Whenever any component is out of alignment, whether it is in the engine, transmission or suspension, unsafe driving conditions can result. In addition, there is an increased possibility of a jackknifing when driving on dangerous roads if the tandem axles of the trailer are not parallel. If all axles of the trailer are parallel, the trailer will travel in a straight line down the road. But if the axles of the trailer are not perpendicular to the longitudinal axis of the trailer, the trailer will tend to move in the direction to which its axles are perpendicular. This makes the vehicle difficult to steer and causes lateral tire slippage in addition to forward motion. This causes premature wear on the rear tires and additional wear on the front tires. Of course, this tire slippage caused by the non-parallel axles will cause increased fuel consumption.
A car with non-parallel axles is certainly 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 tendency of such a car to turn.
The parallelism of the twin axles must be checked every time the vehicle is taken to the workshop for suspension maintenance. This check must be performed no more than after 50,000 km of the tractor and 160,000 to 200,000 km of the trailer using the appropriate equipment.
The need for the next axle alignment may occur before the end of 160 thousand km of run. It may be caused by small impacts (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 axles to become out of alignment.
When driving on the left, the right wheels take more impact loads than the left wheels, because they move more over the uneven edges of the road, and also because of the redistribution of the vehicle's weight along the sides due to the fact that most roads have a slightly convex profile. Normal wear of the bushings or replacement of the torque rod can cause non-parallelism of the tandem axles. It is certainly a rule that checking the parallelism of the axles during any repair work should be the rule.
Radial tires are more sensitive to axle misalignment than bias tires for all axle and suspension types. Symptoms of wear on radial tires often appear after 16,000 to 32,000 km, while for bias tires, they appear after 80,000 to 92,000 km.
Radial tires mounted on steering axles are characterized by wear of 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 maintaining the plane of contact of the tire with the road surface. This wear usually does not affect the durability of the tire, since it is observed only on 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 between the inner and outer tires on dual tires or between the inner and outer treads on single tires. This causes the tire to slip and wear in a wavy pattern, much like a tire mounted on a steer axle. This uneven tread wear can result in tire scalping.
The wear patterns of radial and bias-ply tires are different. For example, the former will show the same tread wear around the inner and outer shoulder area under either too much or too little air pressure. The latter will show increased wear in the center of the tread under high air pressure, and increased wear in the inner and outer edges of the tread under low air pressure. This wear is usually more noticeable in bias-ply tires than in radial tires. A single wide-ply tire has the same contact patch with the road surface as two dual tires of a normal profile.
Wear caused by axle misalignment will probably 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 axle parallelism when installing single wide profile tires is a must.
New trailers need to have their axles checked for parallelism. However, it should be remembered that this adjustment will not be the last one. 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. Usually, the longitudinal axis of the chassis is displaced (relative to the bridge axis) increases over the life of the trailer. The greater the mileage of the trailer, the greater the wear and tear on the suspension guides or cleats.
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 to axle parallelism for safety purposes alone significantly reduce additional costs associated with damage, accidents and vehicle downtime.