Whenever any part is out of alignment, whether it is in the engine, transmission, or suspension, unsafe driving conditions can result. In addition, there is an increased risk of the road train "jackknifing" when driving on dangerous roads if the trailer's dual axles are not parallel. If all axles of a road train are parallel, it will move in a straight line along the road. But if the trailer axles are not perpendicular to the longitudinal axis of the road train, then the trailer will tend to move in the direction to which its axles are perpendicular. This makes the car harder to control and causes the tires to slip laterally in addition to forward motion. Premature wear of the rear tires and additional wear of the front tires occurs. Of course, this tire slippage caused by the non-parallelism of the axles will cause an increase in fuel consumption.
A car with non-parallel axles is undoubtedly much more difficult to control on dangerous roads. If the non-parallelism is great enough, the car can become dangerous even on a good road, since the driver must constantly fight the car's tendency to turn.
The parallelism of dual axles should be checked every time the vehicle is brought into the workshop for suspension service. Such a check must be carried out no more than after 50 thousand km of mileage for the tractor, and for the trailer – from 160 thousand to 200 thousand km using the appropriate equipment.
The need for the next axle alignment may arise before 160 thousand km have passed. It can be caused by minor impacts (when driving over a curb or hitting a corner post) or collision damage. Even normal wear on the suspension reaction rod bushings on one side of the vehicle can cause the axles to become uneven.
When driving on the left, the right wheels experience more impact loads than the left wheels, as they move more over uneven road edges, and also as a result of the redistribution of the vehicle's weight along the sides due to the fact that most roads have a slightly convex profile. Normal bushing wear or torque rod replacement can cause dual axles to become out of alignment. It is certain that checking the parallelism of bridges during any repair work should become a rule.
Radial ply tires are more sensitive to axle misalignment than bias ply tires for all axle and suspension types. Symptoms of wear on radial-ply tires often appear after 16,000–32,000 km, while for bias-ply tires, they appear after 80,000–92,000 km.
Radial ply tires installed on steering axles are characterized by wear on the shoulder areas on the inner and outer sides. When turning, the edges of the tread tend to rise, since, unlike tires mounted on drive axles, in this case there is no force maintaining the plane of contact of the tire with the road surface. This wear does not usually affect the life of the tire, as it only occurs on the first 0.79 mm of tread width.
Axle misalignment usually results in lateral wavy wear on the tire. If the axles are not parallel and the trailer tends to move away from the longitudinal axis of the road train, uneven wear will occur between the inner and outer tires on dual tires or between the inner and outer tread areas on single tires. This causes the tire to slip and wear in a wave-like pattern, very similar to that of a tire mounted on a steer axle. The result of such uneven tread wear can be "scalping" of the tire.
The wear patterns of radial and bias ply tires are different. For example, in the first of them, both too much and too little air pressure leads to the same tread wear near the inner and outer shoulder area. In the second case, with increased air pressure, the wear of the central part of the tread increases, and with reduced air pressure, the wear of the inner and outer edges of the tread increases. This wear is usually seen more quickly on bias-ply tires than on radial-ply tires. A single wide profile tire has the same contact patch with the road surface as both dual normal profile tires.
Wear caused by axle misalignment will likely not appear as quickly on single wide profile tires because they wear less on corners than dual normal profile tires. However, they are more expensive, so checking the parallelism of the axles when installing single wide-profile tires is a must.
New trailers must have their axles checked for parallel alignment. However, it is important to remember that this adjustment will not be the last. There is a period of grinding in of parts, especially when using a four-spring suspension.
After a short run, the chassis should be checked, as some shift to the right is possible due to gaps in the earrings. Typically the longitudinal axis of the chassis is displaced (relative to the bridge axis) increases over the life of the trailer. The higher the mileage of the trailer, the more wear occurs in the suspension guides or earrings.
While tire wear and fuel consumption, which depend on maintaining axle parallelism, can be determined, driving safety, which is an even more important factor, is not so easy to measure. Regular checks and adjustments of axle parallelism for safety purposes alone significantly reduce additional costs associated with damage, accidents and vehicle downtime.