collapse farther than the factory spring. You therefore need a taller spring to keep the front end at normal ride height.
With a lighter-weight spring being compressed farther, the spring has stored (potential) energy, which causes the front of the car to lift quicker and farther during hard acceleration and weight transfer (pitch rotation). As the weight transfer begins removing 200 pounds from a lighter-weight and taller spring it gives more rise to the car than removing 200 pounds from a heaver-weight and shorter spring.
Moroso has a good selection of front drag-race-specific coil springs, which satisfy the needs of most of the original muscle cars at the race tracks. Even then, the help of a good educated guess helps. On the other hand, nothing works better than having your car weighed.
If you have not four-corner scaled your car, take your car to a scale. Usually a recycle yard or a grain elevator or your own race track has scales accurate enough for this task. The proper procedure for weighing your car is detailed on page 102. Once you’re armed with the car’s total weight and know how it’s divided between the front and rear, you’re well-armed to outfit the car with the best-possible components. Get all the information you can before consulting with an expert and purchasing parts. You’ll save time and money in the long run by doing so.
Going to a coil-over setup gives you a better selection of springs and is usually just a few dollars more than a set of conventional springs and a good set of adjustable shocks of equal quality. With coil-overs, it is much easier (and safer) to change springs than with a taller conventional design coil.
That should be enough information to understand the workings of a double A-arm front suspensions car and point you in the right direction to make yours better, faster, and more consistent.
Written by Dick Miller and Posted with Permission of CarTechBooks