"A friend"? You have a friend?.... What's it like? :)
Hi missp2001. Welcome to the forum. Ok, first of all, ask your friend if he unbolted and lowered the engine cradle. That is common practice on some cars to remove the transmission or in some cases the engine, but some people do it for suspension work too. Only on GM front wheel drive cars, it is almost certain you will not get the cradle back in the same place it was before. An alignment can give the appearance of overcoming the mispositioning but there is another underlying problem.
The alignment angle in question is called "camber". It is the inward or outward tilt of the wheel as viewed from the front or rear of the car. Positive camber is tilted out on top. Your car calls for 0.00 degrees of camber, as I recall, which is perfectly straight up, and is common for many GM cars. The adjustment is made by loosening the two lower mounting bolts where the strut attaches to the spindle, and pushing or pulling the spindle in or out. That will change camber and correct it when it is off due to other causes, but it won't fix those other causes. Tires tend to want to pull or steer in the direction they are leaning. Besides each wheel being in specs, they both need to be near equal to offset each other's pull. The left one might be set a little higher positive to offset the effects of "road crown". That means roads lean to the right so rain water will run off.
The top of the strut holds the spindle and wheel in position to maintain proper alignment. If you were to move a strut in, that would tip the wheel and tire in on top too, changing camber. In effect, that's what happens when the cradle is shifted to one side. One strut will be standing up straighter than normal, and the other one will be leaning in on top more than normal. Here's where the problem comes in. Imagine a line drawn through the center of the wheel from top to bottom. That's camber, and in your case it's perfectly vertical. Now imagine a line drawn through the lower ball joint and through the center of the upper strut mount, (the large nut on the strut shaft that you can see under the hood). That angle is called the "scrub radius". It is important for both angles to be the same on each side. If the engine cradle is shifted sideways, scrub radius will increase on one side and decrease on the other. Camber will do that too, but only camber can be readjusted. Camber is one of the three basic alignment angles. Scrub radius is measured automatically by most modern alignment computers, but it is rarely examined unless there is a problem such as you are having. The common causes of unequal scrub radius are bent parts, misadjusted camber on cars where the adjustment is movable upper struts mounts, (common of Ford cars), and on GM cars, a shifted cradle.
The alignment shop should have given you a printout of the alignment readings before they made any adjustments and when they were finished. The clue that I believe what they are saying is that you said it pulls left AND right. Unequal camber will cause a pull toward the wheel with the higher positive value, when you let go of the steering wheel. Unequal scrub radius will typically cause a pull one way during acceleration and the other way during braking. It can be rather miserable to drive.
Look at the printout or ask to see the numbers on the screen of the alignment computer. As I recall, a typical value for scrub radius is around 28 degrees. 0 degrees means the strut would be standing straight up, which they aren't. 90 degrees means it would be laying on its side, which it also isn't. There is no spec given because there is no right or wrong value; only that the two values must be the same. I don't remember the exact tolerance allowable, but a 2 degree difference in scrub radius side-to-side is severe. I seem to recall a 0.5 degree difference is too much. To correct unequal scrub radius, the four engine cradle mounting bolts must be loosened, then a pry bar is usually sufficient to slide the cradle one way or the other until the numbers ARE equal. Once scrub radius is set, camber can be adjusted until it too is equal.
Even if your friend didn't mess with the cradle, scrub radius could be off. That's the first thing to check. Rather than using that as an excuse for a less than perfect alignment, the mechanic should have corrected scrub radius if he noticed a pull or torque steer during the final test drive. The next thing is to question him about when he tightened the control arm bolts after the new bushings were installed. The arms will be hanging down due to that pesky gravity. If the bolts are tightened that way, the bushings will be in a permanent twist when the car is lowered and the control arms move up. Any rubber bushings that twist when the car is jacked up by the frame must not be tightened until the car is sitting on the tires, on level ground. It is smart to drive the car through a parking lot, then crawl underneath or drive it onto a drive-on hoist to tighten the bolts. That will center them. If the new bushings are relatively stiff, one of them being in a permanent twist can actually raise one corner of the car. That can affect camber slightly too. Simply loosening the bolts and retightening them while the car is sitting on the tires will get rid of any twist.
One final thing to be aware of is many GM front wheel drive cars have a lot of torque steer. We had one many years ago that could be driven almost 30 miles on the highway without once touching the steering wheel. To go right, hit the gas pedal harder. To go left, let off the gas a little. To eliminate that, the two half shafts must be the same length which is not possible. Instead, a two-piece half shaft is used on the right side. The outer half is the same length as the entire left shaft so they sit at the same angle relative to the ground. Anything that affects or causes unequal ride height, such as weak springs, or those twisted bushings, will cause different half shaft angles and torque steer.
If you received a printout for your last alignment, what were the final numbers for left and right front camber and scrub radius?
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Monday, July 12th, 2010 AT 2:31 AM