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4x4 Truck Steering System Parts - Steering Theories

Posted in How To: Transmission Drivetrain on December 1, 2005 Comment (0)
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Photographers: Alan HuberCourtesy Of Borgeson

The steering system is probably the most important component of your vehicle to have working flawlessly. And unfortunately, it's probably one of the most ignored systems on modified 4x4s today. We want to help change that. It's easy to get caught up with all the solid axle swaps, locker installs, spring-over conversions, electrical steroids, and custom fabbing we write about. You want the big tires, the lift, the axles, and the motor to push it all, but what about trying to control that three-ton harbinger of terror? You need to be able to properly maneuver it via a well-working (per application) steering system.

Think you know enough about your steering? Maybe you do, but maybe you're not sure if you have rag joints or U-joints on your steering shaft. Ever thought about the caster on your front axle? What about the fact that the inside wheel is at a greater angle than the outside wheel in a turn, or how far off of an ideal Ackerman angle is your steering setup? Have you even heard of an Ackerman angle?

There are many variations of what we're showing you, as well as some one-off setups that probably contradict some of what we tell you, but this will pertain to the majority of us. Read on and we'll steer you straight.

The Hard Parts

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Rack-and-Pinion: A rack-and-pinion steering setup is typically found on independent suspension applications. It consists of a geared shaft (similar to a pinion gear) that moves along a geared track. The pinion gear turns radially, moving the geared track back and forth, moving tie rods that are attached to the rack.

Steering Shaft: The steering shaft joins the steering column to either the steering box or rack-and-pinion. It usually has some type of slip joint built in to absorb shock and compensate for different distances between the steering box and the column. To allow angled movement (because it is almost never a straight shot between components) the shaft usually has some type of joint such as a rag joint or universal joint at both ends.

Power Steering Box (Recirculating Ball): The typical four-wheel-drive steering box is a recirculating ball type that has a worm geardrive to help magnify a driver's input to direct steering linkage one way or another. The steering box gets input from a steering shaft and outputs work through a splined sector shaft. Steering boxes are used in both solid-axle steering systems and on independent suspension setups.

Pitman Arm: The pitman arm moves radially around the sector shaft of the steering box in a single plane. Either a drag link or some type of connecting rod is installed on the other end of the pitman arm, with the pitman arm rotating in an arc, turning the wheels back and forth via the drag link. Dropped pitman arms lower the drag-link mounting point, just as the name implies. These are normally deemed acceptable in most applications and are sold with many suspension systems, but you should realize what is happening when you elongate the distance in between the rotating sector shaft and the tie-rod end of the drag link. The farther away the drag link mounts from the sector shaft, the more sheer force is being put on the sector shaft, and subsequently the more wear you put on the steering box.

Idler Arm: An idler arm is used in IFS steering setups and sometimes in solid-axle steering setups. The idler arm holds one side of a connecting rod that joins to the pitman arm on the opposing side. Tie rods connect to the centerlink and give congruent motion to both knuckles in an IFS configuration. On a solid-axle steering system, using an idler arm is a good way to reduce stress on the sector shaft of a steering box. In a conventional crossover steering setup, the drag link is connected to the pitman arm, putting a massive amount of leveraged stress on the sector shaft whenever force is applied to the drag link. When using an idler-arm setup, the drag link connects to the centerlink, which is attached at both ends to the idler arm and pitman arm. The centerlink is only allowed to move from side to side and therefore puts minimal stress on the sector shaft, preventing premature steering box failure.

Drag Link: On a solid-axle frontend, you typically have a drag link and a tie rod. On a crossover steering setup the drag link moves from side to side, connected to the pitman arm at the steering box, and either to the passenger-side knuckle or directly to the tie rod on the passenger side. Another typical factory solid-axle steering setup was used by Dodges and Chevys, with a drag link moving forward and backward from the steering box to a 90-degree bent steering arm on top of the driver-side knuckle. When lifting a truck with this type of setup, bumpsteer becomes an issue, as the short drag link inhibits the extended travel of modified suspensions.

Automotive Tie-Rod End: A conventional steering tie-rod end has a spherical ball that rides in a race/body that is either connected to the tie rod/drag link or has a shank that threads into the tie rod/drag link. The tie-rod end's spherical ball has a tapered shank that bolts into a tapered hole on the knuckle or steering arm. Generally, higher quality tie-rod ends have zerk fittings to allow them to be packed with grease.

Tie Rod(s): Tie rods control the synchronized knuckle movement of your solid-axle or IFS front end. On a solid-axle truck, a single tie rod connects both knuckles, keeping the tires turning congruently with each other. The tie rod is connected to both knuckles usually in front (but occasionally in back) of the axlehousing. On an independent suspension, two tie rods drop to the knuckles from either a rack-and-pinion or a centerlink connected to the steering box and an idler arm.

