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AUSTRALIAN HEAVY VEHICLE WHEEL ALIGNMENT ASSOCIATION
benefits of a heavy vehicle wheel alignment
Whether you drive a truck, bus, 4WD, road train, crane, people mover or just a family sedan, wheel alignment can have a dramatic effect on steering, road handling, driver fatigue, tyre wear, fuel consumption and general damage to your vehicle.
Safety and Financial Benefits
Apart from the safety factors of improved steering and roadhandling and reduced driver fatigue with increased comfort, correct wheel alignment can bring significant financial benefits to any form of road transport.
Correct wheel alignment can not only extend the life of your tyres, but it can also improve your fuel economy.
Symptoms and Solutions
Here are some of the factors to consider in wheel alignment, with a description of the symptoms, andan explanation of the benefits of correction.
Camber
As the vehicle moves forward, the wheels lean to one side or the other in relation to the axle centre ("positive" or "negative" camber). The angle of the camber is a factor in the design of steering geometry and incorrect camber will not only affect handling, but can also cause excessive wear to the edge of the tyres.
Safety and Financial Benefits
Apart from the safety factors of improved steering and roadhandling and reduced driver fatigue with increased comfort, correct wheel alignment can bring significant financial benefits to any form of road transport.
Correct wheel alignment can not only extend the life of your tyres, but it can also improve your fuel economy.
Symptoms and Solutions
Here are some of the factors to consider in wheel alignment, with a description of the symptoms, andan explanation of the benefits of correction.
Camber
As the vehicle moves forward, the wheels lean to one side or the other in relation to the axle centre ("positive" or "negative" camber). The angle of the camber is a factor in the design of steering geometry and incorrect camber will not only affect handling, but can also cause excessive wear to the edge of the tyres.
Castor
Looking at the vehicle from the side, the axle tilts forward or back in relation to the axle centre. This function is also part of the steering design, and if the castor is set incorrectly, this will affect steering and handling characteristics.
Too little castor causes unstable steering requiring:
Too much castor causes:
Looking at the vehicle from the side, the axle tilts forward or back in relation to the axle centre. This function is also part of the steering design, and if the castor is set incorrectly, this will affect steering and handling characteristics.
Too little castor causes unstable steering requiring:
- constant corrections
- wander and weave
- oversteer
- failure to return to centre after a turn
- road walk
Too much castor causes:
- heavy steering, and when extreme, can cause wheel shimmy
Toe-in and Toe-out
As the wheels face forward, they turn in towards each other slightly (toe-in) or turn outwards (toe-out). If this adjustment is not correct, rubber is scraped off the entire tread of the tyres shortening life and affecting handling.
As the wheels face forward, they turn in towards each other slightly (toe-in) or turn outwards (toe-out). If this adjustment is not correct, rubber is scraped off the entire tread of the tyres shortening life and affecting handling.
Wheel Balance
Wheels and tyres must be evenly balanced as they revolve around the axle.
If they rotate unevenly, the tyre tread will wear unevenly and excessively, causing wheel wobble, and vibration in the steering, which stress the vehicle and tire the driver.
Drive Axle Alignment
Drive axle alignment is very important. Tandem drive axles that are not parallel to each other have a definite effect on steer-tyre wear. Figure (A) shows a model of a tandem-drive-axle tractor with both drive axles in proper alignment. In this case, the driver simply steers the truck straight ahead and neither fast wear nor irregular wear would be expected as a result of the driving axles.
However, Figure (B) is an exaggerated view of a truck with drive axles parallel, but not perpendicular to the chassis centreline. The eight driving tyrescreate a “thrust angle” to the left at the rear of the truck. Turning the steering wheel slightly to the left aligns the steer and drive tyres to run parallel, but the vehicle however will “dog track”. Even though lateral forces on the steer tyres are minimal, the steering geometry is affected, which may result in asymmetrical steer tyre wear.
A more severe case is shown in Figure (C). Here the drive axles are neither parallel to each other nor perpendicular to the chassis centreline. The drive-axle tyres are trying to force the vehicle to turn left and the driver must compensate by turning to the right. This will result in fast and irregular wear and, as recent tests have shown, in a much more severe way than the previous case. This test also indicated that the steer tyre on the same side of the truck on which the drive tyres are closest together will wear intoan out-of-round condition as well.
