By Jerry Cole, Technical Advisor
My daughter took her Mercedes W123 in to get a front end alignment to solve a severe wear problem on the inside edge of the driver’s side front tire. She was presented with an estimate of $276 for repairs to the steering rack and over $100 for one tie rod end before the front end could be aligned. Since neither one of those is going to correct severe wear on the inside edge of one tire and her car doesn’t even have a steering rack at all, it became evident that a little familiarization with the front end and the three major adjustments is in order.
Up until very recently, there have been two types of steering systems: reciprocating ball and rack and pinion. Refer to the above diagram for greater clarity. A reciprocating ball steering system has changed very little in many decades. It employs a power or manual gear box at the end of the steering column. Connecting the steering gear to the center (or drag) link is what is known as the pitman arm. The other end of the center link is connected to the idler arm. Also connected to each end of the center link are the tie rods. The outer ends of the tie rods connect to the steering knuckle at the front wheels. This assembly is required to make the wheels turn together when the steering wheel is turned left or right.
In a rack and pinion system, the steering gear and the center link are one unit. A pinion gear attached to the steering column. The pinion gear moves a toothed piston inside the rack left or right as the steering wheel is moved. The inner end of the tie rods are connected to each end of the toothed piston and the outer ends of the tie rods are connected to arms at the lower steering knuckle at both of the front wheels.
The first of the three major adjustments is toe. Toe is the relationship between the leading edges of the front tires. If the leading edges of the front tires are pointing away from each other, they are said to have positive toe. If the leading edges of the front tires are pointing toward each other, they are said to have negative toe. Toe is adjusted by lengthening or shortening the tie rods in order to make the wheels point straight forward when the steering wheel is centered. Improperly set toe causes the wheels to drag as they roll. This costs power, has a negative effect on cornering and the car’s ability to track straight and causes premature tire wear along the edges of the tires.
The second major measurement is camber. Camber is the angle of the wheel compared to vertical as viewed from the front of the car. If the wheel leans in at the top it has negative camber. If the wheel leans out at the top it has positive camber. Suspension systems are designed to increase the negative camber the harder a car corners. This maximizes the tire’s ability to grip In a nutshell, the camber measurement at rest or running down the road in a straight line is not the same as it is while cornering. Excessive negative camber while running in a straight line will cause moderate to severe wear along the inside edge of the tire and excessive positive camber wears out the outside edge of the tire. Aside from improper adjustment, popular causes of excessive negative camber are worn upper or lower control arm bushings or worn ball joints.
The third major adjustment is the most difficult to picture and that is caster. Caster is the angle that the steering pivot axis is tilted from vertical much in the same way that the Earth is tilted on its axis. A rearward tilt is called negative caster and a forward tilt is called positive caster. Positive caster is designed into steering because it tends to pull the wheels into line when the vehicle is traveling forward. Another benefit of positive caster comes when turning. Since the pivot point is not vertical, the angle increases even more in a turn which aids in cornering.
As you have probably guessed by now, ideal settings for a front end alignment are not a bunch of zero degree settings. Each model has its own range of specs for each measurement. If the front end can’t be aligned within the range of specs, it is most likely steering or suspension component wear or something is bent. Control arms and tie rods can be bent by losing a battle with a highway barrier. I know this firsthand but I won’t give her name here as my health and well-being depends on it. By the way, a quick glance at my daughter’s W123 revealed that the reason for the severe tire wear on the inside edge of the driver’s front tire was caused by a failed lower control arm bushing. The bushing allowed the bottom of the front wheel to move outward, causing severe negative camber.
It wasn’t even mentioned in the estimate. It pays to get to know your Mercedes. It looks like I will be spending some quality time under her car replacing control arm bushings and tie rods before we try the alignment thing again.