Spherical Bearing Rod End: A spherical bearing rod end (aka rod end, Heim joint) is made of either a two- or three-piece design. Rod ends such as these are used in suspension, support bearing setups, for hard linkage, and steering. For most off-road applications, a two-piece rod end is utilized since it will resist a higher axial load before failure (the race can pop out of the body on a three-piece). Rod ends such as these definitely have their benefits such as strength and being able to be stacked or mounted wherever you can thread a bolt through. The downside? Costs can be prohibitive and they will wear more quickly than a conventional automotive tie-rod end, and may therefore not be a good alternative for street-driven vehicles.

Steering Arm: Some knuckles, called flat-top knuckles, have steering arms bolted to the top of them. Steering arms can be found on both the passenger-side knuckle and the driver-side knuckle. In many factory Dodge and Chevy solid-axle applications, a short drag link drops from the steering box to a driver-side steering arm. In many factory Ford and custom steering solid-axle applications, the drag link descends from the steering box across to a passenger-side steering arm. Many times a high-steer arm or a steering-arm block is used to compensate for suspension lifts by raising the mounting point of the drag link to bring it back to a more parallel position much the same as a dropped pitman arm would. This helps keep the tie-rod end from maxing out its total degree of movement as the suspension droops.

Knuckle: Four-wheel-drive knuckles are used in both solid-axle and IFS applications. They are mounted on inner knuckles or A-arms via ball joints or kingpins. The knuckle holds the stub shaft, hub, and brake assembly and rotates on the ball joints (or kingpins) by tie rod movement. Knuckles can be made of aluminum, steel, or cast iron. In solid-axle drivetrains, there are aftermarket manufacturers making flat-top knuckles to accept high-steer arms to mount a crossover or high-steer steering system above the axletubes. Aftermarket knuckles are usually made much thicker and stronger than factory knuckles, so this may not be a bad alternative if you do not have a factory knuckle you can connect a steering arm to.

Hydraulic Ram: Using hydraulic rams to turn wheels back and forth is nothing new in the commercial industry and was even applied to a few vehicles in the '70s, but it hasn't really found its way into today's mainstream passenger vehicles. Some dirt jockeys swap their steering boxes and drag links for full hydraulic setups for their incredible strength and force, but they have largely been deemed too sensitive for street use. We have been in some fullsize trucks with full hydraulic steering at highway speeds without issue, but we have also been in some hydraulically steered rides that left us quivering in our seats. It is possible to successfully set up full hydraulic steering to work on the street when new, but after time the seals and O-rings wear and the hydraulic ram can become sloppy, prohibiting any reasonably safe street driving.

Ram Assist: To gain the advantages of hydraulic rams without inheriting the scary traits, some backyard wrenches started retrofitting their steering boxes with hydraulic ports so an assisting hydraulic ram could be used in conjunction with a steering box and drag link. Now companies like AGR, West Texas Off Road, Howe, ORU, and PSC have made kits to adapt ram assist to almost any 4x4 steering box out there. Ram assist greatly reduces driver effort and stress on the steering box when turning the tires, but it demands modified steering pumps and can lead to premature leaking and steering pump failure if done incorrectly.

Track Bar or Panhard Rod?: Don't worry about it, they're the same thing. It is used to laterally locate an axle and keep it from moving side to side under your 4x4 by attaching one side of the rod to the frame and the other side to the axle. Try to keep it the same angle and length as your drag link.

Typical 4wd Steering Setups

Crossover: Drag Link To Knuckle
A drag-link-to-knuckle type crossover steering is probably the most common aftermarket steering system setups. It's also one of the most ideal designs. The drag link drops from the pitman arm directly to the passenger-side knuckle, preventing any type of radial drag link and tie rod play due to ball joint movement. From the passenger knuckle, the tie rod provides steering control and movement to the driver-side knuckle.

Crossover: Drag Link To Tie Rod
This type of crossover steering is probably the most common factory-produced system. With this setup, the drag link angles down from the pitman arm and connects to the tie rod. The tie rod not only keeps the knuckles parallel, but also controls movement of both knuckles. The design inherently has a minor bit of play, even when brand new. Since tie rod ends are spherical and rotate for knuckle motion, the tie rod is thus allowed to twist a few degrees. With the drag link connected to the tie rod, the drag link will be pulled away from the pitman arm or pushed toward it as the tie rod rotates causing a minor bit of wandering.

Double Crossover
For various reasons, sometimes to eliminate feedback to the steering wheel, stress on the steering box, or to get around a protruding framerail or Panhard rod, double crossover steering is utilized with a bellcrank and a tie rod in between the bellcrank and the pitman arm. From the bellcrank the drag link drops to the tie rod or knuckle, but on the driver side while the bellcrank sits on the passenger-side framerail.

