Torque Arms and Torque Rods for Commercial Trucks
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Torque Arms and Torque Rods

Torque Arms and Torque Rods

What is a Torque rod?

A torque rod can also be called a radius rod, radius arm, torque arm, or dog bone. Torque rods are suspension links that are meant to limit the movement of the axles without limiting suspension articulation. Torque arm suspensions are very common on medium and heavy-duty trucks and trailers.

Torque rods can be longitudinal (running forward and aft), or they can be transverse (running side to side). On truck drive axles the torque rods will keep the axle centered in the frame and control the driveline angles by managing the torque that is running through the driveline and the axles. On trailers, the torque arms will keep the axles centered and aligned to prevent dog-tracking and tire wear. Torque rods are often overlooked, but they play an important role in the longevity of suspension components and tires. They work by resisting change along the length of the assembly. The forces generated by driving down the road will pull or push on the torque rods, and they are designed to bear these stresses instead of other suspension components. Bushings in each end dampen the stresses and allow the torque rod to be mounted to the vehicle.

This video is an in-depth overview on the identification of torque rods and bushings, as well as how to measure them. Hpw to Measure and Identify Torque Rods and Bushings

Torque rod components

Torque rods are simple and have only a few parts. They will have two ends connected by a shaft, and each end will contain a bushing. The shafts are typically made from steel or aluminum with a solid or hollow construction. Most torque rods will be straight, but on certain suspensions, they may be purposely curved to allow clearance to suspension components. It is important to know how the design of the suspension that you are working on before deeming a torque rod damaged by being bent. The bend may be engineered into the torque rod for clearance purposes.

Torque Rod Shafts

Round – The round torque rod shaft is the most common type found, and the shaft will resemble a pipe. The diameter of the pipe can range from 1-3/4” up to 3-1/2”. These may be solid or hollow.

round torque rod

Round Adjustable – Adjustable type torque rods will have threads at each end that allow you to adjust the length of the rod. Both ends will be threaded oppositely so the central tube section turns; both ends will thread in and out evenly just as a tie rod tube is threaded for alignments. Lock bolts will be used on both ends to secure the ends when you are finished adjusting. Adjustable torque rods are used when performing alignments on different types of truck and trailer suspensions. It is common for one side of the suspension to use solid non-adjustable rods, and the other side will use an adjustable rod.

adjustable torque rod

Cast / I-Beam – Cast type torque rods will be solid and resemble an I-beam in shape. The casting process will leave them with a rough surface.

i-beam torque rod

Torque arm bushings

The bushings located in the ends of the torque arms are the only wear areas of the torque arm. As the vehicle moves, the bushings are absorbing the road shock to prevent damage to other suspension components. Over time the material in the bushings will wear out. When the material wears the torque arms will start to allow excess movement of the suspension components, and this may show itself as tire wear or dog tracking. In severe cases, it can cause universal joint wear or even driveline damage. Torque rod bushings should be inspected for wear and excess movement during preventative maintenance. Worn bushings or damaged torque arms should be immediately replaced when issues are found.

Torque Rod Bushing Mounting Types

Various suspension designs will require mounting types that work correctly with the elements of the suspension design. The three main bushing types used in torque rods are straddle, tapered, and sleeve. These will all attach to various torque rod mounts, and it is important to know how to identify these different types to ensure the correct replacement. This is critical whether you are replacing the entire torque arm or just the bushings! The information below will help with identifying different bushing types for replacement, but on all bushings types, you should measure the width of the bushing and the outer diameter of the bushing where it is pressed into the arm.

If the steel on the torque rod is rolled around the bushing and you are unable to measure the OD, this is a high-confinement bushing, and the entire torque arm will need to be replaced.

