Dandy. The first thing that jumped out at me is your mechanic has his alignment computer set to display values to two places after the decimal point, or hundredths of a degree. As late as the early 1980's on old, heavy rear-wheel-drive cars, the mechanical equipment could be read to 1/8th degree, and that was good enough for those cars. When I was the alignment specialist at a very nice family-owned Chrysler dealership through the 1990's, I found I needed to adjust left front camber 0.06 degrees higher than the right side to offset road crown. If I would have had my computer set to read to the tenth of a degree, the numbers would have been rounded off and would not have let me adjust to the needed precision. Shops that have their computers set to read to the tenth of degree are more interested in speed than accuracy.
Camber is the tilt of the wheel in or out on top, as viewed from in front of the car. Perfectly straight up and down is 0.00 degrees. If you could visualize the wheel tipped out so far on top that it is laying on its side, that would be 90.00 degrees. On most cars, the front wheels lean out a little on top, typically between 0.30 to 0.50 degrees. That is not nearly enough to see by eye, but when the movements of the suspension system are added in while driving, that provides the best tire wear. It also places the vehicle's weight directly over the larger load-carrying wheel bearing.
Be aware I left the dealership in 1999 so I never aligned a newer Charger, but with the early 1990's minivans and front-wheel-drive cars, if I tried to let one go with 0.10 degrees pull to the left to offset road crown, that was too much, and the car would pull that way. 0.00 degrees difference was not enough to offset road crown. I needed to be that picky. Most of those vehicles were much more forgiving and I could get away with bigger differences, but at 0.06 degrees, I knew for sure the customer was not coming back with a complaint.
Your car calls for 0.40 degrees camber pull to the left to offset road crown. In my world, that is a lot, and it is more than I would have expected. Also, three of your wheels call for them to be tipped in on top. That is "negative camber" and it adds to cornering ability. Specifically, your left front wheel calls for 0.00 degrees camber, and you have -0.11 degrees. Your right front wheel calls for -0.40 degrees and you have -0.51 degrees. Tires want to pull in the direction they are leaning, so your left tire wants to pull to the right just a fuzz, and the right tire wants to pull the left a lot. Both wheels are tipped in 0.11 degrees too much, but that is insignificant. I am very happy to say you have the exact 0.40 degrees pull to the left you are supposed to have.
Before we started, I should have asked if you have non-standard aftermarket wheels. If they are wider or have a deeper offset than what was designed for the car, all bets are off and we have to start looking at modifying the desired settings. The original wheel dimensions will place the vehicle's weight perfectly centered over the middle of the wheel bearing. Using different wheels upsets a secondary alignment angle we do not usually talk about called "scrub radius". It has a huge effect on handling and braking, but not on pulling.
There is a more basic printout the computer can make that has just a whole screen full of numbers. That will include "steering axis inclination", (SAI). It gets measured automatically by the computer. GM owners run into a lot of trouble with this because the front cross member must be removed to remove the engine or transmission. On Chrysler products, the mounting bolts have a wider area right under the heads that forces the cross member to be reinstalled in the correct position. There is no such provision on GM cars. All that is important with SAI is it must be exactly the same on both sides. Usually there is not even a spec. Listed; it just has to be equal. When it is not equal, you have no idea which way the car is going to dart when you hit even a tiny bump in the road. You can look and feel like you are drunk, and you will not want to drive that car very far. It is very tiring. The reason I brought that up is it is something we look at if we suspect the car was crashed and repaired hastily. If you have another printout that shows all these numbers, a typical value for SAI is around 28 to 32 degrees. Basically, if you have struts on the front, SAI is the angle that strut is tipped at, as viewed from in front of the car. If you have upper and lower ball joints, it is the imaginary line drawn through those joints.