Y-Link
A common OEM solid-axle steering setup consists of a Y-link design where the tie rod ties into the drag link instead of providing a solid link between the two knuckles. Here the drag link drops from the pitman arm to the passenger-side knuckle, and the tie rod picks up straight off the drag link and follows to the driver-side knuckle. This system has one negative aspect: as the drag link and tie rod "flatten out" during suspension compression, the tires will point (or toe) out. As the suspension droops, the tires are toed in. With a stock suspension, the limited amount of travel makes this change in toe negligible.

(IFS) Recirculating Ball And Tie Rod
In an independent front-end setup that uses a steering box, the steering has a centerlink in between the steering box's pitman arm and an idler arm. Two tie rods drop from the centerlink to the knuckles, and control knuckle movement. This is an OEM design that is still utilized today in vehicle manufacturing because of its strength advantages over the newer and more advanced system of a rack-and-pinion.

(IFS) Rack-And-Pinion
Rack-and-pinion steering is a more modern independent-suspension steering design and is slowly making its way into independent-suspension 4x4s. The rack-and-pinion setup has a pinion joined to the steering shaft and its gears meet up to a rack, or geared track that moves from side to side as the steering wheel is turned. Two tie rods leave the rack and end at the steering knuckles, controlling wheel movement.

Steering Geometry

Casteris the angle at which the steering pivot axis is tilted (forward or rearward) from true vertical (to the ground's plane). 4x4s usually have a positive caster, where the pivot axis is tilted rearward (with the upper ball joint or kingpin being farther back than the lower one). Caster can change when you change the lengths of control arms, do a shackle reversal, or change the size of shackles (tilting leaf springs one way or another and therefore rotating the axle). Having too much positive caster on a truck will give it some funny driving characteristics and make the tires feel like they are turning under the truck, not to mention increasing steering effort. Not enough positive caster, and it will feel like the truck is wandering. Caster also helps keep the wheels returning to center and tracking straight when moving forward and no force is applied to the steering system (think about how a shopping cart keeps its front wheels straight when being pushed).

Camber is the angle of the wheel from side to side. Camber isn't really adjustable on solid-axle 4x4s, and if you have a change in it, you should be concerned because you probably have a bent axle. Having too much camber (positive or negative) can prematurely wear tires. Camber has a major effect on the handling of a vehicle because as a vehicle leans, so do its tires, so angling the tire one way or another will put either more or less tire contact patch on the ground when cornering. A tire with a small negative camber angle will have its maximum cornering force.

Scrub Radius (Kingpin Offset) is the distance between the two points derived from the intersection of the wheel centerline and the steering axis with the ground plane. This is also referred to as kingpin offset. The centerline of the wheel is taken at the center of the tire tread's contact patch. The steering axis is determined by the straight line plane passing through the upper and lower ball joints (or kingpins) and the U-joint. Positive scrub would have the wheel centerline outside of the steering axis. Zero scrub would have the two planes (wheel centerline and steering axis) meet exactly at ground level. And negative scrub would set the wheel centerline inboard of the steering axis. The scrub radius is affected by camber, the kingpin angle, wheel offset, and tire diameter.

Kingpin Inclination (KPI, or Steering-Axis Inclination) describes how much the steering knuckles are angled in, putting the top of the kingpin (or ball joint) inboard of the lower one. This is known as kingpin inclination, or steering-axis inclination if you have ball joints and not kingpins. It is a directional control angle measured (in degrees) from the centerline of the kingpin (or ball joint) to true vertical. KPI is not adjustable (a bigger tire or different wheel offset will not change the angle of inclination) and has a direct relationship with a vehicle's camber as one main purpose of it is to reduce the need for excessive camber. Have you ever noticed the outside tire will almost seem to raise the vehicle up in a turn, while the inside seems to drop it down a little? This isn't your imagination: The KPI (or SAI) is designed to move the knuckle down from its highest point when turned to the inside or leading wheel. The outside knuckle will angle the opposite way, raising the knuckle a little. This will in effect raise one side and lower the other side of your vehicle in a turn, putting more opposing force against the weight transfer of your vehicle in a turn.

Ackerman Angle In the 1800s, Rudolph Ackerman figured out a solution that corrected the problem of steering arms that kept perfectly parallel throughout a turn. Parallel steering arms do not work well during a turn since the inside tire wants to travel a smaller circumference arc than the outside tire. This produces an enormous force that causes the wheels to either scrub or "buck" as one or another wheel breaks traction. Ackerman came up with a steering arm design that allows the inside tire to turn at a greater angle than the outside tire. This steering arm angularity is directly related to the distance from the center of your rear differential to the front axle. Ideally you would want to draw a straight-lined "V" from the rear axle's centersection, through the kingpins or ball joints, directly to the tie-rod end mounting points. Obviously, not every vehicle designed has a perfect Ackerman angle (for example, the same steering and solid-axle setup is used on Blazers and Suburbans, which obviously have different wheelbases).

