Right Equipment and Knowing What to Look for Cures Alignment Complaints
Four-wheel alignments are commonplace in today’s service bays. Even light-duty, 4WD SUVs, such as Honda’s CR-V and Toyota’s RAV4, have independent rear suspensions with both camber and toe adjustments. Even solid, rear-axle vehicles require an alignment head on all four wheels. Without considering rear thrust angle, your perfect "front-wheel alignment" might result in an off-center steering wheel.
Many people request an alignment as preventive maintenance during or after buying new tires. But often, the customer has a problem with the way the vehicle handles or the tires wear. Therefore, you might want to add a new category to your alignment menu, a "four-wheel diagnostic alignment."
This would include measurement and evaluation of the diagnostic angles: thrust angle, SAI, included angle, turning angle and setback. This alignment should cost more, and the customer should be informed as to why it’s needed and told of the benefits. This alignment should be sold whenever the customer indicates that there are nagging problems, such as pull or abnormal tire wear.
First and foremost though, always start with the customer’s description of the problem. Is there abnormal tire wear? Does it pull or drift, and if so, which way? Is there any vibration? Are there any unusual noises, and if so, when do they hear them? Where does the noise seem to be coming from? Is there any looseness? Is the steering wheel centered? Has there been a collision or previous repair?
It’s a good idea to use a standard question sheet for every alignment. Different uses can pose different problems, and people often request an alignment only to find out after asking about symptoms that they really need tire balance or brake work. Having a written document eliminates confusion about what the customer needs or expects.
Once you’ve gathered all the pertinent information, a test drive is in order before any inspections are made. It’s imperative to confirm the existence of the problem before you attempt to fix it.
If tire wear is the complaint, find out how old the tires are and if they have been rotated recently. Trying to correct for a worn right front tire that you don’t know just came off the left rear two weeks ago is a recipe for an alignment headache and a customer relations nightmare. What about a recent accident or bump with a curb?
A test ride should be done before and after the alignment in order to uncover a pull or wander condition and confirm its repair. Of course, we need to inspect the usual – tire pressure, ride height and worn suspension components.
Today’s low-profile tires can cause a pull with as little as 4 psi difference from side to side. Different brand tires of the same size may cause a pull due to rolling resistance or diameter differences. You can’t align sagging springs, defective tires, sloppy ball joints or worn bushings.
If you have a pull, it’s a good idea to swap the front tires left to right to see if the tires are causing the pull. If after swapping the tires the pull is to the same side, the tires aren’t at fault. Don’t overlook worn upper strut mounts, which can cause a memory steer condition or that a pull can be caused by a dragging brake.
Always check for previous repairs. Look hard, some people can get very "creative" when trying to align a vehicle with a bent part. Sometimes aftermarket alignment kits are used to compensate for a bent part. If you find a kit installed and its adjustment is maxed out, remove it or center the adjustment, then recheck the diagnostic angles for a clue to the underlying problem area.
Ride height is even more critical on non-adjustable suspensions. Camber can be dramatically altered, particularly on MacPherson struts, by a weak spring. Therefore, you want to be sure that ride height is in spec at all four corners before the first head is mounted to a wheel.
All the Right Angles
Let’s review the angles we should be measuring. For the sake of our discussion, let’s stipulate that we have a four-wheel, independent strut-type suspension at all four corners.
Camber is the inward or outward tilt of the wheel. Positive camber is when the wheel tilts out at the top. Negative is when the wheel tilts in at the top. Keep in mind that this is a very dynamic angle, meaning that your static camber setting can change dramatically with suspension travel when the car is actually driven. Camber plays a role in where the weight of the car is centered on the spindle, how the tires wear and how the car handles.
Cross camber is the difference between the left and right camber readings. Too much cross camber (typically more than 0.5⊄) will cause the car to pull to the side with the more positive camber angle. It’s possible to have individual left and right camber readings within spec, but the cross camber could be out of spec due to one side being near the more negative tolerance and the opposite being near the more positive tolerance.
Caster is the forward or backward tilt of the steering axis, which is a line drawn from the center of the strut mount through the ball joint. Caster is positive when the top of the steering axis tilts to the rear of vertical and negative when the top of the steering axis tilts forward.
Cross caster is the difference in caster readings from side to side. Like cross camber, too much of a variation from left to right can create a pull. More than 1⊄ can cause the car to pull to the side with the more negative reading. Note that this is just the opposite of camber. Remember, ride height, both front and rear, can change caster by changing the relationship of the frame to horizontal.
Steering Axis Inclination (ASA) is a line drawn through the pivot points about which the wheel steers. On a strut suspension, it’s a line drawn from the top of the strut shaft through the lower ball joint as it relates to a vertical line drawn upward from the center of the tire where the tire touches the ground. It is a good diagnostic measurement because it can indicate why a camber reading is out of spec.