The next concern is "caster". That would be the rearward tilt of the strut, or the line drawn through the ball joints, as viewed from the side of the car. Think of the rake of the front fork on a bicycle. It is tipped back as it goes up to the handlebars. When you put your weight on the bike, caster is what makes the fork squirt straight to the front so you can ride no-handed. On a car, caster makes the wheel try to turn toward the center of the car with such force, you would not be able to pull it back by hand. Think of one half of a teeter totter with a two hundred pound person sitting on it. Your job is to twist it right by the pivot point to try to lift that person. You do not have enough strength to do that even without the person on it.
Now add the other half of the teeter totter and another two hundred pound person on the other side. It is real easy now to see that it is balanced, and with one hand you can move it one way or the other. With caster, that wheel wants to turn in real hard, but so does the other one on the other side. When we connect them together with the steering linkage, they become balanced, and offset each other, so the car goes straight, unless they are not equal. The higher the caster value, the harder that wheel wants to turn toward the center of the car. All cars call for caster to be equal, but it is standard practice to make it a little higher on the right front wheel. That makes it want to pull left harder than the left one wants to pull right, and we use that to offset road crown too. For the purposes of this sad story, caster has no effect on tire wear, but camber has a major effect. That is why we like to look to making caster unequal to offset road crown rather than getting too aggressive with camber.
I used to tell my students I had five ways to explain how caster works. This is really difficult because it has to be visualized and imagined. I told them I would explain it a different way each day, starting on Monday. I never made it to Thursday because everyone understood it by Wednesday's description. For this story, all we have to be concerned with is if everything else is equal on both sides, the car will pull toward the side with the lowest caster reading. Also, caster is what makes the steering wheel return to center on its own after you go around a corner. Both wheels and the entire steering system want to come back to where caster is equal on both sides. If we start out with higher caster on the right, both sides will be equal when the steering system is turned slightly to the left, and there is our left-hand pull to offset road crown.
I realize this is getting overly-complicated, and to add to the confusion for anyone else researching this topic, everything I described about caster causing a pull and/or being used to offset road crown, does not apply to ninety nine percent of front-wheel-drive cars and minivans. I do not know why that is. It is still responsible for centering of the steering wheel after rounding a corner, but I had one car with 3.00 degrees more caster on the right side, and it still went straight after I aligned it. (Caster is not adjustable on almost all front-wheel-drive cars, and it does not need to be).
The most common value for caster is around 3.00 degrees. By the way, in the 1950's and earlier, we used negative caster, meaning the upper ball joint was further ahead of the lower one. That makes steering real easy and is why it was not hard to steer a big, heavy truck without power steering. In the 1960's, we started driving faster, and positive caster reduces steering wander and adds to stability. With negative caster, you would be constantly correcting the direction of steering, and the car would be very tiring to drive. Positive caster keeps the car going straight with just one finger on the steering wheel, however, the higher positive caster goes, the harder it is to turn the steering wheel, especially at lower speeds. That is why we added power steering. Try turning the steering wheel with the engine not running. That is what you would be driving if you did not have power steering. You actually would not miss it much at highway speeds, but parallel parking would be so tough, you'd rather walk to the store!
Your car calls for 5.40 degrees caster, plus or minus 1.00 degree. That one degree tolerance does not mean one wheel can be at 4.40 degrees and the other at 6.40 degrees. It means the two readings must be within that range, but they still need to be equal. I remember that Jeeps and Mercedes Benz's call for around 11.00 degrees caster, which is real high and not recommended for other cars. There is not enough range of adjustment on other cars to get close to that much caster. They have some other design variable that makes them more stable with that higher caster, but that leads to another problem known affectionately as "the death wobble". After rounding a corner, the high caster makes the steering wheel return so hard and so fast that it overshoots and goes the other way, then comes back and does that continuously until you slow down to about twenty mph. These vehicles will have a steering stabilizer connected between the frame and the steering linkage. It is just like a shock absorber, except it pulls apart and goes together with equal force both ways. When the hydraulic oil leaks out of those and the death wobble occurs for the first time, it will be a life-changing experience that you will never want to live through again.