Toe In/Toe Out are basic opposites that have inverse effects. When the leading edge of the tires are angled towards each other (no matter how slight it is), the wheels are said to be toed in. If the leading edges of the tires are pointed away from each other, then the wheels are toed out. Too much toe either way isn't good and will cause tire scrub and wear; wear on the outside of the tire if toed in, and wear on the inside if toed out. Why would you purposely set the wheel other than perfectly straight ahead? Well, on most 4x4s you probably won't, but in ideal street conditions, having toe in will make the vehicle track in a straight line, and toeing out slightly will help the vehicle be pulled with the leading tire into a turn during the deflection of the suspension.

Crossover Drag-link Angle With crossover steering, the drag link (at the knuckle) travels in an arc as the suspension cycles. As the drag link becomes parallel to the ground, it reaches its longest parallel distance (to the ground) between the knuckle and whatever the drag link is pivoting off of. This pushes the wheels to the right when the drag link becomes closer to parallel (with the ground), and pulls them to the left when moving away from being parallel (when the suspension droops and drag-link angle becomes greater). Ideally the drag link should be close to parallel with the ground when the truck is stationary. This will guarantee the least amount of distance change in between the vertical plane of the knuckle and the vertical plane of the steering box as the suspension cycles, therefore limiting the amount of knuckle movement and consequently the amount of bumpsteer.

Drag Links and Leaf Springs In a leaf-sprung front end the axle travels up and down and also backwards or forwards as the spring compresses, elongates, and moves towards the pivoting point (the shackle); the axle does not move from side to side (ideally). Since the drag link moves in a side-to-side arc with suspension cycling, it is unlikely one could rid one's vehicle of all bumpsteer with a crossover steering and leaf-spring setup. The best one could do is have the drag link parallel with the ground when the vehicle is at rest. Actually, the "best" one could do would be to put a track bar on the axle that will mimic the arc of the drag link as the suspension cycles. This would almost completely eliminate bumpsteer as explained below in Drag Links and Five-Links. A track bar can also offer a noticeable improvement in handling and stability to 4x4s with very soft front leaves. Sometimes leaf springs don't do a great job of giving lateral support and the front axle has the ability to wander back and forth as forces are put on it (including steering input and road conditions).

Drag Links and Five-Links (Four-Links With a Panhard Rod, Also Pertains to Radius Arm Design) If you have a linked front axle, it is almost certainly a five-link (including the Panhard rod) and not a true four-link. This is for a few reasons, including the sheer difficulty of getting two triangulated upper links not to hit the oil pan. And without having triangulated links, there is a need to laterally locate the axle via a track bar. Luckily this is beneficial with the type of steering that most solid-axle 4x4s run. Much the same as the drag link becomes longest (side to side) when parallel with the ground, the track bar does as well. On that same note, a similar arc is followed with the track bar as the suspension cycles and the track bar pushes the axle slightly from side to side. This can be very good, or very, very bad: if the track bar and drag link follow the same arc, you will have none or almost no bumpsteer, as the track bar's movement of the axle will compensate for the drag link's movement of the knuckle. The easiest way to make this occur is to use a track bar of the same length as the drag link, set at the same angle as the drag link. If you have a track bar that does not follow a similar arc as the drag link, your bumpsteer could be horrendous. The greater the differences in the traveled arcs (of the track bar and drag link), the greater the bumpsteer. Radius-arm design front suspensions use a track bar and two links fixed to the axle. This suspension design will be affected much the same as a five-link would, with the major difference being that an axle can rotate and change caster as suspension cycles, but not on the two fixed radius arms.

Drag Links and Five-Links, but Not Four-Links? To drop off on a tangent for a second, you might have asked yourself why you can't have a four-link with two triangulated links instead of having a four-link with a Panhard rod (truly a five-link, commonly referred to as front four-link) connecting your front axle. Besides the clearance issues of the triangulated links and the oil pan, a triangulated four-link moves the axle straight up and down as the suspension cycles, unlike a five-link (with a track bar) that will move the axle slightly sideways, compensating for the drag link's movement. And since the drag link is still moving the knuckle slightly from side to side with cycling, it will exhibit qualities of bumpsteer. Now, there are always exceptions, and we have seen a front triangulated four-link work well by using a cantilever arm on the axle to feed two tie rods. The cantilever was controlled by a drag link that went from the front to the back of the truck, perpendicular to a normal crossover drag link. This was accomplished by moving the steering box under the floorpan and using a series of pulleys to control it.

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