If SAI is OK, but camber is out, the most likely cause is a bent spindle (or steering knuckle on a FWD car). If camber is OK, but SAI is off on one side, check for an extreme camber adjustment to compensate for a bent part. If the camber adjustment is near center, you may be looking at multiple bent parts, such as a knuckle and lower control arm or control arm mounting point.
If camber and SAI are out of spec by equal amounts on both sides, only positive on one side and negative on the other, the cradle or sub-frame is probably shifted to the side showing the least amount of camber. If there is no bolt-on cradle, you most likely have a bent frame.
Included angle is simply the SAI, plus camber. It really gives us no new information, but may help you think through a set of circumstances depending on how you visualize angles.
SAI, as we’ve said, is the axis about which the wheel pivots when steering. If you continue drawing the steering axis line until it meets the ground, it will fall inside, outside or on the centerline of the wheel. The distance between where this line meets the ground and the centerline of the wheel is the scrub radius. Whether the scrub radius is positive or negative has no effect on tire wear or steering wheel pull in normal situations.
However, it has a great deal to do with how the vehicle responds when one wheel is on ice or sand and the other is on a more tractive surface. Scrub radius is not something that is displayed on the alignment machine, but I mention it because it can be drastically changed by wheels that have non-OE offset, width or backspacing. This can cause some wild torque steer problems with high-horsepower FWD vehicles.
SAI and Struts
Let’s look at SAI as it relates to MacPherson struts and unequal length A-arm suspensions. Keep in mind the line we drew to determine SAI on the strut suspension. Now, remembering that SAI is a line drawn through the pivot points about which the wheel steers, where is the line on an unequal length A-arm suspension?
It’s a line through the upper and lower ball joints. If you think about it, you’ll realize that the SAI angle is intrinsically higher on a typical strut suspension than it is on an A-arm suspension. It has to be to keep the strut out of the way of the top of the tire. Because SAI aids in steering returnability, strut suspensions will usually tolerate cross caster differences that would cause an A-arm suspension to pull. This may be why OEMs often think they don’t always need to make caster adjustable on strut suspensions.
Setback is a little more difficult to describe. Imagine a solid front axle. Now draw a line through the center of the vehicle from rear to front so that it forms a "T" with the solid front axle. If it makes a perfect 90⊄ "T", we have 0⊄ setback. If the solid axle does not create a perfect "T", one front wheel (even if they are both steered straight) will set behind the other. If the angle is such that the left wheel sets further to the rear than the right wheel, we have negative setback. If the right wheel is further back than the left one, we have positive setback.
Think of setback as a caster diagnostic angle just as SAI is to camber. If caster is off spec, setback can help us to determine if the top of the strut is the problem or the position of the lower control arm is the culprit. For example, say we have a negative setback of 0.2⊄. This means the left tire is more rearward than the right one. Let’s say we find that caster on the right side is on spec, but the left side caster is too low. By moving the left lower control arm forward, we would simultaneously increase caster and bring setback toward zero. Don’t forget that cradle shift front to rear can cause setback and caster conditions, just as left or right cradle shift causes camber and SAI problems.
Making the Turn
Toe is the direction that a wheel is pointed. Straight ahead is zero toe. If the front of the wheel is pointed inward, toe is positive. Conversely, if the wheel is pointed outward, toe is negative.
Turning angle, or "toe out on turns" can be described in a couple of ways. When a car makes a turn, the outer tire turns in a circle that is larger in diameter than the inner tire. Stated another way, the inner tire must make a tighter turn than the outer tire or they will fight each other through the turn and scrub the tread. This is called the "turning angle". Uneven turning angle is called the "Ackerman principle" and is accomplished by placing the steering arm at an angle that is not parallel to the centerline of the car.
Specs are usually published for steering angle, so use them. A quick check would be to steer the right wheel 20⊄ to the right as indicated on your turnplate, then look at the reading on the left turnplate. The left wheel should be at about 18⊄. Steer the left wheel to the left 20⊄ and check for 18⊄ on the right. Be sure total toe is in spec and that the turnplates are zeroed before you check turning angle specs.
Rear toe and camber are similar to the front. However, we need to discuss the thrust angle. The thrust angle is the direction that the rear wheels are "steering." If they are pointed straight ahead, we have a zero thrust angle. If they are pointed right, we have a positive thrust angle, and if pointed left, it’s a negative thrust angle. The thrust angle is easily adjusted during rear toe adjustment. If toe is set correctly, the thrust angle will be zero. If a centered steering wheel is to be obtained, any thrust angle must be considered when adjusting front toe. This is why it’s important to do a "four-wheel" or more precisely, a "thrust" alignment on solid rear axle vehicles.