Just saw your reply; thank you. You have 5.52 degrees caster on the left, and 5.84 degrees on the right. That comes out to 0.32 degrees caster pull to the left. Caster has half the effect on pulling as does camber, so this would by like adding another 0.16 degrees camber pull to the 0.40 degrees camber pull to the left that is called for.
This brings me to where I can only suggest what other factors come into play that might be responsible for the pull. We already discussed the tires. They will be the cause ninety nine percent of the time. What follows are just some things we have to think about when diagnosing this complaint. The first one is brake pull. I do not know what system your car uses, but when Chrysler developed the first domestic front-wheel-drive car in the late 1970's, there was around eighty percent of the car's weight in the front where it used to be seventy percent with rear-wheel-drive models. The brake system on the older cars was split front and rear. If a leak developed in a front hose, you still had the rear brakes, but only thirty percent of normal stopping power. With so little weight on the rear of front-wheel-drive cars, with a leak in the front, you would apply the brakes, and they would find you in the next county before the car skidded to a stop! To address that, they developed the "split-diagonal" hydraulic system. That puts one front brake and the opposite rear brake on the same system. When a leak developed, you would still have one working front brake and fifty percent of normal stopping power. If you have ever seen someone pinch off a leaking front line on a rear-wheel-drive car, you would know that when applying the one working brake, it would violently tear the steering wheel out of your hand, and send the car veering to that side. Remember when I mentioned "scrub radius" earlier? That is a non-adjustable angle that cannot really be measured, but it involves the geometrical relationship between all the front suspension and steering parts. Basically what it causes is if you look down from on top of the left front tire where its tread hits the road surface, the right half of that tread wants to tug the tire to the right, and the left half of the tread wants to tug the tire to the left. The engineers modified that angle on front-wheel-drive cars so when only one front brake is working, (lets say the left front one), the left half of the tread is now the left 1/4 of the tread that wants to pull left, and the right 3/4 wants to pull to the right. That increased alignment pull to the right offsets the brake pull to the left. For decades, Chrysler has had scrub radius so well perfected that often a customer only knows there's a brake system problem by the red "brake" warning light on the dash turning on. There can be no other symptom at all. On other car brands, at most you will just see a little wiggle in the steering wheel when the brakes are applied.
My entire reason for sharing that wondrous paragraph is to explain why I am highly doubtful your pull is caused by a dragging brake. When there is pressure on the brake pads, they do not know if it is because a caliper is sticking, (which is not common today), or if it is because you have your foot on the brake pedal. Regardless of the cause, since the engineers have gotten scrub radius so perfect, it only stands to reason they would use their knowledge to design that in to rear-wheel-drive cars too. A sticking brake would act the same as applying only one working front brake, meaning no pull or any other symptom.
The next thing to consider came to light in the late 1970's and early 1980's when what passed for "muscle cars" of that era started coming from the factory with really wide tires. I told you a tire wants to roll in the direction it is leaning, but now imagine laying on the ground in front of the left front tire, and watching where the tread makes contact with the road. As camber is increased more and more, that tire tips out on top further and further, and further, and as you are watching the tread, you will see the inner edge start to lift off the road surface. A normal tire would just fall over, but with these wide tires, it has not gone over center yet. The weight of the car on that tire wants to push the inner edge back down onto the road. In other words, that tire is trying to stand back up straight. That force can push the car to the right. Increasing left front camber even more, from everything theory tells us, should make the car pull more to the left, but with real wide tires, the more we raise camber to pull left harder, the more it will make it pull even harder to the right. (Visualize a little cloud of smoke coming out of our ears)! This gets real frustrating because the more we adjust camber, then go on another test-drive, the more results we get that are opposite of what we expect.
Another way to look at this is there is little weight on the left tire's inner edge. Only the outer edge is scrubbing on the road and creating a road force. The entire right tire is making contact and scrubbing harder, so with its higher road force, or resistance to rolling, it causes a pull to the right.