Straddle Mount Bushing – Accepts two bolts. The bolts will pass through holes in both ears of the bushing and will secure to the suspension or the axle and then to the torque rod mount on the frame of the vehicle. When identifying straddle mount bushings, it is important to note the diameter of the bolts used and the spacing between the two mounting holes.

straddle mount torque rod bushing

Tapered Bushing – features a taper like a tie rod end. The taper will fit into a mount with an opposing taper, usually used on a differential housing. A nut will secure the taper in place. When identifying replacements, it is important to note the major and minor diameters of the taper, the taper length, and the total length of the assembly.

torque rod tapered bushing

Sleeve - A single bolt will pass through a steel sleeve in each bushing. It is important to note the diameter of the bolt being used when replacing the sleeve type.

torque rod sleeve style bushing
Torque Rod Bushing Types

Rubber – Rubber has been the standard material used in most suspension bushings for decades. Rubber is very well suited to minimize vibration and shock, and it is very resistant to damage from torsional loads. Rubber is the most cost-effective bushing material.

torque rod rubber bushing

Polyurethane - Polyurethane bushings are typically stiffer and may transfer more vibration through the material, but the urethane material is extremely resistant to breaking down from oils and heat. Urethane has a very high tear strength, and it is resistant to UV damage and dry rotting. Polyurethane bushings will carry a slightly higher price than rubber but will outlast them in the long run.

polyurethane bushing for torque rods

Full Ball / Sealed type – Full ball torque rod ends are sealed with grease and are not serviceable. The ball and socket ends are built into the torque rod and they are not replaceable. Full ball ends allow unrestricted suspension articulation, but they will allow almost no “give” in length which holds the alignment of the suspension to a tight tolerance. Sealed type is the toughest torque rod and the recommended use is on severe service applications like garbage trucks and tow trucks. The grease in the ends is the only shock absorbing material, so they transfer the most vibration and shock to the suspension. Sealed type torque rods are the costliest.

full ball sealed torque rod bushing

Captured / High Confinement – On some torque rods ends the steel may be rolled around the bushing. The rolled steel confines the material and prevents the bushing from sliding out of the torque rod end. High confinement bushings may be rubber or urethane, and this style will typically be used on suspensions that place a high amount of lateral force on the torque rod. These bushings are not replaceable, and the entire torque arm assembly will need to be replaced.

captured type bushing

Torque arm bushing replacement

We recommended that a shop perform the replacement of torque rod bushings. Replacing the bushings requires a large hydraulic press and correctly sized press dies. Patience and a lot of practice aide in replacing torque rod bushings and incorrectly installing bushings can easily destroy them. In some cases replacement may require a special torque rod bushing tool.

Symptoms of A Bad Borque Rod Bushing

Worn torque rod bushings can cause several issues with the suspension, and if torque rod bushings have enough wear, it may even cause damage to the suspension or driveline components. As the suspension is no longer secured in a fixed position, it may present some strange symptoms.

The main issues that the vehicle will present are:

  • Tire wear
  • Dog Tracking (Trailers)
  • Pull (Trucks)
  • Clunking under acceleration and braking
  • Wandering
polyurethane torque rod bushing

Measuring a torque rod for replacement

torque rod shaft diameter

Shaft Diameter

width & outer bushing diameter

Width & Outer Bushing Diameter

torque rod bushing angles

Bushing Angles

Measure:
  1. Center to Center – The length of a torque rod is measured from the centers of each bushing. Torque rod length is a very important measurement to maintain the vehicle alignment.
  2. Bushing Side A – Start with the width and the outer diameter and then measure the other bushing features depending on the type.
  3. Bushing Side B – Not all bushings will be the same on both sides. Even if it looks the same, verify the measurements!
  4. Bushing Angles – Straddle type bushings may have the bolt holes in line with the torque rod shaft, or they may be set on an angle. Bushing angle is important for mounting as most bushings will not be able to rotate after they are installed in the torque rod and attempting to rotate them may damage them.
  5. Shaft Diameter – The diameter of the shaft may be important if clearance is an issue.

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