100% Employee Owned, Founded 1954

100% Employee Owned, Founded 1954

Technical Guide: Hydraulic Steering 101 for Off-Road Vehicles

Lynn Charlson, self-taught inventor, designer, and mechanical genius, was a pioneer in the field of hydraulic power, and founder of the Char-Lynn Company in 1942. Back in the 60’s and 70’s, low-speed high-torque motors and steering valves were referred to simply as “Char-Lynns,” because, the name was so synonymous with the device. Just like “Coke” was (and still is to much of the world) to all soft drinks of that type.

One of 94 earned patents, he invented and patented the first Steering Control Unit (SCU) for off-road equipment in 1961. Combining a rotary servo valve and gerotor element, he set the standard that is closely followed by Eaton, who acquired Char-Lynn back in 1972, and its competition today. So dominant was Char-Lynn in the steering business then, and now through Eaton, their world market share is over 80%.

Types of Steering Control Units

Torque Generators

Off-road steering can be achieved by a Torque Generator, which is an SCU with an output shaft. With a steering wheel on the input side and a hydraulic boost to the SCU, the resulting rotary output shaft torque is amplified beyond the operator input torque by the hydraulic system – all while maintaining a mechanical link between the steering wheel and existing vehicle mechanical steering components, such as a worm gear type gear box. There are many other uses for a torque generator where a mechanical link requires excellent flow metering and increased torque, beyond which an operator can be expected to provide.

Full Hydraulic Steering Control Units

Full Hydraulic SCUs do not have an output shaft. Instead, they supply metered hydraulic fluid to a vehicle’s linear hydraulic steering cylinder or rotary hydraulic motor driven steering linkages.

Electro-Hydraulic Steering Control Units

Electro-Hydraulic SCUs are being applied to more and more vehicles as a switchable (manual steer or electro-hydraulic) unit or as an all-electro-hydraulic unit that can be mounted virtually anywhere on the vehicle. This type is not linked to a manual steering wheel/column. Intelligent steering functions like parametric steering, auto-guidance/steer systems, joystick control, position sensor based steering wheel, and unlimited software features and potential, provides steering flexibility never before seen. Agriculture and Material Handling are early off-road market leaders for EH steering.

Sizing an SCU For An Off-Road Vehicle

Assuming you are working with an “Ackerman” type steering system on your vehicle, there are a few simple steps to determining the “size” of SCU that will be required. A great resource is the Eaton “Steering Catalog” no. C-STOV-MC001-E2. Specifically the sizing and application section. Take a look at the 8 steps:

Determine the torque required at the king pin to steer the vehicle on its worst surface condition and at its heaviest weight.

Convert that torque to force by dividing the king pin torque by the radius arm. This radius is formed by the minimum distance from the king pin to the hydraulic cylinder axis.

Determine the cylinder area required by dividing the force in Step 2 by the anticipated operating pressure of the hydraulic system. 1000-1500 psi is a normal operating pressure range up to a pressure relief setting of 2000 psi. This is a good design range unless other issues like cylinder space available require a smaller cylinder and higher pressure.

This step is probably best taken with a cylinder catalog as a reference tool. At this stage, we ought to also mention that the use of an equal area or “balanced” cylinder is most desirable. With equal cylinder area on both sides of the piston, the rate of steering is the same for either left or right turning. Two of the same size unequal area “differential” cylinders cross-plumbed (rod end to cap end) will also result in an equal area for left or right turning.

Determine the cylinder diameter by taking the area in Step 3, divide by .7854, and take the square root of that number for the minimum effective bore needed. Do the same Step 3 area calculation for the rod diameter. Choose a cylinder, remembering to always subtract the rod area from the full bore cylinder area to get the actual effective area. Therefore, the cylinder diameter will need to be increased to regain the lost rod area.

Determine the cylinder stroke required, which is a function of the radius arm and the total angle through which the arm turns.