My last comment of little value came to mind when I saw your camber pull left is exactly perfect. It is impossible to achieve that except on rare occasions. I should not even mention this, but if an alignment has to be done when the car is under warranty, the manufacturer paying the bill insists on seeing a perfect set of numbers on the alignment printout, otherwise they will not pay for it. Sometimes the customer has added heavy equipment or some other modification that has to be accounted for when modifying the adjustments slightly from specs. We had to balance the desire to get paid with the need to satisfy the customer. As such, the alignment was adjusted to provide the best handling and tire wear, then the car was manipulated to make the numbers look good on the computer so the printout could be made. Those "tweaks" typically involved opening a door, but in extreme cases it could include letting some air out of a tire momentarily or even leaning on the car. You must remember the alignment is done with the car empty, and everything changes a real lot more when a two hundred pound driver is poured into the car for the test-drive. Opening a door half way lowers that side of the car less than 1/8th inch. Dumping a driver in the car lowers that side a good inch or more.
The last adjustment is "toe". That is the direction the wheels are steering. First we adjust the rear wheels to be parallel to the car body. Next, the steering wheel is centered and locked, then the two front wheels are adjusted to make them parallel to the rear wheels. Even when all four wheels might be turned a little, this insures the steering wheel will be straight. Toe is the other major angle that affects tire wear. Too much camber will cause accelerated wear to the inside or the outside edge of the tread on just that one tire, (although camber can be off on both tires and both will have wear on one edge). Toe always affects wear equally on both tires on that axle. Even if only the left wheel is mis-adjusted too far to the left, the car would tend to steer to the left. We would have to counteract that by turning the steering wheel to the right until both wheels were turned out an equal amount to make the car go straight. That is oversimplifying the explanation, but it results in a choppy tread pattern on both tires, and in this case, an off-center steering wheel. Keep in mind, when replacing steering parts, you can have both sides adjusted wildly wrong, but if they are equal, the steering wheel can still be straight. The only way to know where the correct toe adjustments are is with the alignment computer.
It is also possible to have both wheels mis-adjusted to the same direction an equal amount. The two toe readings taken together is "total toe". That is the number that affects toe wear on the tires. If total toe is perfect, but both wheels are adjusted to the left an equal amount, tire wear will be good, but to bring the wheels back to center, the steering wheel will be off-center to the right. Slightly mis-adjusted total toe will go unnoticed, but as the error becomes greater, the first thing is you will see the choppy wear patterns after a few weeks or months. It is when total toe becomes even worse that unusual things can happen. When you have total "toe out", both wheels are steering away from the center of the car. The car can only follow one of them. That will be the tire with the most weight on it. Usually that is the right one because since the roads lean to the right, more of the car's weight is on that side. The other tire will slide slightly sideways down the road. Even when that isn't bad enough to notice, what you will notice is when a strong wind blows across the road from left to right, it pushes the car over so more of its weight goes on the right side, and guess what, the car follows that tire and drifts to the right. You have to counteract that by turning the steering wheel to the left, then, when the wind stops blowing, or you cross a level stretch of road, as in an intersection, the car suddenly drifts to the left.
Total toe for your car is listed as 0.10 degrees which equates to 1/16". Alignment computers can be set to read toe either way. This spec. Means the edges of the wheels are 1/16" closer together at the front of the wheels than at the rear of them. To say that more simply, both front wheels are steering toward the center of the car by a barely perceptible amount. If no steering or suspension parts are worn, normal road forces will be just right to tug the wheels back while driving so they're perfectly parallel to each other.
Toe for each of your front wheels is shown as 0.04 degrees. That is close enough to a total of 0.10 degrees to be considered perfect.
Judging from these numbers, I'd say the alignment is perfect. We did have some Goodyear tires in the early 1990's that over-exaggerated road crown and needed to have a lot more camber pull to the left adjusted in for the cars to go straight. Other than that, there was nothing wrong with the tires. I went a step further and told the customers I changed some adjustments to compensate for the tires, so when they bought new tires a few years later, they understood the alignment would need to be set back to factory specs. That hopefully avoided them being angry with the tire salesman and unfairly blaming a pull on his product.
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Monday, February 19th, 2018 AT 3:10 PM
