When it comes to correcting wheel alignment, shims are often essential. Shims come in handy for making corrections on both the front and rear wheels of many vehicles, so let’s start up front and work our way to the back.
If you’re aligning an older vehicle that has a short long arm (SLA) front suspension, you may have to add or remove shims from the upper control arm pivots to change camber and/or caster.
In most instances, if shims need to be installed or removed, it is to increase camber to compensate for spring sag. Or, if the vehicle leads to one side, shims may have to be moved between the front and rear shim packs on a control arm to add or subtract caster. Probably 90 percent of the time you’re going to end up moving the tops of the steering knuckles out and back to correct camber and caster.
Just remember that shims are used to make normal alignment corrections. They should not be used to compensate for structural damage or bent parts. If camber or caster readings are more than a couple of degrees out of range, it should raise a red flag. Chances are a spindle or one of the control arms are bent. To identify the problem, check and compare the steering axis inclination (SAI) angle and included angle readings.
If camber is less than specifications, but the SAI angle is correct and the included angle is less than it should be, the problem is a bent steering knuckle or spindle. If the included angle is OK and SAI is less than specifications, it indicates a bent lower control arm. Replace the bent or damaged parts before attempting to make any further alignment corrections with shims.
As a rule, camber and caster should both be within half a degree of each other on both sides. A vehicle will lead towards the side with the most camber or caster if there is too great a difference side-to-side. You may want a little more caster on the driver’s side to counter road crown, but it should be with half a degree of the reading on the passenger side.
Shims for making SLA camber/caster corrections typically come in thickness of 1/64, 1/32, 1/16, 1/8, 5/32 and 3/16 inches. The amount of correction each shim thickness provides will vary somewhat from one vehicle application to another because of differences in the geometry of the control arms and suspension. Generally speaking, a pair of 1/16-inch thick shims will produce about a quarter degree camber change and a 1/8-inch shim will produce about half a degree correction. Even so, figuring out how many shims to install or remove to achieve the desired amount of correction is often a trial-and-error procedure.
Something else that may take some mental effort is figuring out where the shims need to be added or removed to change camber/caster. The shim packs may be located on the inside or outside of the control arm pivot shafts. What side the shims go on is important because of the direction in which they move the control arm.
To change camber, you add or remove an equal number of shims to both the front and rear shim packs on each pivot shaft. To increase camber, add or remove shims to move the upper control arm outward. To reduce camber, add or remove shims to move the upper control arm inward.
To change caster, you add, remove or swap shims between the front and rear control arm shim packs. To increase caster, change shims to move the top of the steering knuckle rearward. To decrease caster, change shims to move the top of the steering knuckle forward.
As a rule, you should make your camber corrections first, then caster. This is because changes in camber affect caster readings.
4X4 SHIMS
On some 4x4 trucks, camber correction shims can be installed between the steering knuckle and spindle. These are steel shims for older Ford, Chevy, GMC and Jeep applications with 60 Spicer axles and six or eight bolt hubs. Corrections of 1/8 to 1-1/2 degrees are usually possible with these shims. Installation requires pulling the wheel and brakes, then unbolting the spindle from the knuckle. Some shims have notches on the outside edge for positioning the shim to achieve the desired amount of camber change.
REAR SHIMS
On rear-wheel-drive cars and trucks, shims are of no help because toe and camber are fixed — unless the vehicle has an independent rear suspension, in which case, some other means of adjusting toe and camber may be provided or required.
The same goes for front-wheel drive cars with struts on the rear wheels. Those that have factory provisions for adjustments typically use camber bolts or offset or sliding control arm bushings to make camber/toe corrections. For those that have no factory adjustments, various aftermarket alignment kits are available.
Where shims come in handy are on FWD cars and minivans that have coil or leaf springs in the rear with some type of axle or trailing arms. On these applications, the rear wheel spindles bolt to the axles or trailing arms. This means some type of shim can usually be installed behind the spindle to make camber and/or toe corrections if no other factory adjustments are provided.
As long as the rear wheels are within specifications, shims shouldn’t be needed — unless the vehicle is experiencing unusual tire wear and you want to set rear toe as close to zero as possible.
Several different types of rear shims are available for making camber/toe corrections on FWD cars and minivans:
• Half Shims — There are "half-shims" in various sizes and thickness for General Motors, Chrysler, Volkswagen and Audi applications. Most half-shims are metal and can be slid into place without having to unbolt and remove the rear spindle or wheel bearing. You just loosen the mounting bolts and slip the shim between the axle and brake backing plate. This can save installation time but creates a small crevice into which dirt and water can penetrate. A partial shim also doesn’t provide as much support as a full contact shim. Even so, they are approved by Chrysler and others. The most common mistake that is made using this type of shim is installing it in the wrong position (between the brake backing plate and hub rather than between the brake backing plate and axle). This can misalign the brake drum with the shoes, causing brake problems.
