Three simple conditions must be met for a four-wheeled vehicle to travel in a straight line:
1. All four wheels must be pointing in the same direction.
That is, all four wheels must be square to each other and square to the road surface (in other words, parallel to one another, perpendicular to a common centerline, and straight up and down).
2. All four wheels must offer the same amount of rolling resistance. This includes the "caster effect" between the front wheels that steer.
3. There must be no play in the steering or suspension linkage that positions the wheels.
If all three conditions are not met, the vehicle will drift to one side depending on which forces are at work. This creates a steering pull which the driver will counteract by steering the other way. Having to constantly apply pressure to the steering wheel to keep the car traveling in a straight line can be tiring on a long trip. It can also be hard on the tires, too.
STEADY PULL
This is a condition where the vehicle continually pulls or drifts to one side while traveling straight. The driver typically has to maintain steady pressure on the steering wheel to keep the vehicle on the road. Possible causes include:
Uneven camber side-to-side. Too much cross-camber can make a vehicle pull or lead towards the side that has the most (positive) camber or away from the side that has the least (negative) camber. The underlying cause may be a bent strut or mislocated strut tower, a bent spindle, collapsed control arm bushing, weak or broken spring, or a shifted crossmember or engine cradle. Check SAI and the included angle to see if these are in or out of specs to diagnose the problem. Also check ride height. Correct by replacing worn or damaged parts, correcting location of strut tower, repositioning engine cradle, and/or reducing cross-camber to half a degree or less by readjusting camber to specifications.
Uneven caster side-to-side. Too much cross-caster can make a vehicle pull or lead towards the side that has the least (negative) caster. The underlying cause may be a bent strut, spindle or mislocated strut tower. Correct by replacing damaged part, correcting location of strut tower, and/or reducing cross-caster to half a degree or less by resetting caster to specifications.
Rear axle steer. The front wheels are with alignment specifications but the vehicle pulls to one side. The underlying cause may be rear toe out of specifications, a bent rear axle, chassis misalignment or a stackup of assembly tolerances in the chassis causing rear axle misalignment. Measure and compare the wheelbase on both sides, check for the presence of a thrust angle, and/or measure individual rear toe. Correct by realigning the rear axle or rear toe, or by performing a thrust angle alignment.
Brake drag. The pull is constant to one side and may get worse with the application of the brakes. Raise the vehicle and spin each wheel by hand to check for excessive drag. Possible causes include caliper sticking, frozen or sticking piston in caliper, overfilled fluid reservoir in master cylinder (does not allow caliper pistons to retract when brakes are released), weak drum brake return springs, misadjusted drum brakes, misadjusted parking brake, misadjusted parking brake pedal switch (creates residual pressure in the master cylinder to cause drag). Readjust or repair brakes as required.
NOTE: If the pull only occurs when the brakes are applied, the problem may be unequal braking not a dragging caliper or misalignment. The vehicle will pull towards the side with the stronger front brake and away from the side with the weaker or inoperative front brake. Uneven braking can be caused by a sticking floating caliper, a frozen caliper piston, the use of different grades or brands of brake linings side-to-side, fluid leaks, or contaminated linings on one side (by brake fluid or grease). Correct by repairing brakes as required. Other causes may include worn or loose control arm bushings or strut rod bushings that allow alignment changes when braking, so be sure to inspect these components before blaming the brakes.
Low tire pressure. The vehicle will lead towards the side with low pressure in the front tire. Correct by inflating tires to recommended pressure.
Mismatched tires side-to-side. The vehicle will pull or lead towards the side that offers the greatest rolling resistance. Compare tire sizes, tread wear, tread styles and patterns, also brands.
Uneven tire wear. If one side of the tread is worn more than the other, the tire develops conicity. The effect is much the same as camber, causing the tire to roll towards the side which is worn most. The uneven wear may be the result of incorrect camber, toe and/or failure to rotate the tires periodically to even out wear. If rotating the tires side-to-side reverses the direction of the pull, the tires need to be replaced.
Ply steer. A manufacturing defect in the way the belts are positioned inside the tire causes the tire to generate a lateral (sideways) force as the tire rolls. To test for this condition, drive the vehicle forward, then in reverse. If the direction of the drift or pull changes, one or more tires at at fault. Rotating the tires front-to-rear or crosswise may help cancel out the ply steer, otherwise tire replacement is necessary to correct the condition.
Memory steer (see memory steer).
Unbalanced power assist. Seal leaks in the control valve or off-center steering may route hydraulic pressure into one side or the other of the boost cylinder piston causing the steering to want to turn itself to one side. This can be checked by raising the wheels with the engine running to see if the wheels turn to one side by themselves. No change would indicate another cause, but if the pull suddenly vanishes an imbalance in the power steering system is to blame. The control valve assembly or steering gear needs to be replaced.
Excessive road crown. Roads are raised or crowned in the middle so rain water will run off to the outside for proper drainage. But the slight slope to the pavement can often make a vehicle drift to the outside. This can be countered by adding a little positive camber and/or negative caster to the left front wheel, but this should only be done if the vehicle spends most of its time on crowned roads and the customer demands a fix.
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Monday, June 15th, 2009 AT 3:04 PM
