Diagnosing Drift

diagnosingdrift.jpg

One of the common handling-related complaints brought to an alignment shop is "drift" - usually meaning the vechicle 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 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 becuase you can chase .2° alignment changes for months only to swap out the tires and it is a completely different truck.

Caster & Camber

Negative caster should never be used on any vehicle. If the question is positive or negative caster, the answer is always, 100% of the time, postive caster. Cross caster can result in the vehicle pulling to the greater side. For instance, if the drive side has 2.6° and the passenger side has 2.3°, this should get the truck to go straight on a crowned road, but could pull to the driver side on a flat road.

After the parts and tires have been ruled out, most alignment techs will take a closer look at cross caster as 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.

 

Caster DiagramCamber Image

*motor.com

 

Toe Diagram

*motor.com

SAI Diagram

*motor.com

Lets clarify drift by 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, about 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 possibly wrong. The high value of SAI has a larger influence on vehicle directional stability compared to the relativley 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 crosscamber by biasing it in the appropiate direction while staying with in the OE specified camber range.

For instance, a vehicle with a left camber of -0.5°, right camber of -0.5° and a drift to the left. This 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 -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.

Super Duty Alignment Specs

caster = min 4°/max 8°, 5° to 6° is optimal,

max cross center caster 1° as needed to reduce pulling.

toe =In each side min .12°/max .18°, .18° is optimal, max toe in is optimal.

camber = ± Min 0°/Max .6°, 0° is optimal,

max cross camber .25° as needed to reduce drift.