The cause of vehicle pull is sometimes on the vehicle side and sometimes due to the road conditions or customer’s habitual usage. When performing troubleshooting for vehicle pull, it is important to understand the cause of it.
Judgement of vehicle pull
It is difficult to judge vehicle pull by using an electrical tester, etc. So, perform a road test and judge whether it is a malfunction or not.
• Compare with another vehicle of the same model
• Measure how much it moves within a certain distance
Measure how many meters the vehicle offsets in 100 meters.
Cause of vehicle pull
• Force generated on the tire
• Influence of alignment
• Influence of road surface
Force Generated on the Tire
The force generated on the tire may cause vehicle pull. The typical phenomenon is shown below.
• Ply steer
The transverse direction force is generated due to the ply direction of the tire belt. When the tires are rotated straight ahead, the force that tries to move the tires in the transverse direction along the ply is referred to as ply steer.
When applying a lateral force to the surface of a standing tire, portion contacting with the ground will alter shape evenly. However, unevenness of the tread, the force that returns it to the original shape is not equal. If rolling the tire in this condition, the tire rolls in the direction where the reaction force is smaller. In this condition, the force in the transverse direction is referred to as conicity. Inspection methods regarding whether the force generating on a tire affects vehicle pull or not.
• Change the left and right tires. Or remove the tire from the wheel and reverse.
Ply steer: The direction does not change at all.
Conicity: The direction changes.
Infl uence of Alignment
Vehicle pull occurs when there is an imbalance between the forces: moving to the right and to the left.
Alignment that has conformity with vehicle pull.
• Difference between right and left of cambers Moves to the positive (+) side
• Difference between right and left of casters Moves to the smaller side
• Difference between right and left of the king pin angles Moves to the negative side when braking
Influence of Road Surface
As for trails, each centre of contact on the ground between right and left are different, so the king pin offset dimension of the right and that of the left differ. Therefore, since the moment around the king pin axis differs between right and left, the steering wheel moves to the side where the moment is larger.
• Depth of the trails:
As the trail becomes deeper, the level becomes larger.
• Width of the tires:
As the tire width becomes wider, the level becomes larger.
• Tire tread rigidity:
As the tire tread rigidity becomes higher, the level becomes higher.
Some roads are slightly sloped for drainage. For this reason, the steering becomes unstable in only a specific place, or the direction differs when going somewhere and returning to the original place.