Power Steering Conversion Basics
All of us have driven cars with power steering and never really understood what makes in all work. You usually know when it doesn’t work, it’s all too obvious, but what about when it sort of works. If you have added power steering to a car that formerly did not have it you may have uncovered a number of challenges to get it to “feel” right. Since SpeedDirect is big on after market steering conversions we are going to look at the basics of a power rack and pinion system and hopefully get a feel for its proper operation.
The powered version of a rack and pinion has several basic components that a manual system does not have as shown in the drawing below. The key elements are as follows:
• A Hydraulic Ram which is “double acting” meaning it is powered in both directions.
• Spool Valve to control when and how hard the ram pushes.
• Torsion Bar to create the desired power boost response.
• A hydraulic pump.
• A Reservoir for hydraulic fluid.
How it All Works Together
The components interact like this: When you turn the steering wheel you are actually twisting the steering shaft. The steering shaft that connects to the rack & pinion unit is in two pieces inside the rack unit which are connected by a torsion bar nested inside the hollow steering shaft. The bar twists before the rack and pinion gears on the bottom start to turn, as it does this it opens passages in the spool valve allowing pressurized power steering fluid from the pump to enter only one side of the hydraulic ram chamber; the rack moves, assisted by the ram in the direction you are turning. As it does this is displaces the fluid in the other chamber opposite the direction you are turning and allows that fluid to escape with very little pressure through a second set of passages in the spool valve that are synchronized with the passages that opened on the pump side. These passages connect to the return line and lead back to the power steering reservoir.
You will notice that since both sides of the ram and lines are filled with fluid. This allows the ram to move quickly either direction you decide to steer. Since this takes a volume of fluid to move the rack; pumps are designed to move volume with pressure being secondary. Pressure does come into affect as we will see a little later, but is clearly volume what is at work.
The torsion bar is there to give some “feel” to the system, it acts to delay response of the ram otherwise you would have over assisted steering that would feel “twitchy” every time you barely moved the steering wheel.
Speaking of Twitchy
The over boosted feeling can happen when you adapt a system using a pump from say a steering box system. It has a different base line pressure; the base line pressure is a minimum pressure where steering response of the torsion bar / spool valve combination starts to assist. Passenger car steering systems range from approximately 30 – 200 psi base line pressure depending on if it is a steering box or rack & pinion pump / valve combination. If you have an over sensitive, twitchy feel here is what you can do to determine one possible solution.
• With the car running, brake set, tires clear of curbs or obstructions and the engine at its normal idle with the steering wheel centered, turn the wheel to the stop one way.
• You should notice some feel or resistance just as you begin to turn.
• As you continue to turn you should feel progressively increasing resistance until you hit the stop.
If this is not the case you may have too high of a baseline pressure. SpeedDirect offers a kit for GM pumps that will allow adjustment of the bypass pressure.
A Little is Good A Lot is Better
Don’t be tempted to lower the pressure as low as you can. The system needs a certain pressure to move the volume of fluid; it is required by how fast you turn the wheel. If the pressure is too low you will start to feel it “catch” or briefly stick until the fluid builds in the ram. Don’t expect a pressure reduction of the by pass regulator in the pump to change the response of the overall rack feel, that is the torsion bars job. But if the base line pressure is too high the kit will affectively lower that pressure and allow the torsion bar to do its job properly.