Determine the cylinder volume in cubic inches by taking the larger (differential type cylinder) of the two cylinder areas in square inches, and multiplying it by the stroke determined in Step 5. This will be a “lock to lock” extension or retraction of the steering cylinder in one direction only.

To select the SCU displacement, divide the cylinder volume in Step 6 by the number of desired steering wheel rotations, (4 is normal) lock to lock. This cubic inch displacement is then used to compare with the SCU’s offered to determine the closest standard unit available.

Calculating needed pump flow requires an understanding of a couple of important issues. The first is to establish a maximum steering wheel RPM. Generally, this is 120 RPM for evasive maneuvering. The next important issue is at what engine (pump) speed to use for 120 RPM. Generally, when evasive maneuvering is required, the foot is taken off the accelerator so the engine RPM will be dropping back to idle. The safe, conservative approach is to use engine idle speed for calculating pump size needed for 120 SCU RPM. Take the SCU displacement in cubic inches determined in Step 7 and multiply it by 120 RPM. Divide the result by the engine idle speed. The result is the pump size needed in cubic inches per revolution.

Hydraulic Steering 101 for Off-Road Vehicles 1

Photo courtesy of Eaton Corporation

For more information, take a chack out the full Steering Catalog from Eaton Powersource


There are some basic hydraulics to be familiar with in order to build a usable model code for an SCU. At this point in the process, it’s best to consult with a certified fluid power specialist offering one of the premium hydrostatic steering product lines like Eaton. If your vehicle has other hydraulic functions, you will need to choose between a fixed displacement pump, (with power beyond or an external flow divider) or a variable volume pressure compensated pump.

An open center SCU is used with fixed displacement pumps and if auxiliary functions require downstream flow, the “power beyond” feature is chosen or you may be installing a separate flow divider between the pump and SCU.

A closed center SCU is used with a variable volume pressure compensated pump. Multiple functions needing varying amounts of flow simultaneously is usually most efficiently handled with this type of pump and SCU.

Load sensing requires a separate “priority valve” and can be used on both fixed and variable displacement pumps. It provides the operator with consistent “feel” irrespective of changing loads brought on by terrain and vehicle loading.

Load reaction steering is what a car has. That is to say, when you are turning a corner, if you release the steering wheel, the axle forces are allowed to return the steering wheel (axle) to its approximate original position. Non-load reaction does not return the steering wheel to center when released, but maintains the steering wheel (axle) position. 

Finally, SCU’s can be equipped with many “integral valves” to simplify the circuit and avoid having to “line mount” similar valves. Examples would be Inlet relief valve, manual steering check valve (this should always be specified in your SCU), Inlet check valve (prevents steering wheel kickback), load sense relief valve, cylinder port relief valves (used where extreme ground forces could cause pressure spikes), and anti-cavitation check valves for the steer cylinder ports. Learn more about steering units with integral valves in this diagram from the Eaton Steering Catalog.

Also, your SCU distributor should be able to design/supply an SCU bolt on the manifold for more custom circuit designs unique to your application. Page 13 of the aforementioned Eaton steering catalog has a chart that illustrates the relationship between SCU displacement and the steering wheel force required to provide steering cylinder pressure in a lost power scenario. Bottom line? Smaller displacements provide for higher cylinder pressure for a given steering wheel manual input torque.

Resources Are Available

There are many other special features available for SCUs that automatically or manually give the operator two steering speeds. This allows for fast maneuvering during work cycles and then back to normal speed when transiting over the road. Also, when larger steering displacements are required to do the application, in order to have manual “lost power” steering, a second smaller displacement can be added integral to the SCU. There are many other special features that are designed for articulated vehicles and the unique problems they represent.

It bears repeating again, have a look at the referenced Eaton steering catalog to get a better understanding of what’s out there and how to apply it. Also, that local certified fluid power specialist is only a phone call away. Contact a Cross Mobile Systems Integration expert to discuss your project and see how our team can help raise quality and improve your efficiency.

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