Tires/Alignment/Suspension/Brakes - Vehicle Pull
Group: CHASSISNumber: 93-50-008
Date: SEPTEMBER, 1993
Model: ALL
Subject:
DIAGNOSIS AND CORRECTION OF VEHICLE PULL
PURPOSE:
Several factors may cause vehicle pull, and may be due to both vehicle related and external conditions. The purpose of this bulletin is to identify the vehicle related conditions that cause pull and how to correct those conditions.
CAUSES OF VEHICLE PULL:
Vehicle pull is the tendency of the vehicle to drift right or left while driving in a straight ahead direction on a straight road at a constant speed with no pressure on the steering wheel. This pull or draft may be gradual and can always be compensated for by inputs through the steering wheel. One or more of the following conditions may cause the vehicle to pull:
Non-vehicle conditions that may cause the vehicle to pull:
1. Cambered road surfaces - Most highways are built with cambered or "crowned" surfaces to drain rain water. Sometimes the road camber is visually noticeable. Other times, it is not. The vehicle may tend to pull to the left or right, depending on which lane of the cambered road surface the vehicle is on.
2. Cross-winds - A sometimes overlooked, yet possible cause of steering drift is cross-winds. Side winds at higher vehicle speeds may cause vehicle pull. Please do not rule this out when diagnosing a vehicle pull complaint.
Vehicle pull caused by these two conditions is normal and no repair is possible.
Vehicle conditions that may cause the vehicle to pull:
1. Tire inflation pressure - As small as a 2 PSI tire inflation pressure difference between the right and left tires might cause a vehicle pull. Be sure to check the tire pressure before the tires are warmed up prior to attempting more extensive troubleshooting.
2. Wheel alignment - Although it is a generally felt that slightly out of specification wheel alignment is a major cause of vehicle pull, improper wheel alignment actually causes very few vehicles to pull. Therefore, if the the vehicle wheel alignment is within specification, DO NOT attempt to correct the vehicle pull by changing the wheel alignment angles by bending suspension components or such other methods.
3. Uneven brake adjustment - If one of the four brakes is dragging, the vehicle may pull to the side of the dragging brake. Verify that none of the brakes are dragging and that they are adjusted correctly prior to attempting more extensive troubleshooting.
4. Incorrect vehicle trim height - If the trim height of the vehicle is not equal at all sides, the vehicle may pull to the side of the lower trim height. Check and adjust the trim height of the vehicle as necessary. Please note that a slight variation in the vehicle trim height seldom causes a noticeable vehicle pull.
5. Tire construction - The way in which the tire is built can produce vehicle pull. An example of this is the placement of the belt. An off center belt on radial tires can cause the tire to develop a side force while rolling straight down the road and the tire will tend to roll like a cone. To correct this condition, please use the flow chart. This chart provides the sequence of procedures designed to be performed for all conditions of vehicle pull.