Helpful information on Camber and Caster Alignment Information for Technicians

Camber or Caster or a combination of the two seem to be one of the most popular settings for alignment technicians. To be clear, Camber is the inward or outward tilt of the tires relative to vertical and Caster is the forward or rear ward tilt of the king pin relative to the same. Camber is built into the axle and caster is accomplished by elevating the mounts for the front or rear of the leaf springs combined with tapered wedges between the springs and the axle. Since the axle is built with little or no difference in caster from one side to the other (no more than .5 degrees) and is intended to remain that way, use of wedges on one spring stack and not the other, or wedges place in opposition to one another is not a recommended practice. Many alignment techs use Caster and Camber to try and control the tendency of the vehicle to pull in one direction or the other. In fact all manufacturers of axles used in North America that I am aware of: “Expressly prohibits bending of axle beams (hot or cold) to change camber or for any other purpose.” This quote is from the Dana Service Manual and similar statements can be found in all other axle manufacturer’s manuals. This is one of the reasons that I do not spend time on these settings. The other reason is that the steer axle is not the dominate axle under the vehicle.

Consider this: In the US, a standard highway tractor will have one steer axle and two drive axles. The steer axle will carry a normal load of about 12,000 lbs and have NO horsepower in it. The drive axles will carry almost three times as much weight, 34,000 lbs and have all the horsepower. This means that the use of Caster and Camber to control the direction of the vehicle require the lightest axle with the least amount of horsepower to counter the heavier and more powerful axle. The obvious result is premature and irregular wear on the steer axle tires. I find it is much more effective to use the alignment of the drive axles to control direction and thereby extend steer tire life. View of drive axle alignments and their effects.

As a final point on Camber: When the TMC Task Force was investigating alignment issues, they followed up on 1,000 camber complaints and on inspection found that none of them, that’s ZERO, NADA, ZILTCH, were actually camber tire wear problems. My experience in the field has had similar experience. The difficulty is that many techs do not know how to identify tire patterns. They just go by “SPECS” and try to make them right and hope the tire will perform properly. If we are not going to use Caster or Camber to control the direction the truck moves then I suggest we use the drive axles. Consider an outboard motor boat. When the motor is aimed to the left the boat steers to the right. Aim it straight ahead and the boat goes straight ahead. On flat ground a truck will do the same thing. However we do not drive on flat ground. In North American, we drive on the right side of a crowned road. In other words the road slopes to the right and because of the effects of gravity, the truck wants to pull to the right.

Back to the outboard motor example, when crossing a current flow like in a stream, if the flow is from your left you have to aim the motor slightly to the right to push the nose of the craft into the current in order to cross the stream. On a single drive axle vehicle the same rule applies. Aim the drive axle very slightly to the right, about 1/32nd inch shim, and the front of the vehicle will push up against the crown of the road and counter the “down hill” effect of gravity. Aim it too far to the right and the vehicle will pull to the right because the drive force is now pushing in the same direction as the force of gravity. We have found there are four drive settings of a single drive axle vehicle. Aim it left and you get a right pull. Aim it straight and you get a right pull. Aim it slightly to the right and it drives straight. Aim it too far to the right and the vehicle pulls right again.

Tandem drive vehicles have another force that needs to be taken into account. The “scrub” angle between the drive axles creates a force that must be addressed. Ideally we would like to set both drive axles perfectly parallel to each other, however in the real world that is virtually impossible. They are held in position with rubber bushing and air bags while less than 17,000 lbs weight per axle and transferring the horsepower from the engine to the road. Depending on the model of suspension the give, flex or compression of the support components can allow small or large changes to the Alignment in dynamic conditions.

Since I cannot be confident that the axles will remain perfectly aligned to each other, we find that setting a slight scrub angle between them is more effective. By aiming the front drive slightly left and the rear drive slightly right we create a “scrub angle” or “cone” between the drive axle that causes the vehicle to push against the crown of the road and counter the gravitational effects of the slope of the road. An example of this is your tapered coffee cup. Lay it on its side and roll it. The cup will roll in the direction of the smaller end of the cup. Similarly, by keeping the narrow gap between the drive axles on the left side of the vehicle, the force created by the “scrub” angle assists in fighting the effects of gravity. None of the settings we use are outside of factory specs and we avoid the issues associated with camber and caster adjustments.

One other point of drive axle alignment, the drives need to be centered under the frame when measured from side to side. Like our old friend the outboard motor boat, if the motor is mounted off center, the drive force is off center, and the push will drive the vehicle in the opposite direction. 70% of steer tire alignment tire wear issues we see are related to drive axle alignment. This means that the most important alignment setting you make is with the drive axle where most of the weight and all the horse power is located. We have explored what I find to be the main issues for alignment on Trucks looked at feathered wear which I consider to be the main tire wear result of miss-alignment. Next we’ll explore the other tire wear pattern on commercial truck tires, cupping. This pattern can manifest in an isolated section of the tire, around 1/3 third of the circumference or all the way around the tire. It can appear on one shoulder or both, or alternate shoulders around the tire. It can appear in the center rib or the 2nd rib.

All of these visual clues are helpful in identifying the source of the problem. The pattern of cupping changes between tread designs and casing structure. For example, a steer tire will not show the same cupping as a drive tire for the same principal mechanical cause. In the same way a Low Rolling resistance tire cups from issues that did not cause cupping in the old Standard tires As engineers change tire construction, compounds and tread designs, suspension designs, brake performance and engine horsepower the tire changes how it reflects the input of force into its footprint. This means that the alignment tech and Maintenance Managers must be open to new evaluations of the performance Characteristics the tire. Nothing remains the same in this area of the industry so we must remain open to new ideas and solutions. There are seven significant causes for cupping in commercial truck tires.

1. Inflation
2. Lack of Balance
3. Miss-Mount
4. Bent rims
5. Feathering
6. Loose Components
7. Tall tire / Short Tire.

As you can see we can go on and on, but we have the ability to correct bad wear patterns in tires. Tires are a major expense to us in the Leasing & Rental industry. We need to be able to control that cost, the best way is to use the best tire for cost per mile. To do that we need to get maximum wear out life from it. By not doing so we increase the units cost per mile exponentially. We need to be able to recognize this issue and to stay on top of it. Don’t be a part changer find the root cause of the problem and solve it there. Look at it as if it’s your truck, your tires and your money what would you do.

Robert Loughlin
Fleet Maintenance Consultant
Bentley Lease and Rental Fleet