Half-shims can be used to make both camber and toe corrections. Installing a shim behind the top two hub bolts will add camber while installing a shim behind the lower two hub bolts will reduce camber. For toe corrections, installing a shim behind the two rear hub bolts will give toe-in while installing the shim behind the front two hub bolts adds toe-out. Using a combination of shims (one installed vertically and one horizontally) will change both camber and toe.
• Full-contact shims — These come in various configurations, thickness and materials (metal or plastic). Different bolt patterns are needed for different applications. Some have slots, other have notches to fit various bolt patterns. The shim itself is wedge-shaped and may provide 1/4 up to 1-1/2 degrees of camber and/or toe correction depending on the thickness of the shim.
With this style shim, how the shim is positioned is critical. The thick part of the shim is installed at the top position to add camber or at the bottom to decrease camber. Installing the shim with the thickest part toward the front will increase toe-out while placing the thick part toward the rear adds toe-in. Some shims can be rotated to produce combination camber/toe corrections. Two shims can also be stacked and offset to produce camber/toe corrections, though most suppliers don’t like to see shims stacked because it adds too much thickness.
Using a full-contact shim usually requires completely removing the spindle or wheel bearing and brake backing plate so the shim can be installed between the backing plate and axle or trailing arm. Care must be used to avoid damaging the wheel speed sensor wiring if the vehicle has anti-lock brakes. Also, depending on the thickness of the shim you’re using, there may be some concern that the hub bolts are long enough to provide adequate support. The bolts should be long enough so that when they are reinstalled and tightened, one or more bolt threads should be visible above the tops of the nuts.
Hub bolts must also be tightened carefully after installation. Use a torque wrench, not an impact wrench, because overtightening a plastic shim may crush it.
• Two-piece adjustable shims — These are the shims that revolutionized rear-wheel alignment because a relatively few number of shims can fit a wide range of applications. Installation is the same as a full-contact shim, but the difference is that adjustable shims must first be set to achieve the desired amount of camber/toe correction.
Most of the two-piece adjustable shims are plastic. There are a couple of different styles, but both use an indexing system to achieve correct positioning and combination camber/toe corrections. If your aligner doesn’t have software that shows you the proper shim pattern, you’ll have to use a reference chart provided by the shim supplier.
If you’ve never used an adjustable shim before, the reference chart may be rather confusing. But once you learn how to use the chart, you should have no trouble adjusting the shims to achieve just the right amount of correction. The most common mistake made here is trying to use an adjustable shim without reading the instructions. Save yourself a call to the tech support line and read the directions. Most charts show the toe and/or camber change desired for one side of the vehicle (driver or passenger side). The shim is then rotated to line up the appropriate combination of numbers and letters using the top (12 o’clock position) as a standard reference point. Some shims then have to be trimmed to clear the hub bolts.
Before using any type of rear-alignment shim, the first thing you need to do is measure rear camber and toe for each wheel, then compare the readings to specifications. Then and only then can you determine how much, if any, correction is needed. More than two degrees out of specification usually indicates worn or bent parts.
If a correction is needed, the next step is to choose a shim that will give you desired camber/toe change. Then you have to figure out how to adjust and position it before you can install it. After the shim is in place and the bolts properly tightened, recheck rear camber and toe to make sure the shim achieved the desired amount of correction. If it didn’t, you get to do it all over again.
One tip here that can save you a lot of wasted time and effort is to check ride height if camber is less than specifications. If the springs are below minimum ride-height specifications, they should be replaced.
On some FWD vehicles, the rear axle can twist over time. This will affect the placement of a correction shim. This can throw the hub bolt pattern out of square with respect to the body and road. The trick here is to use the pinch welds on the body for a horizontal reference when adjusting and trimming the shim.
Thrust angle problems that cause off-center steering and rear-axle steer can also be corrected with shims. In most cases, total rear toe may be within specifications, but the individual toe of each rear wheel will be off an equal amount (one in, one out). The cure here is to shim equally in opposite directions.
Another point you want to pay close attention to when installing adjustable two-piece shims is making sure the shim faces the proper direction (in or out). On one side of the vehicle, the shim may face in while on the other side it may face out. Positioning it the wrong way will give you just the opposite effect you’re trying to correct. Remember, read the directions!
Another problem that can sometimes trip you up when checking and correcting rear-wheel alignment is sticky or worn slip plates. Slip plates must allow the rear wheels to move in and out so the rear suspension will settle properly. If the slip plates bind, are not properly lubricated or are worn underneath, they may not allow free movement and give you false readings.