Whenever a part is misaligned, whether in the engine, transmission or suspension, unsafe driving conditions can occur. In addition, there is an increasing possibility "folding" road trains when driving on dangerous roads, if the twin axles of the trailer are not parallel. If all axes of the road train are parallel, then it will move in a straight line along the road. But if the axles of the trailer 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 driving difficult and causes tire side slip in addition to forward motion. There is premature wear of the rear tires and additional wear of the front. Of course, this tire slip caused by the misalignment of the axles will cause an increase in fuel consumption.
A car with non-parallel bridges is undoubtedly much harder to drive on dangerous roads. If the misalignment is large enough, then the car can become dangerous even on a good road, as the driver must constantly fight against the tendency of such a car to turn.
The parallelism of the tandem axles must be checked each time the vehicle is brought into the workshop for suspension maintenance. Such a check should be carried out no more than after 50 thousand km of the tractor, and the trailer - from 160 thousand to 200 thousand km on the appropriate equipment.
The need for the next centering of the axes may occur before the expiration of 160 thousand kilometers. It can be caused by small bumps (when driving over a curb or hitting a corner post) or collision damage. Even normal wear on the torque rod bushings on one side of the vehicle can cause axle misalignment.
When driving on the left side, the starboard wheels take more shock loads than the left side wheels, as they move more over bumps on the edges of the road, and also as a result of the redistribution of the car's weight on the sides due to the fact that most roads have a slightly convex profile. Normal wear on the bushings or replacement of the torque rod can cause tandem axles to be out of parallel. There is no doubt that checking the parallelism of bridges during any repair work should become the rule.
Radial ply tires are more sensitive to axle misalignment than bias ply tires for all types of axles and suspensions. Symptoms of wear of tires with a radial ply often appear after 16-32 thousand km of run, and for tires with a diagonal ply - after 80-92 thousand km of run.
Tires with a radial cord mounted on steered axles are characterized by wear on the shoulder areas on the inside and outside. When turning, the edges of the tread tend to rise, since, unlike tires mounted on drive axles, in this case there is no force that maintains the plane of contact of the tire with the roadbed. This wear usually does not affect the tire's durability, as it only occurs in the first 0.79 mm of the tread width.
Axle misalignment usually results in lateral wavy tire wear. If the axles are not parallel and the trailer tends to move away from the longitudinal axis of the road train, there will be uneven wear on the inner and outer tire for dual tyres, or the inner and outer tread for single tyres. This causes tire slippage and undulating wear, much like the wear of a tire mounted on a steered axle. This uneven tread wear can result in "scalping" tires.
The forms of wear of a tire with a radial and diagonal ply 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 areas. In the second, 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 detected 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 conventional tires.
Axle misalignment wear probably won't show up as quickly on wide section single tires because they wear less around corners than regular section dual tires. However, they are more expensive, so checking the parallelism of the axles when installing single wide tires is a must.
For new trailers, it is necessary to check the parallelism of the installation of their axles. However, we must remember that this adjustment will not be the last. There is a grinding period for 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. Usually the offset of the longitudinal axis of the chassis (relative to the axis of the bridge) increases over the life of the trailer. The greater the mileage of the trailer, the more wear occurs in the suspension guides or earrings.
While tire wear and fuel consumption, which depend on the parallelism of the axles, can be determined, traffic safety, which is an even more important factor, is not easily measured. Regular checks and alignment adjustments of axles for safety reasons only significantly reduce the additional costs associated with damage, accidents and vehicle downtime.