Making the Job Easier
Let’s look at some things that can make your alignment job go smoother. First is equipment maintenance. If somebody is dropping an alignment head on the floor once a week, how accurate can we expect the alignment machine to be? The alignment machine should be calibrated from time to time to check for accuracy.
On conventional alignment equipment where the electronics are contained in the sensor, an equipment technician is usually required for zero and range calibration. You can purchase a calibration fixture from the manufacturer to use for checking and zeroing calibration.
If you’re careful with the equipment, calibrating the sensors once a year should suffice. Otherwise, use your discretion based on how often the heads are "drop tested." In the long run, it’s faster and cheaper to ensure the accuracy of the equipment than it is to let a few dozen cars go out the door with the toe in the half-inch range!
Some newer equipment uses targets instead of alignment heads. Unlike the alignment heads, targets do not require re-calibration when dropped.
Remember to occasionally check the turntables and slip plates. Accumulated dirt and wear can cause binding, which can result in inaccurate alignments.
Bump steer is a phenomenon where a wheel tends to change toe as it goes over a bump. While bump steer can’t be completely eliminated, it is mitigated by steering linkage design. The most common cause of bump steer is a lack of parallelism in the steering linkage. A steering arm that is bent vertically or a steering rack that is mounted crooked are two possible causes. Some idler arms have slots at their mounting points so that their position can be adjusted. The idea is to adjust the idler arm vertically so the drag link (or center link) is level.
Broken engine, transmission and subframe mounts can contribute to steering and alignment concerns. Fluids, such as oil and transmission fluid, can severely degrade polymer bushings and mounts. If you encounter an fluid-soaked rubber component, don’t replace it without correcting the cause of the leak.
Often the weakest link in a strut is the strut shaft. Lateral wheel impacts can cause the shaft to bend. This can be easily checked by rotating the strut shaft with a wrench on the shaft nut while watching for camber change. There should be no change at all.
While it’s not a common occurrence today, steering pull can be caused by a defective spool valve in a power steering rack or box. This can be easily misdiagnosed as an alignment or tire problem. The check, however, is simple. Raise the front wheels off the ground with the wheels steered in a straight-ahead position. Start the car. The steering wheel should not move. Steer slightly to the right and then the left. The steering wheel should not continue to move on its own after you let go of it. If it does, you’ve found a defective spool valve.
Get The Right Tools
In addition to all the kits available to help adjust the "unadjustable," there are also tools that make alignments easier.
Some vehicles have caster or camber that are easy to adjust incrementally, most often with eccentrics. Others, such as two-bolt, strut-to-knuckle mounting, or slotted, control-arm mounting, can create some difficulty. The problem is that the weight of the vehicle must be on the wheels to obtain accurate measurements. Therefore, when you loosen the adjustment enough to move it, the weight of the vehicle may push your adjustment all the way to its limit, overshooting your desired position, or worse yet, going in the opposite direction. Another problem is, with the wheel in its normal position in the well, the adjustment point may be difficult to reach.
Most of these tools, obviously, are going to be application specific. The tool may fit a range of years, but is most likely limited to one body type.
One type of tool looks like a turnbuckle with hooks on either end. The ends are placed in the appropriate holes in the suspension components, and the turnbuckle is loosened or tightened to move the link in question.
Another tool looks like a pry bar with an "L"-shaped link pivoting from the "working" end. It is used to precisely position the control arm shaft on those vehicles whose control arm bolts move in slots. Another universal tool for strut suspensions is like an air bag. Inflate the bag to position the strut where you want it, then tightened the bolts.
Most modern alignment machines do a great job of walking you through a job. Graphics and photographs of vehicle adjustment points are displayed on screen.
There is one feature that seems to be underutilized by technicians. This feature is often called "jack and hold" or "raise wheels." This feature allows you to make caster and camber adjustments with the vehicle’s wheels hanging in full rebound position (vehicle jacked up by the frame.) This gets the weight off of the adjustment points and the wheel out of the way in the wheel well.
It works like this. Let’s say that you have 0.5⊄ positive camber on the front wheels with the car resting on the turn plates. If you jack the car up, the camber will change significantly. On a strut suspension, it may go to 1.5⊄ positive. What the "raise wheels" feature does, after the car is jacked up, is to revert the alignment machine’s display back to the original 0.5⊄. This way, you can read and make adjustments as if the car were resting on its wheels.
Most machines will also allow caster adjustment this way. Of course, caster is just an approximation, and all readings should be verified when the vehicle is set back down on its wheels. Check the equipment operator’s manual for specific instructions on your machine.