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One of the common handling-related complaints brought to an alignment shop is ‘drift’ — usually meaning the vehicle fails to continue straight when allowed to choose its own path. Many times, an alignment tech is able to easily resolve drift issues, but not always. Making adjustments and then failing to impact a drift issue or satisfy the customer’s complaint can be frustrating.
Tire pressure is the #1 first thing to always check. It is very very common for drivability issues to be caused by tires. Some brands of new, with zero mile tires do not drive nice on solid axle truck and no alignment will fix this. Never rule out the tires for being the problem because you can chase .2 degree alignment changes for months only to swap out the tires and it is a completely different truck.
Negative caster should never used on any vehicle. If the question is positive or negative caster, the answer is always, 100% of the time, positive caster.
Cross caster can result in the vehicle to pulling to the greater side. For instance, if drivers side has 2.6 degrees and the passenger side has 2.3 degrees this should get the truck to go straight on a crowned road but could pull to the driver side on flat road.
After the parts and tires have been ruled out, most alignment techs will take a closer look at cross caster as a directional control angle, which is taught in many wheel alignment classes today. However, cross camber also affects directional stability, but it is usually ignored because camber also impacts tire wear. Some vehicles in production today have steering axis inclination (SAI) values of 9°, or even higher, to improve vehicle stability. On these vehicles with higher SAI values, the cross camber can be more effective in resolving drift than cross caster. Let’s clarify drift by the popular definition: with no driver input, the vehicle will change one full lane in about 1/8 to 1/4 of a mile on a straight, level road. By contrast, a ‘pull’ is when the vehicle changes lanes faster — in less than 1/8 of a mile. The deliberate use of cross camber we’re discussing here is only viable for drifts because a pull would require camber values that are unacceptable for tire wear, and something more serious is probably wrong.
The high value of SAI has a larger influence on vehicle directional stability compared to the relatively smaller caster values. This is why changes in cross caster don’t have much effect on drift on certain vehicles. Instead, try small changes in cross camber by biasing it in the appropriate direction while staying within the OE-specified camber range. For instance, a vehicle with left camber of -0.5º, right camber of -0.5º and a drift to the left could be corrected by moving the left to -0.75° and the right to -0.5°, or the left to -0.75° and the right to -0.25°. Both settings would stay below the typical cross camber maximum of 0.5° and should create enough drift-right to counter the original complaint of drift-left.While working a drift issue with cross camber, keep the less effective, but still relevant cross caster from clouding the results. Keep cross caster within the OEM specified range, which is typically under 0.5°. Also, the actual caster numbers should be close to the OEM specified target value.
Keep in mind the influence that SAI has on vehicle tracking, and be aware that high SAI will cause cross caster to be less effective than cross camber for correcting drift issues. This will save your shop time — and costs — by focusing on the more effective parameter according to the particular vehicle being serviced. If the solid axle front suspension was assembled with the truck’s axle hanging at full droop, and all of the bolts in the radius arms were tightened to full torque, and then the truck was set on the ground, it can cause the suspension to pull. All suspension rubber bushings need to be left snug until the truck is set on the ground. Then the fasteners need to be tightened to full torque when the vehicle is sitting at right height. if you do not do this the bushings will be in a bind and cause drive-ability problems. This includes the track rod frame bolt, the radius arm bolts at the axle, at the frame along with the rear leaf springs. If the OUO ALA's are adjusted incorrectly or if the beam arms or adjusted incorrectly it will cause the truck to pull.