BAC Rear Axle Shims

Bergman Auto Craft Rear Axle Shims correct thrust angle issues by squaring the rear axle to the frame. They’re a simple, low-cost fix for a car that never seems to track straight, even after a front-end alignment.

What thrust angle means

Thrust angle is the direction the rear axle points compared to the centerline of the car. Ideally, it points straight ahead. Typically, though, one side of the rear axle sits closer to the front wheels than the other. As a result, the rear axle steers the car slightly to one side. You may notice a crooked steering wheel, a car that “dog tracks,” or uneven tire wear.

Old leaf spring hangers, worn bushings and past repairs all add up over the years. Therefore, many classic Mopars have some thrust angle, even with fresh springs.

How to use rear axle shims

First, have an alignment shop measure the thrust angle. Then place shims between the front leaf spring hanger and the frame, one at a time. Recheck after each one until the thrust angle reads zero, or as close to zero as possible. Finally, torque the hanger bolts and do a full alignment so the front wheels match the rear axle.

• Available in 1/16″ and 1/8″ thicknesses
• Sold each, so you buy only what you need
• Fits between the front spring hanger and the frame

For most cars, a combination of one or two shims is enough. Still, order a spare or two, because it saves a trip if your car needs more correction. If you’re also upgrading the rear suspension, add these rear axle shims to the same order and set the thrust angle right the first time.

Not sure if thrust angle is your problem? Ask your alignment shop for a printout that shows the rear thrust angle. Any reading well away from zero is worth correcting.

Related: Suspension Parts, leaf spring products. For install and product videos, visit the Bergman Auto Craft YouTube channel.