Debunking the Shim Kit Theory

Have you ever experienced light steering effort or “twitchy” steering that seemed too sensitive? At some point or another, you probably have. Somewhere along the line, a rumor started that the fix to this dilemma is to use a shim kit on the pressure relief valve to lower pressure in the power steering pump. Unfortunately, this is a myth.

Many believe that when a steering system feels over-assisted it means the system has too much pressure and the fix is to install a shim kit. Installing a shim kit will indeed reduce the pump’s pressure capacity. However, if the pump’s pressure capacity is set below the demand of the steering unit control valve, then the system will not operate properly! In order to get the desired feel at high speed, you would have to significantly drop the pump’s pressure capacity, which in turn would result in giving you no assist under low speed maneuvering because the steering unit control valve demands more pressure. Running the pump under these conditions will also dramatically shorten the life of the pump.

In actuality, if your system feels over assisted, it has too much flow. The control valve in the steering rack or box is sensitive to the Gallons Per Minute flow rate (GPM). When the control valve receives more flow rate than what it is designed for, it gets overly sensitive. This results in a system that responds too quickly and gives a light or over-assisted feel. The control valve in the steering rack or box essentially communicates to the pump how much pressure to provide the system. As outlined in our “Pressure vs. Flow” article, it is adjustments to the flow that improve your steering feel.

The proper way to gain steering feel is to use a properly sized Turn One flow restrictor. Not only is this an easier adjustment, but it will have no adverse side effects to the life of the pump. A properly sized flow restrictor will make the pump compatible with the steering unit control valve requirements. This will give the driver the desired steering feel without compromising the steering system.