Accessories & Gear

Tacoma Rear Axle Shims – What, Why, and When

In this second post in our series looking at potential solutions to the driveline vibration problems experienced by owners of lifted Toyota Tacomas, we will move beyond the aftermarket drive shafts that we examined in the first post and delve into the effectiveness of rear axle shims.

What Are Rear Axle Shims?

Rear axle shims are small pieces of metal that are designed to slide in between a vehicle’s axle and its leaf springs in order to adjust a truck’s pinion angle. Typically, these shims come in one to six-degree increments and are sold in pairs. It’s a much better idea to buy a set of three-degree shims than to stack three separate one-degree shims on top of each other under the leaf springs.

Tacoma rear axle shims.

During the course of my conversation with Troy at Tom Wood’s Custom Drive Shafts I was also able to ask him his opinion regarding the use of rear axle shims to reduce driveline vibrations in the Tacoma. His answer was fairly nuanced.

It’s All About Geometry

In Troy’s opinion, the reason these vibrations manifest on a lifted truck has to do with the way the increased ride height affects driveline geometry. Lift kit manufacturers do their best to reduce the angles that are introduced by a taller suspension, but there is only so much that can be accomplished on their end.

Rear axle shims, according to Troy, should be thought of as tools for adjusting driveline geometry so that it provides the optimal angle between driveshaft, output shaft and pinion – an angle that can vary according to the type of driveline components being used. Shims can be part of the answer, but not the entire answer.

Kevin Kelly from KLM Performance agrees. In his experience, rear axle shims can compensate for angle differences related to height differentials between the front and rear of a truck’s suspension system, but that it takes a lift of at least three inches before this particular component becomes truly useful.

Shim Materials Are An Important Consideration

Troy had one more piece of advice to give when selecting a set of rear axle shims. He told me that some of the less expensive shims are made out softer alloys, such as zinc. These metals are quite porous, and over time they crush down to the point where they no longer serve any useful purpose. Crush shims can also loosen leaf spring U-bolts.

Leaf spring shim location.

He recommends going with shims that are made out of steel, and to try and find units which allow the centerbolt to come out of the spring and pass through both the spring and the shim, making the shim part of the spring pack. These offer a much higher quality installation compared to shims which merely slip in between the leaf springs and the axle.

Rear Axle Shims – A Useful Tool, but Not For Stock Trucks

Rear axle shims do appear to be useful for owners of lifted Tacomas looking for a tool to help iron out driveline angle issues introduced by lift kits. However, they should be considered as only one part of a comprehensive suspension solution, not a magic bullet that will automatically solve all vibration issues. Owners of stock ride height Tacomas, unfortunately, will see no benefit in using of these types of shims in terms of reducing driveline vibrations.