Recommendations for drive-axle alignment are as follows:
Tandem axles should be parallel within 1/8-inch difference between the axles centres measured on the left and the right side of the vehicle.
Axles should be perpendicular to the chassis centreline within 1/8-inch measured between axle end and vehicle centreline.
Drive axle alignment is very important. Tandem drive axles that are not parallel to each other have a definite effect on steer-tyre wear. Figure (A) shows a model of a tandem-drive-axle tractor with both drive axles in proper alignment. In this case, the driver simply steers the truck straight ahead and neither fast wear nor irregular wear would be expected as a result of the driving axles.
However, Figure (B) is an exaggerated view of a truck with drive axles parallel, but not perpendicular to the chassis centreline. The eight driving tyrescreate a “thrust angle” to the left at the rear of the truck. Turning the steering wheel slightly to the left aligns the steer and drive tyres to run parallel, but the vehicle however will “dog track”. Even though lateral forces on the steer tyres are minimal, the steering geometry is affected, which may result in asymmetrical steer tyre wear.
A more severe case is shown in Figure (C). Here the drive axles are neither parallel to each other nor perpendicular to the chassis centreline. The drive-axle tyres are trying to force the vehicle to turn left and the driver must compensate by turning to the right. This will result in fast and irregular wear and, as recent tests have shown, in a much more severe way than the previous case. This test also indicated that the steer tyre on the same side of the truck on which the drive tyres are closest together will wear intoan out-of-round condition as well.
Recommendations for drive-axle alignment are as follows:
Tandem axles should be parallel within 1/8-inch difference between the axles centres measured on the left and the right side of the vehicle.
Axles should be perpendicular to the chassis centreline within 1/8-inch measured between axle end and vehicle centreline.
Trailer Axle Alignment
While trailer axle misalignment is not noticeable through the steering wheel, this condition causes uneven tyre wear, increases fuel consumption and stresses the whole rig.
Ackermann Steering Effect on Tyre Wear
Irregular tyre wear patterns can be attributed to many variables. One potential cause for irregular wear on steer tyres may be the truck's "Ackermann" characteristic.
The Ackermann Principle states that for any given corner, the outside wheel should have less turn angle than the inside one, because it is following a larger radius than the inside wheel.
This difference in wheel turning angles is determined by the length and angle of the steering arms that are attached to the hubs of the steer axle.
The theoretical Ackermann angle for a particular vehicle is determinedby drawing a line through the pivoting axis (which is the rear axle of atwo-axle vehicle) to establish a pivot point for a turn then drawing linesto the pivot points of the two steer tyres. The Ackermann, then, is the angle the tyres/wheels need to be turned to form a right angle with each of the lines extending from the turning pivot point to the tyre/wheel pivots.This results in the steer tyres "toeing out" when turning.A vehicle's wheelbase is the most critical variable affecting the "theoretical Ackermann" for a vehicle.
Keep in mind that Ackermann is a purely geometric concept. The argument that the Ackermann Principle was developed in the early 1900s for very slow-moving vehicles and does not consider the dynamic effect of many outside influences on the path a vehicle takes through a turn is somewhat correct.
To complicate the Ackermann Principle further as it applies to trucks, remember that the turning axis must be drawn to determine a pivoting point about which the vehicle turns. It's more difficult to define this axis for vehicles with more than one drive axle. Fifth wheels, depending on their location, can also alter where this line would fall.
The trend is for vehicle manufacturers to provide different Ackermann arms for different wheel bases and different fleet vocations.
Tyre Inflation
One of the most obvious causes of excessive tyre wear and damage is incorrect tyre pressure.
Vehicle manufacturers produce service manuals and technical documentation to provide information about correct tyre pressure. The recommended pressure value apply to the cold tyre, as the inner pressure of the tyre increases during operation.
Tyre pressure should ideally be checked every 2 weeks, 4 at the most, on the cold tyre.
Spare tyres must also be checked.
One of the most obvious causes of excessive tyre wear and damage is incorrect tyre pressure.
Vehicle manufacturers produce service manuals and technical documentation to provide information about correct tyre pressure. The recommended pressure value apply to the cold tyre, as the inner pressure of the tyre increases during operation.
Tyre pressure should ideally be checked every 2 weeks, 4 at the most, on the cold tyre.
Spare tyres must also be checked.
Correct wheel alignment of the entire rig is absolutely essential